Apr 5, 2002

Hitchhiker's Guide to an Asteroid

Learning what near-Earth asteroids are made of and how they're put together is simply prudent. NASA's NEAR spacecraft did just that when it landed on one in 2001.


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April 5, 2002: When NASA's Near Earth Asteroid Rendezvous (NEAR) Shoemaker spacecraft visited the asteroid 433 Eros last year, no one was on board. It was strictly excitement from a distance, exploring the potato-shaped 20-by-8-by-8-mile worldlet through robotic eyes.

But what if you could have hitched a ride on the automobile-sized spacecraft from its launch in February 1996 through its first-ever landing on an asteroid? What would you have seen--and discovered?

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Above: a side-long view of asteroid 433 Eros. Credit: NEAR-Shoemaker. [more]

In early 2000, after nearly four years en route, you would have been excited to be approaching Eros at last--slowly enough to be captured into orbit despite the relatively small body's weak gravity. Then you would have had a year to examine the curious object from all angles, both in darkness and in sunlight, and from distances ranging from 200 miles down to as close as 22 miles. Lastly, in a thrilling four-hour finale on February 12, 2001, you would have clung to NEAR as it oh-so-gently settled down onto the very surface of Eros at merely walking speed, to rest on the tips of two solar panels and the bottom edge of the spacecraft's body--a deliberately planned interplanetary kiss with an asteroid named for the god of love.

Below: These images were captured by NEAR Shoemaker as it descended to the surface of Eros on February 12, 2001. [more]

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Why 433 Eros? As some asteroids in the main belt between Mars and Jupiter are hundreds of miles across, wouldn't they have made more interesting targets?

Bigger, yes, certainly. More interesting? Perhaps not.

433 Eros intrigued NEAR scientists for several reasons.

First, for figuring out what asteroids are made of, smaller is better than bigger. Astronomers have long speculated that asteroids are remnants left over from the original creation of the solar system more than 4.5 billion years ago. But just what was that primoridal material? On Earth, because of eons of volcanism and weathering, it's impossible to find an unchanged rock that old. Asteroids the size of Eros, however, never got big enough to melt and reform, so they are nearly pristine samples of the most primitive stuff in the solar system.

And what ancient secrets might you have seen on Eros as you swooped down close to its surface on NEAR?

Well, Eros may be ancient and geologically unchanged, but it also looks startlingly weathered. Instead of sharp edges where it could have broken off a larger body or been struck by another asteroid, peaks and crater rims are rounded and worn--perhaps by a process called "gardening," in which small impacts from interplanetary dust and gravel over millions of years wear away sharp projections. Its surface is littered with thousands of loose boulders the size of houses--most of them apparently having fallen back to the surface after having been scoured out of the largest impact crater on Eros (some 5 miles across).

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Above: Examples of flat-floored sediment ponds. Most often such features are found on crater floors, offset from the crater center toward the downslope side. [more]

Moreover, Eros is peppered with smaller craters partially filled with flat ponds of fine bluish dust--dust that appears to settle in the craters as smoothly as if it were a fluid. Both loose boulders and dust ponds are astonishing to find on a body whose gravity is so weak that a 200-lb basketball player would weigh only 2 ounces and would risk launching himself into orbit with one good jump shot!

Another reason NASA targeted Eros is that, small as it is, it is actually the second largest of a family of asteroids that can approach within 121 million miles of the sun--that is, possibly within a few million miles of Earth. Aside from the Moon, such near-Earth asteroids (NEAs) are our closest neighbors in the solar system--so it's just plain neighborly to go calling and learn a bit more about them!


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Finding out all we can about near-Earth asteroids is prudent. Although Eros does not come close enough to Earth for a collision, other asteroids in different orbits might. Indeed, many space rocks have struck Earth in the distant past. Even now 40 to 100 tons of smaller interplanetary debris and dust fall into Earth's atmosphere daily. If a rock larger than two-thirds of a mile (1 kilometer) in diameter should slam into Earth, tidal waves, firestorms, and other traumas could spell disaster for civilization and possibly even for all of life on our planet. Right now, one NASA program is scanning the heavens to inventory all near-Earth asteroids; so far, the total is more than 1800, but only 400 or so are rated as "potentially hazardous."

That's another reason for interest in visiting Eros--to find out how it (and other NEAs) are constructed. Are they solid rock? Or are they--like many mountains on Earth--more like compacted piles of rubble? To anyone considering either redirecting an Earth-approaching asteroid into a different (safer for Earth) orbit or blowing it to smithereens with a thermonuclear warhead, it's essential to know where best to give the asteroid an effective push or whether its fragments will disperse as much as you hope.

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Right: Square craters on 433 Eros. [more]

The existence on 433 Eros of craters--and of square-shaped craters at that--seem to suggest the asteroid is one solid body of fractured rock, of one uniform composition similar to the most ancient known meteorites (called chondrites) of material probably older than Earth. At least, that's the prevailing view of most scientists since having seen it close-up through NEAR's cameras and having probed it with NEAR's gamma-ray spectrometer after the craft landed.

Perhaps such solid rock might it make a good bedrock foundation for a mining operation.... (Maybe as the first squatter on Eros, NEAR should file a prospector's claim.) Could there be ice frozen in the fractures? Could asteroids be refueling stations and sources of raw materials for colonies in the inner solar system? NEAR taught us a great deal, yet many mysteries remain.

Oh, just one last hitchhiker's question. Which way to get back to Earth?

Web Links and more...

Near Earth Asteroid Rendezvous mission - NEAR home page from Johns Hopkins University Applied Physics Laboratory

878 YEARS FROM NOW: NASA scientists have announced that asteroid 1950 DA has a small chance of colliding with Earth in the year 2880.

Square Craters -- (Science@NASA) NASA's NEAR Shoemaker spacecraft has spotted square-shaped craters on asteroid Eros, a telltale sign of mysterious goings-on in the asteroid belt long ago.

Gamma-Rays from an Asteroid -- (Science@NASA) Perched on the surface of asteroid 433 Eros, NASA's NEAR spacecraft is beaming back measurements of gamma-rays leaking from the space rock's dusty soil.

NEAR links: landing facts (JHUAPL); NEAR landing, the movie (JHUAPL); FAQ (JHUAPL); Ponds? (JHUAPL); Rocks vs. Rubble (Science News); Rock Garden on Eros (Nature); The Face of Eros (Nature)

Mining asteroids -- (JPL) Are space rocks good for anything?

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