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E arth swingby 0 1 / 2 2 / 9 8


1 ,1 8 6 -km altitude


L aunch 0 2 / 1 7 / 9 6 C3 = 2 5 .9 km2 / s2


Sun


E arth orbit


E ros orbit


E ros arrival 0 1 / 0 9 – 0 2 / 0 6 / 9 9


Deep-space maneuver


0 3 / 0 7 / 9 7 ∆V = 2 1 5 m/ s


> Destination Eros. The NEAR spacecraft w as successfully launched in Feb ruary 19 9 6, tak ing advantage of the uniq ue alignm ent of Earth and Eros that occurs only once every seven y ears. A Delta-II rock et placed NEAR into a tw o-y ear Earth gravity -assist traj ectory . The gravity -assist m aneuver decreased the aphelion distance w hile increasing the inclination from 0 to ab out 10° .


variations, implying a homogeneous composition. Further, the measured albedo was consistent with ground-based telescopic observations. Although significant data were gained by the Mathilde flyby, numerous questions about C-type asteroids remain unanswered. Mathilde’s density was inconsistent with common carbonaceous- chondrite meteorites found on Earth, and the asteroid’s surface appears homogeneous. So, the question remains: what connection, if any, exists between dark asteroids and meteors found in the solar system?


Detecting Gamma Ray Bursts


Gamma ray bursts (GRBs) have remained one of the great mysteries of astrophysics since their discovery more than 30 years ago. NASA’s Hubble Space Telescope made the first observation of an object associated with a GRB that was detected by the Italian BeppoSAX satellite in February 1997.3 3 Scientists believe that GRBs result from massive explosions in the distant universe that release waves of high-energy photons. GRBs seem to occur daily and emanate from random parts of the sky. GRBs represent the most powerful events known in the universe, emitting in one second as much energy as the Sun will emit in its lifetime. Spectroscopic analyses of faint, but long-lasting GRB optical afterglows have, in a number of cases, indicated Doppler shifts in the red spectrum that indicate a cosmological origin of GRBs.3 4


Time is critical in 20 k m


> A q uick look at asteroid Mathilde. This view of 25 3 Mathilde, tak en from a distance of ab out 1, 200 k m , w as acq uired shortly after the NEAR spacecraft’ s closest approach to the asteroid. Show ing on Mathilde are num erous im pact craters, ranging from m ore than 3 0 k m [ 18 m iles] to less than 0. 5 k m [ 0. 3 m iles] in diam eter. Raised crater rim s suggest that som e of the m aterial ej ected from these craters traveled only short distances b efore falling b ack to the surface; straight sections of som e crater rim s indicate the in uence of large faults or fractures on crater form ation. Mathilde has at least  ve craters larger than 20 k m [ 12 m iles] in diam eter on the roughly 60% of the b ody view ed during the NEAR  y b y . ( Im age courtesy of NASA/ J HUAPL. )


follow-up observation efforts, since GRB afterglows fade quickly, in the radio as well as optical spectrum, making it difficult for astronomers to locate the emission source.


28. For m ore on w hile-drilling spectroscopy m easurem ents: Adolph B, Stoller C, Archer M, Codazzi D, el-Halaw ani T, Perciot P, Weller G, Evans M, Grant J , Grif ths R, Hartm an D, Sirk in G, Ichik aw a M, Scott G, Trib e I and White D: “ No More Waiting: Form ation Evaluation While Drilling, ” Oil eld Review 17 , no. 3 ( Autum n 2005 ) : 4 – 21.


29 . Pair production is the chief m ethod b y w hich energy from gam m a ray s is ob served in condensed m atter. Provided there is enough energy availab le to create the pair, a high-energy photon interacts w ith an atom ic nucleus and an elem entary particle and its antiparticle are created.


were obtained during this interval at resolutions ranging from 200 to 5 00 m [ 65 6 to 1,640 ft] (above). Images obtained during the flyby of Mathilde show an asteroid with a heavily cratered surface. At least four giant craters have diameters that are comparable to the asteroid’s mean radius of 26.5 km [ 16.5 miles] . The magnitude of the impacts required to create craters of this size is significant. Scientists suspect that Mathilde did not break apart during any of these impacts


because of the asteroid’s high porosity.


Cratering processes governed by structural properties such as porosity produce craters with steep walls, crisp rims and with little ejecta, similar to those imaged on Mathilde. The images also show Mathilde is remarkably uniform. The NEAR observations revealed no evidence of any regional albedo, or spectral


Laboratory data suggest that cratering in highly porous targets is governed more by compaction of the target material than by fragmentation and excavation.3 2


3 0. Dunham DW, McAdam s J V and Farq uhar RW: “ NEAR Mission Design, ” J ohns Hopk ins APL Technical Digest23 , no. 1 ( 2002) : 18– 3 3 .


3 1. Cheng et al, reference 23 .


3 2. Dom ingue DL and Cheng AF: “ Near Earth Asteroid Rendezvous: The Science of Discovery , ” J ohns Hopk ins APL Technical Digest 23 , no. 1 ( J anuary -March 2002) : 6– 17 .


3 3 . The J ohns Hopk ins University Applied Phy sics Lab oratory – Near Spacecraft Gets Unex pected View of My sterious Gam m a-Ray Burst: http: / / w w w . j huapl. edu/ new scenter/ pressreleases/ 19 9 8/ gam m a. htm ( accessed April 5 , 2006) .


3 4 . NASA– Autom atic NEAR-X GRS Data Processing Sy stem for Rapid and Precise GRB Localizations w ith the Interplanetary Netw ork : http: / / gcn. gsfc. nasa. gov/ gcn/ near. htm l ( accessed April 5 , 2006) .


Spring 2006


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