Class Notes - December 16, 1998

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Asteroid impact

Cheng studies 433 Eros

Johns Hopkins physicist Andrew Cheng '71 is willing to allow artistic license for some of the "fantastic elements" in Hollywood's recent run of asteroid movies. Take Deep Impact, for example, in which a crew goes on a mission to deflect an asteroid headed for Earth by planting detonators on its violent surface to break it up. "You would never blow up an asteroid," says Cheng. "You would gently shove it aside, so that it would miss Earth."

But Cheng's own studies show that a possible impact with an asteroid is not simply a Hollywood fantasy. Cheng is the lead scientist in an upcoming NASA mission that will provide the closest-ever look at an asteroid to determine what it is made of. After its launching next month, the Near Earth Asteroid Rendezvous (NEAR) spacecraft will begin a yearlong orbit of the asteroid 433 Eros. (The number relates to the order in which it was named.)

Cheng says that Eros, which is 40 kilometers long and 14 kilometers across, is a well-studied asteroid. It is one of the first to be observed crossing into Mars's orbit, and in 1975 it passed within 15 million miles of Earth. Scientists also predict that Eros will eventually cross Earth's orbit, possibly hitting it, millions of years from now. "It's the same-size class as the asteroid that destroyed the dinosaurs," says Cheng. "It would cause extinction."

Cheng, who has been involved in the planning of NEAR since 1991, calls the project "the most fantastic ride of my life." Cheng entered Princeton at age 15 and majored in physics, although from the beginning he had doubts about making a career of it. "As I got more into physics," he says, "I wasn't sure that I would like spending my life doing research." Even while he was working on his Ph.D. in physics at Columbia, Cheng thought about possibly becoming a lawyer or studying philosophy. Which is perhaps why he is eager to explain the creation of the solar system to a layperson in simple, nonscientific terms.

The solar system, which is 4.6 billion years old, began as a cloud of gas and dust, with the sun at the center. Dust particles--smaller than the diameter of human hair--continued to stick together until they eventually became planets. Some of the dust particles didn't make it into full-fledged planets--becoming "minor planets," or asteroids, instead. Asteroids formed in the first few million years of the solar system are called "primitive." Asteroids were also created when planets bumped into each other in a too-crowded solar system, breaking off fragments.

NEAR will help scientists determine whether Eros is a primitive asteroid or a fragment. The mission will also help scientists identify sources of meteorites, which are fragments of asteroids. The falling stars that you see from time to time are meteorites that come from either the outer part or the inner part of asteroids. "We'd like to connect this type of meteorite with that type of asteroid," says Cheng. Thousands of meteorites have been found on Earth, including a 26-pound stone that smashed a car in Peekskill, New York, on October 9, 1992. A much larger iron meteorite produced the Barringer meteor crater (named for Daniel Moreau Barringer 1879) near Flagstaff, Arizona, 50,000 years ago.

The NEAR mission will mark the first time a spacecraft has gone into orbit around an asteroid. It's a technical achievement because, says Cheng, the orbital speed of an asteroid is very low, and the spacecraft must go slowly. The speed of an asteroid is "the same as the speed of a fast run on Earth--four meters per second," he says. "If you took a running jump off an asteroid, you would be in orbit."

--Melissa Kaye