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NEARly Done
January 30, 2001
courtesy Johns Hopkins Applied Physics Laboratory

Next month, scientists are going to try something they’ve never done before: land a satellite on an asteroid.

The mission is risky, but the satellite has finished its duty anyway. And at the very least this attempt will give researchers their best look yet at a little understood and possibly dangerous part of our solar system.

Out of time, but not ideas

Somewhere out past the orbit of Mars spins a large rock known as 433 Eros. The peanut-shaped asteroid is 21 miles long and eight miles wide (about twice the size of Manhattan), making it the second-largest near-Earth asteroid.

For almost a year, Eros has had company. The NEAR Shoemaker probe has been conducting the most detailed analysis to date of an asteroid by orbiting around Eros. Instruments on board analyze the asteroid’s chemical composition and magnetic field, while telescopic cameras provide images that let scientists map its surface.

What’s in a name?

NEAR stands for Near Earth Asteroid Rendezvous, while the name Shoemaker was added last year to honor Eugene Shoemaker, an eminent geologist who died in 1997. Together with his wife, Carolyn, and David Levy, Shoemaker discovered the comet Shoemaker-Levy 9, which collided with Jupiter in 1994. He was also an expert on craters, including Meteor Crater in Arizona, and he established the lunar geological time scale that was key to dating features on the moon’s surface. Some of his ashes are even scattered on the moon.

But the spacecraft is almost out of fuel and has already fulfilled the purpose for which it was launched. So, in a maneuver not part of the original mission, scientists will try to land the probe on Eros.

A few days ago, NEAR Shoemaker left its current orbit and began a series of flyovers that eventually brought it to between one to two miles (2 to 3 kilometers) of Eros’ surface, closer than it had ever been before.

It returned to orbit, where it will stay until it begins its final descent, which will be controlled by a series of intermittent engine burns. Researchers aren’t sure the landing scheduled for February 12 will go smoothly, but even if the probe crashes, the project will be deemed a success because it will have given scientists their closest view ever of an asteroid. Before reaching the surface, the spacecraft could send back images showing features as small as four inches (10 centimeters) across.

It’s a bird, it’s a plane…no, it’s an asteroid

Scientists are very interested in studying asteroids for several reasons. For one thing, they could give us clues about the origins of our solar system (since they’re the oldest things in it). Asteroids are thought to be material that would have formed into another planet when the solar system was born 4.6 billion years ago, but couldn’t because of Jupiter’s strong gravity.

NEAR’s low altitude flyover of Eros on October 26, 2000 collected the images to create this movie.
courtesy Johns Hopkins Applied Physics Laboratory
However, there’s another reason scientists want to learn about this asteroid—one that hits closer to home. Most asteroids lie between Mars and Jupiter in a large ring called the Asteroid Belt. But Eros is a near-Earth or Apollo asteroid, meaning it crosses or comes near the Earth’s orbit. Such asteroids can sometimes even crash into our planet.

Meteor Crater in Arizona, the first crater to be identified as an impact crater, is a prime example of what happens when Earth is struck by an asteroid. It was formed when a small asteroid measuring about 80 feet in diameter struck the earth between 20,000 and 50,000 years ago, leaving a hole that’s almost three-quarters of a mile (1.2 kilometers) in diameter.

Many scientists say an asteroid hitting the earth was responsible for plunging it into an ice age and dooming the dinosaurs. They’re worried it could happen again, and some are pushing for better tracking of asteroids and comets. "We’ve reached this rather awkward stage, I think, where we discover things and we’re content to know we’ve discovered something because we’re pretty sure it won’t impact us," said Brian Marsden, director of the Minor Planet Center at the Smithsonian Astrophysical Observatory in a 1998 interview. "But how far can we take that? Really, we should be studying these things."

Although asteroids tend to follow fixed orbits, scientists don’t always know their whereabouts. "In a few cases, the orbits are so imperfectly known that we’ve no idea where they are," said Marsden. "The asteroid Hermes, for example, lost since 1937, is perhaps one of the most dangerous in this respect because we really have no idea where it is."

Earth’s Craters

There are almost 160 known impact craters on Earth, although some of them are buried, eroded, or deformed.

Scientists frantically searched for evidence of the asteroid that was responsible for wiping out the dinosaurs, and they think they found it. The Chicxulub crater in the Yucatan in Mexico straddles the coast and is about 120-180 miles wide (200-300 kilometers). It was difficult to detect because the asteroid crashed so long ago (approximately 65 million years) that it’s buried under limestone.

The oldest known crater, dating back two billion years, is in Vredefort, South Africa, while Meteor Crater in Arizona is only 49,000 years old.

In 1908, scientists believe an asteroid or comet almost struck the earth in the Tunguska region of Siberia, but it exploded before it hit the ground. Although it didn’t leave a crater, it did fell trees over an area of over 1200 square miles (2,000 square kilometers) and its effects can still be seen today.

In addition, Marsden said that asteroids change orbits regularly as they come under the influence of larger bodies like the Earth, the Sun and Jupiter. Scientists hope that making surveys of as many asteroids as possible will help them better track their locations.

Although the largest known asteroid, Ceres, is 623 miles (1,003 kilometers) in diameter, most asteroids are less than a mile across. Still, the damage they could cause if they hit Earth is substantial. Marsden said that a 600-foot asteroid would create a huge tidal wave if it landed in the ocean, inundating cities at the water’s edge. One that’s half a mile across would threaten agriculture by throwing so much dust into the atmosphere that the sun’s rays would be blocked. And a 5-mile asteroid would cut solar radiation for so long that we would starve, he said.

Eros came relatively close to Earth—14 million miles (22 kilometers)—in 1975, but its orbit doesn’t intersect our planet, so there’s no danger it will hit us. But studying it will help scientists get a better idea of how asteroids formed, as well as offer insight about our own beginnings.

Elsewhere on the web

Asteroid Fact Sheet

Chicxulub images

Potentially hazardous asteroids

List of potentially hazardous asteroids

Eros images

Asteroid orbits

by Jill Max

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