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The ASTRO observatory can trace its origins back to 1978, when NASA issued an Announcement of Opportunity for advanced astronomical instruments for carriage on future Shuttle missions. Three were ultimately chosen - the Hopkins Ultraviolet Telescope (HUT), provided by the Johns Hopkins University of Baltimore, Maryland; the Wisconsin Ultraviolet Photopolarimeter Experiment (WUPPE), built in the Space Astronomy Laboratory at the University of Wisconsin at Madison; and the Ultraviolet Imaging Telescope (UIT), sponsored by NASA's Goddard Space Flight Center. The project was to be managed by the Office of Space Science.

By 1982, however, control had passed to NASA's Marshall Space Flight Center and the missions were renamed 'ASTRO'. Two years later, the first flight of the series was tentatively scheduled for the spring of 1986 - exactly the same time that Halley's Comet would visit the inner Solar System - and a special Wide Field Camera was added to permit detailed observations of the celestial wanderer. By the end of January 1986, ASTRO-1 had completed its final checkout and was ready for installation into Columbia's payload bay, when Challenger was lost.

For the next 32 months, the Shuttle and ASTRO-1 were grounded. The telescopes were removed from the Spacelab pallets and stored. Although periodic health checks were conducted, NASA decided to recertify them before clearing them for flight. This included replacing more than 300 bolts in mid-1987. The Wide Field Camera, no longer needed as Halley's Comet was by now long gone, was deleted. In its place, NASA added the Broad-Band X-Ray Telescope (BBXRT), which had also been chosen in 1978 and had hitherto been assigned to another mission called the Shuttle High-Energy Astrophysics Laboratory (SHEAL).

NASA hoped that the addition of BBXRT would shed some new light on a major supernova - known as 1987A - which had first been spotted by astronomers in February 1987. Stars 10 to 100 times more massive than our Sun burn a succession of chemical elements rapidly until their cores collapse and they explode as 'supernovae'; these are among the most powerful events in the Universe. The 1987A event spewed debris into space from a distance of 170,000 light years and one of BBXRT's aims was to examine the different elements present.

Originally, BBXRT had not been expected to fly until 1992, but as it was finished ahead of schedule it was quickly added to the ASTRO-1 payload. A second SHEAL instrument, the Diffuse X-ray Spectrometer (DXS), was eventually shifted onto another Shuttle mission scheduled for the winter of 1992. As work on readying the BBXRT for flight shifted into high gear, preparations were also underway for getting the other three instruments out of storage and back on the Shuttle.

Of these, HUT was kept at KSC throughout the post-Challenger downtime, although its spectrograph was removed and returned to the Johns Hopkins University in October 1988. Checks had confirmed that, although it was protected from air and moisture by a continuous supply of gaseous nitrogen, its ultraviolet sensitivity had degraded and the spectrograph was replaced. When it reached orbit, the 3.6-m-long and 1.2-m-wide HUT, which weighed over 770 kg, was intended to

ASTRO-1: four powerful eyes on the Universe 123

explore objects such as quasars, active galactic nuclei and 'normal' galaxies at far-and extreme-ultraviolet wavelengths.

This region of the electromagnetic spectrum was inaccessible from Earth and even to the instruments on the forthcoming Hubble Space Telescope. To achieve far- and extreme-ultraviolet sensitivity HUT's mirrors were coated with iridium. When the ultraviolet spectrograph returned to KSC in the spring of 1989, it failed its first acceptance test and was again changed; then an ageing television camera had to be removed and replaced.

Meanwhile, the other two ASTRO-1 instruments also underwent recalibration and testing. WUPPE was not shipped back to the University of Wisconsin, but instead a portable vertical calibration facility was built and delivered to KSC. The telescope passed its checks with flying colours in April 1989. UIT also remained in Florida, where the power supply for its onboard image intensifier was replaced in the summer of 1989. By the beginning of autumn, all three instruments had been declared 'flight-ready' and just before Christmas were installed onto the two Spacelab pallets in the Operations and Checkout Building.

When in space, WUPPE would examine the ultraviolet polarisation of hot stars, galactic nuclei and quasars. Any star - with the obvious exception of our Sun - is so distant that it only appears as a far-off point of light in a telescope eyepiece. If its light is polarised, however, it is possible to derive more information about its geometry and physical composition. UIT took wide-field-of-view images of star clusters, planetary nebulae, supernova remnants and galactic clusters. Although Hubble was expected to have higher magnification, UIT could cover much larger areas of the sky at once.

Major targets for ASTRO-1 included red giants which, at the end of their lives, shrink to become dense, hot embers no bigger than Earth, known as 'white dwarfs'. Since this is believed to be the final destination for many stars, they are an important area of study; they also emit most of their radiation at ultraviolet wavelengths, thus placing them squarely within ASTRO-1's capabilities. The observatory was also directed to examine suspected 'black holes' - stars whose density is so high that they collapse under their own gravitational influence to such an extent that nothing, not even light, can escape.

The ASTRO-1 observatory at work during STS-35. Note the cylindrical igloo at the front of the first Spacelab pallet.

It was hoped that ASTRO-1's sensitive ultraviolet instruments, coupled with BBXRT's capabilities, would be able to 'see' hot, swirling material being dragged into a black hole's clutches. Other studies would focus on existing stars: 'binary' systems, in which two stars reside close to one another and sometimes exchange material, and stellar clusters, in which anything up to a million stars reside. In 'visible' light, it is difficult to distinguish the light from each celestial source, but under ASTRO-1's ultraviolet and X-ray gaze they were expected to blaze as individual stars.

More broadly, the observatory would examine the 'interstellar medium' - the enormous expanse of gas and dust between stars - which actually provides building material for future objects. Although the interstellar medium chiefly comprises hydrogen and has a typical density no higher than one atom per thimbleful of space, it was expected that ASTRO-1 would measure its physical properties more closely and explore 'pockets' of it which are much hotter than normal. A wide range of observations of galaxies and objects known as 'quasars', which are supermassive black holes in the nuclei of'active' galaxies, were also among the mission's primary targets.

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