Photo by Rick Geary

In 1983, Challenger collided with a fleck of paint. It damaged the front window badly enough that it had to be replaced before the shuttle could be relaunched.

Dear Matthew Alice: How do spacecraft avoid hitting things as they speed through the universe? Presumably, big things like planets can be planned for and avoided. But what about all the other space junk? It seems like even a pebble could do a lot of damage if collided with while traveling a kajillion miles per hour. — Ginny Jetson, San Diego

According to NASA, space is beginning to resemble the average teenager’s room. Dirty laundry on the floor, potato chips under the bed — junk everywhere. As an example of the potential for disaster, one scientist calculated that an object l/35th the weight of an aspirin would hit a spacecraft with the impact of a .30-caliber bullet (a 30-caliber bullet fired from the average teenager’s room, the way things seem to be going these days). In 1983, Challenger collided with a fleck of paint. It damaged the front window badly enough that it had to be replaced before the shuttle could be relaunched. Every time the shuttle is sent up, NASA figures it has 1 chance in 30 of hitting something.

According to best estimates, we’ve littered space with something in excess of 3000 tons of technology. Shredded rockets, comatose satellites, defunct power supplies, including nuclear reactors and batteries.... This in addition to the dust and debris that occur naturally out there. And of course, space junk doesn’t collide only with rockets and satellites, it bashes into other space junk. One big piece of galactic litter grows exponentially into scores of tiny pieces. By the year 2000, low Earth orbits could be too littered and dangerous to use. To prevent this, spacefaring nations might have to file “orbital impact statements” before they shoot something beyond the atmosphere and make sure they clean up after themselves along the busiest routes.

Most of the worrisome dreck occurs in bunches in a band about 250 miles above Earth, between 28.5 degrees north and south latitudes — a sort of orbital rush-hour freeway. In 1984, NASA launched the Long Duration Exposure Facility that photographed the floating trash clouds, so we actually have a map of where some of the stuff is located. A few of the clumps even have names; the “May swarm” is a particularly nasty cluster, apparently. (The LDEF itself was hit 15,000 times during 50 weeks in orbit.) On radar from Earth, we can detect chunks larger than four inches.

NASA claims that we could police up the area with the technology that exists today, but the cost would outstrip the benefits. Adding a junk shield to space vehicles, like high-tech cow catchers, 1 guess, is one practical possibility. And one wizard at the Johnson Space Center has designed a space sweeper that looks like a huge ceiling fan. Each blade is a quarter-mile long and several hundred feet wide. When radar detects some space junk, the blades intercept it. Theoretically the debris will smash into the blades, which will slow the particles enough to embed them in a blade or knock them out of orbit. The fan blades would be retractable to avoid hitting things we want to stay up there or things so large they’d demolish the sweeper. The riddled blades would have to be replaced every few years. For the moment, the space broom exists only on paper.

From the sound of things, NASA avoids collisions with space junk mainly through a prelaunch ritual during which all concerned cross their fingers and make a wish.