Observation of 433 Eros with the X-Ray/Gamma-Ray Remote Sensing Spectrometer on the NEAR Mission

The NEAR (Near Earth Asteroid Rendezvous) Mission spacecraft will rendezvous with and provide one year of orbital data coverage for the Earth-crossing asteroid Eros. It is believed that Eros is an S-type asteroid, the most common type within the main asteroid belt and may represent either differentiated or undifferentiated mineralogical materials. One of the primary objectives of the NEAR mission is to obtain global elemental composition maps of the asteroid with sufficient accuracy to enable comparison with major meteorite types. Measurements made by the XGRS (X-ray/Gamma-ray spectrometer should allow this objective to be met. Remote sensing X-ray spectrometers are used for elemental analysis of the surfaces (to a depth of tens of microns) of atmosphereless bodies. The X-ray emission spectrum, the source of which is incident X-rays from the Sun, is dependent on both the nature of this solar flux (i.e. the intensity and slope) and the composition of the surface. The intensity of the incident solar spectrum will vary by orders of magnitude, from lowest output during the so-called "quiescent Sun" to highest output during major flares. An increase in solar activity tends to harden the solar spectrum, producing more energetic solar X-rays. This hardening enhances the emissions of the higher Z elements (e.g. Ca, Ti, Fe) relative to the lower Z elements (e.g. Al, Mg, Si) in the planetary surface thus observations must be made of the incident solar X-ray spectrum in order to infer both qualitative and quantitative elemental information. The design of the instrument was influenced by factors such as reliability, cost, weight, and space flight heritage. Three large thin window sealed gas proportional counters with balanced, differential filters, similar to instruments which have been previously flown on successful missions, were the choice for asteroid pointing detectors on NEAR. These specially designed filters will allow clean separation of the three most prominent, characteristic lines: Mg (1.25 keV), Al (1.49 keV), and Si (1.74). To track the incident solar X-ray emission spectrum, a proportional counter and a solid state detector are used. Gamma rays of characteristic energy are emitted by excited nuclei. The natural radioactive elements (K, U, Th) will produce characteristic lines during decay. For other elements, excitation is provided by cosmic-ray bombardment. Except during or just after major solar flares, the response is mainly caused by galactic cosmic ray primary and secondary particles. Secondary high energy neutrons may excite stable nuclei to higher levels by inelastic scatter, the (n, n', gamma) reaction and produce characteristic gamma-ray emission or they may lose energy until they are "thermalized" and captured, giving rise to capture gamma-rays, the (n, gamma) process. The moderation and thermalization strongly depend on the composition of the material, particularly the H content. Gamma-rays are also produced by forming such radioactive nuclei as Al-26 or Be-7 and by several other minor processes. The gamma-rays measurements reflect bulk composition down to the order of tens of centimeters. The NEAR Gamma-ray Remote Sensing Spectrometer utilizes a collimated scintillation detector system. The central detector is a sodium iodide (NaI(Tl)) detector. Bismuth Germinate (BGO) is used as the active shield/collimator. One very important function of the BGO shield is to reject the background signals from both the detector's own structural material as well as the spacecraft. Data from these experiments will allow determination of the meteorite class and sub-class (e.g., Ordinary chrondrite, Type H) of Eros. The degree of differentiation, based on primarily on Mg and Si derived from the X-ray experiment, will be determined on a 5 km or better scale during the course of the mission. Average U, Th, K, Al, and Ti concentrations will also be determined. Surface elemental composition maps will be obtained from Mg, Si, Ca, and Fe.