Debris Precautions Set For Hubble Mission

Frank Morring, Jr.

The space shuttle Atlantis will drop to a lower orbit as soon as it releases the Hubble Space Telescope to reduce the risk from orbital debris on the upcoming mission to service the orbiting observatory.

NASA estimates the risk of a catastrophic debris hit at the 350-mile altitude where the Atlantis crew will upgrade and maintain the telescope at 1 in 221, a calculation that includes the debris scattered when an operational Iridium communications satellite and a defunct Russian military communications satellite collided in February at an altitude of 490 miles.

That is within acceptable risk under NASA's 1-in-200 guideline, but still worse than experienced at the International Space Station orbit more than 100 miles lower. So once Atlantis has dropped off the telescope, probably on Flight Day 9 of the STS-125 mission coming up next month, it will move into an elliptical orbit that averages out at the ISS altitude to lower the risk from debris, according to Tony Ceccacci, lead flight director for STS-125.

During the week of servicing operations at the telescope, Atlantis will fly tail forward, with the payload bay facing the Earth as much as possible given thermal constraints on the telescope and components in the orbiter's payload bay, to afford the four spacewalkers and the orbiter's delicate thermal protection system (TPS) as much shelter as possible from debris, said Leroy Cain, deputy shuttle program manager and chairman of the mission management team.

"People have worked very hard to mitigate this risk," Cain said. "We take it seriously."

NASA canceled the final mission to service the Hubble after the Columbia accident on the grounds that the crew would not have the refuge afforded by the International Space Station if their orbiter suffered the same sort of TPS damage that doomed Columbia. The mission ultimately was revived, but with the added requirement that a second orbiter be ready to mount a rescue mission if the first is damaged beyond repair.

The shuttle Endeavour is already at Launch Complex 39B at Kennedy Space Center to fly the STS-400 rescue mission if needed. Part of the crew of last year's STS-126 mission to the ISS, commanded by Chris Ferguson, has been assigned STS-400 duties during the Hubble mission, now set for launch in a window that could open as early as May 11.

Atlantis will carry 25 days worth of provisions for its crew -- more than twice the nominal 12-day mission duration -- and Endeavour will be at the launch minus seven days configuration. As the STS-125 mission progresses, Kennedy crews will continue processing Endeavour until it is at the launch-minus-three-days configuration, where it will remain until Atlantis lands or it is needed for a rescue.

In a rescue, Endeavour would use its robotic arm to grapple Atlantis from below, and the the rescue crew would bring the telescope-servicing crew across to Endeavour in three spacewalks, with the crews swapping spacesuits as needed to get everyone to safety.

Following its own inspection for damage, Endeavour would then land on its Flight Day 7.

Photo credit: NASA

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