But the problem may be getting more complicated, some say, as more and more of the satellite population becomes controlled by corporate, rather than government, interests. NASA's regulations on limiting debris do not apply to commercial satellites, and although the industry has done much on its own, not all of NASA's suggestions will work in the private sector.
"It's just good business practice" to limit debris said Tim Maclay, manager of flight dynamics for the satellite communications company Orbcomm. "We're users of space, we've got a whole constellation up there...it just makes sense to keep it clean. But it can be very expensive to operate debris mitigation procedures."
The problem is that until there is an international law against orbital littering, "you've got Company A which is adhering to a standard and Company B which is not," said William Ailor, director of the Center for Orbital Debris Studies at The Aerospace Corporation. "The question is whether or not Company A is taking a competitive hit."
But all three scientists agree awareness of the problem is much higher, some definite improvements have been made, and corporations and governments alike are moving in the right direction.
NASA is focusing on the primary source of space junk.
"The vast majority of space debris have come from upper stages and satellites that have blown up," Johnson said. "And they contain thousands of particles."
The best way to avoid this is to make sure that whatever is no longer useful gets sent back towards the earth, where it usually will burn up in the atmosphere. But reserving enough fuel to do that promptly can be expensive, particularly for cash-strapped foreign space programs. So Johnson and his team have worked with the French, Russian, and even the Chinese space agencies to explain to them easy, cheaper ways to avoid orbital explosions.
"It turns out the answer is almost always the same," Johnson said. "When you leave an upper stage in orbit, there's almost always some residual propellant. If you get rid of that residual propellant, there's never a problem. You don't have to understand the specific chain of events for each spacecraft."
Problems can occur in satellites too, since many are equipped with engines and batteries. All U.S. government launches now must "passivate" their spacecraft when they're done with them. That means all systems with residual energy must have that energy drained out. For upper stages of rockets, that usually means venting or burning off any excess fuel. For satellites, it may mean purposefully draining the batteries, or venting any gas used in power cells. Momentum wheels, the fast-spinning internal gyroscopes used to control a spacecraft's orientation, must be allowed to slow down.
Johnson said that every spacecraft that has fully passivated has remained intact in orbit. He said it is "a relatively simple and cheap" method of preventing explosions.
But it isn't that simple for corporations, Orbcomm's Maclay said, because "they're squeezing the last drop out of blood out of every asset."
Maclay, who is one of less than a dozen people in the world with a degree in orbital debris, said that one trouble with NASA's recommendation is that the "end of mission life" is not as easily defined when a company is trying to get the most of its investment.
"Is it when you reach the end of mission life?" Maclay asked. "Is it when the payload is obsolete even though it's still working? Probably not. Is it when the payload is dead? Maybe, but is still might have some value as a testing platform or for training. But if the end of life is when you can't use it at all, then that probably means it's so broken that you can't communicate with it or maneuver it." That makes it impossible to either passivate the satellite or send it back promptly to earth, he said.
Maclay said that Orbcomm uses a different approach, rather than saving fuel to lower the orbit. Orbcomm's 28 satellites weigh only about 90 pounds each, and have a high amount of drag because of their antenna booms and solar panels. That means that they slow down in orbit faster than most satellites, and return to earth sooner.
NASA guidelines call for satellites "to come out of orbit in 25 years after it's life," Maclay said. "We're right on the hairy edge of that."
One necessary step, all three scientists say, is to make passivation an international law. The policy has already been adopted by the world's technical forum on space junk, the Inter-Agency Space Debris Coordination Committee. NASA has also been working on it at the UN for the past five years, and the first comprehensive report on space debris was released in the UN in May of this year, Johnson said.
"We're evolving in a good, responsible way," Ailor said. "I would say that most if not all of the space-using countries are working on it. Everybody realizes it's a potential problem and no one wants to be the bad guy."
The space agency has done a good job of increasing awareness about space debris, Ailor said. Every time the windows on the shuttle have to be replaced, it is a very public reminder that small particles can do big damage.
It's already made a difference, Johnson said. Of the objects tracked by the Air Force, the ones over 10 cm, the percentage that originated in an explosion has dropped from 50 percent during the 1980s to 40 percent now.
But the actual number of explosions has increased this decade, Johnson said, mostly from older spacecraft still in orbit that aren't equipped to passivate. Most of the explosions detected are smaller and have fewer consequences.
"Now, we're getting a lot of really small events that we identified back in 1992," Johnson said, "a problem with the Russian Proton, an upper stage. What we're having now is sort of a residual problem, with things that we can no longer control."
But Ailor thinks the problem may not lay entirely in the past, and noted that there are going to be many more launches in the next few years than there has been in the past, as new companies launch large constellations of small communications satellites.
"As satellites become smaller, they will become more vulnerable to the smaller debris population," the CORDS website reads. "Hundreds of satellites have been proposed for operation in low Earth orbit. Near- term examples are the Iridium satellite constellation with 66 satellites, and the Teledesic constellation with 288 satellites. In fact 1,062 low- and medium-altitude commercial communications satellites are expected to be launched through 2006."
"The thing about all these constellations that are going up is that a satellite in a constellation shares a lot of the same characteristics of orbit, like altitude," Ailor said. "That creates a ring of material for other objects to pass through. And if one of those satellites loses function, it's in the same orbit as others but it loses its station-keeping ability and starts wandering." The chances of a collision in a constellation are therefore more likely, he said.
Satellite constellations are also more susceptible to what he calls "pinch points." When something in orbit explodes, all the debris flies off in different directions, but each piece is in its own orbit. Because those orbits are circular and start at the same point, each explosion has two pinch points, one at the site of the original explosion and one 180 degrees around the globe, through which all the debris must pass at some point.
If a satellite in a constellation exploded, all the other satellites in that orbit would have to pass through the pinch points, and could be hit.
But pinch points only last for a short time, Johnson said, and have become a sort of buzz-word that create a false sense of danger.
"If you go through the area [of the explosion] early, there's a higher chance of collisions," Johnson said. "But we've had 150 break ups since 1961, some pretty severe, some in the midst of constellations, and there have been no consequences. We've had constellations since the 60s. There are problems, but most of the time they are exaggerated by people who don't really understand the mechanics."
More important, Johnson said, is that the new satellite companies are taking steps to avoid creating more space junk. The very fact that Orbcomm has hired Maclay, an expert on the subject, is a good sign.
"This is an example of how we already got to people," Johnson said. For example, Iridium LLC designed its satellites with enough propellant to steer back into the atmosphere at the end of their two-year lives, he said.
"All of them are launching with upper stages that come back immediately or soon," he added.
Although the Orbcomm satellites do not carry any propellant for maneuvering during its mission, they carry some when first deployed to move into the correct orbit. Maclay said that when an Orbcomm satellite reaches the end of its mission life, it will burn whatever fuel is left over to lower the orbit. That will shave off a little bit of time from its return trip to earth and passivate the spacecraft all at once.
"It doesn't get you much, but you might as well use it," he said.
Although Maclay said it may not be easy for companies to comply with all of NASA's guidelines, he thinks the industry has been working hard to approach the problem responsibly.
"We're going to put second generation, third generation satellites up there," he said. "It's an environment we have to operate in."
In an attempt to limit collisions between constellations, Orbcomm and Iridium have written a "memorandum of understanding" that keeps their satellites at different altitudes.
"When we both announced our plans, we wound up planning on using the same orbital regime," Maclay said. "And we decided, gee, maybe that's not the brightest thing. So we talked it over with them and we moved." Orbcomm's fleet now operates at 820 km, while Iridium is between 770 and 790 km.
"There are a lot of smart things going on in industry that they didn't really have to do," Johnson said. "It's the right thing to do, the smart thing to do."
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