Spaceflight Now: Breaking News

Mystery of asteroid Eros: So much rock, so little gravity
Posted: September 24, 2000

  Eros in color
This color image of Eros was acquired by NEAR's multispectral imager on February 12, at a range of 1100 miles. The image shows approximately the color that Eros would appear to the unaided human eye. Its subtle butterscotch hue is typical of a wide variety of minerals thought to be the major components of asteroids like Eros. Photo: JHU/APL.
How could something so small have so much debris lying around? That is the puzzle presented by asteroid 433 Eros in the first major reports on the composition and history of the 21-mile-long body, the solar system's first asteroid to be subjected to close study.

Writing in the latest edition of the journalScience (Sept. 22), Joseph Veverka of Cornell University describes tiny Eros as having a surface "saturated" with tiny craters smaller than 1 kilometer (0.6 miles) in diameter and "abundant" with rocks 30 to 100 meters (33 to 109 yards) across. The craters and the boulders, says Veverka, indicate many violent collisions with the asteroid over time. But the gravity on Eros is so weak "that intuition and calculation tell you that most of the debris produced in a collision would have escaped -- but the surface is full of it."

Veverka explains: "We have several possibilities. One is that we simply don't understand cratering events on small objects, and somehow the debris gets thrown out at very low speeds. Or the ejected material ends up in the same orbit as Eros, and over time the asteroid runs back into its own debris and gathers it up, which is equally bizarre. We simply don't understand this."

Veverka, professor of astronomy at Cornell, is the principal investigator on the multi-spectral imager (MSI), or camera, and the NEAR infrared spectrometer (NIS), two of the five instruments on board NASA's Near Earth Asteroid Rendezvous spacecraft (known as NEAR Shoemaker), which has been in orbit around Eros since Feb. 14. Between that date and April 1, the four teams managing the instrument packages probed the elongated asteroid for its mass distribution, elemental composition and topography and elevation. Their four reports in Science form the most detailed view yet of an asteroid. The Veverka team's report, "NEAR at Eros: Imaging and Spectral Results," notes rock debris, "presumably blocks of ejecta," scattered across the asteroid, but not uniformly. A strong concentration of blocks, the report says, occurs in the complex depression west of the saddle, a 10-kilometer (6-mile) depression. The distribution of blocks shows a low density at high northern latitudes, but the rocks do not seem to have collected in low-lying areas of the asteroid.

"What is striking about Eros," says Veverka, "is that if I look at the moon in great detail, I see lots of tiny craters and fewer blocks of rock. But on this object, when I get down to sizes the size of a car, there are very few craters and lots of boulders." And yet, he says, the surface of Eros shows clear evidence of violent impacts.

The astronomer concedes that little is known about collisions on small bodies with low gravity -- "we have to extrapolate a lot," he says. But calculations indicate that the gravity on Eros is so low that a ball thrown from the surface would escape into space. "Most of the ejecta from a violent collision would be traveling at a reasonable speed, and you would expect it to escape. So we simply don't understand why the surface is littered with so many blocks," says Veverka.

NEAR Shoemaker trained its camera on Eros' large, 5.3-kilometer (3.3-mile) diameter crater for a series of color pictures intended to measure the properties of regolith inside the asteroid's craters. In this false color view -- taken from an altitude of 50 kilometers (31 miles) -- redder hues represent rock and regolith that have been altered chemically by exposure to the solar wind and small impacts. Bluer hues represent fresher, less-altered rock and regolith, such as the bright patches that have been less affected by "space weathering." In that process, during micrometeorite impacts, rock reacts with miniscule amounts of trapped solar wind and is chemically changed. Interestingly, most of the large boulders have been just as affected as the regolith. This suggests either that the rocks are relatively old, or that they are "dirty" from an adhering film of regolith particles. Photo: JHU/APL
The Veverka team also confirms previous reports that Eros -- an S-type asteroid, the most common classification -- is a primitive relic of the emergence of the solar system from a cloud of gas and dust.

"We basically know that Eros is an example of a very primitive body in which nothing much has happened other than formation and cratering. If you want the most pristine material in the solar system that has had the least happen to it, then Eros is a good example," Veverka says.

The imaging team says there is no evidence that Eros has gone through an Earth-like process of heating and segregation of metal from silicates to form an iron core and rocky mantle. From an analysis of surface elements -- by measuring radiation emissions -- and measurements of the gravity field, it was determined that the asteroid is homogenous.

Other institutions on the MSI and NIS team are Northwestern University, the Southwest Research Institute; Rensselaer Polytechnic Institute, the U.S. Geological Survey, the University of Hawaii, Malin Space Science Systems, the University of Maryland, the Jet Propulsion Laboratory, and the Applied Physics Laboratory, at Johns Hopkins University, which designed and built the NEAR spacecraft and manages the mission for NASA.

Other Cornell researchers on the team include Peter Thomas, James Bell, Ann Harch, Maureen Bell, Brian Carcich, Beth Clark, Jonathan Joseph and Colin Peterson. Steven Squyres, professor of astronomy at Cornell, is an author of another of the Science papers, "The Elemental Composition of Asteroid 433 Eros: Results of the NEAR-Shoemaker X-ray Spectrometer."