NEW YORK — 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 Oct. 5 .

“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 News . “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 Sept. 21. Images of the damage site were first published by the Web site, and later released by NASA.

According to a schematic of the damage, the unknown object — either a micrometeoroid or piece of orbital debris — caused a small, 2.5-millimeter puncture in Atlantis’ aft starboard radiator and appeared to leave a 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 damaged a 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 1.25-centimeter -wide panel.

Atlantis’s aft radiator panels each measure about 4.6 meters by 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 return to Earth early depending on the mission’s timeline, he added.

Atlantis’ STS-115 astronauts conducted three in-depth scans of their spacecraft before attempting re-entry, though those were aimed at evaluating the health of the orbiter’s heat shield, 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. “

Just before landing, flight controllers and the astronauts themselves noted several “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, and a camera survey of Atlantis’ payload bay to search for any missing objects. 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 re-entry, and would not have posed a threat to the orbiter during its Earth return, NASA officials have said.

PRIVATE colorchange:<c”Black”>
PRIVATE para:<*p(,12.000,,10.000,,,G,)>
PRIVATE hnjset:<*h”Great H&J’s”>
Watching debris

NASA has kept a close watch on launch and orbital debris since the loss of seven astronauts and Space Shuttle Columbia during re-entry 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 re-entry — was breached by a suitcase-sized chunk of foam insulation from orbiter’s external fuel tank 16 days earlier during launch, leading 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 in July and September.