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NEAR—The Scientific Mission


Prior to the NEAR mission, our knowledge of asteroids came primarily from three sources: Earth-based remote sensing, data from the Galileo mission flybys of the two main-belt S-type asteroids 95 1 Gaspra and 243 Ida, and laboratory analyses of meteorites recovered after impact with the Earth.


Although astronomers theorize that most meteors result from the collision of asteroids, they may not be completely representative of all materials that comprise NEAs.2 2


Clear links


between meteorite types and asteroid types proved difficult to establish.2 3


> Approaching Eros. This im age of the southern hem isphere of Eros offers a long-distance look at the asteroid’ s cratered terrain. ( Im age courtesy of NASA/ J HUAPL. )


Scientists believe that nonmelted S- type asteroids may have preserved the characteristics of the solid material from which the inner planets accreted.


Some S-type asteroids appear to be fragments of bodies that underwent substantial melting and differentiation, while others consist of what appears to be nonmelted primitive materials like chondrites.2 4


The Galileo mission flybys provided the first high-resolution images of asteroids in the early 1990s. Images revealed complex surfaces covered by craters, fractures, grooves and subtle color variations (left).2 5


However, Galileo’s


instrumentation was not capable of measuring elemental composition, so prior to the NEAR mission, scientists continued to be unsure of the relationship between ordinary chondrites and S- type asteroids.


Mission engineers believed that the NEAR Mathilde Gaspra Ida > Asteroids close up. Show n are view s of the three asteroids that had b een im aged at close range


b y spacecraft prior to NEAR’ s arrival at Eros. The im age of Mathilde ( left) w as tak en b y the NEAR spacecraft on J une 27 , 19 9 7 . Im ages of the asteroids Gaspra ( m iddle) and Ida ( right) w ere tak en b y the Galileo spacecraft in 19 9 1 and 19 9 3 , respectively . All three ob j ects are presented at the sam e scale. The visib le part of Mathilde is 5 9 k m w ide b y 4 7 k m long [ 3 7 b y 29 m iles] . ( Im ages courtesy of NASA/ J HUAPL. )


data, when combined with those from the Galileo flybys, would help scientists understand the relationship between S-type asteroids and other small bodies of the solar system. The NEAR mission’s primary objectives were to rendezvous with, achieve orbit around and conduct the first scientific exploration of a near-Earth asteroid.


The NEAR Spacecraft Engineers designed NEAR’s systems to be solar- powered, simple and highly redundant.2 6


Onboard


The large S-type potato-shaped asteroid is one of the most elongated asteroids. It orbits around the Sun, rotating on its axis once every 5 .27 hours, with a perihelion of 1.13 AU and an aphelion of 1.78 AU (top).2 1


NEAR departed Earth for asteroid Eros on February 17, 1996, riding on top of a Delta-II launch vehicle. One year later, on February 18, 1997, NEAR reached its most distant point from the Sun, 2.18 AU, setting a new distance record


for a spacecraft with instrumentation powered by solar cells.


By the end of its five-year mission, NEAR would produce an impressive list of spacecraft firsts: the first spacecraft with instrumentation solely powered by solar cells to operate beyond the orbit of Mars, the first to encounter a C-type asteroid, the first to encounter a near-Earth asteroid, the first to orbit a small body, and the first spacecraft to land on a small body.


NEAR were five instruments designed to make detailed scientific observations of the gross physical properties, surface composition and morphology of Eros. These five were the multi- spectral imager (MSI), near-infrared spectrom- eter (NIS), magnetometer (MAG), NEAR laser rangefinder (NLR) and the combined X-ray, gamma ray spectrometer (XGRS) (next page). The MSI imaged the surface morphology of Eros with spatial resolutions down to 5 m [ 16.4 ft] , while


scientists used the NIS to measure mineral abundances at a spatial


5 0


Oilfield Review


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