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For use With Chapter 8
Universal Gravitation

The Sky Is Falling!
Posted August 1, 1998

Two movies were recently released that asked what we would do if an object was about to crash into Earth. In Deep Impact, the object was a comet the size of Manhattan; in Armageddon, it was an asteroid about the size of Texas. There has also been a TV-movie last year about a killer asteroid headed for Earth. Although this subject matter makes for exciting movies, it is still a very real--and scary possibility.

Space is pretty big, and there's a lot of junk flying around in it. Our solar system is the home for millions and millions of asteroids traveling at 48 300 kilometers per hour or more. Astronomers use Newton's Law of Universal Gravitation and Kepler's Laws of Planetary Motion to estimate the orbits of these bodies. If they pass within 5 million miles of Earth's orbit and are over 200 meters in diameter, these objects are classified as Potentially Hazardous Objects (PHOs). Today, astronomers are tracking over 100 PHOs.

In December 1997, the world got a scare when it was announced that an asteroid called 1997 XF11 could come within 48 000 kilometers of Earth in October of 2028. Some astronomers were saying that a collision was not only possible, but very likely to happen. However, when Eleanor Helin of NASA's Jet Propulsion Lab found some older images of 1997 XF11, they were able to recalculate its path. This time, it was found that the doomsday asteroid would pass within 1 529 500 kilometers of Earth and the chance of collision was "less than zero." The world breathed a collective sigh of relief.

Although a collision with 1997 XF11 will not happen, the event caused many people to start thinking about PHOs. Do we have enough people looking for these objects? What would we do if we discovered one was going to strike Earth? How would we--or could we--stop it?

Astronomers agree that the most likely method of avoiding a collision with a PHO is to change its orbit. Because an asteroid travels a tremendous distance during its orbit, only a slight push would be needed to send it away from Earth. A nuclear explosion on its surface could deflect it enough to pass harmlessly by our planet.

Still, we would need to detect the PHO before we could deflect it. After the 1997 XF11 scare, NASA realized that the earlier the warning, the better. It almost doubled its budget to search for PHOs. NASA also requested that asteroid and comet hunters wait 24 hours before announcing any potential collisions. This would give them a chance to verify the calculations and ensure we don't have any more false alarms.

Activity
Use the Internet to research other PHOs. What other objects fall into this category? Make a list of some PHOs you encounter in your life.

References
"Double-Checking Doomsday." Science, Vol. 280, 24 April 1998, p. 527.

Gordon, Bonnie Bilyeu. "That Asteroid Caper." Astronomy, July 1998, p. 6.

Jaroff, Leon, et al. "Whew!" Time South Pacific, 23 March 1998, p. 46.

Scotti, James V. "Fleeting Expectations: The Tale of an Asteroid." Sky & Telescope, July 1998, pp. 30-34.

Vogel, Gretchen. "Asteroid Scare Provokes Soul-Searching." Science, Vol. 279, 20 March 1998, p. 1843.

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