The X-ray Gamma-ray Spectrometer (XGRS) instrument on the Near Earth Asteroid Rendezvous (NEAR) Shoemaker spacecraft analyzed this X-ray glow and identified the various elements present. Eros is represented as the blue, irregularly-shaped object in the image. The red areas highlight the regions on Eros that glowed in response to the flares. Photo: JHU/APL

Like a shy bride peeking from beneath her veil, the asteroid Eros gave researchers using the NEAR Shoemaker spacecraft the first glimpse of its true nature after massive solar flares briefly lit its surface. The elements magnesium, aluminum, silicon, and iron were observed. These are important elements in studying the formation of solar system bodies.

Two powerful explosions on the Sun, called solar flares, blasted the surface of Eros with X-rays on March 2. Each 300-second blast caused elements at Eros' surface to glow in unique X-ray "colors," depending on the element's classification. The X-ray/Gamma-ray Spectrometer (XGRS) instrument on NEAR Shoemaker analyzed this X-ray glow and identified the various elements present.

"This is the first time anyone has used an X-ray signature to identify the composition of an asteroid, and it's very exciting," said Dr. Jacob Trombka, XGRS Science Team Leader at NASA's Goddard Space Flight Center, Greenbelt, Md. "The interpretation of these types of X-ray signatures will help us determine whether this asteroid can be associated with any of the known classes of meteorites and further our knowledge of the origin of Eros. We only saw a small area of Eros from an extreme angle, and, at more than 180 miles, NEAR Shoemaker is still far from Eros. This first observation indicates that when we get closer, our data will improve."

The ability to record and analyze the X-ray glow at such a distance gives the XGRS team confidence that their mission, scheduled to begin in May when NEAR is only 31 miles from Eros, will be successful.

Eros is seen from two different angles because the NEAR Shoemaker spacecraft progressed in its orbit around Eros between the occurrences of the flares. Eros is represented as the blue, irregularly-shaped object in the image. The red areas highlight the regions on Eros that glowed in response to the flares. Photo: JHU/APL

"Eros is a Rosetta stone that can return us to our cosmological roots by determining what happened during the solar system's formation, and the XGRS is a key to translating its language - that of geochemistry," said Dr. Pamela Clark of Goddard.

When a forming planet reaches a certain size, its heat and gravity become sufficient to cause differentiation of its elements - the planet becomes molten, and heavy elements, like nickel and iron, tend to sink to the center, while light elements, like silicon and carbon, rise to the surface. If Eros is composed mostly of light elements, this indicates that it is a fragment from near the surface of a larger body. If it is rich in heavy elements, it is likely from the center of a larger object. If instead it is undifferentiated, like the chondrite meteorites found on Earth, it probably never was exposed to much heat. This implies that it is a very primitive, unprocessed object, probably an aggregate of smaller objects from the early solar system nebula.

"The last successful observation of characteristic X-ray emissions from a solar system body, the moon, was carried out by our Goddard group during the Apollo 15 and 16 missions over twenty five years ago," said Trombka.