On the eve of the first crew arrival to the International Space Station (ISS), number one on the list of priorities among scientists and officials at NASA and around the international space community is the station's safety and well-being. But there's one seemingly low-profile threat that could someday cause big trouble for the ISS: space junk.

Near-Earth orbit is full of space junk bits and pieces of metal and other materials left over from rocket shells, exploded satellites and other aged spacecraft. In fact, scientists last month were for the first time able to link a cloud of orbital debris to an exploded rocket part, adding fodder to the notion that what we put up into space could quite literally have great impact on satellites we rely on, and even threaten the lives of space explorers.

The debris cited in last week's announcement came from a Chinese Long March 4 projectile, which exploded on March 11 after five months in near-Earth orbit and was detected by the University of Chicagos space dust instrument, SPADUS. Although the detected particles were sub-millimeter in size, they are nevertheless representative of the type of space junk that can be hazardous to sensitive parts of orbiting spacecraft including the space shuttle and the ISS.

Indeed, it may not sound like much of a threat, but small particles can puncture and seriously harm spacecraft parts. In fact, space-shuttle ground crews routinely find evidence of tiny space particles having dug themselves quite deeply into the shuttle's windows after a mission.

"We get hit regularly on the shuttle," said Joseph Loftus, assistant director of engineering for NASA's Space and Life Science Directorate. "We've replaced more than 80 [shuttle] windows because of debris impacts."

Of course, tiny particles are just the tip of the iceberg when it comes to orbital debris. Thousands of other "junk" objects, ranging in size from inches to feet (millimeters to meters) across, loom several hundred miles (kilometers) above Earth. And these objects arent standing still. Theyre orbiting Earth just like other satellites, often at speeds of up to several miles (kilometers) per second. Crash one or more of these objects into a spacecraft or an astronaut out on a spacewalk, and you could end up with Swiss cheese serious damage.

Today, the ripest target for orbital debris appears to be the ISS, as it is slated to be the largest and most sophisticated object ever assembled in space. Add to that fact engineers expectation that the ISS will function for at least 20 years, and some experts say the station may be a disaster waiting to happen.

"When you roll it all up, the probability of the ISS getting hit by something other than very small stuff becomes nonzero," said Nicholas Johnson, chief scientist for orbital debris at NASAs Johnson Space Center in Houston, Texas. "That means you really need to take some countermeasures."

Fortunately, both NASA and other space agencies have already made tremendous efforts to curb space-junk encounters. At the most rudimentary level, the U.S. Space Command keeps track of many of the objects orbiting Earth, including debris. As of June 2000, the total number of trackable space objects including 90 space probes, 2,671 satellites and 6,096 pieces of space junk was a whopping 8,927.

If any of the more than 6,000 space-junk objects gets too close for comfort to a satellite or inhabited spacecraft, the Space Command sends out a warning. Already, the ISS has had to move out of the way of an oncoming piece of space debris; in October 1999, ground controllers successfully maneuvered the ISS out of the way of a piece of debris that was headed right for the station. The space shuttle has also had to make way for a number of pieces of space junk.

But the Space Command can only detect particles larger than the size of a typical melon. So although engineers have developed special shielding materials to protect against the tiniest of debris, intermediate sized objects, from about half an inch to five or six inches (1 to 15 centimeters) across, become the biggest hazard of all for orbiting spacecraft.

"That gap represents the true risk to the space station," said Johnson. "Those [intermediate-sized] objects are too small to be tracked but too large to be protected against with shielding. So if we get hit by a particle in that size regime, some damage will occur."

Just how much damage depends on where and how the impact takes place. The most likely event, according to Johnson, is a small penetration, which poses a low risk to those living inside. But a worst-case scenario could easily cause cosmonauts and astronauts occupying the station to immediately evacuate the damaged module or else risk suffering from depressurization, loss of oxygen and, eventually, unconsciousness.

Though several major schemes have floated around since the Orion Project first began, the most feasible seems to be a ground-based laser system that would hunt down space junk and shine its laser at the particle for several minutes. The energy of the laser light would actually ablate a tiny layer on the debris surface, thus creating a thrust for the object as the molecules in the thin layer expand away from the object while theyre being vaporized.

"We have demonstrated that a laser pulse can indeed move orbital debris-like material in a vacuum," said project scientist and Orion program manager Jonathan Campbell. "If we can work on a piece of debris for several minutes, then we can tease it into the atmosphere and let the atmosphere take care of it for us."

Another idea was to have an in-situ laser on board a craft such as the ISS, which would essentially work the same as a ground-based laser, but would have the added benefit of being much closer to the actual debris.

However, two major problems with this model (referred to by some as a "laser-broom model") suggest the approach would never work: the system would take more power than the entire space station has and humans wouldnt have enough time to detect and hunt down an object coming over the horizon before it either hits or passes the craft.

"Doing it in orbit is theoretically possible, but from a systems-development management point of view, its not very practical," said Loftus.

Accordingly, scientists and engineers are working primarily on the former Orion model, and are hopeful they will soon get to test out the concept using a special laser system in Hawaii. "If we can demonstrate that we can detect a tennis-ball-sized object 2 kilometers (1.2 miles) away, and we can hold a laser beam on it through turbulent atmosphere, then that should be enough to prove to even the greatest skeptics that this is something that can be done," said Campbell.

Of course, the other obvious way to reduce space junk is to send less of it up in the first place. Currently about 200 objects per year are added to the space environment around Earth. But with more and more emphasis placed on space business and science, it seems likely that the upward trend toward more space debris will only increase.

For now, NASA and other members of the international space community are trying to fight space junk by keeping the debris in check as much as possible. Already, there are a number of design requirements and procedures for each launch and space mission that ensure the least amount of space junk, including controlled deorbiting of aging spacecraft. But people working with the Interagency Debris Corps Working Group, who look at managing orbital debris, are already looking ahead to create even better standards.

"Were trying to get international consensus," said Loftus. "So far were making very significant progress."