Press Release

NEAR’s spectrometer measures Eros

By SpaceRef Editor
February 13, 2000
Filed under

NEAR spectrum of Eros

NEAR’s spectrometer measures Eros

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.

(MSI images and NIS spectra 125212660-125232784)

Image archive

SpaceRef staff editor.