# National Aeronautics and Space Administration

## Library of Past Questions

### The Solar System

 We no longer answer questions on this topic, to concentrate our limited resources on questions on space based astrophysics and closely related areas (see this page for more explanation). Check out the resource links. These are the sites we used to point to in answering your questions. Browse through the library of questions below. We may have already answered your questions. Also try our Ask an Astrophysicist Search Engine!.

The Sun

### How many times would the Earth fit into the sun? (Submitted May 18, 1997)

The Question

This is not high priority but would you please tell me how many times the Earth would fit inside the sun?

Your question was forwarded from StarChild to our Ask a NASA Scientist service on Imagine the Universe!

There are two ways to think about the size of the earth with respect to the sun.

First, the Sun's diameter is about 100 times that of the Earth. So, you'd have to line up 100 Earth's end-to-end to stretch across the face of the sun.

The second way to think about it is your question: how many Earth's would fit inside the sun. Imagine the you had a big, round, fish-bowl and a bunch of marbles. Now imagine that the diameter of the fish bowl was 100x the diameter of the each marble. That way, 100 marbles would stretch end to end across the fish bowl. Now, how many marbles will fit in the fish bowl? This is like asking your question about the Earth and the Sun.

As it turns out, we can stretch the marbles across the fish bowl in three directions, up/down, in/out and left/right, and everywhere in between. So,to fill the bowl we would need 100x100x100 = 1,000,000 marbles. Or, about 1,000,000 Earths would fit inside the sun.

Jonathan Keohane and Jim Lochner

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Question ID: 970518a

### Does the Sun rotate? (Submitted January 08, 1997)

The Question

Does the Sun rotate? Are we seeing the same face of the Sun all the time?

Yes, the Sun does rotate. We can observe this by observing sunspots. All sunspots move across the face of the Sun. This motion is part of the general rotation of the Sun on its axis. Observations also indicate that the Sun does not rotate as a solid body, but it spins differentially. That means that it rotates faster at the equator of the Sun and slower at its poles. (The gas giants Jupiter and Saturn also have differential rotation.) The movements of the sunspots indicate that the Sun rotates once every 27 days at the equator, but only once in 31 days at the poles.

for Imagine the Universe!

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Question ID: 970108a

### What manages to keep the Sun so hot all of the time? Could it burn out? (Submitted November 07, 1996)

The Question

Is it possible that the Sun could just burn out? What manages to keep the Sun so hot all of the time?

The Sun is basically a thermonuclear bomb with a built-in thermostat. Just as in a hydrogen bomb, hydrogen atoms are fusing together to make helium atoms and this nuclear reaction produces heat (along with the light that we see). If the reactions go on too fast, the Sun expands slightly (just like a balloon expands when you heat up the air in it). This slows down the reactions and then the Sun cools and contracts. If it contracts too much, the nuclear reactions speed up, and then the Sun heats up and expands again. So the Sun stays at the same temperature, burning its nuclear fuel at a steady rate. At the rate it is going, we have about 4 billion years left until the Sun burns out

Andy Ptak and the Ask an Astrophysicist team

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Question ID: 961107a2

### Is the sunspot cycle and Jupiter's orbital period related? (Submitted January 27, 1998)

The Question

I am probably way out of my league here, as I am barely qualified to call myself an amateur astronomer. But I was contemplating the Sunspot cycle and was wondering what the odds were that it could be a product of Jupiter's orbit? Would appreciate any comments you might have on this.

This is a really excellent question. The sunspot cycle is caused by the flip of the solar magnetic field approximately every 11 years (close to the 11.86 year period of Jupiter's orbit). The exact reasons why the Sun's field flips are not known, but it has it's basis in irregularities in the plasma dynamo at the core of the Sun that generates the magnetic field. It seems plausible that tidal effects from Jupiter are one of the perturbations that cause these irregularities, but not the only one. The earth's magnetic field flips as well, although not as regularly as the Sun's, and lunar tidal effects probably contribute. For more information on the solar cycle, you can look at:

Eric Christian

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Question ID: 980127d

### Are there any satellites currently studying the sun? (Submitted November 14, 1997)

The Question

In regards to the Solar Maximum mission, I was wondering if there have been any similar missions lately. If not was there a mission or something that was started in 1980 during February and was continued later and in the present. Or if they were not continued, was there anything similar?

Solar Max was launched in Feb. 1980, and continued operating until it reentered in December 1989 (except for a few months in 1983-1984, until it was repaired by Space Shuttle astronauts). http://heasarc.gsfc.nasa.gov/docs/heasarc/missions/solarmax.html

Two space observatories looking at the Sun right now are SOHO and Yohkoh http://sohowww.nascom.nasa.gov/

http://solar.physics.montana.edu/sxt/ (one of several Yohkoh instruments)

SOHO is an ESA/NASA collaboration, Yohkoh is Japanese.

The latest pictures of the Sun from these and other solar observatories are at http://umbra.nascom.nasa.gov/images/latest.html

David Palmer

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Question ID: 971114c

### What would happen to the Earth if the Sun was at Pluto's distance? (Submitted January 06, 1997)

The Question

What would happen to the Earth if the Sun was by Pluto?

We will assume for the sake of argument that you mean if the Sun were farther away, say at the distance of Pluto, which is about 40 times farther away from the Earth as the Sun is. At such a distance, the Sun would appear as little more than the brightest star in the sky, similar in size and brightness to the way Venus looks in our night sky.

The big difference (apart from needing lamps to read by all day) is the fact that since the Sun would be 40 times farther away, the Earth would receive MUCH less energy, which drops off as the square of the distance. This is called an inverse-square law. An object twice as far away from the Sun receives only 1/(2*2)=1/4 as much energy. For an object 40 times farther away, the energy is 1/(40*40)=1/1600. This reduced energy would have severe effects on Earth. It would lead to a drastic cooling in the atmosphere, lowering the temperature down to about -228 to -238 C. (Water freezes at 0 C.) At this new temperature, all the nitrogen in our atmosphere would turn into a liquid, while all the water would be solid.

Since the Earth is naturally radioactive, it has a good store of energy which would still be released, mostly through the radioactive decay of Uranium. This heat might lead to the development of vents on the frigid surface.

This could possibly lead to the development of life at the vents on the ocean floor where the ice in the oceans would be melted into water. Life as we know it on the surface of the planet would not be possible. (We got some information for this answer from the web pages at the University of Arizona: http://seds.lpl.arizona.edu/nineplanets/nineplanets/pluto.html)

Cheers,
for Imagine the Universe!

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Question ID: 970106c

Planetary Motions

### How does the Sun just stay in one spot and how do all of the other planets stay on course/track? (Submitted November 07, 1996)

The Question

How does the Sun just stay in one spot and how do all of the other planets stay on course/track ?

The planets in our solar system stay on track because of the force of gravity. This is identical the same force that holds us on the Earth. Imagine shooting a rocket--the faster you fire the rocket, the farther it goes. If you fire it fast enough, it will escape the Earth's gravity and go into outer space. You could also fire the rocket into an orbit around the Earth by propelling it fast enough that it doesn't hit the ground, but not fast enough to escape the Earth's gravity. This is what NASA does when it launches the space shuttle into an orbit. In the same way, all of the planets, along with other things like comets, are in an orbit around the Sun. Just like the Sun's gravity keeps the planets into a stable orbit, the planets exert an equal force on the Sun (gravity is a two-way street), but the Sun is so massive that it hardly budges.

Andy Ptak and the Ask an Astrophysicist team

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Question ID: 961107a1

### Why aren't the orbits of the planets circular? (Submitted January 04, 1998)

The Question

I want to know why aren't the planets' orbits circular?

Many ancient astronomers thought that the orbits of the planets should be circular, because it's simple and it seemed very natural to them.

But when Newton discovered the laws of motion and gravity, the natural answer turned out to be elliptical orbit. A circular orbit is allowed, but requires a very special set of conditions. A circular orbit occurs when the kinetic energy of an orbiting body is exactly equal to half of its negative potential energy, and its direction of travel is exactly perpendicular to the direction from the orbiter to the thing orbited. The only way to get an exact circular orbit is by carefully fiddling with the parameters, e.g. by firing rockets. Nature does not usually fiddle, so natural orbits will all be elliptical to some extent.

Best wishes,

David Palmer and Koji Mukai

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Question ID: 980104a

### How can astronomers determine the Astronomical Unit (average Sun-Earth distance)? (Submitted January 22, 1998)

The Question

I want to learn how to calculate the astronomical unit. How can I calculate it? I don't know formula and I want to learn.

The Astronomical Unit is the average distance between the Sun and Earth. Its value is 149,597,870 km (about 93 million miles).

There are a variety of ways to measure it, but the most accurate is to fly spacecraft to various planets. Kepler's laws relate the period of a planet's orbit (in years) to the average distance of the planet to the Sun in (astronomical units).

Using these laws, you can determine the distance from Earth to a planet in astronomical units. By sending radio signals (which travel at the speed of light) to a spacecraft, and measuring how long it takes for the spacecraft to return those signals, you can determine how far away a spacecraft is in kilometers. Thus a spacecraft in orbit around another planet has its distance known in both kilometers and astronomical units, and you can just divide one by the other to get the number of kilometers in an astronomical unit.

David Palmer

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Question ID: 980122b

### Did Venus and Earth ever swap orbits? (Submitted March 09, 1997)

The Question

Could Venus and Earth have, at one time, switched orbits?

No, it is not really a possibility that Venus and Earth have ever switched orbits. There are some folks who have proposed outlandish ideas about Venus actually being a part of Jupiter that was blown off somehow, wandered around in the solar system for awhile (sometimes near Earth), and then settled down into its present orbit. This is not supported by scientific evidence, however.

Venus is sometimes regarded as Earth's sister planet. In some ways they are very similar, e.g. they are roughly the same size, their densities and chemical compositions are similar, they both have relatively young surfaces. Because of these similarities, it was thought that below its dense clouds Venus might be very Earth-like and might even have life. But, unfortunately, more detailed study of Venus reveals that in many important ways it is radically different from Earth. You can learn more about this at http://seds.lpl.arizona.edu/billa/tnp/venus.html

Regards,
Laura Whitlock
for the Ask an Astrophysicist Team

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Question ID: 970309b

### Since Neptune and Pluto's orbits cross, will they eventually collide? (Submitted March 26, 1997)

The Question

Pluto's orbit crosses that of Neptune's, on what date will they eventually collide?

Pluto "crossed" Neptune's orbit on January 21, 1979, and temporarily became the 8th planet from the sun. It will cross Neptune's orbit again on Feb. 11, 1999 to resume its place as the ninth planet from the Sun for the next 228 years.

Despite the fact that Pluto and Neptune temporarily change places in their distance from the Sun, they will never collide. This is due to two reasons: First, Pluto's orbit is inclined to the ecliptic. by 17 degrees. (To see an illustration of this, take a look at http://nineplanets.org/overview.html.) So even though we say their orbits "cross", Pluto is actually quite a distance "above"Neptune. Secondly, Pluto orbits the Sun twice for every three orbits of Neptune. The two planets are said to be in a "resonance orbit". For such orbits, the two bodies never get close to each other. In fact, the closest the two planets come to each other is 2 billion kilometers.

Jim Lochner & Karen Smale

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Question ID: 970326c

### Why is the orbit of Pluto so different than the other planets? (Submitted October 27, 1998)

The Question

Recently, in physics class we learned that the orbit of Pluto was not co-planar to the other eight planets, in fact it was 17 degrees off! I want to know why this is? If you would be so kind as to answer my question and e-mail me back I would be much obliged.

The other important thing about Pluto's orbit is the 3:2 resonance between Pluto and Neptune; that is, in the time it takes Pluto to orbit around the Sun twice, Neptune will have orbited the Sun three times. It is clear from this that there is a special relationship between the orbits of Neptune and Pluto, and also probably the origins of Neptune and Pluto.

Starting in 1992, astronomers (such as Dr. David Jewitt at University of Hawaii) have began to discover a large number of small objects in the outer Solar System, the so-called Kuiper Belt. It turns out that a surprising number of these are also in 3:2 resonance with Neptune. These are nicknamed "Plutinos", or little Plutos:

Many Plutinos share properties of the high eccentricities and significant inclinations with Pluto. It is therefore natural to associate the presence of so many Plutinos with their interesting orbital properties.

According to one theory (which Dr. Jewitt calls 'as good as any, and better than most'), Neptune migrated outwards during the accretion stage of solar system formation. During this process, Neptune trapped many planetesimals in the 3:2 resonance and carried them outwards with it. Although these planetesimals were originally in more or less circular orbits on the same plane as the planets, their eccentricities and inclinations increased as they were pushed outwards.

Clearly, this is not a settled question, and more theoretical and observational works are necessary.

Here are a selection of other web sites related to this question:
http://www.seds.org/nineplanets/nineplanets/plutodyn.html
http://www.seds.org/nineplanets/nineplanets/pluto.html
http://www.boulder.swri.edu/~buie/pluto/pluto.html

Best wishes,

Koji Mukai & Maggie Masetti

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Question ID: 981027c

### How unique is the retrograde rotation of Venus? (Submitted October 26, 1998)

The Question

Why is the rotation of Venus retrograde, or East to West - unlike nearby planets?

We got two good answers to this question from our distribution of scientists. Here they are:

"The standard answer to this question and things like Neptune's tilt is that there was a large collision early in the planetary formation process. The models of planetary and solar system formation have the angular momentums of the planets and their orbits in the same direction as the initial angular momentum of the gas cloud. You need something like a collision to get anything else." - Eric Christian

"Of the nine planets, a bare majority (Earth, Mars, Jupiter, Saturn, and Neptune) rotate in a way we consider 'normal'. Mercury and Venus are slow, Venus, Uranus, and Pluto are retrograde, Uranus and Pluto are highly inclined. Mars' inclination varies chaotically over long (billion-year) time scales, so it is not always 'normal' either. It is only parochialism that makes us point and laugh at the zany antics of the other planets.

"How a planet rotates is related to how it was formed from the accretion of planetesimals. If more impacts occur on one side than the other, then it will tend to rotate accordingly. But the impacts are largely random. Tidal effects can also change the rotation." -David Palmer

Maggie Masetti & Koji Mukai

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Question ID: 981026a

### When will the next alignment of the planets be? (Submitted December 07, 1996)

The Question

Perhaps you could settle a bet between a friend and me. I would like to know if there has ever been a time when all planets in our solar system were aligned ? Also isn't that supposed to be one of the highlight of the next century when it does happen? My friend at work says it happened recently, as in, in my lifetime. We are 25 and 26. If I remember right it's supposed to be May 5, 2005. Do you know when it will happen? Any response would be much appreciated.

Given that the solar system has been around for several billion years, the odds are that the planets have been reasonably well lined up any number of times. There are a number of things to keep in mind, however. First, there is a difference between having the planets aligned with respect to the Sun or as viewed from Earth. If the planets are lined up from the Sun, they also appear lined up from the Earth. If they are lined up from the Earth they need not be lined up from the Sun. Second, there is the question of just how well aligned they are. Perfect alignment is hard. All just being in the sky at the same time is much easier. Anyway, there is software available for you to check it out if you need a more exact answer.

• http://www.fourmilab.ch/cgi-bin/uncgi/Solar. This page will give you a map of the solar system for any date you input. (Eg., in June of 1988, all the planets except Jupiter were on the same side of the Sun, and, excepting Pluto, formed kind of a line, as well)
Cheers,
Steve Snowden, Jonathan Keohane and Gail Rohrbach
for Imagine the Universe!

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Question ID: 961207a

### Can an alignment of the planets affect the Earth? (Submitted October 29, 1996)

The Question

Is it possible that on May 5, 2000 the alignment of the planets can have enough gravitational effect on the earth to cause it to tip slightly causing mass destruction.

We are not solar system astronomers. So we don't know which, if any, planets align on May 5, 2000. But we know for sure that what the other planets in our solar system do cannot have any profound effects on the motion of the Earth. Let us explain.

Long ago Sir Isaac Newton gave us a mathematical description of how one object affects, and is affected by, the gravitational force of another object. Many, many years of observations have proven this description to be accurate (at least for masses like those of the planets). Newton's Law of Gravitation states: The force between any two objects having masses M1 and M2 separated by a distance R is an attraction along the line joining the objects and has a magnitude

F = (G x M1 x M2) / (R x R)

G is the universal gravitational constant which has a value of 6.6732 x 10-11 newton-meters2/kg2 for all pairs of objects. A "newton" is a unit of force that physicists use. It is defined to be the amount of force needed to accelerate a 1 kg mass at 1 meter per sec2. The important thing to remember here is that a newton, as a unit of force, is fairly small... like a millimeter is a small unit of distance or a microsecond is a small unit of time.

So let's examine the pull of the planets on the Earth.

We know the following:

PlanetPlanet MassMinimum Distance from Earth
(Earth Masses)(106 km)
Mercury 0.054991
Venus 0.807 41
Mars 0.106 79
Jupiter 314.5 629
Saturn 94.1 1,277
Uranus 14.4 2,720
Neptune 16.7 4,346
Pluto 0.00218 5,751

The Earth has a mass of about 6 x 1024 kg.

So... let's hypothesize that we can miraculously put all the planets at their minimum distances from Earth, all in a straight line in one direction -- so their gravitational forces add. This, of course, can never actually occur, but it will give us the maximum possible gravitational pull of the other planets on the Earth. So let's pretend that we can do this. What do we get?

If you put all the numbers in Newton's law (and make the units compatible), you get that the maximum force that all the other planets can exert on the Earth is roughly 3 x 1018 newtons. What sort of result will this have on the Earth... well, we use Newton's First Law which says that if a force F is applied to a mass M, the mass is accelerated by a value A. (This is the famous equation F=ma). A force of 3 x 1018 newtons acting on the Earth causes the Earth to accelerate by 5 x 10-7 meters per second per second. In other words, the planets in our solar system -- aligned or not -- cannot cause a shift in the movement of the Earth which will lead to mass destruction.

To compare, let us look at the gravitational force that the Sun exerts on the Earth. The mass of the Sun is about 329,400 times that of the Earth. They are separated on average by approximately 149,000,000 km. Thus, the Sun exerts a force of ~3.5 x 1022 newtons on the Earth. So you see, that the other planets in our solar system don't matter at all compared to the Sun!

To be entirely correct, we must tell you that the laws of physics involved actually show that the Moon is the one of the most important objects to the Earth... gravitationally speaking. It is the Moon which is responsible for things like ocean tides and such. All the other planets in our solar system added together do not have as large a gravitational effect on the Earth as the Moon does.

Here is a table of tidal forces of the Sun, Moon, and Planets. With the Sun's tidal force equal to 1.00, the following values are given in Thompson (1981):

 Moon 2.21 Sun 1 Venus 0.000113 Jupiter 1.31e-05 Mars 2.3e-06 Mercury 7e-07 Saturn 5e-07 Uranus 1e-09 Neptune 2e-09 Pluto 1e-13

A last thought: alignments of various planets in the solar system occur all the time. It seems that every time one comes along, doom-sayers arise to cry that this is the end of life on Earth as we know it. The laws of physics, however, cannot be denied in this Universe of ours. If life on this Earth ends, it will not be because of planets aligning in our solar system.

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Question ID: 961029b

### Do the moons of other planets have synchronous rotation? (Submitted February 18, 1998)

The Question

The Earth's moon orbits the earth with one side always facing the Earth. Do other moons orbit their respective planets in the same manner?

Most of the satellites in the solar system rotate synchronously like our moon (see http://www.seds.org/nineplanets/nineplanets/luna.html). An example of one that doesn't is Saturn's moon Hyperion. Its rotation is actually chaotic. You can find out more about it at http://www.seds.org/nineplanets/nineplanets/hyperion.html

, which is a table of orbital and rotation periods, among other things.

Damian Audley and John Cannizzo

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Question ID: 980218b

### What keeps planetary rings in place? (Submitted October 27, 1998)

The Question

What keeps planetary rings in place?

You can think of rings as a bunch of tiny moons, all in a similar orbit around the planet. Each particle is in free-fall, like the space shuttle in orbit --- so one could ask why would a ring not stay there forever instead.

They don't because they collide with each other, and are subject to forces other than the gravity of its parent planet (gravity of the bigger moons, magnetic field etc.). Such collisions and extra forces tend to spread rings out, and at the lowest altitude, friction with the tenuous outer atmosphere will cause particles to drop onto the planet.

Sometimes planetary rings are kept in place by the gravitational force of shepherd moons. Saturn has a very intricate ring system with lots of moons helping to keep its rings together. According to http://ringmaster.arc.nasa.gov/neptune/neptune.html, Neptune's rings are probably confined by one of its moons. In the case of Jupiter's rings, particles in them probably don't stay there for long (due to atmospheric and magnetic drag). Galileo (the spacecraft) found clear evidence that the rings are continuously resupplied by dust formed by micrometeor impacts on the four inner moons, which are very energetic because of Jupiter's large gravitational field. The inner halo ring is broadened by interactions with Jupiter's magnetic field. (from http://www.seds.org/nineplanets/nineplanets/jupiter.html) Uranus, like the other gaseous giant planets in our solar system, also has a ring system.

And more about the planets here: http://www.seds.org/nineplanets/nineplanets/

Hope this helps!

Maggie Masetti & Koji Mukai

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Question ID: 981027a

Solar System as a Whole

### When were all the planets discovered? (Submitted May 07, 1997)

The Question

When were all of the planets discovered?

Mercury, Venus, Mars, Jupiter, and Saturn have been known for a very long time...but no one knows when they were first seen. Being such bright objects in the sky, they have been known and observed by ancient civilizations.

Uranus was first recorded in 1690, but thought to be a star. William Herschel discovered it as a planet in 1781.

Neptune was discovered in 1846....by James Challis (who recorded it first, but did not realize what it was) and Johann Galle (who knew exactly what he was observing).

Pluto was discovered by Clyde Tombaugh in 1930.

Laura Whitlock and Tim Kallman
for the Ask an Astrophysicist team

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Question ID: 970507c

### How large is the solar system? (Submitted March 17, 1998)

The Question

How large is the Solar System?

The size of the solar system is about 40 AU (40 times the distance between the earth and the sun), which is the orbit of Pluto. One AU equals 1.5 x 10^11 meters (150,000,000,000). But the gas from the Sun (the solar wind) goes out about five times that distance, and there are comets that are part of the solar system that extend even further.

Eric Christian

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Question ID: 980317a

### Why are the densest planets closest to the sun? (Submitted May 20, 1998)

The Question

Why are the densest planets closest to the Sun?

It is because of the temperature gradient that was in the solar nebula, from which the planets formed. The inner (hotter) parts could only condense out the silicates, forming the terrestrial planets. The cooler part further out could condense out the more volatile compounds, forming the Jovian (or gas) planets.

Best wishes,

Koji Mukai & Karen Smale

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Question ID: 980520a

### Why do some planets have more moons than others? (Submitted October 26, 1998)

The Question

Why do some planets have more moons than others?

1) The giant gas planets have many more moons than the terrestrial planets (like Mercury, Venus, Earth, and Mars) - this is because of their larger mass, which means they have a correspondingly greater gravitational field. Whether moons are captured by the gravity of a large planet or formed in place when our solar system was created, larger planets have the *potential* to have more simply because their gravitational "reach" allows them to "control" more space and hold more mass around them.

Your question, a very good one, by the way, is one that has no single correct answer - it is one that is being actively researched now!

Hope this helps!

Gail Rohrbach, Maggie Masetti & Koji Mukai

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Question ID: 981026b

### Why aren't the gas giant planets stars? (Submitted March 10, 1998)

The Question

My first grade son is doing a project on Jupiter. We have been able to find the answers to most of his questions using the internet or reference books, however, I can't explain to him why Jupiter and the other gas giant planets are not stars. Is there a difference in the way they were formed? Will they eventually become stars? Is it possible to explain this to a first grader?

You have asked an excellent question!

A "gas giant" would only be considered a "star" if it generated its own energy through nuclear reactions in its core and Jupiter is just not massive enough to do this. Jupiter would have to be about 50 times more massive than it is to undergo nuclear reactions, and thus be a star and not just a planet.

Steve Bloom

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Question ID: 980310a

### What are some substances not found on Earth, but found on other planets or comets? (Submitted April 08, 1997)

The Question

What are some substances not found on earth but are found on other planets, or comets?

I.
If you are asking about Elements or Isotopes that are found in space, but not found naturally on Earth, there are many: In the center of stars and after supernova explosions many isotopes and elements are created. Some of these are radioactive, which means that they are unstable and will turn into other elements over time. Now, all the material that made the Earth and solar system (except hydrogen and helium) was created in stars and supernovae. However, that was a long time ago, so the radioactive elements have mostly already turned into stable elements.

When we observe supernovae, we see the signature of these elements decaying. So, we know they exist out there -- but not here.

II.
Now, if you are asking about chemical compounds in our solar system that are found on other planets but not here, that is another question. Some substances are abundant on some planets but scarce on others. For example, water is abundant on Earth, but scarce on most other planets.

Your question, on the other hand, is about stuff that does not exist naturally here at all, but does exist naturally off of earth. To answer this, I asked a colleague here at NASA/Goddard Space Flight Center who is a chemist (Dr. Susan Hallenbeck).

She writes:

"Some gases exist in the hydrogen rich environments of the gas giants (Jupiter, Saturn, Uranus and Neptune) such as silane (SiH4), arsine (AsH3), and phosphine (PH3) which do not occur naturally on Earth but can be easily produced in the laboratory."

"Also, there are some highly oxidized compounds in the top layers of the Martian soil, which again do not occur naturally on Earth but can be easily produced in the laboratory."

Jonathan Keohane
-- for Imagine the Universe!

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Question ID: 970408b

### Can you describe two or more aspects of the solar system that are not common knowledge? (Submitted June 09, 1997)

The Question

Can you describe two or more aspects of the solar system that are not common knowledge?

Depending on how you define 'commonly known' there are a lot of interesting things about the solar system that fit into this category. Did you know, for example, that the rotation of the planet Mars has been found to be chaotic over the lifetime of the Solar System? What about the fact that the tidal interaction between the Earth and the Moon is causing the Moon to slowly spiral farther away from Earth? Long ago, the Moon was much closer to the Earth, and looked larger in the sky. Did you know that the Sun contains 98% of the entire mass of the Solar System? Did you know that the planet Jupiter's atmosphere contains a Great Red Spot: a storm system that has remained remarkably stable since the first astronomers observed it with a telescope in the early 17th century?

Regards,

for the Ask an Astrophysicist Team

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Question ID: 970609b

### Where can I find Internet information on Planet X? (Submitted August 05, 1997)

The Question

I am having trouble finding information regarding Planet X. Could you please advise of an Internet site or an e-mail address that I could inquire regarding this fascinating subject.

I can understand why you had troubles finding Internet sites about Planet X ! It seems to be a popular name for a number of non-astronomy topics (comic books among them !).

I did eventually find, however, a very interesting article at

by Paul Schlyter. The section entitled "Planet X" discusses at some length the search for a trans-Neptunian planet, the discovery of Pluto, and the subsequent continued search for a tenth planet. It discusses the use of Halley's Comet, Voyager 2, Pioneer 10 and Pioneer 11 to search for any deflections from unknown gravity sources. The article concludes that although a trans-Plutonian planet has never been found (and likely doesn't exist), the search has instead yielded a new class of asteroids at the outer reaches of the solar system.

I hope this helps.

Jim Lochner

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Question ID: 970805a

### Will Planet X end life on Earth in 2012? (Submitted February 9, 2009)

The Question

I recently read on an organization's website about a newly found planet called "Planet X" and how it will end life on Earth in December of 2012. The group is called "The Institute for Human Continuity," and they are supposedly dedicated to seeing that human life is continued and that the people who survive can repopulate the earth. They have some sort of lottery entry that people can enter in "to ensure their survival" after this massive disaster (as if money is really going to matter then). You can find this info at www.instituteforhumancontinuity.org. Could you please tell me if their story has any merit and if it's possible that our exsistence on Earth could end on that date? Thanks!

Ashlee

Hi Ashlee-

Thanks for your question. To be blunt, this is a complete hoax designed to get people's money. There have been several "Planet X"'s in science fiction over the years, but the only so-named object with any astronomical merit was a hypothetical planet with more mass than Pluto long thought to orbit the sun somewhere beyond Neptune. The search for such a planet has recently come to a fruitless conclusions, given that its "missing mass" is no longer deemed necessary to account for the orbital motions of the planets we do recognize:

The site you mention appears to be a part of an advertising campaign for the upcoming Sony Pictures movie "2012" starring John Cusak, and indeed is linked from the official home page of the movie:
http://www.sonypictures.com/movies/2012/

So the "Institute..." site is as trustworthy as the plot of, say, "Independence Day." It remains to be seen if "2012" is as entertaining.

Many serious scholors of the Mayan civilization are very annoyed about all these bogus predictions:
http://www.cnn.com/2009/TECH/science/01/27/2012.maya.calendar.theories/index.html

Hope this helps,
Laura & Koji
for the "Ask an Astrophysicist" team

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Question ID: 090209a

### What will happen to the Solar System when the Sun evolves? (Submitted February 18, 1998)

The Question

I was wondering what would happen to our solar system when the Sun goes into the next phase in its life cycle.

You ask a very intriguing question that scientists still discuss.

First, have you read the information in our site about the Sun and the evolution of stars.

If you look at those topics on the above page, you will find a lot of information about what may happen in the VERY distant future. The Sun should burn normally (though gradually getting hotter) for the next 5 billion years or so, which means most elected officials won't even call for studies of the problem for about the next 4.99 billion years or so.

contains an archive of past questions, some of which also ask about the life cycles of stars. You may find some interesting information there.

I may also point out that human beings (Homo Sapiens) as a species are only about 1 Million years old. According to present theory the Sun should use up its Hydrogen fuel, and start to evolve into a red giant star in about 5 Billion years. In such a long period of time it seems strange to even consider humans as being around. Some other species, possibly directly descended from human beings, would have evolved to best adapt to the environment where they are living.

There was an Astrophysical Journal (ApJ) paper a few years back which dealt with the future evolution of the Sun (1993, ApJ, 418, 457). You may be able to find this at a large library, or a college library. It is also available via the Internet at:

They write:

..... The Sun eventually reaches a luminosity of 2300 L(sun) and a radius of 170 R(sun) on the RGB, shedding 0.275 M(sun) and engulfing the planet Mercury. ... the Sun climbs the AGB, encountering four thermal pulses .... the Sun reaches its largest extent at 0.99 AU ... However, at this point the Sun's mass has been reduced to 0.591 M(sun) and the orbits of Venus and Earth have moved out to 1.22 and 1.69 AU, respectively - they both escape being engulfed. ...

As the Sun sheds mass, the gravitational attraction that the planets feel will decrease and their orbits will get larger.

Mars will definitely become more comfortable (Of course that is a relative term, for me comfortable is about 20 degrees F, with snow falling at a rate of 12 inches per hour) but it will be warmer. To actually guess as actual conditions is pretty tough since predicting the exact weather 1 week in advance is still pretty hard here on earth where we have a lot of information.

Hope this helps,
Mike Arida and John Cannizzo
for the Ask an Astrophysicist team

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Question ID: 980218c

### What can you tell me about the Nemesis Star that supposedly had a hand in the extinction of the dinosaurs? (Submitted November 30, 1996)

The Question

Do you have any new information on the Nemesis Star,the so called companion star to our Sun ?

To recap the story of Nemesis (see, e.g., 1990 October issue of Scientific American): in 1984, Raup & Sepkoski claimed that mass extinctions, like the one that killed the dinosaurs 65 million years ago, occurred every 32 million years. Since the favored theory for the demise of dinosaurs is an asteroid or cometary impact, the periodicity would suggests some mechanism to disturb the comets in the Oort cloud every 32 million years. Richard Muller and others hypothesized that a faint companion star, nicknamed Nemesis, that orbits the Sun every 32 million years, could explain this.

However, many geologists are not convinced that mass extinctions are periodic, so they see no need for such a star. Nevertheless, Muller and colleagues have embarked on the difficult search for a possible, dim companion to the Sun. The most recent report I could find on this was a conference paper from 1994 (Carlson et al 1994 in "New Developments Regarding the KT Event and Other Catastrophes in Earth History", Houston Univ., p19-20). Here are some sentences off the abstract: "Unfortunately, standard four-color photometry does not distinguish between red dwarfs and giants. ... Every star of the correct spectral type and magnitude must be scrutinized. ... We are currently scrutinizing 3098 fields, which we believe contain all possible red dwarf candidates in the northern hemisphere. ... The software is now completed and we are eliminating stars every clear night." I presume the search is still on-going but have not yielded a positive detection.

A good description and more references can be found at: http://www.nineplanets.org/hypo.html#nemesis

You may also want to check out the article in the 1990 October issue of Scientific American

Koji Mukai and Eric Christian
with help from Drs. Chen, Loewenstein and Snowden
for Imagine the Universe!

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Question ID: 961130b1

Individual Planets

### When can I best view Mercury? (Submitted August 05, 1997)

The Question

A co-worker and myself have been talking about Mercury.I have a computer program that will show me the position in the sky, but I've yet to see it. When I looked for the position in the sky for him some time ago, I thought that it appeared that Mercury rose near the time of sunset and set some 45 minutes later. During our conversation, he said, "But Mercury is always near the Sun and should therefore be in the sky at approximately the same time." This makes sense to me, but I was still wondering about my previous suggestion. I speculated that perhaps at some time of the year Mercury would be behind the Sun, and at some day would peek out from behind, and if this occurred at or near sunset, Mercury would pop above the horizon for a brief period before setting again later. Is this possible? Or is there any other explanation that would have Mercury come above the horizon then drop below after a brief time?

I own binoculars and a f=1000mm D=114mm Newtonian telescope... will I ever be able to see Mercury with these or my naked eye?

Your co-worker is right. Mercury, the innermost planet in our solar system, is never far away from the Sun as seen from Earth. The maximum separation is about 28 degrees. This is about 60 times the apparent size of the Sun, so it is rare for Mercury to be actually hidden behind the Sun. It is, however, very commonplace for Mercury to be hidden by the glare of the Sun (and of the daytime sky) --- it impossible to see Mercury unless it is more than about 10 degrees away from the Sun.

If your computer program shows (or can be set to show) which stars and planets are up in the sky during the day, you should almost always be able to find mercury very close to the Sun.

Once every 4 months or so, there is a period when Mercury can be seen shortly after sunset in the western sky; there is another period during which Mercury can be seen in the eastern sky shortly before sunrise. Your program should be able to help you figure out exactly when. At these times, Mercury is bright enough to be seen with your naked eyes, no binoculars or telescopes are necessary.

Best wishes,
Koji Mukai

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Question ID: 970805c

### Who discovered Venus, and when? (Submitted December 05, 1996)

The Question

Well, I've searched the net quite a bit and was hoping you could help me out in some manner or way. I am trying to find out who discovered Venus and when it was discovered...and how it was discovered, whether telescope, naked eye or such. I do hope that you may be able to help me out in some way and would appreciate any information you might have in this subject. Thank you very much.

It depends on what you mean by discover. Because Venus is so bright, the first caveman who went outside at dusk or dawn surely noticed it. The ancient Greeks knew enough about it to know that it was different from other "stars". Mercury, Mars, Jupiter, and Saturn were also known to the ancients. This probably includes among many others the Chinese, Babylonian, and early American cultures which were relatively sophisticated.

Galileo in 1610 was first to observe that Venus had a visible disk and that it had phases like the moon so perhaps he could be considered to have discovered the modern Venus. Captain James Cook made observations of a "Transit of Venus" in 1776-1779.

Steve Snowden
for Imagine the Universe!

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Question ID: 961205a

### When are the next transits of Venus? (Submitted December 20, 1996)

The Question

I have heard that Venus will make a rare transit across the face of the sun. My question would be, is there anyone who could tell me exactly when that will occur? I don't need the exact hour or even day. Month and year would be very helpful.

When transits of Venus occur, they occur in pairs separated by 8 years. The last set of transits occurred in December 1874 and December 1882.

The next set of transits will occur on June 8, 2004, starting at approximately 1 AM, EST, and lasting until approximately 6 AM. (That is, on the East Coast it will be just ending at sunrise.) Another transit will occur on June 5, 2012, starting at approximately 5 PM EST and lasting until about midnight. So the East Coast will see the beginning of that one. The precise start times and duration of the transit depend upon one's location on the earth.

I seem to recall that a recent issue of Sky and Telescope magazine had a nice article about the upcoming transits of both Venus and Mercury.

Jim Lochner
Imagine the Universe!

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Question ID: 961220

### Do you have information about the oppositions of Mars? (Submitted February 17, 1997)

The Question

Hi. My friend and I are working on a small project about the oppositions of Mars and we had a few questions. We calculated the synodic period, and we were wondering why some intervals are longer than others? Also, we know that favorable oppositions of Mars occur in August, but why is this so? Is the month of unfavorable oppositions in March? And last of all: Mars will be at opposition on March 17, 1997. On March 21, the Sun is viewed from the earth in the direction of the Vernal Equinox. What would the Mars-Earth separation be (the distance) in March 1997, and would this be a favorable opposition? We hope you can help us! Thanks for your time.

I'm not sure exactly why some opposition intervals are longer than others. I can tell you why some oppositions are "favorable" and some are "unfavorable", and why the favorable ones occur in August. It is likely that the same reason also explains why the opposition intervals differ.

The orbit of Mars has a larger eccentricity than the Earth's. So its maximum and minimum distances from the Sun varies much more than the Earth. Hence, the distance between the Earth's orbit and Mars' orbit varies as well. Picture now the two orbits in space (say, looking down on the solar system). The locations where the minimum and maximum distances between the orbits occur are fixed in space. Earth arrives at where the two orbits are closest sometime in mid-August through mid-September. Of course, Mars usually isn't there. But when it is, there is a favorable opposition of Mars. The unfavorable oppositions occur on the opposite side of the orbit, hence in mid-Feb or mid-March. The fact that this is near the time of the Vernal equinox is unrelated.

The Mars-Earth Distance on March 17, 1997 will be 98 million kilometers (or 0.66 Astronomical Units). During favorable oppositions, the distance is 57 million kilometers. The next favorable opposition will occur in Aug, 2003.

To "see" the oppositions take place, try using an orrerary or a desktop planetarium for your computer (or possibly on the web). You might also be able to figure out why the intervals between oppositions differ.) For more on the oppositions of Mars, I'll also suggest you take a look at Sky and Telescope magazine. They usually have an article or two a few months in advance of an opposition.

Or you could take a look at "Solar System Live" at:

This is an electronic orrerary you can play with to see the planets in their orbits. Using the "Inner system" display mode, it shows the orbits of Earth and Mars quite nicely.

Jim Lochner
for Imagine the Universe!

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Question ID: 970217a

### Is there water on Mars? (Submitted November 10, 1997)

The Question

Is there any water on Mars?

There is water on Mars. More specifically, there is ice in and on the ground (especially at the polar caps) and water vapor in the atmosphere. There may be liquid water underground, but liquid water on the surface is probably very rare and only temporary, requiring weather conditions (temperatures above freezing, high humidity, and high barometric atmospheric pressure) which never occur in most places on Mars, and occur only rarely in others.

It was not always so. In earlier times, the air pressure on Mars was higher, and you can still see the river beds and other features that water carved out.

David Palmer

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Question ID: 971110c

### Does Mars have precipitation? (Submitted April 03, 1998)

The Question

Reading about Mars, I noticed that it has .03% of water vapor in it's atmosphere, and my question was if it has water vapor why doesn't it have precipitation?

Mars does have precipitation.

A volume at a given temperature can hold a certain amount of water vapor. (The amount of water vapor the volume can hold is almost independent of any other gases the volume also holds, so if you are looking at the amount of water in the atmosphere, you can ignore the atmosphere and look only at the water.) The colder it is, the less water it can hold. The ratio of the amount of water it does hold to the amount of water it CAN hold is called the Relative Humidity (R.H.). When the volume is cooled so that the R.H. would be greater than 100%, the water vapor in the volume turns into solid or liquid water, depending on whether the temperature is above the freezing point.

At around -75 Celsius, a volume can hold 0.03% of 7 millibars = 2 microbars worth of water, which is about the typical content of Mars's atmosphere. (It is more complicated than that because the amount of water vapor and atmospheric pressure vary a lot from place to place, just like on Earth). At about this temperature, the relative humidity reaches 100%. So when it gets cold, Mars has precipitation.

However, this precipitation most likely takes the form of frost, rather than rain or snow. The ground is likely to be colder than the air (especially on cold clear nights), and so air hitting the ground cools and the water freezes to the ground as frost. Viking II (a Mars lander in the 1970's) saw frost on the ground some mornings.

A part of the polar ice caps of Mars is made of precipitated water ice (the rest is made of carbon dioxide as 'dry ice').

David Palmer and Kevin Boyce

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Question ID: 980403c

### If all of the planets are named after Roman gods, why is it that the moons of Mars (the Roman god of war) are Deimos and Phobos (the sons of the Greek God of war)? (Submitted May 20, 1997)

The Question

If all of the planets are named after Roman gods, why is it that the moons of Mars (the Roman god of war) are Deimos and Phobos when those are the sons of the Greek god of war Ares?

Aseph Hall, the discoverer of the moons of Mars in 1877, named them: see, for example

The names of the planets came down to us from the Romans, i.e. from Western Civilizations significant roots in the Roman Empire. However, the names of the satellites were chosen by their discoverers, and while they often chose to stay within the mythic traditions, they were at a bit more liberty to choose names (including some taken from English literature). Galileo was the first to discover and name satellites orbiting another planet, using one of the early telescopes. While observing Jupiter, he found 4 moons of the big planet, which he named Io, Europa, Ganymede, and Callisto. These were all names from Greek myths associated with Zeus, or Jupiter to the Romans.

Since these people were Greeks, it is reasonable to use the Greek names. When the Romans adopted the Greek myths, they would have changed their names, but most of their best stories involved Greeks.

Jim Lochner, Gail Rohrbach, Koji Mukai and David Palmer

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Question ID: 970520d

### Is the 'face on Mars' really only just a hill? (Submitted April 07, 1998)

The Question

From recent publications in the mass media I understand that the face of Mars seems to appear as nothing more than "some hill". From listening to alternative media I understand that the recent pictures were taken in poor resolution. My question is why did NASA use poor resolution when it is capable of taking better pictures?

That is a very good question, and unfortunately it doesn't get the detailed attention it may deserve in the popular or alternative press.

The recent scan of the area containing the "face on Mars" is of a much higher resolution than any taken before. The Viking 1 image had a resolution of 43 meters/pixel, while the Mars Global Surveyor resolution was 4.3 meters/pixel, a factor of 10 increase.

I would look at: http://antwrp.gsfc.nasa.gov/apod/ap980407.html

for good links comparing the two images.

Since the Global Surveyor hasn't reached its final orbit yet, which will be closer to the surface than the altitude at which it took these images, the resolution will increase a bit at that time, not because of the instrumentation as much as the decreased distance to that point. In other words, the 1998 April pictures were taken at the best resolution possible at the moment. When the Global Surveyor reaches its final orbit in 1999 March, it will start the detailed mapping of all Martian surfaces in unprecedented details. The Cydonia region will be photographed again at an even higher resolution in the course of this mapping phase.

It is also important to remember that the first released images were not "poor resolution" but RAW data. It was basically a data dump --- no cleaning, no processing, just formatting into a TIFF file. This allows the so-inclined to perform their own analyses and draw their own supportable or insupportable conclusions. JPL released its cleaned images about 5 hours later. This was an attempt to show that NASA is not hiding any information from the public on this issue.

Now you may ask why the CLEANED data looks so much different than the RAW data. Data cleaning is a bit like tuning your television to remove the static and make sure the picture is the right color and centered on the screen. Every image has some signal and much noise, and cleaning helps remove the noise so the signal (the accurate picture of what is there) stands out more clearly.

The JPL web site listed above gives a "recipe" for what steps were taken to refine and clean the image - you can perform these same steps using a personal computer and some commercially available software. Try it, and see what you think.

To answer the other part of your question, I believe that the alternative media have a very big financial stake in keeping any controversy they can in the public consciousness, just like the mass media have their own agenda. It is often important to keep this in mind when reading any second (or third) hand accounting of a situation. One of us (Mike) found after living in Africa for 2 years that any popular news report he saw on the situation over there was so oversimplified as to be erroneous on many specific issues.

NASA (and the American taxpayers) did not send this mission to Mars to look at the 'face' but to map out the entire surface, to gather geologic information, and in preparation for future missions to the planet. However, as any public agency, it tries to be accountable to the people who pay for it, and hence attempts to answer questions that are important to the American people.

If you have further questions on the Mars Global Surveyor mission please check out the JPL web pages at: http://mars.jpl.nasa.gov/mgs/

Hope this helps,

Mike Arida, Mark Kowitt, Pat Tyler, Sandy Antunes, and Gail Rohrbach

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Question ID: 980407b

### Is the Jupiter referred to in the New Testament the planet or the Roman god? (Submitted October 27, 1997)

The Question

I am an adult Sunday School teacher and I am getting ready to do a lesson on the passage in the bible of Acts 19:35. Regarding the idol worship of Diana. The passage references the name of Jupiter. Is this the same Jupiter in the solar system? Also the book of Acts is dated between A.D. 61 and 63. Does this mean that Jupiter was known about even then? I am curious to hear your response. Please email whatever information you have regarding the planet Jupiter and the first recorded discovery of it.

Jupiter was a Roman god, and the planet was named after him. Perhaps the verse in the Bible doesn't refer to the planet, but to the god.

The planet was named after the deity, who was the leader of the Roman pantheon, which may be more relevant. A careful reading, and perhaps comparison to the original text and other translations, may be allow you to distinguish between the god and the gas giant in this context. Jupiter the god is mentioned in 2 Macabees 6:2, and earlier in Acts (14:13). (KJV Bible searches are available at http://www.hti.umich.edu/relig/kjv/ )

The history, or the discovery, of the planet Jupiter dates so far back in antiquity that we can't trace it. All of the naked eye planets were undoubtedly familiar to the earliest civilizations. The details of the passage you refer to is well beyond our expertise. However, there can be little doubt that the planet Jupiter was known long before the time the passage was written.

I hope this helps!

Tim Kallman, Karen Smale, and David Palmer
for the Ask an Astrophysicist team

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Question ID: 971027a

### Can you walk on Jupiter? What causes its radioactivity? (Submitted August 21, 1996)

The Question

I understand Jupiter is not dense enough to walk around on, and that it has a deadly radioactivity about it. That doesn't seem reasonable to me, because it's too far from the Sun. How do you account for this? Is the radioactivity stable, or does it fluctuate? And what is its Source? Just curious. Thanks a lot. I NEVER heretofore have had any interest in Astronomy--that is, until I had a couple of sons to talk with. Now Mom has fun also.

It is true that you cannot walk on Jupiter. This is because the atmosphere is very deep, perhaps comprising the whole planet (which would mean there is no solid surface to walk on!). It is composed mainly of hydrogen and helium, with small amounts of methane, ammonia, water vapor and other compounds. At great depths within Jupiter, the pressure is so great that the hydrogen becomes metallic.

This metallic hydrogen core generates a huge magnetic field around Jupiter. It's like the magnetic field of Earth, but Jupiter's field is 10 times stronger. This magnetic field traps high energy particles --- the particles come from the Sun, from outside the Solar system, and from the volcanoes on Io, one of Jupiter's many moons. The trapped particles then creates the radiation belt. This is similar to the Van Allen belts around Earth, but can be up to 10,000 times more intense. These levels of radiation near Io are so high that a human would absorb a lethal dose in just minutes.

It's not quite correct to call this "radioactivity", which is different from radiation. Radiation can refer to a lot of things: sunlight, broadcast signal from a radio station, microwaves in your microwave oven, ... Some are dangerous, some are not. Radioactivity refers to the properties of certain atoms and subatomic particles that spontaneously decay and emit particular types of radiation (gamma-rays, alpha particles etc.) that are often dangerous and sometimes deadly.

We hope you and your sons will continue to enjoy astronomy!

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Question ID: 960821b

### How big is Jupiter's Great Red Spot? (Submitted April 19, 1998)

The Question

How big is Jupiter's red spot?

The GRS is an oval about 12,000 by 25,000 km, big enough to hold two Earths.

Tim Kallman

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Question ID: 980419a

### How long will the Great Red Spot on Jupiter last? (Submitted April 20, 1997)

The Question

How long will the Great Red Spot on Jupiter last?

The Great Red Spot on the face of Jupiter has been in place since the early 1600's when telescopes were first used to observe surface features on the Sun, Moon and planets, thus is it very stable. The dynamics of the Jovian atmosphere, and all turbulent systems, are quite complicated, and no one can say for sure how long a structure such as the Great Red Spot will last. However, it has been a very active area of fluid dynamics research lately. The spot is what is known as a "coherent vortex structure". It has undergone only minor variations in size and color. The vortex is a type of cyclone in Jupiter's upper atmosphere, and the wind speeds within it can be up to 360 km/hour. The clouds are rotating counter-clockwise in the southern hemisphere, indicating it is an area of of high pressure with respect to the clouds around it. The vortex is maintained by forcing from the opposite shearing motion from the cloud bands on the sides of the vortex and from the very fast rotation of Jupiter, once every 9.6 hours. For more details on the dynamics of the Great Red Spot, see http://www.jpl.nasa.gov/galileo/ganymede/092396.html. The short answer to your question is that it is not known how long the spot will persist.

Neptune also has large spots on its surface, but they are not as long-lived as Jupiter's Great Red Spot.

Regards,

for the "Ask an Astrophysicist" Team

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Question ID: 970420e

### What is the average thickness of Saturn's rings? (Submitted November 11, 1997)

The Question

What is the average thickness of the rings of Saturn?

According to http://starchild.gsfc.nasa.gov/docs/StarChild/solar_system_level2/saturn.html
>The thickness of the rings ranges from 10 to 100 meters and the rings vary >in brightness.

According to http://www.seds.org/billa/tnp/saturn.html

```>   Though they look continuous from the Earth, the rings are actually
>composed of innumerable small particles each in an independent orbit.
>They range in size from a centimeter or so to several meters. A few
>kilometer-sized objects are also likely.
>
>   Saturn's rings are extraordinarily thin: though they're 250,000 km or
>more in diameter they're no more than 1.5 kilometers thick. Despite
>their impressive appearance, there's really very little material in the
>rings -- if the rings were compressed into a single body it would be no
>more than 100 km across.
```

The 300 km number is probably from before space probes such as Voyager visited the planet.

David Palmer

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Question ID: 971111a

### Do scientists still believe that diamonds can be found at the core of Uranus? (Submitted April 16, 1997)

The Question

Do scientists still believe diamonds may be "found" at center of Uranus?

The theoretical consideration of a diamond at the core of the gas giant planets of the solar system has been around for over a decade.

One of the first papers I can find is:

Title: High pressure cosmochemistry applied to major planetary interiors: Experimental studies
Authors: NICOL, M. F.; JOHNSON, M.; KOUMVAKALIS, A. S.
Affiliation: California Univ., Los Angeles.
Journal: Status Report California Univ., Los Angeles. Dept. of Chemistry and Biochemistry.
Publication Date: 11/1984

though a coworker and I both recall a 1982(?) paper entitled "Lucy in the Sky with Diamonds" which I couldn't find a reference to.

I believe that the possibility was first applied to Jupiter, but later extended to all the outer gas giants of the solar system.

Most of the chemical research involving diamonds in the solar system (outside of the Earth) deals with microdiamonds found in asteroids (and hence meteors and meteorites). These are created in the high heat and pressure of the collisions which create these rocky fragments.

Galileo, a recent NASA mission to the planet Jupiter, fired a small probe into Jupiter's atmosphere. The probe was never meant to survive long enough to reach any possible solid core, so it couldn't really provide any proof of the theories that diamonds might exist at the center of planets like Jupiter. Still, you might want to check out their homepage at:

Regards,
Mike Arida
for the Ask an Astrophysicist Team

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Question ID: 970416a

### Why are Uranus' moons named after Shakespearean characters rather than something related to Uranus? (Submitted April 30, 1997)

The Question

We is studying about the planets and their moons. Neptune's moons are named after sea creatures. Why are Uranus's moons named after Shakespearean characters rather than something related to Uranus?

John Herschel, son of William Herschel (who discovered Uranus), and William Lassell named the moons after characters from Shakespeare and from Alexander Pope's 'Rape of the Lock', according to http://wwwflag.wr.usgs.gov/USGSFlag/Space/nomen/append7.html

The planets and moons in our solar system have been named by astronomers (usually ,led by whoever discovered the particular object) suggesting the name -- sometimes they tried to mythologically match moon names to planet names, but there is no rule that this must be done. Names must be submitted to and approved by a "governing board" of scientists before they are accepted by the scientific community. Toward this end, you can have a look at:

Rule 5 has been invoked by the IAU when establishing a theme for naming features on newly discriminated satellites or planets. Thus, newly discovered Uranian satellites and features on previously discovered satellites continued the theme established by William Lassell when he named the first four satellites for characters (mostly bright and dark spirits) from Shakespeare and Pope; names for satellites of Neptune continue the "watery" theme established by the names of the planet and first two satellites.

You might also have a look at

Regards,
David Palmer, Damian Audley, and Laura Whitlock

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Question ID: 970430c

### Is Pluto a Planet or Not? (Submitted January 20, 1999)

The Question

I'm a High school Senior and have a fair amount of knowledge of the outer planets. At one point in my career planning, I wanted to be an field expert for You (NASA). As of this morning I heard on GOOD MORNING AMERICA that Pluto is no longer considered a planet, just a big ball of ice. Is this true, and if so what is Pluto's current status in regards to its classification?

There have been a lot of reports, such as http://news.bbc.co.uk/hi/english/sci/tech/newsid_259000/259767.stm about this subject, with varying degrees of accuracy.

The group who will decide the official status of Pluto for the professional astronomers world-wide (as they do all official questions related to objects in the Universe) is called the International Astronomical Union; in this particular case, IAU Division III (Planetary Systems Sciences) is taking the lead.

Pluto has been known as the ninth planet of our solar system since it was discovered by Clyde Tombaugh at Lowell Observatory in 1930. On the other hand, it has been clear for decades that Pluto does not fit in with the pattern of the other planets. Over the last few years, the accumulated information on Pluto and the discovery of an increasing number of other objects in the outer solar system with orbital characteristics very similar to those of Pluto have been discussed within the community of astronomers called "minor-planet researchers". The question of the official status of Pluto has recently come to the forefront because the orbits of some of these other objects are now sufficiently well determined that it is reasonable to begin including them in the catalog of orbits of what are now generically known as "Trans-Neptunian Objects" (TNOs).

IAU Division III has already recommended that Pluto be included as number 1 in a catalog of TNOs.

Does this mean that Pluto has been demoted? The answer is no. Pluto will have dual classification as a planet and a TNO, at least for the time being.

Currently, the definition of a planet (as opposed to an asteriod or a TNO) is rather arbitrary. If astronomers reach a consensus on what the defintion of a planet should be, then IAU may reclassify some Solar System objects. However, in the absense of such a consensus, the definition is historical and arbitrary; moreover, many people outside the professional astronomy community have an interest in this issue, as the media attention attests. "Until there is a consensus that one of the physical definitions is clearly the most useful approach in thinking about the solar system, the IAU will not 'demote' Pluto or 'promote' Ceres," says the IAU.

Brian Marsden, head of the IAU's Minor Planet Center, has also addes his voice, as quoted in a press release.

"There is no plan to 'downgrade' or 'demote' Pluto. It will stay as a planet."

Allie Hajian, John Cannizzo, Laura Whitlock
and the Ask an Astrophysicist team

Note added in January 2007: the above answer correctly describes the situation in 1999. However, the discovery in 2003 of the object Eris, which is almost certainly bigger than Pluto, prompted a new round of discussion among astronomers. In August 2006, the International Astronomical Union decided to re-classify Pluto as a "dwarf planet," a category separate from the 8 planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune). Not everybody is happy with this decision, however, so this may not be the end of the story.

You can also read about Eris, Pluto, and the definition of planets on the web pages of Eris's discoverer, Dr. Michael Brown:
http://www.gps.caltech.edu/~mbrown/

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Question ID: pluto

Asteroids and Comets

### Are there pictures of the Asteroid Belt? (Submitted April 25, 1997)

The Question

I want a picture of the asteroid belt!

The individual asteroids are small and do not reflect a lot of sunlight, so it is not possible to view them all at once... you have to point a telescope at one at a time. Also, the NASA satellite Galileo took some nice pictures of asteroids, see:
http://antwrp.gsfc.nasa.gov/apod/ap951020.html and http://antwrp.gsfc.nasa.gov/apod/ap950630.html
The latter picture shows an asteroid that has its own little moon.

More pictures and information on asteroids is available at: http://seds.lpl.arizona.edu/nineplanets/nineplanets/asteroids.html

Andy Ptak
for the Ask an Astrophysicist Team

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Question ID: 970425a

### Could the Asteroid Belt be a destroyed planet? (Submitted August 10, 1998)

The Question

I noticed that the orbits between Mars and Jupiter (where the asteroid belt is) looks as though there ought to be a planet there instead of the asteroid belt. Has it ever been considered that the asteroid belt was maybe a destroyed planet?

Thank you for your question! The fact that the asteroid belt has such a well-defined, high concentration of asteroids suggests two things. One, that they are fragments of a planet that broke-up long ago, or two, that they are rocks that never managed to accumulate into a genuine planet. Currently, scientists tend to favor the latter explanation. According to Eric Chaisson and Steve McMillan, the authors of the text book "Astronomy Today", 1993 edition, "There is far too little mass in the belt to constitute a planet, and the marked chemical differences between individual asteroids strongly suggest that the asteroids could not all have originated in a single planet. Instead, astronomers believe that the strong gravitational field of Jupiter continuously disturbs the motions of these chunks of primitive matter, nudging and pulling at them, thereby prohibiting them from aggregating into a planet. The existence and composition of the asteroid belt joins the general properties of the planets and their moons on our list of features that any theory of solar system formation must explain."

Here is a good web page about the asteroids: http://nineplanets.org/asteroids.html

Hope this helps!

Maggie Masetti & Koji Mukai

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Question ID: 980810a

### When was the earliest known sighting of a comet? (Submitted November 12, 1997)

The Question

When was the earliest known sighting of a comet made?

Unfortunately, none of us here know when the earliest surviving record of a comet observation was. Comets have existed for longer than life has been on Earth, so the first time one was seen by a human was shortly after the time of the first human.

Aristotle (b. 384 BC) wrote about comets in his _Meteorology_ (~350 BC). http://webatomics.com/Classics/Aristotle/meteorology.html However, this talks about comets in general, I haven't read the whole thing to see if he discusses any specific apparitions. According to Plutarch, a comet was seen in the sky after Julius Caesar's death in 44 BC (mentioned in Shakespeare) and Tacitus talks of a comet in the late days of Nero (still later). This demonstrates how a good search engine can be more useful than erudition. I knew only of Caesar's comet before using the search engine:
http://webatomics.com/Classics/Search/index.html

Seneca wrote about comets, apparently compiling a catalog. However, the oldest specific recorded comet we have found (on the Web at least) was the Chinese observation of Halley's comet in 240 BC http://cometography.com/pcomets/001p.html

To find the oldest recorded comet sighting, you should look for the oldest astronomical records. This can include mythology and other traditional literature, astronomical records from China, Egypt, Mesoamerica and other ancient cultures etc.

David Palmer

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Question ID: 971112e

### How can ice, which is a form of water and which has a finite vapor pressure, exist in the near vacuum of space? (Submitted May 25, 1997)

The Question

This question is about comets in space. How can ice, which is a form of water and which has a finite vapor pressure, exist in the near vacuum of space?

Comets are usually in the outermost regions of the Solar system (the Oort cloud), where it is extremely cold. Water ice can survive billions of years in the Oort cloud.

However, the comets we observe -- those that come into the inner Solar system --- do lose a lot of volatiles. This is a part of the process that creates the tails, the signature we associate with comets. Comets that are trapped in the inner Solar system will soon (astronomically speaking) exhaust all their volatiles and become extinct (i.e., rocks with no cometary activities).

Koji Mukai for

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Question ID: 970525

### What happens if a comet is hit by a solar flare? (Submitted April 14, 1997)

The Question

I would like to know what would happen if a comet was hit by the full force of a solar flare from the Sun?

Thanks for your question on comets and solar flares. A large solar flare hitting a comet like Hale-Bopp would certainly cause a noticeable brightening of the comet for a period of time. A solar flare contains high energy photons and particles, and is released from the Sun in a relatively short amount of time (a few minutes). The ion tail of comet Hale-Bopp, which is comprised largely of charged particles as opposed to the heavier dust particles making up the dust tail, could possibly show complex structure or discontinuities, depending on the structure of the magnetic field in and around the flare. There are some helpful figures of the solar wind interacting with a comet in the review of the magnetosphere which can be found at http://www-ssc.igpp.ucla.edu/ssc/tutorial/magnetosphere.html

If a comet got close enough to the Sun to pass "through" a solar flare, it would most likely be vaporized and wouldn't have fared much better even without a flare. Flares don't really reach out very far (in an astronomical sense) from the surface of the Sun. On the other hand, the particles that flares release stream outward to well beyond the orbit of the Earth. It's these particles that can cause the effects mentioned in the previous paragraph.

Another type of solar event, a coronal mass ejection, reaches far out into space and could also cause all of the effects mentioned above. They are generally associated with filament liftoffs. You may have seen videos of long tendrils of matter lifting off from the "edge" or limb of the Sun - these are filaments.

There are also some excellent animations from LASCO, one of the instruments on the ESA/NASA SOHO Sun observing satellite that show the dynamic behavior of the solar wind and, in one case, a sungrazing comet. These are located at http://sohowww.nascom.nasa.gov/data/realtime/realtime-c3.html

Regards,
for the Ask an Astrophysicist Team

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Question ID: 970414e

### Can you give me information about the instruments and measurements that are made of comet's tails? (Submitted May 18, 1997)

The Question

I am 34 years old and have recently completed Astronomy 101 in college. I am writing a novel in which scientists are studying a comet whose tail will cut across the orbital path of Earth. I wish to be accurate about the instruments and measurements to be used to determine our rate of velocity and possible chemical composition of this tail.

With recent detections of Comet Hyakutake in the x-rays, we high-energy astronomers are just beginning to become more deeply acquainted with the instrumentation needed to study comets. You can find an article about the x-rays detected from Hyakutake in

However, the real experts are the planetary scientists. So I'd recommend you take a look at the JPL home page:

Jim Lochner

P.S. One of my colleagues, Casey Lisse, has provided further information about instrumentation used to study comets and the results about the composition of the tail. Here's his reply. (We think he mis-remembered the name of the Japanese satellite that flew by Halley. It was Suisei. Another Japanese satellite that flew by Halley but didn't get close was Sakigake.)

I hope this is helpful to you.

Jim Lochner (with Japanese satellite help from Koji Mukai)

What a question! The range of instruments now used to study comets is huge. Comet astronomers are using everything from the VLA and BIMA in the radio to CSO and JCMT in the sub millimeter to the NASA/IRTF and UKIRT and KECK (and now HST NICMOS) in the infrared to HST and KECK and all the other optical telescopes in the world (including amateur small telescopes in the backyard), to HST and EUVE (IUE is dead) in the UV to ROSAT and ASCA and BeppoSAX and RXTE in the x-ray. Orbits are determined by multiple observations over weeks in the optical, typically, and solving Kepler's equations - we have two premier scientists doing this, Brian Marsden at Harvard-SAO and Don Yeomans at JPL. Occasionally we get an in situ flyby, like the ICE, Vega, Giotto, and Sukui flybys of P/Halley, and the upcoming ROSETTA, DS-1, and STARDUST missions to P/Wirtannen and P/Wild2.

Our understanding of the composition of comets is a long one, too - the short answer is that comets are bid dirty snowballs, very roughly ~30% dust (rock forming elements like silicates) and the rest, ~70%, volatile materials like water ice and methane ice and ammonia ice and carbon monoxide ice and carbon dioxide ice. Comets are usually a few km in radius (Hale-Bopp was VERY large), and some of the smallest bodies in the solar system, while also having some of the largest structures in the solar system, the dust/ plasma tails and dust trails.

The intro to my thesis, published 1992 , UMD, available on microfilm
The wonderful books "The Mystery of Comets" By Fred Whipple, and a
"History of Comets" by Gary Kronk
The web pages by Kronk and JPL for comets

Hope this helps!

Casey

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Question ID: 970518b

Life in Space (Solar System and Beyond)

### What are the remote indicators of life on Earth from space? (Submitted April 23, 1998)

The Question

Imagine you were an alien roaming around in Space and you were interested in finding a planet that has life. What is the evidence that Earth has human life? What are the unambiguous indications of life on Earth?

There are a combination of features of the Earth that you can detect from space that would be very hard to explain without invoking the presence of life. These are: the abundance of molecular oxygen; the presence of chlorophyll, the pigment used by plants in combining carbon dioxide and water to form sugar; and a trace amount of methane (which is hard to maintain with all that oxygen around).

It so happens that the Galileo spacecraft (see, for example http://www.jpl.nasa.gov/galileo/ ) reached its ultimate destination, Jupiter, via a circuitous route (for technical reasons), including two flybys of Earth. NASA attempted, and succeeded, in detecting the abovementioned signs of life on Earth during one of them.

(Also if you are talking about intelligent life forms, radio and TV broadcast signals would be a conclusive evidence.)

Of course, there is no guarantee that every planet with living beings will look like the Earth (it's hard to be sure when you have only one example!). In fact, Europa, a moon of Jupiter, seems to have a liquid ocean underneath its icy crust, and some scientists speculate that there could be life on Europa. In this case, the ice on the surface makes it difficult to prove or disprove this idea from a distance. (The JPL Galileo pages above have a lot of information about Europa.)

Best wishes,

Koji Mukai

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Question ID: 980423a

### Has evidence been found for life on Mars? (Submitted December 23, 1996)

The Question

Has evidence been found for life on Mars?

We are a center for research in high-energy astrophysics and have no institutional experience in the search for life in Mars (though back in 1976 I was a student intern with the Viking Project at JPL!). There is a lot of very good material available on the Internet concerning the recent discoveries within Martian meteorites of possible evidence for past life on Mars. A good starting point is:

• http://www.jpl.nasa.gov/snc/ which has links for each Martian meteorite, as well as to the various current Mars missions and many news stories concerning the recent discoveries.

Although each piece of evidence (morphological, mineralogical, chemical and isotopic) presented so far can be explained by non-biological as well as biological processes, their presence in close proximity to each other, and in two quite different Martian meteorites (the old ALH84001 and the much younger EETA79001) makes further investigation a clear priority. The two NASA spacecraft now on route to Mars - Mars Pathfinder and Mars Global Surveyor - will prepare the way for future sample return missions. In the meantime, a lot of work can be done using the Martian meteorites already available.

As of October, 1998, the evidence against microfossils in AH84001 is now even stronger, with the discovery that some lunar meteorites have objects similar to the 'microfossils' found in the Martian meteorite.

Details can be found off the JPL Mars Meteorite Home Page at: http://www.jpl.nasa.gov/snc/

Best wishes,
Paul Butterworth
Imagine the Universe!

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Question ID: 961223b

### What would an alien be like on Europa? (Submitted October 07, 1996)

The Question

What would alien life be like on Europa? I understand that the icy moon does have an atmosphere, though a very thin one, and that it contains oxygen obtained differently than on earth. Could this atmosphere support life? Could the dramatic temperature changes affect life (if there is) when Europa is blocked from the Sun because of Jupiter?

As I am sure you already know, Europa is one of Jupiter's moons that Galileo discovered using a small (and very crude by modern standards) telescope.

Remember, there is one place in the solar system that we know life exists now -- Earth. So, you might want to think about comparing Europa to Earth as much as possible.

Europa's atmosphere does contain oxygen but it is extremely thin so it is hard to see how it could support life. However, Europa might have oceans under its icy crust. These would be kept liquid by heat from tidal friction. These hypothetical oceans would be more hospitable to life than the surface. Since they would be below a layer of ice and their main heat source would be tidal friction, having the Sun blocked by Jupiter wouldn't make much difference to them.

• Is there an atmosphere? If so, what is it made of?
• What is the surface gravity? (on Earth it is 1"g" = 9.8 m/s/s = 32 ft/s/s )
• What is the temperature? How different is it from day to night?
• How long are the days?
• Does Jupiter block the Sun often?
• Is there volcanic activity? If so, how does this affect it?
• Are there more questions I should be asking?
• Where does it get its energy? (We get ours from eating and breathing and then "burning" the food, to be very simplistic.)
• How does it keep its body temperature about right? What is "about right" and why?
• How does it move around -- or does it not have to? How does it reproduce?

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Question ID: 961007

### Is there any other form of life out there? (Submitted November 19, 1996)

The Question

My question is what is really out there? Is there any form of life out there?

Your question is a good one and, since I am not a real expert in any of these subjects, I think the best way for you to learn more is if you look at some of the web pages which have answers. I can say that many scientists think it is possible that there is life elsewhere in the Universe, but that finding it may prove difficult. First of all, we don't know exactly what form life on other planets outside our solar system might take, so it is difficult to know how to search for it. Second, the universe is a big place, and radio signals (and space vehicles) travel too slowly to allow us to find life that is very far away. We do know something about the conditions on Mars, the planet in our solar system which is most likely to have life, and it turns out to be similar to the conditions in Antarctica (except for the lack of oxygen). So we can guess what forms of life might be able to survive there, and there are projects under way that will someday send machines to Mars which can look for these kinds of life. There is more information about the exploration of Mars, for example in

Also, the SETI ("Search for ExtraTerrestrial Intelligence") institute has some pages which may be useful:

I hope this is helpful to you!

Sincerely,
Tim Kallman
(for the Ask an Astrophysicist team)

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Question ID: 961119b

### What are the chances of life existing outside our solar system? (Submitted September 24, 1997)

The Question

What are the chances of life existing outside our solar system?

This is a question that astronomers first started to quantify in the early 1960s. In 1961, a radio astronomer named Frank Drake developed an equation to stimulate discussion of the Search for Extraterrestrial Intelligence (SETI). This equation, which is now called the Drake equation, states that the number of communicating civilizations in our galaxy likely depends on a number of factors which must combine to yield a habitable planet where life has the chance develop to a certain level of technological know-how. These factors include the rate of formation of stars like the Sun, the fraction of those with planets, the fraction of Earth-like planets, the fraction of such planets where life develops, the fraction of those where the life becomes intelligent, the fraction of intelligent species who can communicate in a way we would detect, and the lifetime of the communicating civilizations. As you may imagine, there is a lot of debate about reasonable values for most of these factors. As we learn more about the likelihood of planets around other stars, we are able to better estimate one of these parameters. For the other parameters, the estimates vary widely. Frank Drake's own current estimate puts the number of communicating civilizations in the galaxy at 10,000.

You can find out more about the Drake Equation from

Cheers,

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Question ID: 970924

### Where can I find information on extraterrestrial life? (Submitted March 17, 1997)

The Question

I'm an undergraduate student in Physics and I would like to have some information on extra-terrestrial life for my astrophysics project.

It would also be a help if you could propose some web sites.

There has been quite a bit of work on extraterrestrial life, both from a theoretical point of view and also based on the known properties of the planets in our solar system, notably mars. A place to start is the SETI institute.

We are interested primarily in X-rays, gamma-rays, and cosmic rays in astronomy, and so can claim no particular expertise in this area. I hope that these references are some help.

Tim Kallman
for the Ask an Astrophysicist team.

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Question ID: 970317b

### Could life based on an element other than carbon exist? (Submitted February 21, 1998)

The Question

I am a student currently in the 10th grade. I have been curious to whether life could exist on Europa (or any other unexplored space body) in different forms. For example, since Earth is shaped around the element, carbon, could life exist revolving around a different element?

Scientists have occasionally speculated that life could be based on an element other than carbon. Silicon, being the lightest element with an electronic structure analogous to that of carbon (having a half-filled outer shell with 4 unpaired electrons), is the most likely candidate mentioned. However, carbon's tendency to form the long chains and rings that form the basis for organic compounds that at some level of complexity begin to self-replicate is unique. Also, because older stars naturally produce carbon, along with nitrogen and oxygen (its neighbors on the periodic table), it is relatively abundant in the universe. Many astrophysicists who study the spectra of stars believe that complex chains and even rings of carbon appear in such unlikely places as stellar envelopes (e.g., in the form of PAHs, polycyclic aromatic hydrocarbons). When such compounds reach cooler regions of space where they can bond with readily available hydrogen, organic compounds as we know them are naturally formed.

Although other elements may form complex, covalently bonded structures, none has the rich molecular variety of carbon. It is the chemistry of carbon that allows us to consider the possibility of life "as we know it" in other parts of the Galaxy and the Universe beyond. We do not know whether Earth-like conditions exist elsewhere; but if they do, it is highly likely that life forms, if they exist, will be based on carbon.

One more point: The organic types of structures appearing in stellar envelopes are very hot and probably stripped of hydrogen, so that they are not themselves alive; it is only when carried off to a more hospitable environment, such as a much cooler planet 100 million miles away or so, that the kind of chemistry required by life becomes possible on a scale large enough to allow for stable development and replication. Water is also a factor, causing the hydrophobic proteins to clump together at all, and serving as a medium a conduit for new material, protection from temperature changes and harmful stellar radiation, etc.

Mark Kowitt and Damian Audley