A: The Minor Planet Center maintains a list of "the predicted encounters by Potentially Hazardous Asteroids (PHAs) to within 0.05 AU of the earth from the start of this year through the 21st century."
(See http://cfa-www.harvard.edu/iau/lists/PHACloseApp.html).
Any known asteroid that will come remotely close to the earth within the next century is on this list. One AU (Astronomical Unit) is 150,000,000 km or about 94,000,000 miles. For reference the Moon is 0.0026 AU - that is 384,000 km, or about 240,000 miles - away. Currently (October 1998) the closest predicted approach for any asteroid during the next century is 0.0059 AU, which is 890,000 km, 553,000 miles, or more than twice the distance to the Moon.
Q: I'm really worried that we are going to be wiped out by an asteroid.
A: In my job I'm thinking about asteroids all the time and I'm not worried. Nor have I ever met a planetary scientist who is loosing sleep worrying about an asteroid or comet impact. The "impact hazard" is an important area of scientific study. Yes, large impacts have happened in the very distant past. However, consider that in all of recorded human history there is not a single case of anyone being killed by an asteroid impact.
Q: But what about 1997XF11 that the papers said might hit Earth in 2028.
A: When an asteroid is first discovered it is difficult to calculate its orbit with much accuracy. As time goes on and astronomers can watch its movement across the sky, the orbit can be determined with ever greater accuracy. In the case of 1997XF11 the initial orbit indicated the possibility that it could hit Earth in 2028. Very soon, however, additional observations allowed improved calculations showing that this was not the case. http://neo.arc.nasa.gov/news/1998/jun/12.html has more information.
Q: What is the difference between a comet and an asteroid?
A: The web pages http://www.hawastsoc.org/solar/eng/comet.htm
and http://www.hawastsoc.org/solar/eng/asteroid.htm
give a good overview of the characteristics of comets and asteroids.
Briefly we tend to think of asteroids as rocky and/or metallic "minor planets"
while comets are "dirty snowballs" that develop a diffuse cloud when heated
by sunlight. However the distinction between comets and asteroids
seems to have steadily eroded. For example, some asteroids are probably
extinct comets.
Q: What happens if an asteroid hits the earth?
A: That depends on the size of the asteroid. If the asteroid is D kilometers across, the energy it would release at impact would be equivalent to around 100,000(D3) Megatons of TNT. For comparison consider the USAF B61 nuclear bomb which is "one of the most devastating weapons of mass destruction known to man. This 10-Kiloton bomb is capable of decimating everything within a 3 mile radius of the detonation point" (quote from http://www.airpower.maxwell.af.mil/FactSheets/b61-fs.html - note: this web site apparently no longer exists). A one-kilometer asteroid would release ten million times the energy of a B61!
What would be the result of the equivalent of 10,000,000 nuclear bombs going off at once? Nobody really knows, but the educated guess is that it would have a devastating effect on our ecosystem.
Where does all this energy come from? The key is that asteroids are heavy and are moving really fast. The calculation is as follows. An object of mass M moving with velocity V has a kinetic energy MV2/2. A "typical" asteroid impact velocity might be 20,000 meters per second. For comparison the speed of sound in air is about 340 meters per second, so an asteroid impact would happen at roughly "Mach 60," i.e., about 60 times the speed of sound, far faster than "a speeding bullet." If the asteroid is a ball of diameter D, its mass is rpD3/4 with r the density. Using a "typical" density of 3 grams per cubic centimeter we get a mass of 2,360,000,000(D3 ) metric tons where D is the diameter in kilometers. (A metric ton is 1,000 kilograms or about 2,200 pounds, a bit more than an English ton.) Putting that all together, and using the fact that one Megaton of TNT releases 4.2e15 Joules, we get the 100,000(D3) Megatons TNT figure.
A good source of additional information is the Asteroid and Comet Impact Hazards page.
Q: I think I may have seen an asteroid on such-and-such a night. Which asteroid was it.
A: Asteroid's can't be seen with the naked eye. (One occasional exception is the very large and bright asteroid Vesta, but even it is very faint.) Even with a telescope they are not easy to distinguish from dim stars as they do not move fast enough for their motion to be immediately apparent. So, almost certainly you didn't see an asteroid. You may have seen a meteor.
Q: Won't the radiation from the comet or asteroid make us sick?
A: Asteroids and comets are probably no more radioactive than other solar system bodies like Earth. So, this is most likely not a problem.
Q: What is the biggest asteroid that ever hit the earth?
A: That's hard to say because the evidence of most collisions has probably been erased by erosion and other geological processes. One of the larger asteroids to ever hit the earth was about 10 km ( about 6 miles) across and hit about 65 million years ago off the coast of Mexico. There is good evidence that this impact is what killed off the dinosaurs. See http://www.man.ac.uk/Geology/special/stehoy.htm
Q: How large is the biggest asteroid?
A: The largest asteroid is Ceres at about 913 km (about 570 miles) across. Discovered at Palermo, Italy by G. Piazzi on Jan. 1, 1801, it was the first asteroid ever found. It's orbit lies safely in the asteroid best beyond Mars. The largest "potentially hazardous" asteroids are about 5 km (3 miles) across. Among these are Toutatis and Geographos.
Q: How are asteroids formed?
A: This has always been one of the biggest questions in asteroid astronomy. It is generally thought that asteroids (and comets) are left overs from the formation of the solar system, basically building blocks that didn't get used for making the planets. One form of evidence for this are the chondrules found in many meteorites, most of which are pieces of asteroids. (Go to http://www.meteorlab.com/images/rchfld2.gif to see an example.) Chondrules are millimeter-sized spheres that formed during the early stages of the Solar System.
Q: Where are asteroids found
A: Most asteroids are found in the asteroid belt between Mars and Jupiter. However some have orbits that bring them into the inner solar system.
Q: Are there any healing or spiritual properties associated with asteroids?
A: Not that I am aware of. The idea that celestial objects influence us (astrology) is an ancient one. By the way, it's not at all unreasonable to me that people should have considered this possibility. After all, we are certainly influenced by our environment, and the motion of the planets through the night sky is a striking natural phenomenon. However, as humankind has come to understand the physical laws that govern nature, it's difficult to imagine the physical mechanism by which celestial objects would have any influence on us. Nor is there any scientific evidence for any such mechanism.
Q: I've heard that an asteroid is going to hit Earth, but the government is keeping this information secret to avoid panic. Is this true?
A: Even though I don't particularly trust "The Government," it is difficult for me to imagine how an impending asteroid impact could be covered up. Even if it wanted to, no single government could have control over astronomical data. There are astronomers and telescopes all over the world and the skies are open to everyone. These days even serious amateur astronomers can make research quality astronomical measurements. As discussed above, the Minor Planet Center keeps lists on the web of all predicted close approaches. Of course, my answer could be part of the cover up!!! [grin.]
Q: Is the movie Armageddon realistic?
A: To tell you the truth, I haven't seen it. I'll just say that I'm not aware of any technology available today that could destroy or rapidly change the orbit of an asteroid.
Q: Where can I get information about the ephemeredes (location in the sky) of asteroids?
A: The Jet Propulsion Laboratory (JPL) has an excellent web-based ephemeris generator called Horizons. Please see http://ssd.jpl.nasa.gov/cgi-bin/eph
Q: How is it possible to calculate where an asteroid will be many years into the future?
A:
Q: Where can I get models of the asteroids Ida and Gaspra. They aren't on your website.
A: Those models were produced by other folks. Please see the Planetary Data System at the Jet Propulsion Laboratory (http://pds.jpl.nasa.gov/).
Q: Could an asteroids' stable orbit be perturbed by a gravitational influence of a comet and make it unstable?
A: Probably not. Most asteroids and comets are so small that their gravitational influence on other bodies is next to nothing. Usually it's the gravitational influence of planets, Jupiter especially, that can change and asteroid's orbit over a long period of time.
Send questions (that aren't answered above) or comments to toutatian@yahoo.com. Please note that, due to my teaching and research schedules, I usually only answer this e-mail once a month. Also, I don't think it's appropriate for me to answer questions along the lines of "I have a school report on asteroids due next week. Can you answer the following questions ..." I would encourage you to seek out information on the web using a search engine such as AltaVista or an index such as Yahoo.
