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A Collision Worth a Thousand Boulders

NEAR Mission News

The first detailed global mapping of an asteroid - conducted as part of NASA's Near Earth Asteroid Rendezvous (NEAR) mission - has found that most of the larger rocks strewn across 433 Eros were ejected from a single crater in a meteorite collision perhaps a billion years ago.

"One big impact spread all this debris," says NEAR team member Peter Thomas, a senior researcher in Cornell University's Department of Astronomy. "This observation is helping us start answering questions about how things work on the surface of an asteroid."

Thomas' report on the crater - which has the proposed name of Shoemaker - as a major source of ejected rocks on Eros appears in the Sept. 27 issue of the of the journal Nature. Thomas' fellow authors are NEAR team members Joseph Veverka, imaging team leader and professor of astronomy at Cornell; Mark Robinson of Northwestern University; and Scott Murchie of The Johns Hopkins University Applied Physics Laboratory, which managed the NEAR mission for NASA. The paper is one of three detailing the first findings from the NEAR Shoemaker spacecraft's controlled landing on the surface of Eros on Feb. 12, 2001.

Before landing, NEAR Shoemaker had orbited Eros for a year, taking thousands of high-resolution images of the 21-mile-long asteroid. From the global map of the surface the team assembled, Thomas and his colleagues counted 6,760 rocks larger than about 16 yards across (15 meters) strewn over the asteroid's 434 square miles (1,125 square kilometers). They found that nearly half (44 percent) of these rocks were inside the Shoemaker crater, positioned near one end of the potato-shaped asteroid. And most of the rocks of this size along the asteroid's equator appear to have been ejected from Shoemaker, Thomas says.

"We know they came from Shoemaker because the mapping of the geography of the pattern [of the rocks] on the surface closely matches the predicted paths from the one impact event that made Shoemaker," he says.

Eros is estimated to be more than 4 billion years old, probably the remnant of a larger asteroid broken up by a collision with another asteroid. Perhaps a billion years ago, Eros itself was struck by an object - a meteorite or small comet - creating a crater nearly 5 miles (7.6 kilometers) wide and shattering into rocks of all sizes. Some of these rocks "went straight up and straight down," says Thomas. Most of the remainder traveled as far as two-thirds of the way around the rotating asteroid in either direction (the asteroid rotates once every 5 1/4 hours), finally coming to rest on the surface.

The mystery posed by the Eros maps for the researchers is why the same thing didn't happen with two other large craters on Eros: Himeros, the saddle-shaped depression on the body's convex side, and Psyche, on the concave side. Either the rocks have been buried, have been eroded or weren't made in the first place, says Thomas.

One of the big surprises from the maps, Robinson reports in his Nature paper, is that Eros' surface appears to have a global cover of "loose fragmental debris." The surface appears to be blanketed with a fine material, some of which has created flat deposits, particularly in depressions, such as craters. These fine deposits, Robinson's paper reports, appear to have been sorted from the upper portion of the asteroid's regolith, or soil.

These so-called "ponded" deposits were visible in the final images NEAR Shoemaker transmitted before it touched down. In fact, as Veverka reports in his paper, "A strong argument is that the last image shows that the spacecraft landed on or within a few meters of a pond, a landform known to occur predominantly on the floors of craters."

How has this sorting occurred? Robinson's paper postulates an electrostatic effect, similar to that indicated on the moon's surface by the Surveyor spacecraft. Particles can build up photoelectric charges with long exposure to the sun, and this charge might separate out finer particles, says Thomas. But he concedes, "This requires a lot of assumptions, and does not explain all the mechanisms."

The big question for researchers is: Do these observations of the surface mechanics of Eros indicate that similar processes are under way on other astronomical bodies? Veverka notes it is difficult to make comparisons because no other such distant body has been so closely mapped. There are high-resolution views of the asteroids Gaspra and Ida and of Phobos, a satellite of Mars. Phobos, he writes, does show groupings of rocks in the vicinity of the crater Stickney that are comparable to those on Eros. "Nothing comparable to the flat 'pond' deposits has been noted on Gaspra, Ida or Phobos, even though Phobos coverage is certainly adequate to show such features if they were present," he writes.

In assessing the rock distribution on Eros, Thomas counted about 30,000 rocks. He was able to do this by using software created by Cornell analyst Jonathan Joseph. The software allows a researcher to mark a rock in an image, calculate from a shape model the rock's location and size, and then record this information in a data file.

Thomas' report in Nature is titled "Shoemaker crater as the source of most ejecta blocks on the asteroid 433 Eros." Veverka's report, which has several co-authors, is titled "The landing of the NEAR Shoemaker spacecraft on asteroid 433 Eros." Robinson's report, co-authored by Thomas, Veverka, Murchie and Brian Carcich of Cornell, is titled "The nature of ponded deposits on Eros."

NEAR Shoemaker launched on Feb. 17, 1996 - the first in NASA's Discovery Program of low-cost, scientifically focused planetary missions - and became the first spacecraft to orbit an asteroid on Feb. 14, 2000. The car-sized spacecraft gathered 10 times more data during its orbit than originally planned.

(From a Cornell University news release.)

The first in NASA's Discovery Program of low-cost, scientifically focused planetary missions, NEAR conducted a close-up, yearlong study of asteroid 433 Eros. The Johns Hopkins University Applied Physics Laboratory in Laurel, Md., designed and built the NEAR Shoemaker spacecraft and managed the NEAR mission for NASA. For the latest news and images visit the NEAR Web site at http://near.jhuapl.edu. For more on NASA's Discovery Program visit https://planetarymissions.nasa.gov/.