Improved odds ease NASA's concerns about space debris
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: April 16, 2009
Even factoring in a recent satellite collision, an analysis of the threat posed by space debris at the Hubble Space Telescope's 350-mile-high altitude shows the crew of shuttle Atlantis, scheduled for launch May 12 on a mission to service the observatory, will not face a dramatically higher risk of potentially catastrophic damage, a NASA official said today.
"It's not going to keep us on the ground," Steve Stich, manager of the orbiter project office at the Johnson Space Center, said in an interview. "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 consideration of the shuttle's orientation in space during the Hubble mission. 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 they took some more measurements and they 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 man-made 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. Those numbers don't take into account post-Columbia inspection and repair techniques, not to mention the agency's plan to have a second shuttle ready for launch on an emergency rescue mission if necessary.
"The bottom line is, since return to flight this one is in the ball park" with past Hubble missions, an official said.
The Hubble servicing mission originally was planned for last October, but the flight was delayed to May when a data processing system on the telescope broke down. Going into that launch campaign, shuttle Program Manager John Shannon told reporters NASA planned to do everything possible to reduce exposure to MMOD damage.
"Because we recognize it as a significant risk, we have already taken all the actions we can as far as attitude timeline in putting the vehicle in a position where if we get micrometeoroid or orbital debris pieces coming at the vehicle, the come typically along the velocity vector," Shannon said.
"So you'll see with our attitude that we'll typically put the shuttle main engines toward the velocity vector (in the direction of travel). It protects the windows and the payload bay and the Freon loops and the RCC (nose cap and wing leading edge panels). So they have optimized the attitude timeline as much as they can for this mission. And we'll do our inspections, so we will know by the end of the mission if anything is required to go repair or not."
To put the MMOD numbers in perspective, the MMOD risk for the most recent space station assembly flight was 1-in-332. The two flights before that came in at 1-in-333 and 1-in-339 respectively.
"The 1-in-200 is a fairly arbitrary number that was decided upon kind of by consensus to make sure we have the discussion and that the discussion takes place at the right level," Shannon said. "When you get to a risk greater than 1-in-200, it was decided that decision should be made at the agency level."
Based on the latest analysis, that will not be necessary for Atlantis' upcoming flight. But the analysis highlights the increased risk the Atlantis astronauts will face because of the unique nature of their mission.
Unlike missions to the International Space Station, the Atlantis astronauts cannot seek safe haven aboard the lab complex if some sort of major problem develops that might prevent a safe re-entry. The telescope and the space station are in different orbits and Atlantis cannot change its trajectory enough to reach the lab complex.
As a result, the shuttle Endeavour will be moved to pad 39B Friday for work to ready the ship for a quick-response launch on an emergency rescue mission if necessary. The rescue mission would be known as STS-400.
But the Atlantis crew will only have enough supplies to last about 25 days on their own, even with extreme power downs. That means a rescue mission would need to approved relatively early in the Atlantis mission. If a problem is discovered during the late inspection toward the end of the mission, it's not clear Endeavour could be launched in time to pull off a rescue.
As a result, Shannon said, "we worked very hard to develop repair capabilities for micrometeoroid/orbital debris damage, we've got plugs we can put in the reinforced carbon carbon (wing leading edge panels), we've got the non-oxide adhesive we can put over any cracks or any kind of holes. I think 400 is there more for an ascent debris kind of situation, some kind of a really gross ascent problem like we had on Columbia. 400 would be very effective for that kind of case.
"For the MMOD case where we saw something during late inspection, if it were a case where we did not think we had repair capability for it, it's questionable whether 400 could get off the pad in time to go do any kind of a rescue. ... I think our protection for MMOD lies in our repair capability. We've spent a lot of time doing hypervelocity impacts on RCC materials, doing our repairs and putting them in the arc jet facility and we've had outstanding results. So we feel very comfortable about what we have.
"It would take a very rare, and very significant, large-size damage from MMOD in a critical area to cause us to have to consider 400 for that kind of case."
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