ASTRONOMERS hoping to a solve a riddle that has dogged them for two decades had their hopes dashed last week, when a rocket launching a pair of astrophysical satellites into orbit hung onto the spacecraft instead.
The High Energy Transient Experiment (HETE) satellite, built for NASA by the Massachusetts Institute of Technology, was supposed to study gamma-ray bursts, mysterious explosions of energy that come from all parts of the sky at a rate of about one a day. The bursts were first detected in 1973.
HETE was launched by a Pegasus XL rocket. The craft鈥檚 companion was the Scientific Applications Satellite-B (SAC-B), the result of a collaboration between NASA and Argentina鈥檚 National Commission of Space Activities, which was designed to observe the Sun and other stars.
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The problem came after the third stage of the Pegasus rocket, which is launched from beneath an aircraft, reached the planned orbit. The explosive device which should have released the satellites failed to ignite, sending the rocket and satellites tumbling in an orbit that is expected to decay within months, leaving the spacecraft to burn up in the atmosphere.
HETE was unable to deploy its solar arrays and lost battery power within hours. After deployment of the SAC-B solar array, scientists initially thought that four of the five SAC-B instruments would work, but the tumbling motion and shadowing by the rocket鈥檚 third stage prevented the array generating enough power to keep SAC-B鈥檚 batteries charged. Less than 24 hours after launch, both satellites were dead.
The loss of HETE is particularly disappointing as scientific review panels have ranked the project as second in importance only to the Hubble Space Telescope. HETE鈥檚 instruments were more sensitive than those on other gamma-ray satellites, and the spacecraft would have analysed X-rays and any ultraviolet radiation accompanying the bursts. It would also have been able to scrutinise the source of the bursts almost immediately after each event. This may have allowed astronomers to identify the objects responsible for the bursts.
The failure is also bad news for Orbital Sciences of Dulles, Virginia, which makes Pegasus. Three out of 14 Pegasus launches have failed. Nevertheless, both Orbital Sciences and NASA remain optimistic about the prospects for the rocket, which can launch satellites more cheaply than ground launchers. It can also be used anywhere in the world, allowing satellites to be launched into orbits that are inaccessible from the northern hemisphere.
鈥淚f you look back at the history of early expendable launch vehicles, the failure rate is not all that unusual,鈥� says Donald Miller, a NASA official who is responsible for launch systems.
鈥淚t was a failure from the customer point of view in that we were not able to service the mission as planned,鈥� says Orbital Sciences spokesman Barron Beneski, 鈥渂ut from our own internal perspective it reaffirmed that the Pegasus programme is very good at delivering the satellites into their proper orbit.鈥�
NASA has suspended launches of payloads aboard Pegasus XL until the fault has been identified and corrected. Orbital Sciences and NASA have both set up investigations into the failure. One possible cause is a loss of power in the electronics responsible for firing the explosive release mechanism. 鈥淲e鈥檙e fairly confident we can locate [the failure] to just a few elements of the vehicle,鈥� says Miller.