Space Junk Threat Invisible to Astronauts in Orbit

Space junk littered around Earth poses an ever-present threat to astronauts in orbit, but the crew of NASA?s space shuttle Discovery said the danger, to them, is mostly an invisible one.

Discovery commander Alan Poindexter told students on Earth Saturday that the space debris whizzing around Earth is mostly imperceptible to the unaided eye of astronauts working in orbit. The astronauts are at the International Space Station to deliver tons of new supplies and equipment to the orbiting lab.

?We don?t really see any space junk,? Poindexter said. ?Anything we see floating outside usually comes from the shuttle.?

Every now and then, astronauts dump wastewater overboard through a nozzle on the side of their space shuttle, Poindexter said.

?It looks like a snowstorm because it instantly turns to ice,? he said. ?It looks beautiful when the light hits the ice particles.?

But just because the astronauts can?t see the space junk, doesn?t mean no one is watching.

?There are a lot of people on the ground who are watching that very closely ,and they?ll let us know if we have to move the shuttle or station out of the way,? Poindexter said.

NASA typically moves the space station if there is a 1-in-10,000 chance of an object striking the $100 billion orbiting laboratory. So far, Mission Control has seen no signs of any bits of trash near the station and Discovery.

NASA and the Department of Defense's Space Surveillance Network routinely monitor the space debris environment for potential impact risks for satellites, spacecraft and astronauts in orbit. An unprecedented crash of two satellites last year increased that impact risk, about a 1-in-300 chance for the space station, for all spacecraft.

At last estimate, there are than 20,000 pieces of space trash in orbit, according to the Space Surveillance Network.

There are 13 astronauts living and working aboard the linked shuttle and space station. They spent Saturday hauling cargo out of a transport module delivered to the space station by Discovery and speaking with reporters on Earth about the spaceflight. A false smoke alarm, caused by dust while an astronaut was cleaning an air filter, interrupted their work briefly.

The astronauts are gearing up for their mission?s second spacewalk, scheduled for early Sunday.

Discovery?s STS-131 mission is one of NASA?s last few planned shuttle flights before the space shuttle fleet retires in September. After this mission, only three more shuttle flights will be left.

Poindexter said he hopes to not see any space debris warnings during Discovery?s 14-day spaceflight.

Space station flight director Ron Spencer told reporters that there hasn?t been anything on the radar to worry about yet.

?So far, the skies have been clear,? he said.

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SPACE.com is providing complete coverage of Discovery's STS-131 mission to the International Space Station with Managing Editor Tariq Malik and Staff Writer Clara Moskowitz based in New York. Click here for shuttle mission updates and a link to NASA TV