Since the launch of the Soviet satellite Sputnik in 1957, humanity has sent thousands of spacecraft into orbit. Some of those were boosted into orbit by multi-stage rockets, and parts of those rockets are still in orbit too. Some satellites have long since spiraled back to earth, but not before leaving behind frightening things like nuclear power plants circling the Earth. And a surprising number of satellites and rockets, about 150, have exploded in orbit, sending showers of debris flying at high speeds in all directions.

There are millions of man-made items circling the globe, ranging in size from millimeter flecks of paint to foot-long hunks of rocket wreckage to the multiple tons of the massive Mir Space Station. And with the rapid growth in the commercial satellite market, hundreds more are coming.

"You can pollute a stream or an ocean for a long time and not see any consequence," said NASA scientist Dr. Nicholas Johnson. "By the time you see something, it may be very difficult or very costly to remedy the environment."

Johnson is director of NASA's Orbital Debris Research Center at the Johnson Space Center in Houston. He's been studying the problem for 20 years, and is the space agency's chief scientist on the subject. He said that while debris is a factor in any space operation, the situation is under control.

"I don't think anyone would say it is a significant problem," said William Ailor, a scientist for the non-profit research company The Aerospace Corporation. "It could be a problem if we continued business as usual." Ailor is director of the company's Center for Orbital and Reentry Debris Studies (CORDS), which does research for the Department of Defense and other national agencies.

Orbital debris, sometimes called simply space junk, comes in a variety of sizes. Scientists usually divide it into three different categories: under 1 cm, 1 to 10 cm, and over 10 cm.

The smallest category of particles is the most common, Johnson said, with millions of them in LEO, or Low Earth Orbit. Most can be found in the altitudes used for most unmanned satellites, anywhere from 600 to 2,000 km above the Earth. Although these particles can sometimes be less than a millimeter in diameter, their speeds make them dangerous.

The average relative velocity for impacts is 10 km per second, or 21,600 miles per hour. At that speed, specks of paint have caused craters in the space shuttle's windows. NASA often replaces the windows because of the cumulative damage of micro-debris and micrometeoroids.

All critical systems of the International Space Station are designed to repel impacts of this size, Johnson said. Over 200 types of shields will cover the pressurized modules, propellant tanks and lines, momentum wheels, and other sensitive, critical parts. Most are aluminum sheets, called bumpers, set apart from the actual structure. In some cases, Johnson said, they "stuff the bumper" by sandwiching in many layers of a shock-resistant material like Kevlar between the bumper and the station.

"It's the same technique that has been used for micrometeoroid protection since the 1960s," Johnson said. "There have been some new materials, but nothing exotic. It's mostly engineering."

The station, scheduled to be completed in 2006 and with an initial mission lasting at least 10 years, will be better protected than any previous spacecraft against debris and meteoroids, Johnson said. It has to be.

"When you look at the space station, it will be by far the largest spacecraft ever assembled," he said. "It will be up there 15, 20, 30 years. Probabilities that you used to ignore because they were one in a million now become one in a thousand, so you know have to come up with a countermeasure. We're going to be hit several times, many times, by particles smaller than one centimeter."

The station is not specifically armored against larger objects. Debris between 1 and 10 cm are less common, but more dangerous. The CORDS website gives a dramatic analogy: "An aluminum sphere which is 1 cm in diameter is comparable to a 400-pound safe traveling at 60 mph. A fragment which is 10 cm in its long dimension is roughly comparable to 25 sticks of dynamite."

Most space debris of this type is made up of fragments from previous explosions, Johnson said, and is therefore in a higher orbit than the space station or other manned missions. The station is 400 km above the earth, below the dangerous radiation of the Van Allen belts. Most unmanned satellites operate well above that, Johnson said, mostly between 800 to 1,200 km. But Ailor said there's now a lot more of this class of debris floating around, because old nuclear reactors from deactivated Russian satellites have started leaking drops of liquid metal coolant into orbit.

"They freeze and form spheres up to five centimeters across," Ailor said. He said there may be 70,000 of these metal spheres in orbit. "Anything larger than a centimeter is damaging, just depending on where it hits of course."

Both Ailor and Johnson said there is not any "significant risk" to commercial satellites from smaller debris. Space is still so vast that damaging collisions are rare. When debris does strike a satellite, it's usually in a solar panel or some other large, non-critical component, Johnson said.

If a piece of debris over a centimeter in size did hit the space station in a crucial place, Johnson said, the crew would have to deal with it. He said NASA is working on a "repair kit" for in-orbit work, and there are already plans to maintain submarine-like protocols that would limit any loss of pressure to just one section of the station. "It could be anything from a pinhole leak to something more catastrophic," Johnson said.

Of course, the bigger the piece of junk, the more damage it can do. That's why the Air Force actually tracks all objects in orbit over 10 cm. There are about 10,000 of them. About 8,600 of them are individually identified and catalogued, Johnson said, and that includes both operational satellites and space junk.

The Air Force does not make regular predictions about object collisions, Johnson said. However, "whenever the shuttle or station is up, we do that," he said. "We always look about three days in the future." If there's a chance of a collision, the spacecraft will move. And with the ISS costing an estimated $53 billion, NASA is playing it safe.

"We're pretty ultra-conservative at NASA," Johnson laughed. "The threshold for concern for us in a 1 in 100,000 chance [of collision with a tracked object]. Once the chance gets down to 1 to 10,000, we maneuver the station."

Johnson said a much-publicized event last June, when the Russian controllers in charge of ISS navigation failed to move the station at NASA's request, was never really a crisis. The object was "nowhere near" the station, but NASA has "instituted procedures to make sure it doesn't happen again." At the time, he said, the Russian facility was not staffed around the clock. When the request to move the station came through from NASA on the weekend, no one was there.

The shuttle, less armored against impacts and more mobile than the station, moves if there is a 1 in 100,000 chance. It has had to move six times in the history of the program to avoid possible collisions, Johnson said. There have only been three collisions of tracked objects since the space age began, and two of those were planned tests of the U.S. Air Force anti-satellite system. The third was a collision in June 1996 that cut a 20-foot boom off what Johnson called "the very first, and only, French military satellite." The culprit -- a piece of a French rocket that blew up in orbit 10 years earlier.

"It was a very unlikely set of circumstances," Johnson said. But Ailor was more cautious, saying that the French collision is an example of what can happen as LEO space continues to get more crowded.

"That really provides the context of what's going on up there," he said. "There are probably going to be others, it's only a matter of time," said Tim Maclay, Orbcomm's manager of flight dynamics and one of less than a dozen people with a graduate degree in orbital debris. Maclay thinks the issue may be a bit more complicated now with the rapid growth of the commercial satellite population, particularly the growth of large constellations of smaller satellites like Orbcomm's.

"We're doing the same thing to space that we've done on the ground," Maclay said. "We tend to do the same thing with every new frontier. Mostly it's economics -- you're starting a business, you do everything that you have to do, and you don't really consider any consequences and then your practices become standard practices. At some point it's going to come back to bite you."

While national and international policy are certainly headed in the right direction, Maclay said, the problem bears watching. Commercial satellite operators are laboring under different economic conditions than NASA or the Department of Defense, and some techniques that work for the government may not be as feasible in the private sector.

"I think it's still an issue," Maclay said. "You can bring this to your management, and say 'We've got to worry about debris, we could get hit and you'll lose your asset.' And they say, 'Wow, that's a problem, what are the chances of that happening?' And you say, 'Oh, about one in 10,000.' And they look at you like you're crazy. 'Why are you bothering me with this, there's a one in ten chance that my launch vehicle is going to blow up before the debris even gets above the clouds.'"