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Satellites & Space Technology

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Library of Past Questions and Answers

Rockets and propulsion

The Question

I would be grateful if you could answer this question for me.

I have heard about three methods for propelling spacecraft, these are conventional rocket, ion propulsion and anti matter/matter collision. Are there any other methods for propelling spacecraft both theoretical and actual?

If so, could you please outline them.

To tell you a bit about myself, I'm 18 and go to college in Southern England. I study physics at advanced level. Physics is a hobby for me, I will join the Merchant Navy in September as a deck officer, in my spare time on the tankers I hope to study an Open University course in astrophysics or cosmology.

The Answer

There are certainly a number of different methods for propelling spacecraft beyond conventional rockets, ion propulsion, and anti-matter/matter collisions (though I've never heard of the last ever discussed in a practical system).

Among those already in use are aerobraking and gravitational assists. Aerobraking consists of using a planet's atmosphere to slow down a spacecraft when it arrives at its destination. The currently operational Mars Global Surveyor has been using this scheme to lower its original orbit around Mars into a lower one suitable for its mapping experiments. I believe MGS is the first space mission to use this technique. MGS was sent to Mars by conventional means, but use of aerobraking instead of standard retrorocket systems saved lots of fuel and launch costs.

Gravitational assists have been used by a great number of different missions. In this scenario, a space probe approaches a planet from a carefully planned orbit such that the planet's gravity transfers some of the kinetic energy of its orbit to the probe. A probe like Voyager would come in behind say Jupiter, and get an extra kick of energy while slowing Jupiter's orbit around the Sun. Since Voyager is miniscule compared to Jupiter, the difference in Jupiter's orbit afterwards is very very very very small. But the extra velocity the probe gets can be very significant, trimming many years off the voyage time to more distant places. Both the Galileo and Cassini missions were planned so as to swing those probes around planets in the inner solar system (Earth and Venus in both these missions) to work up enough velocity to orbit out to much more distant planets.

On the drawing board side, one idea often discussed is solar sailing. light has momentum, so if you put up a giant mirror, sunlight being reflected off it would impart a slight force. So you could put up giant mirrors to generate small amounts of thrust. You don't get much in the way of velocity change this way... unless you keep those sails up for a long time! But you could get a probe to pick up a lot of velocity if it spread out sails and kept them up. If the probe was heading towards the outer solar system, however, the amount of sunlight drops off in inversely proportional to the square of the distance to the Sun, so the same sail would receive only 4% as much sunlight at Jupiter as near the Earth, so solar sailing would largely only be useful for exploration within the inner solar system.

Another drawing board idea is something called a mass driver. If say for instance you were using an asteroid as a space vehicle, you could set up a launch rail to throw pieces of the asteroid off in one direction. There are already ways to set up such electromagnetic sleds to throw things off in one direction. So if you've got a lot of spare material you can give the old high-speed heave-ho, you can get some thrust out of the deal. I don't think this one is off the science fiction book rounds, but it is actually a physically plausible way to go about it.

You might also find some useful information on the longer-term planning and ideas sections of the Office of Aeronautics and Space Transportation Technology and also the Office of Space Flight, both at nasa Headquarters. Their WWW addresses are


Hope this provides the insight you were looking for!

Jesse Allen
for Ask an Astrophysicist

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Question ID: 980204a

The Question

I am trying to find information on the propellants used in the Thor-Delta Rocket for some educational material I am preparing. I would like to be able to use the stoichiometry, mechanism, and kinetics of the propellant reactions in teaching my chemistry classes.

I would be interested in similar information for the Space Shuttle and Apollo rockets as well. All I could find at the NASA KSC web site was the composition of the shuttle systems.

The Answer

Unfortunately, none of us are 'rocket scientists' as such --- our group works on instrumentation on, and data from, astronomical satellites, and not directly with the launch vehicles. Still, we used one of the search engines on the Web and found the following site:

According to this, the Delta II first stage uses refined kerosene and liquid oxygen, and the second stage uses Aerozine 50 and nitrogen tetroxide.

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Question ID: 960925a

The Question

What kind of fuel do you use to make the space craft move?

The Answer

A discussion of the Thor-Delta rocket can be found on our web site at:

A discussion of the shuttle solid rocket motors can be found at:

The propellant mixture in each SRB motor consists of an ammonium perchlorate (oxidizer, 69.6 percent by weight), aluminum (fuel, 16 percent), iron oxide (a catalyst, 0.4 percent), a polymer (a binder that holds the mixture together, 12.04 percent), and an epoxy curing agent (1.96 percent). Oxidizer (choose one of LOX, hydrogen peroxide, red fuming nitric acid etc.) and Kerosene hydrazine monopropellant pressurized nitrogen used in space-walking jet packs

An example:

Orbital Maneuvering System (OMS)

The nasa Space Shuttle orbiter carries two OMS pods (name coined by Aerojet), each housing a single Aerojet OMS engine for orbit insertion, maneuvering, and re-entry initiation. They are capable of 100 missions and 500 starts in space.

Applications Space Shuttle orbit/de-orbit insertion, circularization
First Flown April 12th, 1981, on the Orbiter Columbia
Number Flown 14, to end of 1993
Dry Mass 118 kg
Length 195.6 cm
Maximum Diameter 116.8 cm
Mounting gimbaled ( 7 degrees yaw ( 6 pitch by two electromechanical actuators for thrust vector control
Engine Cycle pressure-fed (improvement underway for pump-fed)
Oxidizer 6743 kg nitrogen tetroxide in each pod (pods can be cross-linked)
Fuel 4087 kg of monomethyl hydrazine in each pod (pods can be cross-linked)
Mixture Ratio 1.65:1
Thrust 26.7 kN vacuum
Isp 316 sec vacuum
Expansion Ratio 55:1
Combustion Chamber Pressure 8.62 atm
Cooling Method fuel regenerative for chamber, radiative for nozzle
Burn Time qualified for 500 starts, 15 hr/100 mission life, longest firing 1250 sec, de-orbit burn typically 150-250 secs

David Palmer, Allie Cliffe and Tim Kallman
for the Ask an Astrophysicist team

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Question ID: 970509c

The Question

Do today's rockets use the same fuel as the Apollo missions used in the late 60's and early 70's? I am in the ninth grade. I was wondering about this because you don't hear very much about changes in technology in the space program. I assume basic technology must have changed a lot in 30 years, but maybe not.

The Answer

You are basically correct. There have been no fundamental changes in rocket engines for the last 30 or so years. For more on the subject check:

However, Aerospike technologies are being developed for the X-33 experimental spacecraft. An Aerospike is like a traditional bell-shaped rocket engine turned inside out.

For non-chemical propulsion systems, there are several alternatives in various stages of development. None of these have enough thrust to get a spaceship off the ground and into orbit, but once in space, a small amount of thrust over a long time period can produce a large velocity change.

Ion drives (which use electric power rather than chemical power to eject a propellant at high speeds) will be used by the Deep Space I mission, scheduled to launch next July

By using a tether (long wire) between two masses, orbital changes can be made. If current is run through the tether when it is in Earth's magnetic field, the system acts as an electric motor which can climb to a higher orbit. This technique has been tested several times in space, although it has not yet been used for practical purposes.

Lightsails are even further in the future. When light hits something, it gives it a tiny force. If you make a very big, very lightweight mirror, you can get a very small thrust on your spacecraft.

Jeff Silvis, Leonard Garcia and David Palmer
For Ask an Astrophysicist

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Question ID: 971019b

The Question

How fast can we travel in Space?

The Answer

The Cassini probe heading toward Saturn will reach speeds of 5.2 kilometers per second (11,700 mph). That gives you a rough idea of the types of velocities that we can obtain.

Hope this helps,
Jeff Silvis
For Ask an Astrophysicist

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Question ID: 980308b

The Question

Is a solar powered rocket possible?

The Answer

One kind of solar-powered spacecraft which will probably be built well within you lifetime is the solar sail, which uses radiation pressure from the Sun much like sailing ships use the wind. For details see:

Koji Mukai
for Ask an Astrophysicist

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Question ID: 980401c

The Question

Helium particles sound strong. Could they push a solar sail better than sunlight pressure? In other words, what's their energy per sq. meter near earth?

The Answer

We know of two kinds of energetic particles near the Earth, cosmic rays and solar wind. In both cases, the densities are rather low, so even though the individual particles are energetic, they do not create a strong pressure.

Cosmic rays are from outside our solar system, and come from all directions; so they cannot be used for propulsion.

Solar wind (stream of particles from the Sun) is usually around 0.001 joules per square meter per second -- about a million times less than sunlight.

Best wishes,
Koji Mukai & Jonathan Keohane
for Imagine the Universe!

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Question ID: 970127

The Question

I have an idea for space exploration. The idea in theory is simply to piggy back a satellite or probe to a comet with a travel time of 10 years or less. If there is such a comet. When the comet returns, download any and all information gathered. I haven't quite figured out how it would be attached.

I would like the e-mail address of someone I can correspond with on this. I need to know if this would work, or why it would not, in layman's terms, for I am not a scientist of any sort. Can you direct me to the right place?

The Answer

What an interesting idea! There are indeed short period comets. Comet Enke has a period of 3.3 years, and comet Wild-2 takes 6.15 years to make one complete trip around the Sun. In fact, there is a mission called STARDUST which is a collaboration of NASA/Jet Propulsion Laboratory, Lockheed Martin, and University of Washington, that will launch a low-mass probe to fly through the tail of comet Wild-2 and collect samples of that material. You should check out the web-page for this project, at

They have a lot of detailed information about the project, both the scientific goals and the mechanics of the orbit, as well as a good amount of educational material related to the project.

As far as actually landing a probe on a comet, that may be pretty tricky. The nucleus of the comet can be tumbling around, due to the release of gases on the surface of the comet. This can make the attempted landing very difficult. In addition, once the probe was on the surface of the nucleus it could possibly be damaged by the outgassing. If this outgassing is not what you were planning to study, it could disturb the results of the experiments you were interested in. There is an ESA mission being planned that will land a probe on the surface of comet Wirtanen (see This mission is called ROSETTA.

It would be possible to place your probe into an orbit that is similar to the comet's orbit, either in front of it somewhat, or lagging behind the comet. By the time you've got the probe near the comet, you've already placed the probe in the comet's orbit and it will make its way round the inner solar system with the comet.

Comets are important to study because they are the oldest, most primitive bodies in our Solar System, and are thus the earliest record of the formation of the solar system. The material that comets are composed of is very rich in organic material; it is for this reason that comets are thought to possibly be involved in the origin of life on Earth. I encourage you to stay enthusiastic about comets, and new, never-been-done-before ideas about exploring them.

Here at Goddard's Exploration of the Univsrse Division, we have experience mainly with spacecraft that are launched into orbit around the Earth to observe stars and galaxies in the X-ray and gamma-ray regions of the spectrum. We don't have expertise at planning and designing comet encounter missions, so what I say below is mainly an educated guess. The folks at JPL who are working on the STARDUST mission will know much more, and I encourage you to talk to them about your idea.

So for further information, try sending your question to, or to the questions and comments section of the JPL main page at: .

Padi Boyd,
for Imagine the Universe!

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Question ID: 970106b

The Question

Please update me on theoretical propulsion systems and any current plans for the new space shuttles. I will soon be a student of physics and one day a space systems engineer.

Thank you for your time and response.

The Answer


The nasa KSC homepage has links to a lot of Shuttle-related information, including the upgrade program and the X-33, X-34 and X-38 programs, and links to pages for many other rockets:

Speculation on future means of propulsion can be found at:

Paul Butterworth and Mark Stollberg
for Ask an Astrophysicist

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Question ID: 980826h

The Question

I was rather interested in the new type of propulsion mentioned on television. Please e-mail me about the subject.

The Answer


Thank you for writing to "Ask an Astrophysicist." It is great that you are interested in astronomy and in space travel; the technology you are probably referring to is an "ion thruster." From a web page on a course in plasma and Electric Thrusters given at the University of Wisconsin comes this definition of Electrostatic or Ion Thrusters:

This class has a single member, the ion thruster. Its key principle is that a voltage difference between two conductors sets up an electrostatic potential difference that can accelerate ions to produce thrust. The ions must, of course, be neutralized--often by electrons emitted from a hot filament. The three main stages of an ion-thruster design are ion production, acceleration, and neutralization.

This page appears to have disappeared.)

An ion thruster is being used by Deep Space 1, which uses solar panels as a source of energy to build up the necessary electrostatic potential to accelerate the ions:

They are also used for some new commercial communications satellites (Hughes offers this option) for altitude and station keeping.


J. Allie Hajian
John Cannizzo
for the "Ask an Astrophysicist" Team

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Question ID: 980927f

The Question

Could you send me some info on Anti-Matter and on Nuclear Powered Spacecraft Engines?

I've seen these two things on the internet or FOXTEL and would like to know some more about them.

The Answer

Our website contains a lot of information on antimatter. If you enter 'antimatter' into the search feature on our homepage you will get a list of links to all the places the topic is discussed.

There is a book called 'Mirror Matter' by Robert Forward which talks about engineering with antimatter and using it to make rockets. It is currently out of print, but it may be in a local library.

Hardcover, 262 pages,
Published by John Wiley & Sons,
Publication date: May 1, 1988,
Dimensions (in inches): 9.50 x 6.65 x .98,
ISBN: 0471628123,

nasa does not have plans or designs at present on either nuclear powered rockets or anti-matter engines. Anti-matter is very very very expensive to make (a gram of antiprotons would cost several hundred billions of dollars to make...). And you thought the price of gasoline was bad! It's really not a good choice for fuel right now nor into the easily foreseeable future. Science fiction authors do like it, however. Nuclear engine designs are, however, somewhat more realistic. The real bugaboo is safety: what happens if the rocket fails or explodes. I know of only one particular design for nuclear rockets (although I'm certain many exist) called "Orion" which consists of a very large metal plate lofted to orbit by exploding low yield atomic bombs underneath it. As you can imagine, the "rocket exhaust" for an Orion vehicle would pose some serious environmental hazards...

That said, there are far more interesting things NASA REALLY IS DOING! New rocket designs like aerospike engines, supersonic combustion ramjets (often called "scramjets"), solar/electric propulsion, ion drives... For the scoop on some of these fascinating and REALISTIC ideas, check out the Breakthrough Propulsion Physics Program page at

The truth is even more exciting than fiction!

You also might want to search the www for information on new rockets such as the X-33 program.

Jesse Allen, with help from David Palmer and Paul Butterworth
for the Ask an Astrophysicist Team

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Question ID: 971126a


The Question

How many satellites (roughly), both active and obsolete, are currently orbiting the earth?

Roughly how many countries of the world are using the active satellites for any purpose?

Roughly how many countries of the world have the capability ($'s and/or technology) to launch satellites?

I am a college graduate who is trying to settle a recent discussion at dinner party. I need to get the facts.

The Answer

There have been about 4000 launches (some with multiple payloads) and my guess is that several hundred of the satellites involved are still active. Nine individual countries have launched satellites (USA, Russia, Japan, China, France, India, Israel, Australia, UK) as well as international consortia (ESA, the European Space Agency, being the most important of those). A large number of countries have enough money and could easily acquire the technology to launch satellites, but choose for economy and convenience to either have another country launch their payloads or to participate in the space projects of other nations.

Paul Butterworth and David Palmer
for the Ask an Astrophysicist team

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Question ID: 980202e

The Question

Could you tell me the minimum height of that a satellite can be at in order to remain in a geo-stationary position? I have been told it is as low as 400 km and as high as 40,000 km. Thanks.

The Answer

Thank you for your question. A geostationary orbit is one that appears to stay above one point on the Earth. This means that it has a period of almost a day.

Please forgive us a digression about the length of a day. We usually think of a day as having 24 hours: the amount of time it takes for the Sun to go from highest in the sky (noon) on one day, to highest in the sky (due south in the USA) on the next. In fact, the Earth makes one complete rotation on its axis in slightly less than 24 hours (23 hours + 56 min., to be exact). The reason noon tomorrow is not 23 hours and 56 min. after noon today is that the Earth has moved a little bit in its orbit around the Sun during this time. Because of this, the Sun is now in a slightly different direction from the center of the Earth, and the Earth has to turn slightly more than one rotation to bring the Sun to due south again. (This, of course, takes 4 minutes longer, making the familiar 24 hour day.)

Now back to the actual orbit of a satellite around the Earth: Since geostationary satellites remain over the same point on the Earth, their orbits must have a period equal to the Earth's rotation on its axis = 23h56m. They also must go around the equator (or else they would appear to move North and South throughout the day), and go in a circular orbit (or else they would appear to move East and West throughout the day).

Now from these constraints, we can calculate the one specific height above the Earth where a geostationary satellite has to go. If we put it too high, the satellite would move too slow. If we put it too low, it moves too fast.

This distance from the center of the Earth is given by:

R = (G x M x period2/(4 x pi2) )(1/3)

where G is Newton's constant of gravity (6.61x10-11 m3kg-1s-2), M is the mass of the Earth (5.93x1024 kg), and period is 23h56m = 86160 s.

If you subtract the radius of the Earth from this answer, you get the height above the Earth for a geostationary satellite:

We calculate 35,000 km

Therefore, whoever told you 40,000 km was correct.


Jonathan Keohane and Gail Rohrbach
-- for Imagine the Universe!

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Question ID: 970408d

The Question

What are the conditions and requirements for a Sun-synchronous satellite orbit? To which orbits with which orbital parameters does this fit. Don't hesitate to go into mathematical details! Are there some references on the web?

The Answer

A Sun-synchronous orbit is an interesting orbit. Since the Earth is not an exact sphere (it has a slight bulge at the equator) satellite orbits can be effected by the extra gravitational pull. A Sun-synchronous orbit uses this feature of the Earth's shape. An orbit that is close to being polar (passing within about 1000 km of the North and South poles of the Earth) will be affected asymmetrically by the bulge at the equator. This asymmetry acts to slowly rotate the plane of the orbit about the axis of the Earth. When the inclination is chosen just right (about 8 degrees off the polar orbit) the motion matches the motion of the Sun across the sky. The plane of the orbit executes one full rotation about the axis of the Earth in one year. They are typically orbiting about 800-1000 km above the Earth.


Padi Boyd,
for the Ask an Astrophysicist

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Question ID: 970613a

The Question

I am presently completing a degree at Caledonia University in Glasgow and as part of a presentation I request information on the newest mission. I need information on the current project as my aim is to promote this to businessmen in order that they sponsor and advertise on the the new project. This is purely fictitious but my interest in this new project is such that I want it to be as realistic as possible.

The Answer

We are a group mainly of High Energy Astrophysicist (X-ray and gamma-ray astronomers) and so we know best about astronomical satellites. Our own lists of X-ray and gamma-ray satellites can be found at:


For nasa launches over the next year or so, the Kennedy Space Center web-site is a good place to check (they have Shuttle information and Expendable Launch Vehicle pages). Another good resource is the "Space Calendar" at JPL.

This page (with voluntary contributions) tries to maintain a complete list of upcoming launches for the next year.

Hope this helps,

Koji Mukai
for Imagine the Universe!

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Question ID: 970319a

The Question

I am looking for information on two Soviet satellites, Cosmos 207 and 208. I'm particularly interested in dates and locations for their reentry into the atmosphere.

The Answer

You found everything we know about the Cosmos satellites on our missions page:

You can find out information about just about every satellite ever launched in Jonathan's Space Report Satellite Catalog at

Kosmos-207 Launch 1968 Mar 16.00 Landed 1968 Mar 24
Kosmos-208 Launch 1968 Mar 21.00 Landed 1968 Apr 2

I have no idea where you might find out WHERE these satellites actually landed. Perhaps Jonathan (his homepage with info on how to contact him is at can help you.

Hope this helps,

Laura Whitlock
for Ask an Astrophysicist

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Question ID: 970624d

Future Possibilities

The Question

Would you have by any chance any information on possible nasa projects as regards observing from the Moon? Would the dark side of the moon be a good place for a telescope?

The Answer

There are many advantages to putting telescopes on the Moon.

proposes an optical telescope on the Moon.

Optical telescopes can be on either the Nearside or the Farside of the Moon. (The term 'dark side' is no longer used because it confuses people into thinking that the Sun doesn't shine there. In fact, the Sun shines on both sides equally.) There is very little atmosphere to scatter light from the Sun or Earth, so you can use the telescope all day and all night (14 Earth days each).

radio telescopes are best placed on the Farside, to block out the radio noise of Earth and its increasingly chatty retinue of satellites. Radio bends around small obstacles so it is harder to block out. Half a mile from the point where you can no longer see any of Earth would not be enough. (Besides which, an effect called 'libration' means that Earth wanders slightly in the sky over the course of a month.) Data communications from the observatory to Earth should be done by laser through a Lunar satellite to further avoid noise.

As for whether you would want a manned observatory, probably not. Even on Earth, there is a tendency to lower the staffing of telescopes for budgetary reasons, and let the astronomers control the telescopes through the international computer networks from their own offices. And it costs a lot more to send an astronomer to the Moon than it does to send her to Arizona. In addition to being expensive, people are noisy, smelly, and filthy. Even when she's not kicking dust onto the mirror, she'll be talking on the radio, stomping around, and venting gas. That's the sort of thing we're trying to get away from.

A good source of information on NASA's future plans is the following WWW page:

In general, the idea of putting observatories on the Moon has been around for quite some time. A few years ago, the Bush administration suggested that NASA look into putting a man on Mars. One step in this process would be to produce a lunar station as a testing ground.

Assuming that this lunar station would happen, many astronomers discussed building observatories on the Moon. In their calculations, this would be cost-effective only because the prior infrastructure would already be there.

As it happened, it was later decided that manned missions would be too expensive, so the lunar station concept was abandoned (at least for the near future). Without an already existing lunar station, the cost of building lunar observatories becomes prohibitively expensive.

As I understand it, the only near-future missions to the Moon or mars are unmanned missions to study the Moon or mars respectively.

Jonathan Keohane and David Palmer
for Ask an Astrophysicist

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Question ID: 970528a

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