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Asteroid: Expert Q&A

In October 2006, NASA Senior Research Scientist Don Yeomans answered questions about Apophis and the asteroid threat in general.


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Q: What is the likelihood that a global killer asteroid could be predicted and stopped? Tony, Wisconsin

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Don Yeomans: An asteroid large enough to cause global consequences would have to be larger than one kilometer in diameter, and NASA-supported searches have already found about 75 percent of them—and none of them is Earth-threatening. So long as the threatening asteroid is found many years before a predicted Earth impact, current technology could be used to change its velocity a bit and hence deflect the asteroid so that it would miss the Earth completely.

Q: How much power or energy would be required if we were to use rocket boosters to attempt to change the orbit of Apophis? Ann Stanley, Mount Airy, North Carolina

Yeomans: The energy required would depend upon the unknown mass of the asteroid. If we assume that Apophis will hit the Earth's center 10 years from now, we have 10 years to move the asteroid one Earth radius (6,400 km). So, roughly speaking, we'll have to change the asteroid's velocity by 0.7 cm per second now to prevent a hit 10 years from now.

We could use any of several technology options to deflect the motion of an asteroid that threatens Earth. These include running into the asteroid with a spacecraft at high speed to slow it down a bit with an impulsive push, slowly pushing on the asteroid with a thrusting spacecraft, or using a solar-concentrating mirror to vaporize material on the front side of the asteroid, thus introducing a rocket-like thrust on the surface of the asteroid itself.

Q: Is it possible for us to miss or overlook an object the size of, let's say, five by two miles given the almost infinite, omni-directional paths towards our planet and be hit right as you read this? Can an object that size be deflected if detected on time? Thank you for taking the time to answer my question. David Rivera, Bayamon, Puerto Rico

Yeomans: It's possible but very unlikely. Astronomers have discovered almost all of the near-Earth asteroids that are two to five miles in diameter, and none is on an Earth-threatening orbit. If one was found to be threatening, and we had a warning time of several years, we could use current technology to deflect it on time.

Q: What are societies on Earth doing to prevent a huge asteroid from potentially destroying our planet? Jonathan, 11th grade, Eagle Academy for Young Men, New York, New York

Yeomans: NASA is supporting five full-time telescopic search programs to discover Near-Earth Objects as well as a Near-Earth Object Program Office that has the responsibility to compute their orbits, extrapolate their motions into the future on the computer, and compute the likelihood of future impacts for those few objects for which an Earth impact cannot yet be ruled out (see

Q: Is a collision between Earth and a very big rock/asteroid inevitable, i.e., not if but when? Rick Freeman, Louisville, Kentucky

Yeomans: Asteroidal rocks the size of basketballs hit the Earth's atmosphere twice a day on average, and Volkswagen-sized asteroid fragments hit twice a year. They do not survive the atmospheric entry, but they do provide impressive fireball events. There are far more small asteroids than large ones, so as you consider larger and larger asteroids, an impact is less and less likely. For example, one would expect an impact by an asteroid larger than one kilometer across only every 600,000 years on average.

Q: In a worst-case scenario—if an object that is set to impact us directly cannot be steered off course—are there any feasible or realistic preparations that we can make to mitigate the terrible loss of life on Earth? And are any such plans being discussed right now? Victoria P., Jacksonville, Florida

Yeomans: In the unlikely event that we had little in the way of warning time for a sizeable Earth-impacting asteroid, authorities could evacuate regions along the possible impact path to reduce the losses. Mitigation options for dealing with an Earth-threatening object are currently under consideration.

Q: Gravitational Tractor vs. Moon Laser?

It seems to me that the gravitational tractor hovering with a reaction rocket would send its thrust into the asteroid, imparting momentum and resulting in no net movement of the pair (spacecraft and asteroid). On the other hand, a moon-based laser aimed at a pole of the spinning asteroid might excite enough particles to leave the surface and slowly push the asteroid to a new course. Does this make sense?

Also, although the government has a small program to track Near Earth Objects in the plane of the ecliptic, would it be good insurance to expand the search to find and monitor parabolic comets from outside the ecliptic plane? John Furber, Gainesville, Florida

Yeomans: Reaction rockets on a gravity tractor would have to be angled outward so that the thrust did not push on the neighboring asteroid.

Experts have suggested space-based lasers, or solar-concentrating mirrors, for vaporizing one side of a threatening asteroid, with the resulting thrust pushing the asteroid out of harm's way.

Close Earth approaches by the more numerous asteroids are 100 times more frequent than close approaches by nearly parabolic comets. Nevertheless, the current telescopic search surveys do discover many long-period comets.

Q: Where is this asteroid [Apophis] now, and how close to Earth would it have to get before any intervention could be done? Thank you. Kathy Nugent, Fort Myers, Florida

Yeomans: You can view the current position of Apophis or any other comet or asteroid in a nifty interactive Web site at:

Type in "Apophis" then hit "search" and there it is....

Q: If they blew up the asteroid into small pieces, wouldn't they burn up in the atmosphere? Jacob, Mobridge Middle School, Mobridge, South Dakota

Yeomans: If one could blow the asteroid up into pieces no larger than about 50 meters across, they would not be expected to survive atmospheric entry, nor would they be expected to cause any ground damage. We would expect pieces larger than about 100 meters across to punch through the atmosphere and cause considerable ground damage. Those whose sizes are between 50 and 100 meters across would probably not hit the ground but would be expected to cause an air blast explosion with considerable ground damage.

Q: I have heard for a number of years that the event that caused the sudden disappearance of the dinosaurs was an asteroid impact. I had gotten very comfortable with that theory, but then recently I started hearing about another type of event that was probably responsible for the dinosaurs' demise. How certain are we that a heavenly body suddenly wiped out dinosaur life on the Earth? Anonymous, Calumet City, Illinois

Yeomans: Fairly conclusive evidence exists of a major asteroid impact some 65 million years ago, about the time the dinosaurs became extinct. Scientists discovered a large (180-km-in-diameter) asteroid impact crater near Chicxulub on Mexico's Yucatan Peninsula, and it has been dated to about 65 million years ago. However, some dispute remains as to whether or not this was the only cause of the dinosaurs' demise or whether major volcanic events—perhaps triggered by the blast—were also responsible.

Q: I want to know how stars fall. Iulios (age 5), Belle Harbour, New York

Yeomans: While stars don't fall, the Earth in its orbit about the sun does run into tiny particles of asteroids and comets, and these sand-sized particles do fall into our Earth's atmosphere, burn up, and cause so-called "shooting stars" (also called meteors).

Q: Earth's moon is scarred with an incalculable number of impact craters, and yet it stays in an unchanged orbit with the Earth. How is it that after so many large impacts that could shift its orbit that it has not itself collided with the Earth? David Devin, St. Louis Park, Minnesota

Yeomans: The moon is far too large and massive to have its motion significantly affected by asteroid impacts. Due to Earth and moon tidal effects, the moon is actually receding from the Earth at about 3.8 cm per year.

Q: What would happen if an Apophis-like asteroid hit the sun instead of the Earth? What could be the outcome, and how would it affect our planet? Thank you. Edwin, Los Angeles, California

Yeomans: Small comets currently hit the sun rather frequently without any apparent effect. Our sun could swallow Apophis with no trouble at all, and there would be no affect to Earth.

Q: How large is Ceres, and how was it formed? Kerry Kertes, Cape Elizabeth, Maine

Yeomans: Ceres is about 975 km across in its largest dimension, and it was formed between the orbits of Mars and Jupiter by gravitationally drawing to itself smaller neighboring asteroids about 4.6 billion years ago.

Q: How do you find asteroids? Anonymous, Kenai, Alaska

Yeomans: Modern search telescopes look at a particular region of the dark sky and record an image of the region of the sky with a charge-coupled device (CCD) camera, just like the detector in your digital camera. Fifteen minutes later, a second image is taken of the same region of the sky, and another 15 minutes later a third image is taken of the same region. The three images are then examined to see if any object in the telescope's image field has moved with respect to the background stars. (The background stars remain fixed in position in all three images, of course.) Any object that is found to be moving from one image to the next is an asteroid candidate, and the faster the object's apparent motion between the images, the closer the object is to Earth.

Q: How do you measure distances and trajectory and size? And if we have so many small particles hitting daily, how come I never see any falling? David

Yeomans: Astronomers measure angular distances between stars and asteroids and then determine the angular positions of the asteroids on the sky from the known star positions. Once a group of these angular positions is available, we can compute the asteroid's orbit about the sun and determine its predicted position and distance at any time. This latter process is called computing an ephemeris for the object.

Since an asteroid's ephemeris will provide its distance from both the sun and Earth, the size of an asteroid can be estimated from its measured apparent brightness and by assuming a reasonable value for its reflectivity (i.e., albedo).

Any time you see a falling star, or meteor, you are witnessing a tiny piece of a comet or asteroid hitting the Earth's atmosphere.

Q: Is it true that in the Southern Hemisphere scientists are not searching for asteroids and comets due to the lack of telescopes available to search for them? If true, this prospect is quite frightening! Stu, Las Vegas, Nevada

Yeomans: One of NASA's supported Near Earth Object search programs (the Catalina Sky Survey) operates a 0.5-meter-aperture telescope in Australia. The under-representation of search telescopes in the Southern Hemisphere is not as serious as it first appears, since eventually almost all Near-Earth Objects will be observable from the Northern Hemisphere.

Q: If a meteor the size of a football stadium hit San Diego, would I be able to feel its effects all the way in Alabama? Dillon Haney, Dora, Alabama

Yeomans: You probably would not feel the shaking, but seismometers certainly would.

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