v1.0.1 / chapter 7 of 10 / 01 aug 08 / greg goebel / public domain
* By the beginning of the 1990s, space astronomy had come of age, with a new generation of much more capable space observatories placed in orbit in that decade, including the Compton Gamma Ray Observatory and the Hubble Space Telescope.
* Space astronomy missions picked up pace during the 1990s, with NASA in particular performing missions of unprecedented scale under the agency's "Great Observatories" program. The Great Observatories envisioned flight of a gamma ray observatory, an optical telescope, an X ray telescope, and an infrared telescope, and NASA flew the first three in the series during the decade.
The pioneer in the series was the "Compton Gamma Ray Observatory", also known as "Compton GRO" or just "CGRO". The project was formally initiated in 1977, with the spacecraft launched into low Earth orbit by the NASA space shuttle ATLANTIS on 5 April 1991. CGRO was a huge satellite, a rectangular box about 4.6 x 4.6 x 7.5 meters in size, fitted with solar power panels on each side, with a launch mass of 15,620 kilograms, almost half of that payload. It was one of the biggest scientific satellites launched to that time. The payload suite included four instruments:
Each detector was built around a sodium iodide crystal, with photomultiplier tubes to pick up the light scintillations in the crystal caused by gamma rays. The eight detectors were mounted on the corners of the spacecraft, allowing BATSE to pin down the location of a burst to within a few degrees. The instrument could also obtain spectral data on the bursts. BATSE featured a high-speed data storage memory to capture burst data, and could measure bursts to within a tenth of a millisecond. When a burst was detected, BATSE alerted the other three instruments so they could observe the event.
OSSE could observe gamma rays up to about 10 MeV. Each detector was is shielded to provide a narrow 3 by 11 degree field of view, and rotated along a single axis of rotation. By scanning the detectors across a source, OSSE could achieve a high degree of spatial resolution, localizing sources to about a 10-arc-minute square, a third of the apparent diameter of the full Moon.
The German-built COMPTEL had a field of view of 60 degrees; could build an image with one-degree resolution; and also provided gamma ray spectral data on the source being imaged, over a spectral range above that covered by OSSE.
The instruments were necessarily large, since the capability of instruments like a spark chamber is inescapably proportional to size, no matter how sophisticated the design and implementation. Each instrument was surrounded by plastic scintillation counters that were sensitive to the high-energy particles that make up "cosmic rays", but were not sensitive to gamma ray photons. If these anti-coincidence counters went off, the event measured by the CGRO's main instruments was discarded as a cosmic ray event.
CGRO was one of the first science satellites to be designed for in-orbit refueling, though this capability was never used. Data was supposed to be stored on two data tape recorders, but they failed in the spring of 1992. Compton was still able to relay data through the NASA Tracking & Data Relay Satellite System (TDRSS). Gradual degradation of CGRO's gyros rendered it increasingly unstable, and NASA had to deorbit before it became uncontrollable; the big satellite had heavy structural elements that could survive reentry and possibly cause damage if they fell into a populated area. CGRO fell to Earth on 3 June 2000.
CGRO conducted the first all-sky survey in the gamma ray region of the electromagnetic spectrum. The spacecraft's observations opened new doors into a set of mysteries. One discovery was that the distant quasars are brighter in the gamma ray region of the spectrum than they are at longer wavelengths. Since quasars were already known to have an enormous energy output, this discovery made the quasars seem even more impressive and puzzling. The EGRET instrument was able to provide closeup details for four quasars.
CGRO's BATSE instrument also provided a clue to the riddle of the gamma ray bursters by showing they were distributed evenly all over the sky. If the bursters were mainly occurring in our own Galaxy, they would have been preferentially distributed along the galactic plane. Since they weren't, they were likely to be extragalactic, which meant that the power output of the bursts was tremendous.
* One of the dreams of astronomers since the beginning of spaceflight was to put a telescope into orbit. Before the development of modern adaptive optics systems, there was no way to get around the optical interference of the atmosphere that degraded "seeing" except to put a telescope above the atmosphere.
NASA began work on a "Large Space Telescope" project in the 1970s, with the project approved by the US Congress in 1977. It was originally to be launched in 1982 by the NASA space shuttle, but development difficulties pushed the launch date out to 1986. Unfortunately, the shuttle Challenger was lost in a launch accident in that year, grounding the shuttle fleet. The "Hubble Space Telescope (HST)", or just "Hubble", was finally put into orbit by the shuttle Discovery on 24 April 1990.
The Hubble's main structure was a cylinder 13 meters long and 4.27 meters wide in its launch configuration; it featured twin solar panels that spanned 13.7 meters when deployed. The HST had a launch weight of 11.3 tonnes. The telescope element of the spacecraft was supported by a suite of instruments and a "support systems module". The telescope itself was of Ritchey-Chretien / Cassegrain configuration, with lightweight but sturdy composite structure, a 2.4 meter primary mirror, and a 30 centimeter secondary mirror. An actuator system was fitted to the mirrors to allow them to be realigned in space, though this wasn't an adaptive optics system.
The space telescope's attitude control system was designed to point to a target with an accuracy of 0.007 arcseconds, and keep it on target for up to ten hours. The pointing control subsystem used a number of different sensors to achieve this alignment, including sun sensors, star trackers, a magnetic sensing system, six attitude sensing gyros, four reaction wheels, and three fine guidance sensors. Position targeting was based on a catalog of 15 million guide stars. The fine guidance system was so precise that it was planned to use it for stellar astrometry studies as a secondary mission.
Hubble's original instrument suite included five systems covering the range from the high infrared, through the visible region of the electromagnetic spectrum, into the low infrared. They consisted of:
Although the other Hubble instruments obtained their imagery from the telescope's focal point on the far side of the hole in the main mirror, WF/PC obtained its imagery from a pickoff mirror inserted between the secondary mirror and the focal point. The imagery was picked up by a set of CCDs that had a spectral response from the high infrared up into the low ultraviolet, with the ultraviolet sensitivity obtained from a special coating known as "Coronene". The WF/PC included a cooling system to reduce thermal noise in the CCDs.
The delays in the Hubble had been frustrating for astronomers, partly because it meant the instrument suite ended up lagging well behind the state of the art, and more so because the costs escalated dramatically during development, with some estimates claiming that the final costs were $1.6 billion USD when the original cost estimate was only $300 million USD. Partly this was due to management problems: Perkin-Elmer worked on the telescope itself, Lockheed (now Lockheed Martin) worked on the spacecraft bus, and the two organizations didn't communicate well. At one point, the schedule was slipping a month per month, and NASA was finally forced to step in and restore order.
Of course, NASA was overselling the space shuttle at the time the Hubble program was initiated, which also left the telescope hostage to the shuttle's troubles. Even if the shuttle program had gone more smoothly, launch by the shuttle meant that the Hubble had to be placed in low Earth orbit. The problem was that meant the telescope's view would be blocked on a relatively short interval; although the pointing system could keep the satellite boresighted on a target for ten hours, there was no way to keep a target in view continuously for that period of time.
In compensation, the shuttle was supposed to be able to recover the Hubble and return it to Earth for refurbishing. However, long before the telescope was launched, this was determined to be a bad idea, since the recovery was likely to damage the satellite. More practically, the Hubble was designed with in-orbit servicing in mind, which would prove to be critically valuable. Despite the difficulties, astronomers were still happy to get the Hubble into orbit. Unfortunately, they found out they had not yet reached the promised land.
* As ground controllers began to adjust the Hubble's optics as the satellite orbited the Earth, they found that they couldn't bring them to a focus. Although Perkin-Elmer had run the telescope through an extensive series of tests, one test instrument was defective, throwing off the main mirror's figure. The telescope was still partly operational, with the spectrometers affected less than the Faint Object Camera and WF/PC. In addition, the spacecraft's solar panels vibrated every time they went from light to shadow or the reverse during each orbit, throwing off the precision pointing system.
The fact that the Hubble had been designed for on-orbit servicing meant that the spacecraft could be rescued. Fortunately, Perkin-Elmer had kept test data that showed exactly where the distortions in the main mirror actually were and permitted design of a compensating system.
The NASA space shuttle Endeavour was launched on a repair mission on 2 December 1993 to provide fixes. The fixes included replacement of the WF/PC instrument with the "WF/PC-2" that featured corrective optics, and a module named "Corrective Optics Space Telescope Axial Replacement (COSTAR)" installed in the place of the High Speed Photometer. COSTAR featured ten coin-sized corrective mirrors, with four each for the Faint Object Camera and Faint Object Spectrometer, and two for the High Resolution Spectrometer. The solar panels were replaced with stiffer new panels, and two gyro modules and related gear were swapped out.
Although the repair mission was seen as dodgy, since there were worries that exhaust fumes from the shuttle's thrusters might foul the Hubble's optics, it all went according to plan. The HST was finally able to reach its potential, returning spectacular imagery of deep space objects.
* There were several more Hubble servicing flights. Shuttle Discovery was launched on 11 February 1997 to perform a second servicing mission, with this flight removing the old High Resolution Spectrometer and Faint Object Spectrograph, replacing them with the new "Space Telescope Imaging Spectrograph (STIS)" and the "Near Infrared Camera & Multi-Object Spectrometer (NICMOS)". In addition, a fine guidance sensor was replaced, as was one of the "reaction wheels" used to help point the satellite, and a data tape recorder was replaced with a "solid-state recorder (SSR)".
The third Hubble service flight was also performed by shuttle Discovery, on a launch on 19 December 1999. This flight was driven by failure of the Hubble's guidance gyros, three of the six on board the satellite having failed, with the shuttle crew replacing all six gyros. Other kit installed in the flight included a new guidance sensor, a new computer, a new transmitter, an electronics update for the Hubble's battery power system, new thermal insulation blankets, and a new SSR, with ten times as much memory capacity as the old one. None of the Hubble's main instruments were updated.
The fourth Hubble service flight was performed by the shuttle Columbia, launched on 1 March 2002. Fixes included installation of the high-resolution "Advanced Camera for Surveys (ACS)", which replaced the FOC; a new cooler system for the NICMOS instrument; new solar panels; and new power system electronics.
There was indecision for a time about a fifth service flight. After shuttle Columbia was lost on reentry in 2003, the decision was made to phase out the shuttle by 2010, and new NASA Administrator Sean O'Keefe was unwilling to fly the shuttle any more than absolutely necessary until it was retired. However, the Hubble was to be phased out at about that same time, with the new Webb Space Telescope (described later) replacing it. The Hubble needed to be fitted with a rocket system to allow it to be properly deorbited -- it was much too big a spacecraft for NASA to allow it to fall to Earth on its own -- and there were studies for using a robot flight to install the booster on the Hubble and possibly add a few improvements to keep the Hubble in service.
The studies went nowhere, but when O'Keefe left NASA in 2005 his replacement, Michael Griffin, was not as cautious about using the shuttle up to its retirement, and planning for a Hubble service mission went forward, with the flight currently scheduled for 2008. As it turned out, the mission plan didn't not involve adding the deorbit booster, at least not directly, with the agenda instead focusing on:
As far as the deorbit booster goes, the mission will attach a new NASA "Low Impact Docking System (LIDS)" ring and a retroreflector homing target to the base of the Hubble, permitting an unmanned mission to attach the booster later.
The fixes will keep the Hubble operational to at least 2013. It is expected to remain in orbit until at least 2022 or at latest 2028. With the docking ring fitted, the option remains open to perform more servicing missions to the Hubble with the new Orion crew vehicle that will follow the shuttle, but at present there is no perceived need for yet another servicing mission, since by the time it would be ready, ground-based telescopes with adaptive optics will have outpaced the Hubble's capabilities.
* There were several major X ray / extreme ultraviolet (EUV) astronomy missions in the 1990s. The first was the "Roentgen Satellite (ROSAT)", which was a collaboration of German and British researchers with NASA, and was launched from Cape Canaveral on a Delta II 6925 booster into low Earth orbit on 1 June 1990.
The payload of the 2,424 kilogram ROSAT consisted of a German-built grazing incidence soft X-ray telescope (XRT), and a British-built EUV grazing-incidence telescope called the "Wide Field Camera (WFC)". The XRT featured two instrument systems, including a German-built dual "Position-Sensitive Proportional Counter (PSPC)" with a field of view of 2 degrees and a resolution of 25 arc-seconds; and a US-built "High Resolution Imager (HRI)" -- an improved derivative of an instrument built for Einstein -- with a field of view of 32 arc-minutes and a resolution of 3.7 degrees. The EUV WFC featured a selectable field of view of 2.5 or 5 degrees and a resolution of 1.7 arc-seconds.
With its advanced instruments, ROSAT was able to sweep in X ray sources a hundred times fainter than those located by the Einstein observatory, cataloguing 100,000 sources. ROSAT's EUV observations were regarded as more dramatic, being the first sky survey performed in that band, following a simple EUV experiment flown on the US-Soviet Apollo-Soyuz joint manned mission in 1975 to demonstrate that such observations were practical. The mission finally ended in early 1999.
* The ROSAT observations paved the way for the first dedicated EUV satellite, the NASA "Extreme Ultraviolet Explorer (EUVE)" satellite, launched by a Delta II 6925 booster in low Earth orbit from Cape Canaveral on 7 June 1992.
EUVE was in planning roughly in parallel with ROSAT. The spacecraft featured four grazing-incidence telescopes, with three of them pointing in the same direction and making up a "survey telescope", covering a wide range of the EUV spectrum through use of filters. They were pointed outward from the satellite's axis of rotation, allowing them to scan the sky over 360 degrees on a continuous basis, producing a full-sky survey in six months. The fourth telescope, the "deep survey" telescope, was mounted parallel to the axis of rotation, allowing it to focus on a small section of sky. Instrumentation included EUV imagers and spectrometers.
EUVE provided detailed data on a range of phenomena, such as hot stars, high-energy stellar flares, and even on a plasma torus around the planet Jupiter. The mission ended in early 2001.
* The next major high-energy astronomy mission was the third of NASA's Great Observatories, the "Advanced X-Ray Astrophysics Facility (AXAF)", which was launched by the NASA space shuttle Columbia on 23 July 1999. After deployment, it was boosted by a "kick stage" into a highly elliptical orbit that took it a third of the way to the Moon; NASA had learned the lesson from Hubble that putting a space telescope designed to perform close observations into low Earth orbit was a bad idea.
Once in space, the AXAF was named "Chandra", after the great Indo-American astrophysicist Subrahmanyan Chandrasekhar. Chandra was designed to create the first high-resolution X ray map of the heavens. The Einstein observatory's resolution was about that of one of Galileo's telescopes, while ROSAT's resolution was comparable to that of a good amateur telescope. Chandra, in contrast, could obtain X ray images with a resolution of half an arcsecond -- as good or better than a modern large ground-based optical telescope.
AXAF was originally proposed in 1976, but formal development didn't begin until 1988. Funding proved troublesome, with AXAF split into an imaging satellite -- essentially what would become Chandra -- and an X-ray spectroscopic satellite. The spectroscopic satellite was killed off in 1993, leading some jokers to call the mission "AX-Half". The spectrometer technology to be used on the cancelled satellite was leveraged into the Japanese Astro-E satellite, discussed later.
Despite the cutbacks, the development team managed to add improvements to make Chandra a better observatory, for example coating Chandra's grazing-incidence X ray telescope with iridium instead of gold, as initially planned. Chandra had a launch weight of 4,620 kilograms and a length of 13.8 meters, over four times the length of ROSAT. The longer length was due to the desire to pick up short wavelength / high energy X rays with a shallow grazing incidence angle, such as emission lines of iron that ROSAT couldn't see.
Dual spectroscopic transmission gratings were included, one for low energy X rays and the other for high energy X rays, as well as two cameras -- the " Advanced CCD Imaging Spectrometer (ACIS)" and the High Resolution Camera (HRC). The ACIS, as its name implied, used a CCD array with a resolution of over a million pixels, while the HRC used microchannel image intensifiers. The HRC had a field of view of about half an arc-minute, provided half-arcsecond spatial resolution, and 16 microsecond timing resolution. However, it only had about a tenth of the spectral resolution of the ACIS instrument. Chandra had twin solar panels with a span of about 20 meters, providing two kilowatts of power.
* Chandra was followed into space later in the year by the ESA "X Ray Multimirror Mission (XMM)", launched by an Ariane 5 booster from the ESA launch center at Kourou in French Guiana on 10 December 1999. It was named the "Newton XMM" after arrival in orbit, of course in honor of Isaac Newton.
XMM had a launch mass of 3.9 tonnes (4.3 tons) and a length of 10 meters (33 foot), with a general configuration much like that of Chandra. Like Chandra, Newton was placed in a highly elliptical orbit to give it long dwell times on targets. The instrument payload featured three instruments on a common boresight, including two grazing-incidence telescopes and a small optical telescope or "Optical Monitor (OM)", all of which could perform observations at the same time. The imagery of the X ray telescopes was picked up on the dual "European Photon Imaging Camera (EPIC)" system, which was supported by the "Reflection Grating Spectrometer (RGS)".
The X-ray telescopes had large apertures, allowing the satellite to acquire about five times more photons than Chandra, permitting observations of fainter sources with comparable timing resolution and field of view, though much coarser spatial resolution. The OM only had an aperture of 30 centimeters (one foot), but was capable of performing multicolor imaging, low resolution spectroscopy, and millisecond timing. It permitted immediate optical validation of X ray sources spotted by the other instruments, a particularly valuable feature for dealing with transient events.
* The ESA performed a major space infrared astronomy mission in the 1990s. An Ariane 44P booster was launched from Kourou on 17 November 1995 to put the ESA "Infrared Space Observatory (ISO)" into orbit. It was an improved successor to the pioneering IRAS, with a launch weight of 2.4 tonnes and featuring a cryogenically cooled 60 centimeter infrared telescope, feeding imagery to an imager, two spectrometers, and a photopolarimeter. Although ISO's telescope was the same diameter as that of IRAS, ISO's instruments had much better resolution and sensitivity. ISO also operated for much longer, finally losing its coolant in the spring of 1998.
* The USSR launched the "Gamma" gamma-ray observatory on a Soyuz booster from Baikonur on 11 July 1990. Gamma was based on the Soyuz manned space capsule and had a launch mass of 7,385 kilograms. The payloads were developed in cooperation with France and included three gamma-ray instruments. The project had actually been initiated in the mid-1970s, and by the time Gamma actually flew, it was too far behind the times to return much new information of any significance.
* The Japanese "Astro-D" X-ray satellite was launched from Kagoshima on an Mu-3S-11 booster on 20 February 1993. It was also known as the "Advance Satellite for Cosmology & Astrophysics (ASCA)", renamed "Asuka (Flying Bird)" once it was safely in orbit, partly as a pun on the acronym -- the Japanese tend towards soft pronunciation of the "su" phoneme and so "Asuka" was pronounced "As'ka". The satellite had a launch mass of 420 kilograms and carried four X-ray telescopes, with the associated instrument suite including four spectrometers, two based on proportional counters and two based on CCD imagers, the first time such devices had been used for space X-ray astronomy. Astro-D was shut down in 2000 and reentered in 2001.
* A Delta II 7925 booster was launched from Cape Canaveral on 30 December 1995 to put the "Rossi X-ray Timing Explorer (RXTE)" into orbit. RXTE had a launch mass of 2.7 tonnes and carried a proportional counter system, a high precision X-ray timing instrument, and an "all-sky monitor" system that swept the sky to alert provide alerts of "transient" events for observation. XTE was intended to provide detailed timing analyses of high-energy events. At last notice, it was still in operation.
* An Atlas-Centaur I booster was launched from Cape Canaveral on 30 April 1996 to put the Italian "BeppoSAX" X-ray satellite into orbit. "SAX" was the Italian acronym for "X-Ray Astronomy Satellite", and the "Beppo" was in honor of the Italian experimental physicist Guiseppi "Beppo" Occhialini.
BeppoSAX had a launch mass of 1.4 tonnes and carried four narrow-field X-ray telescopes, working with two spectrometers, a proportional counter array, a gamma-ray detector, and a wide-field X-ray camera. The spacecraft was primarily intended to hunt down the sources of GRBs. It was a highly successful mission, quickly pinning down several GRBs. BeppoSAX was finally turned off in 2002, and fell back to Earth at the end of April 2003.
* The Japanese ISAS space science organization launched the MUSES-B radio astronomy spacecraft on 12 February 1997, using an ISAS M-V booster from the Kagoshima space center. MUSES stood for "Mu Space Engineering Satellite", where "Mu" was the class of launch vehicle; once in orbit, MUSES-B was renamed the designated the "Highly Advanced Laboratory for Communications and Astronomy (HALCA)", rendered in Japanese phonetics as "Haruka" -- a personal name, usually of women, also meaning "faraway". It was part of the "VLBI Space Observatory Program (VSOP)".
MUSES-B was mentioned earlier in the installment on radio astronomy, providing the space-based arm of a VLBI experiment. The satellite, which had a launch mass of 830 kilograms and an 8-meter folding antenna, was placed into a highly elliptical orbit that took it as far as 21,400 kilometers from the Earth. Observations ceased in late 2003.
* The NASA "Submillimeter Wave Astronomy Satellite (SWAS)" was launched by a Pegasus XL booster from over the Pacific Ocean on 6 December 1998. SWAS was a NASA "Small Explorer (SMEX)" satellite, with a launch mass of 288 kilograms and featuring a 60 centimeter (2 foot) submillimeter telescope and spectroscope. SWAS was used to study the cooling of molecular cloud cores, the sites of star formation in our galaxy, by measuring lines from molecular oxygen and water. The spacecraft was put into hibernation in 2004, but has been occasionally revived for special observations.
* The NASA "Far Ultraviolet Spectroscopic Explorer (FUSE)" was launched from Cape Canaveral on a Delta II 7925 booster on 24 June 1999. FUSE had a launch mass of 1,360 kilograms and carried an array of four ultraviolet telescopes, all on a common boresight and sensitive to different ranges in the ultraviolet. The mirrors of two of the telescopes were coated with silicon carbide, which reflects short ultraviolet wavelengths, while the other two were coated with aluminum and lithium fluoride, which are sensitive to longer wavelengths. The instrument suite included both imaging and spectroscopic capabilities. The primary mission ended in 2003, but FUSE remained in partial service until it finally failed for good in 2007.
* The NASA space shuttle flew a number of minor space astronomy payloads during the 1990s, and also performed a dedicated ultraviolet astronomy mission. The shuttle Columbia was launched on 2 December 1990 to carry the "Astro-1" space astronomy payload into orbit. The payload consisted of four telescopes, mounted on ESA Spacelab pallets in the shuttle cargo bay, including the:
The mission was a success, though loss of shuttle data displays forced the crew to rely on ground control for pointing the ultraviolet telescopes. BBXRT was to be aimed under ground control all along.
* There were a few mission failures in the 1990s as well:
