IMAGING IN SPACE: Instruments trace the history of Eros

Images taken by the Near Earth Asteroid Rendezvous (NEAR) mission are giving scientists a close-up view of 433 Eros, the largest asteroid that comes close to Earth's orbit. The NEAR spacecraft settled into orbit around the 40 x 14 x 14-km, potato-shaped rock on February 14, where its instruments began studying the asteroid, looking for clues to the asteroid's origin that will tell scientists more about the history of the solar system. This picture is a mosaic of four images.

Images taken by the Near Earth Asteroid Rendezvous (NEAR) mission are giving scientists a close-up view of 433 Eros, the largest asteroid that comes close to Earth's orbit. The NEAR spacecraft settled into orbit around the 40 x 14 x 14-km, potato-shaped rock on February 14, where its instruments began studying the asteroid, looking for clues to the asteroid's origin that will tell scientists more about the history of the solar system. This picture is a mosaic of four images.

One of the instruments, the Multi-Spectral Imager (MSI), is responsible for recording surface features and mapping the distribution of minerals in the asteroid. The device consists of a five-element refractive telescope with a passively cooled silicon charge-coupled device (CCD), a wheel with a broadband filter and seven spectral filters, and a computer. The MSI has a field of view of 2.26° x 2.95° in 244 x 537 pixels, equivalent to 3.9 x 5.1 km when the spacecraft is 100 km from the surface. At that distance, the resolution is 9.6 x 16.2 m. The CCD is sensitive to wavelengths from 400 to 1100 nm, with filters at 450, 550, 760, 900, 950, 1000, and 1050 nm.

The instrument will allow scientists to determine the shape, size, and volume of Eros and to study the surface in detail. Already, pictures showing a large number of craters reveal that the asteroid is very old. Uniform grooves in the craters and ridges suggest that the whole body has the same makeup and may have underground layers. That could mean Eros was once part of a larger body that was big enough to support volcanic activity.

Also onboard is the Near-Infrared Spectrometer, which is measuring spectra of reflected sunlight over regions as small as 300 m. The scanning spectrometer contains a germanium array that detects wavelengths from 804 to 1056 nm in 21.6-nm increments, as well as an indium gallium arsenide array that detects from 1348 to 2732 nm in 43.1-nm increments. A gold scan mirror rotates over 140°. The instrument has picked up variations in the mineral composition of the asteroid. Determining the proportions of such minerals as pyroxene and olivine will give clues to the geologic history of the body. Combining the spectrometer's measurements with the multispectral images, which have revealed some bright spots, will let scientists construct the first-ever mineral map of an asteroid.

"We want to correlate the changes in color with the geologic features," says Scott Murchie of Johns Hopkins University (Laurel, MD). "If we see a crater, for example, is it different on the outside than on the inside? Is the face of a cliff different than the ridge? These data will eventually tell us about the asteroid's history."

A laser rangefinder consists of a diode-pumped Nd:YAG delivering 12-ns, 15-mJ pulses at 1.064 µm and an enhanced hybrid silicon avalanche-photodiode detector. It will use time-of-flight measurements to map the entire surface of the asteroid and to measure its rotational dynamics. This will tell scientists about the internal density of the asteroid and help them decide whether it formed as one piece or was created when smaller pieces smashed together. The asteroid Mathilde, which NEAR photographed as it flew by in 1997, is a pile of rubble bound together by gravity, but Eros appears to be a fairly solid body, scientists said.

Other instruments aboard NEAR include an x-ray spectrometer, a gamma-ray spectrometer, and a magnetometer. The spacecraft will collect data for a year, and scientists back on Earth will continue to analyze the images.

Neil Savage

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