NEARs spectrometer measures Eros
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NEAR's spectrometer measures Eros

October 22, 2009
Throughout NEAR's approach to Eros, both the Multispectral Imager (MSI) and the Near-Infrared Spectrometer (NIS) have acquired measurements of the asteroid's light-reflecting properties. One of the standard ways that astronomers study asteroids is to measure their "light curves", or how their brightness varies as they spin because of shape, orientation, spin axis, and surface markings.

This plots shows light curves of Eros measured both by MSI and by NIS, on February 6 when NEAR was about 4200 miles (6800 kilometers) from Eros. The plot shows the relative brightness of Eros over the course of one Eros "day" (5.27 hours). Eros was brightest near the beginning of the observations, then dimmed by about 20% over the course of the next 2 hours, and by an hour later it brightened to about 5% brighter than at the start. Then it dimmed again by about 40% at 4 hours from the start of observations, and finally returned back to its original brightness at the start. This pattern of two brightness cycles per Eros "day" is characteristic and results from alternating viewing of Eros's long and short dimensions. Observations like this, taken from telescopes, provided the first clues that Eros has an elongated, peanut-like shape.

As of the time of this posting, NEAR is conducting what is called a "low-phase angle flyover" of Eros. That is, the spacecraft passes between the asteroid and the Sun. To an observer on the spacecraft, Eros would appear at a "full" phase in contrast to the partly illuminated gibbous asteroid that has been seen during NEAR's approach. The low-phase angle flyover provides the optimal condition for NIS to spectrally map the surface to determine what types of minerals are present.

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