20 May 2005
On Jan. 17, the engine from a Thor rocket launched 31 years ago was soaring southward, 550 miles over the African continent. At the same time, a fragment of a Chinese rocket that blew up five years ago was high over the Pacific Ocean, also headed south.
Incredibly, the two chunks of metal flew into the same spot over Antarctica at the same instant.
The high-speed collision, reported last month by NASA's Orbital Debris Quarterly News, created even more orbiting space junk. It also drew renewed attention to the litter that surrounds our planet - and efforts to keep the neighborhood from becoming even more cluttered and dangerous.
"It's sort of a classic environmental problem, not unlike air pollution or water pollution," said Nicholas L. Johnson, chief scientist and program manager for NASA's orbital debris program. "If you wait until you start seeing negative consequences, then the environment is pretty far gone already, and cleaning it up can be very, very difficult."
When the shuttle Discovery is finally launched, most likely in July, it will join more than 13,000 pieces of orbiting hardware that the U.S. military tracks 24 hours a day.
Among them are hundreds of working satellites - and thousands of dead ones - along with spent rockets and other odd scraps set loose by decades of breakups, explosions and collisions.
The junk pile includes about a ton of radioactive fuel from defunct reactors launched into orbit before the practice ended in 1988, according to a recent report to the Fourth European Conference on Space Debris.
Much of the material is concentrated in "low-Earth orbit," which extends to about 1,200 miles and is home to the International Space Station and hundreds of communications, environmental, scientific and spy satellites.
A thousand more satellites - about half of them working - cluster in a slender ring, like a bicycle tire, about 22,000 miles above the equator. They orbit once a day, hanging above the same spot on the ground as the Earth spins. These "geosynchronous" orbits are ideal for communications satellites, which must stay in view of fixed dish antennas on the ground.
Everything larger than a softball is tracked 24 hours a day by the 1st Space Control Squadron. It's a part of the U.S. Air Force Space Command, tucked deep into the Cheyenne Mountain in Colorado Springs, along with the nation's aerospace and ballistic missile warning centers.
Satellite tracking began in 1957, with the launch of the Soviets' Sputnik I - the first manmade object ever orbited. Today, data from a worldwide network of 21 telescopes and radars called the Space Surveillance Network stream into powerful computers deep inside the Cheyenne Mountain fortress. The system can register 350,000 contacts per day, according to Michael E. Stringer, the squadron's technical director.
Every year, some objects fall back to Earth, while new ones are launched. On average, the squadron logs a net increase of 200 objects per year.
Uncounted are hundreds of thousands, perhaps millions, of bits of space litter too small to be seen - ranging from nuts and bolts to paint chips. They may be small, but with closing speeds up to 12 miles per second, Stringer said, they pack tremendous energy.
Craft hit by particles
Inspections have shown that the space shuttles and the International Space Station are hit routinely by particles too small to track. "We can't determine where it came from, but we can tell if it was titanium or aluminum, or a paint flake or plastic," Johnson said.
In 1999, Discovery landed with evidence of 64 impacts, at least 10 caused by manmade debris. So far, nothing bigger than 0.08 of an inch has struck a shuttle. But even such tiny particles can damage thermal tiles and windows. "We wind up replacing the outer panes of at least two windows every mission because of impacts with small debris," Johnson said.
It's the big hunks of space junk that really worry NASA. On May 29, 2003, for example, military trackers warned NASA that debris would pass less than a mile from the space station. The station is shielded against objects 1 centimeter wide or smaller, but the heft and trajectory of this one alarmed Houston, and the station moved to a slightly higher orbit.
"We do about one maneuver per year on average. But we get lots of warnings; they come in more than once a month," said Johnson.
NASA estimates that there are hundreds of near-misses - tracked objects passing within a kilometer of each other - every day. But at least for now, collisions are rare. The first one ever detected occurred July 24, 1996, when the French Cerise spacecraft smacked into a piece of the third stage of a European Ariane rocket that blew up in 1986. A search of historical data recently revealed a previously unnoticed smashup, in late December 1991, between one defunct Russian Cosmos satellite and debris from another.
Eventually, most objects in low-Earth orbit disappear when they re-enter the atmosphere, at the rate of three to five a month. And while most of them melt and vaporize on the way down, components with high melting points can survive. Impacts in densely populated areas, while unlikely, do pose a risk of deaths or property damage.
Since Jan. 1, 2001, 10 egg-shaped titanium casings from rockets that launched Navstar global positioning satellites have fallen to Earth. At least three of the 110-pound casings struck land - in Saudi Arabia, Argentina and Thailand.
In January 1997, a 551-pound stainless-steel propellant tank from a Delta 2 rocket landed near Georgetown, Texas.
Understandably, NASA and others would like to clean up Earth's orbiting junkyard. Some have proposed giant "nerf" balls or laser beams to slow speeding debris and bring it down, Johnson said, "but we do not yet have the technology to economically make any substantial improvements."
Some limited efforts are afoot, however. NASA will someday send a robot craft to latch onto the Hubble Space Telescope and drag it safely into the ocean.
And an international team is developing an "electrodynamic tether" that could be unreeled by satellites at the end of their lives. The wires, tens of kilometers long, would speed the satellites' plunge to Earth.
"It would be the most cost-effective way of doing it," Johnson said. A test flight on a Russian spacecraft is in the works.
For now, space planners are focusing on prevention. In the 1990s, NASA issued the first comprehensive guidelines for preventing the creation of new debris. Other nations have joined in similar cooperative efforts, and new United Nations guidelines are expected by 2007.
Spent rockets have been the biggest problem. Years after launch, they can deteriorate to a point where unused fuel and oxidizers come into contact and explode, spraying debris in all directions.
Today, once satellites are deployed, operators are required to burn off any fuel left in their boosters. When satellites in low-Earth orbit are shut down, operators are supposed to release any compressed gases and propel them into orbits that bring them down within 25 years.
The strategy is working, he said - except when operators lose control of their aging satellites before they're made safe.
"Most of the threats to the shuttle and the International Space Station today arise from the explosions in the '60s, '70s and '80s. That debris is long-lived," Johnson said.
Old satellites in geosynchronous orbit are too high to be dumped into the atmosphere. Instead, operators are supposed to push them into a sort of "junkyard" orbit 186 miles higher - ensuring that dead satellites don't clutter up the vital but finite real estate in geosynchronous orbit.
But shoving old satellites aside requires keeping fuel in reserve. "To do that, you're giving up an extra few months of a mission, ... a nontrivial penalty," Johnson said. Some operators have simply not followed the rules. Others have lost control of their spacecraft before they could move them.
As a result, of 103 satellites that expired between 1997 and 2003, 34 were abandoned in the geosynchronous ring, according to a recent study by Rudiger Jehn, at the European Space Agency. Of those, 24 were Russian.
For satellite operators, cooperating should be a simple matter of self-interest.
According to a warning in NASA's Orbital Debris Quarterly News, "If future spacecraft and rocket bodies are not removed from [low-Earth orbit] within a moderate amount of time after the end of mission, within 25 years, the rate of accidental collisions will increase markedly later in this century."
The current positions of more than 500 Earth satellites can be viewed at http://science.nasa.gov/Realtime/JTrack/ . Click on J-Track 3D.