Genesis Of Chandra

Chandra was originally conceived in the 1970s when NASA envisaged a Large Orbiting X-ray Telescope, which Weisskopf described as ''a one-metre-class, arc-second-class X-ray observatory. That got descoped and became the Einstein Observatory.'' The success of the latter prompted the agency to include an X-ray telescope on its wish list for a four-spacecraft flotilla of Great Observatories. Early plans called for it to be launched into a low-Earth orbit and periodically serviced by spacewalking Shuttle astronauts for a projected 15-year lifespan, but in 1991 escalating costs and technical problems forced NASA to rethink the mission.

''What [we] did,'' said Weisskopf, ''was to take the first complement of instruments, which was very powerful, and break it up into two missions.'' One would transport a high-resolution camera and imaging X-ray spectrometer into a highly elliptical orbit which, although out of reach of Shuttle repair crews, would be able to gather data for no fewer than 55 hours of each 64-hour circuit of Earth. Meanwhile, the second mission, fitted with a super-cooled X-ray spectrometer, would be launched into a lower orbit; this was cancelled by NASA in 1993 in another round of cost-cutting.

A computer-generated graphic of Chandra when operational.

Even with the cancellation of the second mission, Chandra - known at the time as the Advanced X-ray Astrophysics Facility (AXAF) - was still expected to cost the better part of $3 billion during its first eight years of operation, including the spacecraft itself, its Shuttle and IUS launch costs, annual mission-control and data-analysis fees, use of NASA's TDRS communications network and a one-time charge for the use of an X-ray mirror-testing facility in Alabama. Nevertheless, even this steep pricetag was $4 billion cheaper than the originally planned, all-in-one AXAF in low-Earth orbit.

''The alternative we were looking at was Chandra in this orbit, or no Chandra at all,'' Weisskopf said, ''and so we took the risk to reduce the cost because the option scientifically was infinitely better to have a Chandra in orbit [than] no Chandra at all. We saved billions of dollars with that decision.'' The decision to launch the observatory into an orbit that was beyond the capabilities of Shuttle repair crews was, however, a tough sell. Already, Hubble had been sent into orbit with a flawed primary mirror and only the skills of spacewalking Shuttle astronauts had saved it.

Still, the unusual orbit - which would bring Chandra as close as 10,000 km and as far as 140,000 km, or a third of the way to the Moon, from Earth - offered several important advantages. ''The orbit'', said Tananbaum, ''is a lot more benign in the sense that you're away from Earth most of the time, so you're not cycling from light to dark, warm to cold, every hour and a half. We don't have to deal with the temperature cycling, which tends to wear out both electrical and mechanical components.''

Additionally, the observatory would spend 85% of its time above the radiation belts, allowing it a large, uninterrupted portion of each orbit for celestial observations, and avoiding interference from energetic particles that could otherwise overwhelm its sensitive instruments. ''Once it's up there and working, there's reason to be hopeful that it will work for 10 to 15 years anyway in that orbit,'' said Tananbaum, ''[but] we've got to get it right the first time.''

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