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made after landing and the GRS instrument can probe tens of centimeters below the surface, these measurements reflect a volume of about 1m3 [ 35 .3 ft3]


around the detector. From the


GRS data alone, scientists could not determine whether the Fe depletion is a global composi- tional property of Eros or a localized property of the landing zone.


Although the XGRS system observed Eros during a one-year orbital period, the useful time for data collection was considerably shorter. Engineers were limited by the angular require- ments of the solar panels relative to the sun, telemetry time and periods when the surface of Eros was properly lit by the Sun. In the end, scientists found that the best quality composi- tional data were acquired during low-altitude orbits and after landing on Eros (right). Once NEAR was on the surface, the gamma ray spectrometer obtained in-situ measurements of the regolith for a period of about 14 days.3 9 The surface composition of Eros suggests that the asteroid is similar in bulk composition to a range of meteorites that have experienced minimal thermal alteration since their formation at the birth of the solar system. Scientists believe that Eros is primitive in its chemical composition and has not experienced differentiation into a core, mantle and crust. Differences between XRS and GRS data in Fe/Si ratio and an apparent deficiency of sulfur at the surface of Eros could reflect either alteration processes in the regolith during the last millions to billions of years or partial melting in the first 10 million years of solar system history.


1 2


These spectral measurements provided scientists with a new set of questions. While the spectral observations are consistent with an ordinary chondritic meteorite composition, the measurements did not establish an undisputed link between Eros and a specific meteorite type. The question remains whether Eros is unrelated to any known meteorite type, or is actually a chondrite type at depth, below the surface layers that may have been altered by weathering processes.


3 5 . NASA– Am ateurs Catch a Gam m a-Ray Burst: http: / / science. nasa. gov/ headlines/ y 2000/ ast14 m ar_ 2m . htm ( accessed April 5 , 2006) .


3 6. Trom b k a J I et al: NASA Goddard Space Flight Center: http: / / w w w . dtm . ciw . edu/ lrn/ preprints/ 4 63 1trom b k a. pdf ( accessed April 5 , 2006) .


3 7 . Dom ingue and Cheng, reference 3 2. 3 8. Dom ingue and Cheng, reference 3 2. 3 9 . Trom b k a et al, reference 3 6.


1 0 6


N E A R S urf a c e G R S S p e c tra I ron


P otassium Silicon


1 0 5 Silicon 1 0 4 1 0 3 0


O uter detector I nner detector


2 ,0 0 0 4 ,0 0 0 E nergy, keV F inal E ros images: range 1 ,1 5 0 m ( 3 ,7 7 3 ft) 3 F inal E ros images: range 2 5 0 m ( 8 2 0 ft) 6 ,0 0 0 8 ,0 0 0 1 0 ,0 0 0 O xygen I ron O xygen I ron


F inal E ros images: range 7 0 0 m ( 2 ,3 0 0 ft) 4


L ast, closest image of E ros


> Landing on Eros. The location of NEAR Shoem ak er’ s planned landing site ( top right) is show n in this im age ( y ellow circle) m osaic tak en on Decem b er 3 , 2000, from an orb ital altitude of 200 k m [ 124


m iles] . NEAR’ s im aging sy stem s w ere recording ( b ottom 4 im ages) as the spacecraft perform ed a controlled landing on the surface of Eros. At a range of 1, 15 0 m , NEAR captured an im age that spans 5 4 m [ 17 7 ft] of the asteroid’ s surface ( 1) . The large rock at the low er left corner of the im age


m easures 7 . 4 m [ 24 ft] across. NEAR then recorded im ages at ranges of 7 00 m ( 2) , 25 0 m eters ( 3 ) , follow ed b y the last im age b efore landing ( 4 ) at a range of 120 m [ 3 9 4 ft] . The  eld of view in this  nal im age m easures 6 m [ 20 ft] across. The large rock at the top of the im age m easures 4 m [ 12 ft] across. The streak y lines at the b ottom indicate loss of signal as the spacecraft touched dow n on the asteroid during im age transm ission. Once on the surface, the GRS sy stem generated gam m a ray spectrum data for a period of seven day s ( graph, top left) . These scienti c data w ere the  rst ever collected on the surface of an asteroid. The gam m a ray instrum ent has tw o sensors ( red and b lue lines) that detected clear signatures of k ey elem ents in the com position of Eros. These data, w hich surpass in q uality all the data accum ulated b y this instrum ent from orb it, helped the NEAR science team relate the com position of Eros to that of m eteorites that have fallen to Earth. ( Im ages courtesy of NASA/ J HUAPL. )


Spring 2006


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R elative intensity


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