ROBERT SIEGEL, host:
NASA has cleared the shuttle Atlantis to land tomorrow.
Mr. WAYNE HALE (NASA): Nothing was found to be missing or damaged on the thermal protection system, the heat shield of the space shuttle Atlantis or, in fact, any other part of the space shuttle Atlantis. And so we feel very confident that we're in for a good landing opportunity tomorrow morning.
SIEGEL: That was Wayne Hale, the space shuttle program manager, dispelling concerns about the junk that the Atlantis crew have seen floating outside their orbiter. They saw an unidentified object on Tuesday. NASA thinks it was a very small piece of plastic.
On Wednesday they saw three more pieces of debris, but the word from NASA is no big deal. In any case, the Atlantis experience raises the question of how much stuff is floating around up there and what hazard it might pose.
And Dr. Mark Matney, who works for NASA at the Johnson Space Center in Houston, is a space debris scientist. Welcome to the program.
Dr. MARK MATNEY (NASA): Thank you.
SIEGEL: Obviously the fact that one can be a space debris scientist and that NASA needs one suggest that there's a lot of debris up there. How much is there and what is it?
Dr. MATNEY: Well, it all depends on how you define the size of the debris. Objects that are bigger than about four inches in size, 10 centimeters, the space surveillance network tracks those and there's more than 10,000 objects that they track. But as you get down to smaller sizes, down to make a centimeter or a millimeter in size, you get up into the hundreds of thousands of objects we believe are in Earth orbit right now.
SIEGEL: Where did they come from?
Dr. MATNEY: A variety of different places. But it turns out one of the biggest hazards is that when people launch things into space, satellites, scientific satellites or communication satellites, they often leave rocket body upper stages in orbit with some residual fuel. And some time later, maybe a month or a year or a decade later, the rocket body will explode and spread hundreds or thousands of pieces of shrapnel into orbit.
SIEGEL: You're saying conceivably a decade later it might explode?
Dr. MATNEY: That's right. And one of the things that we do as part of our job here at the Orbit Debris Program Office is educate spacecraft manufacturers that they really ought to do something about perhaps venting the fuel when they're done with their rocket bodies so that they don't explode at some later time.
SIEGEL: So if astronauts see bits of stuff outside or their cameras pick it up, should they assume it's come off their own craft? Is that the most likely explanation or no?
Dr. MATNEY: Yes. Things that come off your spacecraft come off at very low velocity. And so they will stay in your general region for a period of time. The things that we worry about actually come in at very, very high velocities. They're in different orbital planes than your spacecraft.
And the hazard is not that there's a piece of debris. The hazard is that it hits you at a very, very high rate of speed, many, many times the speed of a bullet and that's where the concern is. Because an object about the size of a Tylenol tablet can do tremendous damage to a spacecraft if it hits at those high rates of speed I was talking about.
SIEGEL: And the shuttle program is accustomed to seeing craft, the orbiters return somewhat pitted from hitting various things on their way down?
Dr. MATNEY: Yes. We get very small nicks in the windows and in the radiators. Those are the two places we examine every mission. And actually the shuttle program generally replaces a window after every mission because it's been damaged by orbital debris or meteoroids.
SIEGEL: Is the space station itself a producer of much debris?
Dr. MATNEY: It does produce some debris. But because of the low altitude where we fly the space station and the space shuttle, the debris that comes off has a very, very short lifetime from atmosphere drag. In a few months it will reenter. For instance, I believe in the last shuttle mission, one of the astronauts lost a spatula, and that will probably be reentering here in a few weeks.
SIEGEL: Spatulas in space, you're saying.
Dr. MATNEY: Spatsat, we call it.
SIEGEL: It's Dr. Mark Matney who is a space debris scientist at the Johnson Space Center in Houston.
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