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resolution on the order of 300 m [ 984 ft] . The MAG was used to define and map intrinsic magnetic fields on Eros.


Scientists used the NLR to enhance the surface morphology profiles derived from NEAR’s imaging camera. The NLR is a laser altimeter that measures the distance from the spacecraft to the asteroid surface by sending out a short burst of laser light and then recording the time required for the signal to return from the asteroid. The ranging data were used to construct a global shape model and a global topographic map of Eros with a spatial resolution of about 5 m.


The XGRS was the primary tool used for


surface and near-surface elemental analysis of Eros. Scientists combined data from the XGRS, MSI and the NIS to produce global maps of Eros’s surface composition.


Development of the complex XGRS system began about three years prior to launch. The instrument was designed to detect and analyze X-ray and gamma ray emissions from the asteroid surface from orbital altitudes of 35 to 100 km


[ 22 to 62 miles] . Although spectroscopy of remote surfaces is possible during spacecraft flyby operations, measurements made while orbiting allow longer observation times and produce higher quality spectral data. X-rays emitted from the Sun shining on Eros produce X-ray fluorescence from the elements contained in the top 1 mm [ 0.04 in.] of the asteroid’s surface. In the absence of any significant atmosphere that might otherwise absorb X-ray emissions, elements fluoresce at energy levels that are characteristic of specific elements. Scientists used the X-ray fluorescence energy detected in the 1- to 10-keV level to infer surface elemental composition.


The XRS subunit consists of three identical gas-filled proportional counters that provide a large active surface area and therefore the sensitivity required for remote sensing. Similar detectors have been used on lunar orbital missions and most recently on Apollo missions. The X-ray gas tubes are not particularly sensitive to temperature change, since the multiplication effect depends more on the number of gas molecules than the gas pressure. However, the gain in the gas tubes is sensitive to voltage variations. Gamma ray spectrometry provides a complementary measurement of near-surface elemental composition. The gamma ray spectrometer (GRS) detects discrete-line gamma ray emissions in the 0.1- to 10-MeV energy range.


Aft deck M ultispectral imager


N ear-infrared spectrometer


X -ray spectrometer N E AR laser rangefinder


> NEAR spacecraft sy stem s. NEAR’ s b asic design and prim ary sy stem s are show n. ( Im age courtesy of NASA/ J HUAPL. )


Solar panel


X -ray solar monitors


P ropulsion system F orward deck Side panels


G amma ray spectrometer


At these energy levels, oxygen [ O] , silicon [ Si] , iron [ Fe] and hydrogen [ H] become excited, or radioactively activated, from the continual influx of cosmic rays. The GRS also detects naturally radioactive elements such as potassium [ K] , thorium [ Th] and uranium [ U] . The measure- ments have been used for years in oil and gas well logging to determine the physical and elemental composition of reservoir rock.


21. Asteroids are classi ed b ased on re ectance spectrum and light-re ection characteristics, or alb edo, w hich are indicators of surface com position. S-Ty pe ( silicaceous) asteroids are m ore prevalent in the inner part of the


m ain asteroid b elt, w hile C-Ty pe ( carb onaceous) asteroids are found in the m iddle and outer parts of the


b elt. Together, these tw o ty pes account for ab out 9 0% of the asteroid population.


Perihelion and aphelion are the orb ital points nearest and farthest from the center of attraction— in this case, the Sun.


22. A m eteorite is a solid portion of a m eteoroid that survives its fall to Earth. Meteorites are classi ed as stony m eteorites, iron m eteorites and stony iron m eteorites, and further categorized according to their m ineralogical content. They range in size from


m icroscopic to m any m eters across. Of the several tens of tons of cosm ic m aterial entering Earth’ s atm osphere each day , only ab out one ton reaches the ground.


23 . Cheng AF, Farq uhar RW and Santo AG: “ NEAR Overview , ” J ohns Hopk ins APL Technical Digest 19 , no. 2 ( 19 9 8) : 9 5 – 106.


Unlike the low-energy X-rays, gamma rays are not as easily absorbed and therefore can escape from regions beneath the surface, allowing the GRS to reveal elemental composition to depths as much as 10 cm [ 4 in.] below the surface. By comparing elemental analysis from the XRS and GRS, scientists inferred the depth and extent of the dust layer, or regolith, covering the surface of Eros.2 7


24 . Chondrites are a ty pe of stony m eteorite m ade m ostly of iron- and m agnesium -b earing silicate m inerals. Chondrites are the m ost com m on ty pe of m eteorite, accounting for ab out 86% that fall to Earth. They originate from asteroids that never m elted, or underw ent differentiation. As such, they have the sam e elem ental com position as the original solar neb ula. Chondrites derive their nam e from the fact that they contain chondrules— sm all round droplets of olivine and py rox ene that apparently condensed and cry stallized in the solar neb ula and then accreted w ith other m aterials to form a m atrix w ithin the asteroid.


25 . Cheng et al, reference 23 . 26. Cheng et al, reference 23 .


27 . Regolith is a lay er of loose m aterial, including soil, sub soil and b rok en rock , that covers b edrock . On Earth’ s


m oon and m any other b odies in the solar sy stem , it consists m ostly of deb ris produced b y m eteorite im pacts and b lank ets m ost of the surface.


Spring 2006


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