Imagine the Universe!
Imagine Home  |   Ask an Astrophysicist  |  
Ask an Astrophysicist

Questions on this topic are no longer responded to by the "Ask an Astrophysicist" service. See http://imagine.gsfc.nasa.gov/docs/ask_astro/ask_an_astronomer.html for help on other astronomy Q&A services.

(Submitted September, 1998)

Due to the high number of questions we've received about the threat of an asteroid striking the Earth, we've decided to dedicate a web page to this issue. Most of these concerns about asteroids are probably due to the recent summer movies "Armageddon" and "Deep Impact".

First of all, the most important thing to remember, is that these movies are works of fiction intended to entertain audiences. They are not real! Oftentimes, movies distort reality (especially the reality of science) to make it more exciting.

But, sometimes there is a kernel of truth behind a movie, though it is often hard to discern what that is. So here, we will attempt to bring you the truth about asteroids and what they mean for Earth, as well as give you some references to check out.

First and foremost, we know of nothing currently on a collision course with Earth! It is true that Earth has been hit by meteors or asteroids before and will be hit again someday. However, currently, we are not in any immediate danger. In particular, we now know that the asteroid which was originally announced to possibly pose a small risk of an impact in 2028 will miss us completely. http://impact.arc.nasa.gov/news_detail.cfm?ID=60


  • What are the chances of us being hit and when would it happen?

    The first thing to remember is that space is big and empty. Which makes the chance that we will be hit by anything from space very small. In much of space, for example, large-sized objects are hundreds or thousands of light years apart. Even the asteroid belt has so much space in it, that we can send space probes through it without any problems. The asteroids in the belt are spread over a ring that is more than a billion kilometers in circumference, more than 100 million kilometers wide, and millions of kilometers thick.

    Here's what JPL's Near Earth Asteroid Tracking team has to say:

    "The most dangerous asteroids, capable of a global disaster, are extremely rare. The threshold size is believed to be 1/2 to 1 km. These bodies impact the Earth only once every 1,000 centuries on average. Comets in this size range are thought to impact even less frequently, perhaps once every 5,000 centuries or so."

    The Asteroid and Impact Hazard page says:

    "The threshold for an impact that causes widespread global mortality and threatens civilization almost certainly lies between about 0.5 and 5 km diameter, perhaps near 2 km. Impacts of objects this large occur from one to several times per million years.

    "Because the risk of such an impact happening in the near future is very low, the nature of the impact hazard is unique in our experience. Nearly all hazards we face in life actually happen to someone we know, or we learn about them from the media, whereas no large impact has taken place within the total span of human history... It is this juxtaposition of the small probability of occurrence balanced against the enormous consequences if it does happen that makes the impact hazard such a difficult and controversial topic."

    This is a difficult issue because an impact would pose enormous risk, yet because the odds of it occurring within our lifetimes is so low, it is unnecessary to run around believing that the sky is falling. There are two things to consider: one is that there are many organizations with telescopes trained to the sky, watching and tracking asteroids and comets, compiling a list of potentially hazardous objects to keep an eye on. Many of these objects are decades away from approaching the Earth which gives us a lot of time to track them in order to accurately predict their orbits.


  • If tomorrow, we discovered an object that will intersect Earth's orbit, what would happen?

    The Near Earth Asteroid Tracking Team replies:

    "Actually, some 100 bodies have already been discovered on orbits which take them so close to the Earth's orbit, that they could hit in the far distant future. This is because the orbits of these bodies change slowly with time. Although their orbits do not intersect Earth's orbit at present, they could hit in a few thousand years or more.

    "The scenario you have in mind is most likely to unfold as follows. In the course of our search for Earth-crossing asteroids, we could find one that will hit not in the next year, or even in the next ten years, but might hit in the next hundred years. We believe that the chance that we will find such an object is only 1 in 1,000, even after a complete search. If we do find such an object, we will have plenty of time to track it, measure its orbit more precisely, and plan a system for deflecting it from its current orbit (hopefully away from the Earth's). There will be no great hurry, and no great panic. It would be a project for all the world's nations to take part in. It could be a globally unifying event. Because we will have found it long before it actually hits the Earth, it probably would take only a small impulse (chemical rockets, or perhaps mass drivers) to divert it from a threatening path.

    "There is a much smaller chance that we would find one that could impact in the next 10 years. The chance of that happening is 1 in 10,000. If this were to happen, we would probably still have time to launch a crash program of scientific and technological research, with the goal of characterizing both the structure of the menacing asteroid, and the best means for diverting its orbit."


  • Risk Analysis

    Now would be a good time to look at some of the risks we face in our daily lives - risks that we don't think twice about taking. For example, according to the Independent Insurance Agents of America:

    "Today a motor vehicle accident occurs every second. Auto accidents cause an injury every 14 seconds, and every 13 minutes a car accident results in a fatality. More than 31 million accidents occur per year, at an annual cost of almost $100 billion."

    Yet most people continue to drive their automobiles regardless. For the same reason, that we can't live our lives paralyzed by the fear that something bad may happen, we shouldn't let the remote possibility of being struck by a meteor or asteroid rule our lives.

    Risk analysis is the process of determining the risks associated with an act (driving a car to Florida for a spring break vacation, for example) or a product (such as a food additive), etc. When considering risks, and determining whether something is "worth" the risks it has, many factors must be considered. For example, an event could be fairly likely to happen, but have only mild consequences (such as the risk of getting a ticket if you park for 2 hours and 15 minutes in a two hour zone). Or, it could be somewhat common and moderately serious (such as the chances that you will get in a car accident of some sort during your driving lifetime). Or, it could be very, VERY unlikely, but have devastating consequences if it did (such as the chances that a 5 km comet will hit the Earth next year). Sometimes the chances are so small that something will happen that we are willing to accept the risk, the consequences if it does happen.

    Many factors enter into a person's (or a society's) decision of what are "acceptable risks". Some people are unwilling to consider any risks acceptable, but it is hard to live life like that, since breathing and eating and living on the Earth all have a certain amount of risk. The important thing is to keep it all in a balanced perspective, and to realize that risk is a part of life. It is important to minimize it as much as possible, of course, but carrying it to an extreme can do more harm than good, too.


  • Who is watching the skies?

    There are agencies, NASA among them, that realize that although we are in no immediate danger, asteroid impacts are something that could pose a problem somewhere in the future. Panicking about something that could happen some day is not constructive at all. However, it is in everyone's best interest not to ignore this, but to be watchful in order to provide the maximum warning time should a possible threatening situation occur. The following organizations are doing just this:

  • The Near Earth Asteroid Tracking Team

  • SpaceWatch

  • The Asteroid and Comet Impact Hazard Group


  • Facts about asteroids

  • SEDS Asteroid Page
  • Asteroid Page
  • Asteroid Fact Sheet
  • What are asteroids?


    Questions on this topic are no longer responded to by the "Ask an Astrophysicist" service. See http://imagine.gsfc.nasa.gov/docs/ask_astro/ask_an_astronomer.html for help on other astronomy Q&A services.

    Previous question
    Prev
    Main topic
    Main
    Next question
    Next
  • If words seem to be missing from the articles, please read this.

    Imagine the Universe! is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center.

    The Imagine Team
    Project Leader: Dr. Barbara Mattson
    Curator: Meredith Gibb
    Responsible NASA Official: Phil Newman
    All material on this site has been created and updated between 1997-2014.
    This page last updated: Wednesday, 13-Apr-2011 15:32:13 EDT