|Debris Is Biggest Risk to Hubble Mission
Aviation Week & Space Technology
09/15/2008 , page 18
The biggest risk to the crew of the space shuttle Atlantis during next month’s mission to service the Hubble Space Telescope will come from micrometeoroid/orbital debris (MMOD) instead of the falling insulation that doomed the shuttle Columbia and has been the main concern with subsequent shuttle flights. NASA has modified the shuttle’s external tank extensively to lower the risk from the foam that insulates it. But a combination of factors—including debris from last year’s Chinese anti-satellite weapon test, the Hubble’s high orbital altitude and an unusually quiet Sun—contributes to the risk from MMOD. At the moment, shuttle risk analysis sets the danger high enough—one chance in 185 of a catastrophic impact—to require top-level authority to launch the STS-125 mission to Hubble. The risk assessment for a typical shuttle mission to the International Space Station is one chance in 300 of vehicle loss. But the ISS orbits at about 200 naut. mi., while the debris environment at the telescope’s 300-naut.-mi. orbit is more severe. Shuttle program managers plan to fly the orbiter in an attitude that places its main engines facing the velocity vector, where the most dangerous debris would appear. Other risk-mitigation steps being taken include the standard late inspection of the orbiter’s thermal protection system, training the crew in the TPS-repair techniques developed since the Columbia accident, and lowering the orbiter’s altitude into a more benign environment once the telescope is released after servicing.