Space junk risk for Hubble crew: 1 in 221

Even factoring in a recent satellite collision, a threat analysis found that the crew of space shuttle Atlantis will not face a dramatically higher risk of catastrophic damage due to space debris when it travels to the Hubble Space Telescope in May, according to NASA.

The overall risk of impact damage is higher for a mission to Hubble, which is 350 miles from Earth, than it is for a flight to the International Space Station, which orbits at a lower, less debris-choked altitude. However, the actual numbers are better than flight planners initially expected, a NASA official said Thursday.

"It's not going to keep us on the ground," Steve Stich, manager of the orbiter project office at the Johnson Space Center, told CBS News. "Obviously, we know we're accepting a little higher risk for this flight. That's why we've tracked it very carefully."

Including the threat posed by debris from a satellite collision in February between a defunct Russian Cosmos satellite and an Iridium telephone relay station, the mean odds of a catastrophic impact during the Hubble mission are on the order of 1 in 221, which is below the 1-in-200 threshold that requires an executive-level decision by NASA's leadership.

A preliminary analysis put the odds at 1 in 185, but the numbers improved after recent radar observations and the shuttle's orientation in space during the Hubble mission were taken into account. The planned orientation, or attitude timeline, reduces the crew's exposure to impacts that could damage critical areas of the ship's heat shield, the coolant loops in the shuttle's cargo bay door radiators, and cockpit windows.

"The numbers changed recently from three factors," Stich said. "One, they went back and looked at the radar data and took some more measurements and found the debris environment isn't quite as severe. So that led to a reduction in the number.

"Two, we got an attitude timeline update that had higher fidelity breakdowns of the periods of time where we're going to be in attitudes to protect Hubble from the sun, and that was a factor in reducing that number. The third thing was we actually were able to model HST in the payload bay, and sometimes the HST actually provides a shield for the wing leading edge. Those three things combined took the risk from where we were last week, at 1 in 185, to 1 in 221 as of today."

For perspective, the overall odds of a catastrophic failure from all sources, including launch, orbital operations, re-entry, and landing are around 1 in 80.

Analysts took a conservative approach to the February satellite collisions, factoring in twice the amount of debris predicted by computer models. As it turns out, the amount of wreckage from the Iridium satellite was, in fact, roughly twice the predicted value. But radar tracking shows debris from the Cosmos matches the computer model's prediction. The overall risk was reduced accordingly.

Taking all that into account, the analysis shows a 1-in-141 chance of a potentially catastrophic problem from micrometeoroids/orbital debris--MMOD--if the crew simply flew the mission and made no attempt to inspect the shuttle for heat shield damage. The odds improve to 1 in 151 taking into account a planned inspection the day after launch, and to 1 in 243 with an inspection on the ninth day of the mission, after the Hubble Space Telescope is redeployed.

In those cases, the odds reflect the crew's post-Columbia ability to repair minor impact damage that might otherwise cause a catastrophic problem during entry. The mean value, 1 in 221, assumes a late inspection on flight day nine and an 86 percent chance of damage that could be successfully repaired.

"When you fold all that together, the residual risk of loss of crew and vehicle for the entire mission now is 1 in 221," Stich said. "That's MMOD, both the manmade and micrometeoroids, for the entire mission."

While an executive-level decision on what to do about the MMOD risk is not required in this case, shuttle program engineers will brief agency managers during a flight readiness review April 30 at the Kennedy Space Center.

"We've looked at Hubble very closely and we've done everything we can to mitigate the risks, the attitudes that we're flying, of course we've got our repair capability, we have launch on need (emergency rescue mission) ready, and we've got late inspection," Stich said. "And for late inspection, for the hot (wing leading edge) panels, we've actually improved that inspection to get better resolution for panels 8 through 11 that actually drive the risk. So we've done everything we can to mitigate the risk."

MMOD risks for previous Hubble servicing missions covered a wide range of values, from 1 in 150 for a flight in 1993 to 1 in 761 for a mission in 1999. For the most recent mission in 2002, the MMOD risk was 1 in 365.

"The bottom line is, since return to flight, this one is in the ball park" with past Hubble missions, an official said.