An unknown object pierced a radiator mounted to one of two cargo bay doors aboard NASA's shuttle Atlantis during its September spaceflight, but did not endanger the vehicle or its six-astronaut crew at any time, the space agency said Thursday.
"Although it's small, by comparison it may be the second largest impact we've been able to detect on a payload bay door," NASA spokesperson Kyle Herring told SPACE.com. "It did not do any other damage to the vehicle other than penetrate the radiator."
The impact occurred at some time during Atlantis' STS-115 mission to install new trusses and solar arrays at the International Space Station. The 13-day spaceflight ended with a predawn landing on Sept. 21. Images of the damage site - dubbed "Ding 18" were first published by the website collectSPACE.com, and later released by NASA.
According to a schematic of the damage, the unknown object - either a micrometeoroid or other piece of orbital debris - caused a small, one-tenth of an inch (2.5-millimeter) puncture in Atlantis' aft starboard radiator and appeared to leave a 0.031-inch (0.7-millimeter) exit hole and a nearby crack.
"The nature of the object that hit the shuttle radiator isn't known," a NASA update said, adding that shuttle processing crews discovered the hole while working on the spacecraft.
The impact also damaged a one-inch (2.5-centimeter) area in the radiator's honeycomb-like aluminum mesh, but did not sever any of the panel's 26 vital coolant tubes as it passed through the half-inch wide panel.
Atlantis's aft radiator panels each measure about 15.1 feet (4.6 meters) by 10.5 feet (3.2 meters) and house 26 tubes for Freon-21 coolant.
Even if the breach had severed one of the coolant tubes, Atlantis carries two redundant cooling lines, Herring said, adding that a Freon loop breach in one loop would have led the shuttle's onboard software to shut down the damaged system.
About 70 percent of the orbiter's cooling needs could be met by one cooling loop, though the situation would prompt discussions of whether to power down non-vital electronics or returning to Earth early depending on the mission's timeline, he added.
"It becomes a discussion and not an emergency situation," Herring said.
Atlantis' mystery objects
Atlantis' STS-115 astronauts - commanded by veteran shuttle flyer Brent Jett - conducted three in-depth scans of their spacecraft, though most of them were aimed at evaluating the health of the orbiter's heat shield and not its payload bay.
"This is exactly why we instituted the late inspection in this program," Herring said. "We've known for the life of this program that [micrometeorite and orbital debris] was a top risk to the space shuttle. Definitely in the top five, and maybe in the top three."
Just before landing, flight controllers and the astronauts themselves noted several items - later dubbed "mystery objects" - that appeared to have floated out of Atlantis' payload bay after a series of thruster tests. The debris prompted a third and final round of heat shield inspection.
That scan also included a camera survey of Atlantis' payload bay to search for any missing objects, and mission managers later gave the shuttle a clean bill of health and cleared its crew for landing.
Mission managers did not report any signs of the radiator impact after the payload bay survey. The region is not subjected to the severe heating stresses of atmospheric reentry, and would not have posed a threat to the orbiter during its Earth return, NASA officials have said.
NASA has kept a close watch on launch and orbital debris since the loss of seven astronauts and their space shuttle Columbia during reentry on Feb. 1, 2003.
The vital heat shield on Columbia's left wing leading edge - an area that sees some of the highest temperatures during reentry - was breached by a suitcase-sized chunk of foam insulation from orbiter's external fuel tank 16 days earlier during launch, and led to its destruction. NASA has since taken great strides to prevent foam insulation debris during launch, an effort that led to two, near-flawless launches this summer.
The damage seen on Atlantis' radiator - while not a threat to the spacecraft or its crew - is indicative of the potential impacts from micrometeoroids and orbital debris (MMOD) to spacecraft in flight. It is that hazard that prompted shuttle managers to add the second heat shield inspection to July's STS-121 mission and subsequent spaceflights.
"In simple terms, there is a risk that you could be struck my MMOD," Atlantis' lead shuttle flight director Paul Dye said last month during the STS-115 mission, adding that the risk is small but always present for any piloted spacecraft. "So we think it's a prudent thing to take a look."
Meanwhile, NASA shuttle engineers continue to prepare Atlantis' sister ship Discovery for its STS-116 mission to continue ISS construction. Commanded by experienced NASA astronaut Mark Polansky, the STS-116 will deliver a new piece of framework to the ISS, stage several spacewalks to rewire the station's power grid and ferry a new crewmember - first-time flyer Sunita Williams - to join the outpost's Expedition 14 crew.
The spaceflight is slated to launch no earlier than Dec. 7 at 9:38 p.m. EST (0138 Dec. 8 GMT) and mission managers discussed the mission's planned night launch, which would be the first since the Columbia accident, in a Thursday meeting.
"The consensus was that restoring night launch capability for the shuttle does not impose any additional risk to the safety of the vehicle or the crew," Herring said. "Clearly launching in daylight is desirable, but everyone agreed it's not required."
- Complete Space Shuttle Mission Coverage
- Complete Coverage: ISS Expedition 14
- NASA's STS-115: Shuttle Atlantis to Jump Start ISS Construction
- The Great Space Quiz: Space Shuttle Countdown