NEAR Shoemaker Observations Link Eros To Primordial Solar System

The NEAR team gathered the information after a powerful explosion on the sun zapped Eros with X-rays on May 4. The half-hour solar flare caused elements on Eros' surface to emit X-rays, a type of radiation invisible to humans. Instruments on NEAR Shoemaker analyzed the intensity of X-rays produced by the asteroid at different wavelengths, getting a fingerprint of the asteroid's chemical composition.

"Analysis of X-rays from an area roughly 3.7 miles [6 kilometers] across on Eros indicates it has an elemental composition similar to the most primitive rocks in the solar system, the chondritic meteorites," says Dr. Jacob Trombka of NASA's Goddard Space Flight Center, who heads the XGRS instrument team. Trombka presented the result May 30 during the American Geophysical Union's spring meeting in Washington, D.C.

"Chondrites are the building blocks of terrestrial planets," says Dr. Tim McCoy of the Smithsonian Institution's National Museum of Natural History, a participating scientist on the XGRS team. "If more data confirm Eros is primordial, Eros will be a link between the chondrite meteorites found on Earth and the history of the solar system's formation. With Eros, we could be looking at the structure of the solar system during a time no longer recorded on Earth."

Astronomers and geologists believe chondrites formed from the nebula of gas and dust that comprised the nascent solar system. Collisions between these dust particles eventually formed larger bodies, called planetesimals, which then collided under the influence of their mutual gravity to form planets.

Asteroids are relics of this earliest period in solar system history. The gravitational tugs exerted on asteroids by Jupiter kept a large planet from ever forming in main asteroid belt. Even today, collisions continue and fragments of asteroids occasionally fall to Earth as meteorites.

The most primitive meteorites, called chondrites, are a homogeneous mixture of heavy and light materials, a sign that the asteroid from which they originated was never subjected to melting. The process of heating and cooling that changes the nature and distribution of these materials is referred to as differentiation. If Eros is undifferentiated, like the chondritic meteorites that fall to Earth, it probably never was exposed to intense heating. In contrast, asteroids that were extensively melted have a layered structure – like that of the Earth – resulting from the separation of materials of different density.

Heavy materials, like nickel-iron metal, tend to sink to the center while lighter materials, composed mostly of silicon and oxygen, rise to the surface. If Eros were composed mostly of light materials, it would probably be a fragment from near the surface of a larger body, while an abundance of heavy materials would point to the origin of Eros as the core of a larger, broken-up asteroid.

The latest XGRS data show the surface of Eros does not have an excess of heavy or light materials, and that the relative abundance of elements is similar to that in chondrites. This indicates that the particular region observed during the solar flare on May 4 is undifferentiated and likely primordial.

"It's possible that other areas on Eros were exposed to some melting and differentiation, or that the surface composition has been altered by the constant hail of micrometeorites in space," says Trombka. "As we get closer and take more data, we will gradually unveil Eros' character. July will be particularly exciting, because we plan to descend to within 12 to 18 miles [19 to 29 kilometers] from the surface, allowing us to take higher-quality data, and our orbit will take us directly over a few large craters. Since the XGRS instrument can only identify surface composition, looking inside these large craters should reveal much about the asteroid's interior."

More than 94 million miles (152 million kilometers) from Earth, NEAR Shoemaker is currently observing Eros from an orbital altitude of 31 miles (50 kilometers). On July 7, the spacecraft will start a gradual descent into orbit 22 miles (35 kilometers) from the asteroid's center – its closest look at Eros so far. Since beginning its historic yearlong orbit around Eros on February 14, 2000, NEAR Shoemaker has circled the rotating space rock 37 times at various speeds, angles and altitudes.

The first in NASA's Discovery Program of low-cost planetary missions, NEAR was launched from Cape Canaveral Air Station, Fla., on Feb. 17, 1996. The car-sized spacecraft is observing Eros from various distances and will come within several miles of its surface before the mission ends in February 2001. The Johns Hopkins University Applied Physics Laboratory in Laurel, Md., designed and built the NEAR spacecraft and manages the mission for NASA.

The NEAR XGRS team includes members from Goddard Space Flight Center, the University of Arizona, Cornell University, and Max Planck Institute for Chemistry, and participating scientists from the Smithsonian Institution National Museum of Natural History, Harvard-Smithsonian Center for Astrophysics, Los Alamos National Laboratory, University of California at Berkeley, University of California at San Diego, and Computer Sciences Corporation.

For the latest images and mission information, visit the NEAR Web site at http://near.jhuapl.edu