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Space debris impacts on shuttle mission
Started by E.B.E., June 9, 2006
E.B.E. 0
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#1 Posted June 9, 2006

The run-up to the next space shuttle mission has been dominated by the hazards posed by the external fuel tank's foam insulation, but NASA and its astronauts also have their eye on another lurking threat - micrometeoroids and space debris.
The mission's top priority is to inspect the heat shield panels and tiles on the orbiter for any damage caused by falling foam during launch. But second on the priority list is inspecting the heat shield to look for damage from micrometeroids and space junk.
The shuttle Columbia was ultimately destroyed by a piece of foam insulation that broke from the external tank and put a hole in the orbiter's wing, allowing hot plasma to enter the breach during atmospheric re-entry. But the Columbia Accident Investigation Board said that NASA also needed to do more to protect against space junk and micrometeoroids that could also create holes in the orbiter. Before the Columbia accident, NASA documents said the risk of a critical penetration from orbital debris and micrometeoroids was acceptable as long the chance of it occurring was less than 1 in 200.
The current estimate of the likelihood of a space debris impact that might lead to the loss of the shuttle is 1 in 210. If the heat shield is inspected shortly before its return through the Earth's atmosphere, the risk of flying back with a hole in the heat shield drops to at least 1 in 280, says orbiter project manager Steve Poulos. Indirect view
On the tenth day of the 13-day mission - due to launch between 1 to 19 July, 2006 - the crew will do the first ever in-orbit inspection devoted to micrometeoroids and orbital debris damage. They will use the shuttle's robotic arm and a 15-metre boom extension to scan the left wing.
But the task is not easy, warns Tony Ceccacci, the lead flight director for this mission. He says it is comparable to operating a crane but with only TV screens to guide you: "You don't have the actual visibility." The scan will retrace the path to be followed on the second day of the mission, when the left wing will be examined for hits from foam or ice shed by the external tank. The before-and-after images will then be compared using computer software for any space junk damage, a process taking just a few hours.
After undocking from the ISS on day 11, pilot Mark Kelly will skip the usual "fly around" of the station. "It saves us a little propellant and it also saves some time," Kelly told New Scientist. Instead, he will fly the shuttle 74 kilometres away from the station. The crew will use that extra time to examine the orbiter's right wing and nose cap. Safety first
Kelly sketched out a hypothetical scenario of a space-debris impact for reporters on Thursday: "You're on the space station for a week, and a little tiny rock goes flying through the leading edge of the wing. You undock and do a de-orbit burn. And you come home with a hole in your wing and you don't make it home - you have a Columbia type of accident." "So in our case," Kelly said, "we undock, we inspect the leading edge of the wing, and if we see a hole we can re-dock with the space station, have the rescue crew come and pick us up, or we can decide to repair the damage." As an added improvement, Discovery will fly with nine heat shielding tiles of a new type that more than double the kinetic energy the tile can withstand from orbital debris and foam impacts. These tiles are sited around the nose landing-gear door - a known weak spot. Future missions are expected to fly with more of these improved tiles. The US Air Force Space Command catalogues and tracks about 9000 objects orbiting Earth, including active and dead satellites and space debris as big as 10 centimetres across. Micrometeoroids no larger than a grain of sand can still cause serious damage to the orbiter as they zip around Earth at over 32,000 kilometres per hour.
NASA considers this flight to be the second of two test flights following the Columbia accident. Most of this crew's time will be spent restocking the International Space Station and making repairs to the orbiting outpost.