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Astronomers had never before found an asteroid that had left the main “belt” between Mars and Jupiter and approached earth’s orbit…until now.

Published March 1, 2002

By Robert Zimmerman

On ordinary days, the control room for a deep-space mission is rather sedate: data stream in, routine commands stream out, no one need raise his voice. But February 12, 2001, was no ordinary day for the technicians controlling NASA’s Near Earth Asteroid Rendezvous (NEAR)-Shoemaker spacecraft. Some punched calculators madly, while others ran from computer monitor to computer monitor, shouting numbers, trying to find out what was happening. Nearby, television crews aimed cameras at the scrambling engineers, capturing their every motion. Pandemonium had replaced the serene orderliness.

The NEAR team had brought this chaos on themselves. In a bold flourish to end their successful mission, the spacecraft’s science and engineering teams at the Johns Hopkins University Applied Physics Lab in Laurel, Maryland, sent NEAR-Shoemaker toward a landing on the surface of Eros, the asteroid it had circled for a year. Never mind that the probe had been built as an orbiter and had no landing mechanism of any kind. Even if NEAR wound up shattering into a thousand pieces, the images it would send in its final moments would make the stunt worthwhile.

Two members of NEAR’s imaging team, Joseph Veverka, professor of astronomy at Cornell University and Mark Robinson of Northwestern University, huddled in front of a computer to marvel at the high-resolution images coming from space. Veverka was amazed by the absence of craters in the close-up pictures of the asteroid’s surface, and Robinson was impressed at the numerous boulders of all shapes and sizes.

Hungrily Consuming Information

The spacecraft descended at a leisurely four miles per hour, and the images grew in detail and complexity. The investigators hungrily consumed each bit of information, fully expecting the data stream to end abruptly at the moment of impact. Several technicians watched as their computer programs counted the altitude down to zero. Then one of the flight engineers yelled, incredulously, “Totally nominal––we’ve got a signal!” Robert Farquhar, the mission director, shouted, “Hold that signal!”

NEAR-Shoemaker had not only touched the surface of Eros, it had come through the impact seemingly whole and in operation. It was as if the controllers had rolled an egg across a gravel field without even cracking the shell. Although no more images could be transmitted, NASA allowed the mission an extension of several weeks to enable the craft to gather and radio back additional data about the chemical make-up of the spacecraft’s landing site.

After accomplishing the first rendezvous with an asteroid, the first orbit of an asteroid and the first landing on an asteroid, the investigators in charge of the NEAR-Shoemaker mission now have compiled a wealth of information about a heretofore shadowy subject –– the bits of planetary debris that inhabit the middle reaches of the solar system.

The data and images from the mission have already helped answer innumerable questions about asteroids and how they figure in the birth and formation of the solar system. But more interesting, perhaps, was what NEAR-Shoemaker did not tell scientists. As extraordinary as the landing was, the last-second images paralleled many of NEAR-Shoemaker’s other discoveries. For every question that was settled, another conundrum was unexpectedly uncovered.

“These [images] leave us with mysteries that will have us scratching our heads for years to come,” Veverka said.

A Place in Space

Even before the NEAR-Shoemaker mission, Eros had been one of the most studied asteroids. Its orbit ranges from 165 million miles to 105 million miles from the sun; that means on occasion it comes within 10 million miles of the earth. Astronomers have long used those close approaches as a valuable measuring stick. The earth’s distance to the sun and the mass of the earth-moon system were measured using positions triangulated with the help of Eros. What’s more, the regular visits enable astronomers to study the asteroid from the earth with relative precision.

The first asteroid was discovered in 1801 by Giuseppe Piazzi, a professor of mathematics and astronomy at the University of Palermo in Sicily. Piazzi had been surveying a part of the solar system between Mars and Jupiter in hopes of spotting a planet thought to lie there. Those hopes were based upon the Titius-Bode Law, a simple mathematical routine that could produce the orbital distance of the first eight planets with surprising accuracy; that law predicted a planet at a distance of 275 million miles.

After tracking a bright object across the background stars for more than a month, Piazzi calculated its position and found that its orbit closely corresponded with the location of the “missing planet.” On February 12, 1801 –– 200 years to the day before NEAR’s landing on Eros –– Piazzi announced his discovery. A new planet had been found, one he called Ceres, after the Roman goddess of the harvest.

A Point in the Sky

Piazzi’s fame was short-lived. Once other astronomers began observing Ceres they discovered that, unlike other planets, this one presented no discernable disk. It was, like a star, a point in the sky. The name “asteroid” (meaning “starlike”) stuck. The next year the German astronomer Heinrich Olbers found another asteroid in much the same orbit as Ceres. Hundreds of asteroids had been spotted by the time the German Gustav Witt and the Frenchman Auguste Charlois independently discovered Eros on the same night in 1898.

Eros, however, marked a first: Astronomers had never before found an asteroid that had left the main “belt” between Mars and Jupiter and approached earth’s orbit. And it is large, measuring some 21 miles long and eight miles wide. Although the total number of known asteroids exceeds 10,000, astronomers have identified only 250 or so near-earth asteroids, as those with orbits like that of Eros’s are called.

No asteroid is known to be on a collision course with earth, but impacts have occurred throughout geological history –– asteroid impacts are implicated in large extinctions and with creating the craters that formed lakes in Canada and elsewhere. Very small bits of asteroids hit the earth all the time. They’re called meteorites once they land.

Geologists have collected thousands of meteorites. Some meteorites are composed of carbon-rich minerals and look like soot; others are almost pure iron. But the majority –– some 80 percent –– are what geologists call ordinary chondrites. Such rocks are stony in appearance and largely made up of silicate ores, such as olivine and pyroxene.

A Model Mission

Rather than get fleeting images of many asteroids, the NEAR mission, launched in 1996, was designed to gain an extraordinary amount of information about just one. (The name of the mission was changed to NEAR-Shoemaker to honor the planetary geologist Eugene Shoemaker, who died in 1997.) The mission also was to be a model of efficiency: rather than roar to the target asteroid in one quick arc, the spacecraft would swing past the earth to get a gravitational boost. Along the way, NEAR-Shoemaker zipped through the asteroid belt and past Mathilde, a C asteroid.

Mathilde proved to be a bit of a surprise: a jagged, irregularly shaped 33-mile-wide body, darker than charcoal, was found to be only slightly denser than ice. Since the carbon-rich material that the asteroid is thought to be made of is far denser than this, planetary geologists believe Mathilde is nothing more than a gravel pile of primordial material loosely stuck together. But the fly-by of Mathilde was too fast to obtain detailed spectra.

NEAR-Shoemaker approached Eros in December 1999, and controllers sent the command that would slow it enough to be captured in an orbit. With so little gravitational pull (an astronaut on the surface could throw a rock fast enough to reach escape velocity) Eros was more of a point to maneuver about than a world to orbit. But at the very moment the spacecraft was supposed to settle into orbit around Eros, an engine failed to burn and the probe shot past.

That could well have been the end of the mission. But engineers found a way to correct the engine problem and re-aim the spacecraft. NEAR made an extra orbit of the sun so its path could be brought back to Eros 14 months later, on February 14, 2000.

Unprecedented Challenge

After settling into an orbit around the 21-mile-long, peanut-shape asteroid, NEAR-Shoemaker kept a careful distance. It was a matter of wise discretion, since orbiting such a strangely shaped object with such a tiny gravitational field was in itself an unprecedented challenge. And because of Eros’s bent and elongated shape and its rotation through a five hour and 15 minute “day,” the relative speed between spacecraft and asteroid ranged between two and 15 miles per hour, and was never the same from orbit to orbit. If ground controllers were not careful, the spacecraft could get whacked as the nose of the asteroid swung by.

For about two months, then, the spacecraft circled more than 100 miles above the surface, employing its camera, laser altimeter, magnetometer, infrared, and x-ray/gamma-ray detectors to obtain a comprehensive view of the end pointed to the sun. In mid-April the spacecraft moved inward, spending the next five months in orbits as low as 22 miles. Then, in August, ground controllers lifted NEAR-Shoemaker upward to a higher orbit so that scientists could get global views of the other end, now in daylight.

Of the many intriguing and distinct geological features spotted during this orbital reconnaissance, the most notable was the giant saddle-like feature. Data from the laser altimeter suggests that the feature is actually a crater, though strangely shaped. A more normal-looking large crater –– some three miles in diameter and a half-mile deep — dominates the asteroid’s other side.

Few Small Craters

Indeed, the size of the large craters gouged into Eros’s surface was perhaps less surprising than the absence of small ones. Unlike the moon and other solar system bodies –– where the relative number of differently sized craters remains constant as you get closer –– Eros lacks many craters less than 100 yards in diameter. “I am amazed at how devoid the surface is of small craters,” Veverka said.

Instead of small craters, investigators saw just the opposite: boulders everywhere, in all sizes and shapes. Some are rounded. Others have sharp angular facets. In fact, the entire surface of the asteroid seems covered with a layer of pulverized dust and debris of unknown depth. In some areas, such as in the large saddle, the layer appears thick enough to  completely blanket and fill older craters. The photographs also revealed grooves, troughs, pits, ridges and fractures, similar to what was seen on Ida.

“These are generally very old features, and suggest the existence of fractures in the deep interior,” says Veverka. The ridges, one of which wraps one-third the way around the asteroid, average about 30-feet high and 300-feet wide. Their existence suggests that Eros has an internal structure and is therefore a consolidated body and not a rubble pile like Mathilde. In other words, if you gave Eros a push, it would move away from you as a unit, rather than dissolve into a cloud of gravel.

The strangest features spotted by the close-up photos were what appeared to be extremely smooth ponds of material at the base of some craters, as if the dust and dirt on the crater slopes had flowed downward and pooled at the bottom. “Some process we don’t understand seems to sort out the really fine particles and move them into the lowest spots,” notes Veverka.

New Conclusions

Not only is Eros a solid hunk, close-up views reveal that its composition is remarkably uniform and evenly distributed. In fact, Eros appears incredibly bland, with little color variation anywhere on its surface. “The very small color differences lend support to Eros being all the same composition,” says the planetary scientist Clark Chapman of the Southwest Research Institute in Boulder, Colorado, and a member of the NEAR-Shoemaker science team.

That means the ground-based spectroscopy suggesting that Eros was a differentiated body –– with hemispheres composed of minerals that had separated due to melting –– was wrong. In fact, the data NEAR-Shoemaker has collected calls into question many of the conclusions that have been made about the composition of asteroids. Astronomers believed that Eros and all other S-type asteroids were geologically distinct from ordinary chondrite meteorites; on close inspection, NEAR has shown Eros to be nothing more than one large ordinary chondrite.

Many investigators now believe that such S asteroids –– which make up the majority of asteroids in the inner part of the solar system –– might well be the source of most meteorites. In fact, the difference in spectra between S asteroids and ordinary chondrites might be more a function of rotation than substance: as asteroids rotate, their irregular surface distorts their spectrum.

A Daring Finish

Rather than simply shut NEARShoemaker off, mission director Robert Farquhar suggested a more daring finish: Why not try to land the orbiter on the surface of Eros? Not only would such a landing enable investigators to get some high-resolution images that would have been impossible to obtain otherwise, the feat would teach ground controllers the best techniques for landing spacecraft on such low-gravity objects, a skill that future space navigators will surely need.

On its way down, NEAR-Shoemaker snapped 69 high-resolution images of Eros’ surface, resolving details less than an inch across. Just before impact, the last two pictures caught the edge of one of the sand ponds. Though the pond appeared smooth –– as in more distant photographs –– small stones were seen peeking up through the fine dust. Those final photographs raised more questions than they answered.

NEAR-Shoemaker recorded 160,000 photographs –– imaging surface features as small as a foot. It will take years for the investigators working on the mission to digest it all. Just as the VIKING missions to Mars informed the study of that planet for a generation, it may take decades before planetary scientists get a set of asteroid data that is richer or more detailed. Even so, NEAR-Shoemaker gave astronomers a wealth of data on just one asteroid. Whatever conclusions astronomers may draw from NEAR must be tempered with the knowledge that asteroids come in many sizes, shapes and compositions. Any definitive conclusions can be said only of Eros.

The First Close and Detailed Look at an Asteroid

Nonetheless, this first close and detailed look at an asteroid gave humanity its first tantalizing glimpse at the very earliest birth pangs of a planet. The flow of material down the slopes of craters, the crumbling of boulders, and the pooling of material into sand ponds are merely the processes by which an irregularly shaped object slowly rounds itself off into a spherical planet.

Ancient and worn by its billion-year journey through the black emptiness of space, Eros has slowly been chiseled by impact after impact, then shaped by the slow, inexorable pull of its tiny gravity. In this dim, dark and silent environment, nature has –– like the seed in an oyster from which pearls will grow –– relentlessly built Eros up from nothing. From a similar seed grew our earth.

As things stand now, however, the best summary of what we really know about Eros and asteroids comes from Veverka, who spoke freely at a press conference immediately after the landing. Again and again, Veverka told reporters, “We really don’t understand what’s going on.”

Also read:To Infinity: The New Age of Space Exploration

About the Author

Robert Zimmerman is author of Genesis, the Story of Apollo 8, published by Four Walls Eight Windows, and The Chronological Encyclopedia of Discoveries in Space, published by Oryx Pres.