This hardy spacecraft may have burned up in the Earths atmosphereThis hardy spacecraft may have burned up in the Earths atmosphere

48 July 2000 Sky & Telescope

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Comptons Legacy

by Peter J. T. Leonard and Christopher Wanjek

The highly productive and long-lived ComptonGamma Ray Observatory mission which documenteda previously little-known world of gamma-ray bursts, unusualpulsars, and black-hole-fueled particle jets has come to anend with the failure of one of the satellites three gyroscopes.On March 24th NASA announced the decision to direct thesatellite back to Earth. If all goes according to plan, Comptonwill glide into Earths atmosphere during the first days ofJune, less than a month after this issue of Sky & Telescope goesto press. At 17 tons, Compton is too large to burn up entirelyon reentry. Therefore it must be politely guided toward the

isolated waters of the Pacific Ocean.But what a trip its been! Launched in 1991, Compton was the

second of NASAs four planned Great Observatories, establishingnew heights for the gamma-ray realm. What the Hubble SpaceTelescope (the first Great Observatory) and the Chandra X-rayObservatory (the third) are doing for optical and X-ray astrono-my, Compton has done for gamma-ray astronomy. (The SpaceInfraRed Telescope Facility, or SIRTF, Part Four in the Great Ob-servatory story, wont be launched until 2001.) Compton provid-ed NASA with an unprecedented chunk of the electromagneticpie, covering a broader range in energy than any other observa-

Highlights from the Gamma Ray Observatory

Launched on April 5, 1991, and deployed twodays later from the Space Shuttle Atlantis, theCompton Gamma Ray Observatory (CGRO) hasenabled astronomers to scrutinize the high-energy sky for nearly a decade. The second ofNASAs Great Observatories, the spacecraft wasnamed after Arthur Holly Compton (18921962),who explained how high-energy photons arescattered by electrons. NASA photograph.

Truth be told, Compton still had some juice left. All of itsinstruments functioned more or less perfectly. NASAs decisionto deorbit was based mainly on the 1-in-1,000 chance of some-one being injured should the spacecraft come down in a com-pletely uncontrolled manner. Plan A called for a deorbit whilethere are still two working gyroscopes, for two were deemednecessary to steer the massive satellite safely. Deorbiting withone gyro, should the other fail, would be a bit riskier.

NASA space-flight engineers, an ever-resourceful and auda-cious breed, said they could safely steer the spacecraft back toEarth without any gyros. This group spent several months per-fecting Plan B while some Compton scientists argued that thesatellite could be useful for observing the now-ongoing solar-activity maximum. In the end, NASA didnt want to chance it.

A tough loss, yes. But the Compton team realizes that itssatellite was wildly successful, a workhorse that lived many yearslonger than expected. Reflecting on the mission, the teamboasts that Compton brought our understanding of gamma-raybursts, quasars, and pulsars to new levels. Here we summarizebut a few of the many hits from Comptons stellar career.

Comptons greatest achieve-ment, in many peoples minds,was its work on gamma-raybursts (GRBs). Although these

bursts were discovered in 1967, Compton wasthe first satellite that truly enabled an in-depthstudy of the phenomenon.

GRBs are the most energetic events knownin the universe, second only to the Big Bang inpower. During a GRBs flash as short as afew milliseconds or as long as a minute ormore the burst can outshine the rest of thegamma-ray universe. Then it disappears forev-

er. The nature of the bursting objects remains unknown.The 300-odd GRBs known prior to Compton were thought

to be associated with neutron stars within the plane of the MilkyWay. Comptons BATSE instrument, many assumed, would sim-ply confirm this scenario, and GRBs would be relegated to an in-significant place in high-energy astrophysics research.

Instead, BATSE showed the GRB distribution to be isotropic(favoring no direction over another) and spatially limited (thatis, the distribution has an outer edge). This rules out the galactic-plane hypothesis and favors the notion that GRBs originate atcosmological distances, vastly beyond our galaxy.

by now, but its discoveries still reverberate among astrophysicists.by now, but its discoveries still reverberate among astrophysicists.

tory six orders of magnitude, nearly a million times widerthan the visible portion of the spectrum.

Compton managed this feat with four main instruments(BATSE, OSSE, COMPTEL, and EGRET), each handling a dif-ferent patch of the gamma-ray spectrum. Compton neededfour instruments because the gamma-ray electromagneticband is so broad. Also, gamma rays of different energies inter-act with matter in different ways, so different types of technol-ogy must be employed to collect them.

Compton needed to be in orbit because the Earths atmos-phere, which wonderfully protects our body tissue from harmfulgamma-ray radiation, makes the gamma-ray light show impos-sible to see. Ground-based gamma-ray observatories can observeonly the very most energetic cosmic gamma rays (those with en-ergies measured in trillions of electron volts), and indirectly atthat, via cascades of secondary particles created when those sin-gular photons slam into the Earths atmosphere (S&T: Septem-ber 1995, page 20). Although imperative for certain types of sci-ence, ground-based observatories cannot witness anywhere nearthe range of gamma-ray activity that a satellite can.

Sky & Telescope July 2000 49

Gamma-RayBursts

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How gamma-ray bursts (GRBs) may comeabout. Two city-size, Sun-and-a-half-massneutron stars spiral together at an ever-increasing pace (left panel), eventually merg-ing (center panel) and forming a black holewith jets fueled by a transient accretion disk(right panel). Billions of years later, the explo-sion briefly manifests itself as a burst ofgamma rays in our sky.

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Comptons four scientific instruments were designed for gamma raysof different energies and optimized for different kinds of studies:light curves of transient phenomena like gamma-ray bursts (BATSE);detailed spectra of sources as diverse as the Sun and active galacticnuclei (OSSE); and all-sky maps at moderately high (COMPTEL) andextremely high (EGRET) gamma-ray energies. Sky & Telescope dia-gram based on NASA material.

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50 July 2000 Sky & Telescope

Accretion disk(cross-section)

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According to one model, a blazars gamma rays emerge chiefly froma shock front within the objects Earthward-pointing jet. Adaptedfrom a diagram by Alan Marscher (Boston University).

When combined with observations spanning the electromagneticspectrum, gamma-ray data have made it clear that these highest-energy photons constitute most of the quasar 3C 279s radiant ener-gy, especially during flares. Inset: This false-color image shows howvery energetic gamma rays (those above 100 million electron volts)from 3C 279 and 3C 273 (both in Virgo) appeared to ComptonsEGRET instrument in February 1996. The crosses represent the quasarsoptical positions. Courtesy the EGRET team and NASA.

North Galactic pole

South Galactic pole

Two-thirds of the way through Comptons career, the Italian-Dutch BeppoSAX satellite discovered that many GRBs give offX-rays hours after the gamma-ray flash. Furthermore, Bep-poSAX has been able to determine the positions of these X-rayafterglows with a precision of a few arcminutes roughly 50times more precise than BATSE. Follow-up observations at vi-sual and radio wavelengths have nailed down the cosmologicalinterpretation: redshifts for GRB counterparts range fromroughly 0.4 to 4.0, implying distances measured in billions oflight-years (S&T: February 1998, page 32). Thats way out there.

While BATSE provides only crude coordinates for GRBs, itrelays them to the astronomical community in near-real time.The idea is that someone, somewhere will record the burstwith something before it fades from view. On January 23,1999, BATSE helped a robotic camera catch the first visible-light GRB counterpart seen to flare at the same time as thegamma-ray flash. The burst briefly reached 9th magnitudeand would have been visible with good binoculars (S&T: May1999, page 54).

BATSE may be in ashes, but there is gold in its treasurechest of data. Characteristics of the bursts light curves mayenable astronomers to determine GRB distances, allowing thebursts to be used as cosmological probes even when no visi-ble-light counterpart is seen. This and other archival uses ofComptons data may someday help astronomers figure out theultimate cause of GRBs.

Quasars are the extraordinarilybright cores of very distant galaxies,

and they often are visible at radio and X-rayenergies as well as in ordinary light. This emis-sion is likely produced by a supermassive blackhole accreting copious amounts of interstellargas (S&T: May 1999, page 40). Along withgamma-ray bursts, quasars are among the mostdistant objects known to science.

When Compton was launched, quasars werenot well understood. The only quasar seen ingamma rays before Compton was 3C 273, de-tected in 1976 by the European Space AgencysCOS-B satellite. When Comptons EGRET in-

Blazars

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The positions of 2,408 gamma-ray bursts detected with BATSE sug-gest that the bursts took place billions of light-years away and aremore luminous than supernovae. Courtesy the BATSE team.

Radio waves, visible light, and gamma rays are all composed of photons particles of electromagnetic radiation that differ onlyin wavelength. Gamma rays cannot penetrate Earths atmosphere, soastronomical sources of them must be studied from orbit. Adaptedfrom Discovering the Cosmos by R. C. Bless.

Compton showed that gamma-ray bursts originateCompton showed that gamma-ray bursts originate

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strument stared at 3C 273 in 1991, it found another quasar inthe same field of view. This other quasar, named 3C 279, wasmany times brighter than 3C 273 to EGRETs eyes. It just sohappened that 3C 279 was undergoing a flare that made it oneof the brightest sources of high-energy gamma rays in the entiresky at the time, despite its distance of 4 billion light-years.

Quasars visible at gamma-ray energies are now called blazars,and EGRET established them as a class of astronomical objects.The Third EGRET Catalog contains 66 high-confidence blazarsand 27 lower-confidence ones. Blazars represent the largest well-defined class of nontransient gamma-ray sources.

As with quasars, each blazar likely harbors a central super-massive black hole with a pair of relativistic jets emanating inopposite directions. The bright, highly variable emission charac-teristic of blazars can be seen when the observer looks almostalong one of the jets that is, down the barrel of the gun.

Many of the unidentified gamma-ray sources EGRETfound at high galactic latitudes may be blazars. A multitude ofblazars also may account for a diffuse, isotropic, high-energy,gamma-ray background first observed by NASAs SAS-2(Small Astronomy Satellite 2) in the 1970s and subsequentlyconfirmed by EGRET at photon energies of 30 million elec-tron volts or more.

Supernovae reveal them-selves to Compton in a uniqueway through gamma rayscreated by the radioactive de-

cay of trace elements in their fiery ejecta.A supernova occurs after a massive star ex-

hausts its nuclear fuel, allowing the core of thestar to collapse suddenly, then explode. The ex-ploding stars outer layers are thrown off intothe interstellar medium and the mix is visibleas a supernova remnant (SNR). For a few daysto a couple of weeks, a single supernova canoutshine its host galaxy.

On February 24, 1987, Supernova 1987Awent off in the Large Magellanic Cloud (LMC). The Solar Max-imum Mission saw the radioactive decay of cobalt-56 (56Co) sixmonths later. This was the first direct confirmation of the wide-ly held belief that heavy chemical elements are forged by super-novae. Shortly after Comptons launch in 1991, the OSSEinstrument detected gamma rays from SN 1987Aspawned cobalt-57 (57Co), which has a half-life of272 days. The data were used to determine thatthe abundance ratio of iron isotopes (56Fe/57Fe)in the LMC was 1.5 times that of our Sun, argu-ing against a much more substantial ratio,which had been reported previously on thebasis of a less-reliable analysis.

Gamma rays from the decay of titanium-44(44Ti) and aluminum-26 (26Al), both present in su-pernova remnants, are food for COMPTEL. Titanium-44 has a half-life of around 60 years; aluminum-26s is700,000 years. As such, these isotopes serve as tracers for new

Sky & Telescope July 2000 51

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Some 10,000 light-years away, Cassiopeia Ais the expanding remnant of a supernovaexplosion that Earthbound astronomerscould have seen around 1680 if they hadhad X-ray or radio telescopes at their dis-posal. These false-color X-ray (top left) andradio (bottom left) views, from the ChandraX-ray Observatory and the Very LargeArray, respectively, detail the interaction ofthe dead stars castoffs with surroundingmatter. Comptons gamma-ray spectrum ofCas A (above) measured the amount of ti-tanium-44, a short-lived radioactive prod-uct of supernova explosions.

Below: This false-color all-sky map from the COMPTEL instrumentshows the brightness of gamma rays from aluminum-26 nuclei. Witha half-life of 700,000 years, these long-lived nuclei trace the galaxysstar-formation history over the last several million years. The gridrepresents galactic coordinates, with the North Galactic Pole on top.Courtesy the COMPTEL collaboration and NASA.

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supernovae and old ones, respectively. A prime example of avery young supernova is SNR GRO J0852-4642 near the VelaSNR. This remnant, approximately 680 years old and 650 light-years away, was discovered independently by COMPTEL andthe X-ray-sensitive Rosat satellite. The extremely bright VelaSNR dominates the region and kept the younger, closer SNRhidden until recently. Why this supernova wasnt seen (or atleast recorded) by astronomers 680 years ago is a mystery in itsown right (S&T: April 1999, page 22).

COMPTEL also detected 44Ti emission from the CassiopeiaA supernova remnant, enabling estimates of the isotopesyield. Cassiopeia A is likely about 300 years old, but the super-nova that spawned this remnant also wasnt recorded by as-tronomers (if indeed it ever was visible to the eye).

In 1979, the HEAO (High Energy Astronomy Observatory) 3spacecraft was the first to see gamma-ray emission due tonucleosynthesis: the radioactive decay of small quantities of26Al produced by massive stars and expelled into the interstel-lar medium. Aluminum-26 emission traces the galaxys recentstar-formation history. COMPTEL mapped this emission withunprecedented angular resolution and found that it is indeedconcentrated in regions of star formation. The data have beenused to calculate that there are roughly 1 to 2 solar masses of26Al in the galaxy.

Comptons legacy in theworld of pulsars was the

revelation that some may be pulsing primarily(if not only) in gamma rays, and not at thetelltale radio wavelengths preferred by mostpulsars. This revelation will radically altertodays pulsar census as tomorrows high-reso-lution gamma-ray instruments find more ofthese gamma-ray pulsars.

A pulsar is a rotating neutron star with astrong dipolar magnetic field. Created during asupernova explosion, a neutron star packs amass slightly greater than the Suns into a spherewith a 10-kilometer radius. As it spins, the neu-

tron star produces a beam of radiation from charged particlestrapped in its intense magnetic field. An observer fixed in spacesees pulses of this radiation as the beam periodically sweepsthrough his or her line of sight, hence the name pulsar.

Since their discovery in 1967, pulsars have been largely thedomain of radio astronomers. There are hundreds now identi-fied in our galaxy. Before Compton, only two the Crab andVela pulsars were known to emit gamma rays, and they didso in conjunction with their radio pulses. Compton found fivemore gamma-ray-emitting pulsars. These objects tend to beyoung and rapidly rotating. The Crab pulsar, for example, ro-tates 30 times per second. Pulsars rotating at these speedsseem to efficiently accelerate particles to very high energies.

A mysterious object called Geminga (for Gemini gamma-ray source) has turned out to be a pulsar. Initially Gemingacould be seen only at gamma-ray wavelengths, and in an Italiandialect the word has a second meaning: It is not there. Eventu-

ally X-ray pulses were seen from this region, and EGRET founda pulse period identical to that seen in X-rays, confirming thatGeminga is indeed a pulsar. Geminga has only recently been de-tected at radio wavelengths, where pulsars have been tradition-ally discovered (and some astronomers find the radio data un-convincing). Several of the 170 unidentified EGRET sourcesmay turn out to be Geminga-like pulsars.

Soft Gamma Repeaters (SGRs) are another class of neutronstars that Compton has scrutinized profitably. SGRs sporadical-ly emit short bursts of soft gamma rays, those with energiesbelow 10,000 electron volts. Before Compton, many thoughtSGRs had something to do with neutron stars that have very

52 July 2000 Sky & Telescope

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A spinning neutron star with a tilted magnetic field can generatetwo opposing beams of particles and electromagnetic radiation; ifat least one of the latter can be seen from Earth, astronomers detectthe spinning star as a pulsar. Adapted from a diagram by JoshuaWinn (MIT).

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Pulsed gamma rays have been detected by Compton from seven pul-sars. But Geminga, on the Gemini-Orion border, stands out becausefor years its pulses were seen only by X-ray and gamma-ray satellites.(Other gamma-ray pulsars, like the Crab, generally were discoveredand first characterized with radio telescopes.) Courtesy the EGRETteam and NASA.

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strong magnetic fields, and perhaps even with GRBs. Comptonhelped demonstrate that the former belief was true (in spades)and that the latter belief was completely false.

The three SGRs known before Compton were all discoveredin 1979. The March 5, 1979, outburst of SGR 0526-66 in theLarge Magellanic Cloud released more energy in gamma raysin one-tenth of a second than the Sun has released at all wave-lengths over the past 1,000 years. A powerful outburst fromSGR 1900+14 briefly disrupted communications on and nearEarth on August 27, 1998, even though the object lies roughly20,000 light-years distant (S&T: January 1999, page 22).

BATSE discovered a fourth SGR in June 1998, now calledSGR 1627-41. Observations from the Rossi X-ray Timing Ex-plorer (RXTE) satellite and the so-called InterPlanetary Net-work (IPN) of spacecraft helped link this SGR to a supernovaremnant called G337.0-0.1 for its galactic coordinates in theconstellation Ara. Many now agree that SGR outbursts are dueto starquakes on magnetars, neutron stars born with ex-tremely strong magnetic fields (1014 gauss). SGRs also appearto be related to anomalous X-ray pulsars, or AXPs. But SGRshave nothing to do with GRBs, which are vastly more power-ful events that take place in distant galaxies.

Solar flares are explosionsof energetic particles and electro-

magnetic radiation in the outer atmosphere of the Sun. Lessons learned from solar explo-sions apply to much larger explosions that we see elsewhere in the universe. Closer tohome, these solar particles can cause commu-nications and electrical problems on Earth(March issue, page 50).

Compton was launched just after the lastpeak in solar activity, or solar maximum, butfortunately the Sun was still active and Comp-ton got itself a tan with several large flares inJune 1991. In this regard, Compton was fol-

lowing in the footsteps of OSO (Orbiting Solar Observatory)7 (197174) and the Solar Maximum Mission (198081 and198489), the only other space missions to see gamma-rayemission lines from solar flares.

Comptons spectrometer, OSSE, detected several nuclearemission lines from a solar flare on June 4, 1991, includingthose of iron, magnesium, neon, silicon, carbon, oxygen, andnitrogen. These give information about the abundances of ele-ments in the ambient coronal gas. EGRET detected a high-energy afterglow from a solar flare on June 11, 1991. No spec-tral cutoff was detected, so presumably the flare produced pho-tons with even higher energies than those picked up by EGRET.

For its part, COMPTEL detected neutrons from a solar flareon June 15, 1991. (The instrument is able to discern whenneutrons, rather than gamma rays, have collided with its in-nards.) This resulted in the first particle image of any astro-physical object. The Sun may be the only object that will everbe imaged this way, since neutrons decay with a half-life ofonly five minutes when they are not bound up within atomic

nuclei. COMPTEL also detected a gamma-ray afterglow fromthe same flare. In this case, the particles were likely acceleratednot just during the impulsive phase at the beginning of theflare but over an extended period of time.

Many questions about solar flares remain unanswered. Toobad Compton will not be around to observe them during thissolar maximum.

It is hoped that several upcomingmissions will continue where the

Compton Gamma Ray Observatory left off. Eachzooms in on a particular swath of gamma-raybandwidth that Compton had covered.

NASAs HETE (High-Energy Transient Ex-plorer) 2 is a small Explorer-class mission thatwill localize gamma-ray bursts more preciselythan BATSE and BeppoSAX and relay that in-formation to the ground very quickly. Nowscheduled for a July or August launch, HETE-2will also monitor X-ray and gamma-ray flaresfrom a variety of astrophysical sources.

HESSI, the High Energy Solar SpectroscopicImager, will observe solar-flare gamma rays with better energyresolution than Compton could provide. With a launch oncescheduled for July 2000, HESSI would have been perfect forstudying the present solar maximum. However, the spacecraftsuffered serious damage during vibration tests in March. Themission has been postponed for at least six months,making it all the more frustrating that Compton wont be in orbit anymore.

The European Space Agencys International Gamma-Ray As-

Sky & Telescope July 2000 53

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The evolution of three solar flares that took place just after the lastsolar-activity maximum. Each curve is derived from gamma rays at anenergy of 2.223 million electron volts; such a photon is emitted froma helium nucleus that has absorbed an extra neutron. Inset: Whilecoarse, this false-color COMPTEL image shows neutrons, not gammarays, from the June 15th flare, making a unique particle picture ofthe Sun. Courtesy the COMPTEL collaboration.

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censuses of pulsars based only on radio surveys.censuses of pulsars based only on radio surveys.

The Future

Future missions will follow where Compton has led.Future missions will follow where Compton has led.

The gamma-ray sky, as seen today by Comptons EGRET instrument(top) and as might be seen one day by the proposed Gamma-rayLarge Area Space Telescope, or GLAST (bottom). Both false-color im-ages are oriented with the plane of the Milky Way running horizon-tally through the center. The GLAST image is only a simulation, in-tended to illustrate how expected gains in sensitivity and resolutionmight reveal myriad new sources of energetic gamma rays.

trophysics Laboratory (INTEGRAL) will be launched in 2001.INTEGRAL will concentrate on the high-energy X-ray andlow- and medium-energy gamma-ray bands, essentially re-placing OSSE and COMPTEL. And NASAs Swift mission,scheduled for 2003, will search for gamma-ray bursts andother explosive phenomena, filling BATSEs shoes. The nameSwift reflects this spacecrafts ability to rapidly locate bursts,relay this information to Earth, and follow up with its own ul-traviolet and X-ray observations.

Finally, the Gamma-ray Large Area Space Telescope (GLAST),

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scheduled for a 2005 launch, is a collaboration among NASA,the U.S. Department of Energy, and international partners.GLAST will continuously probe the high-energy gamma-raysky with 50 to 100 times the sensitivity of EGRET. GLASTsability to study relativistic particle jets from black holes makesthe mission particularly alluring to both astronomers and par-ticle physicists.

Comptons undeniablelegacy is in the numbers.The mission observed ap-proximately 400 gamma-

ray sources (not including GRBs); beforeCompton, only about 40 were known. BATSEdetected more than 2,600 GRBs; before Comp-ton, only about 300 had been logged. Scientificjournals publish roughly 180 Compton-specificarticles per year, about one every other day.

The Compton era will have ended as it began,in suspense. The nail biting began back in April1991, a few hours after Comptons release fromthe Space Shuttle Atlantis. Comptons high-gain

antenna would not erect itself properly, and its loss would havecrippled the mission. Astronauts Jerry Ross and Jay Apt per-formed an unscheduled space walk to physically shake it loose.

The nail biting resumed last December with the news thatone of Comptons gyros had failed. Months of debate overComptons fate culminated with the decision to bring er onhome. If the players follow the script, during the first week ofJune the mighty observatory will burn to silvery dust highover the Pacific, south of Hawaii, where it may well be visibleto skywatchers along the path. A few stubborn chunks willsink quietly to the bottom of the ocean.

No Compton spacecraft will adorn the halls of the Smith-sonian Institutions National Air and Space Museum. But theCompton legacy will remain on permanent exhibit in thoseminds graced with fantastic visions of gamma-ray bursts,black-hole jets, and all the other things that go boom no,make that KABOOM in the night.

Peter Leonard and Christopher Wanjek work for Raytheon In-formation Technology and Scientific Services in support of NASAspace-science missions.

Ending in a Blaze of Glory

Just as the 17-ton Compton Gamma Ray Observa-tory loomed large in the bay of the Space ShuttleAtlantis, its legacy will loom large in the annals ofhigh-energy astrophysics. NASA photograph.

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myText: 2000 Sky Publishing Corp. All rights reserved.