In a warehouse in the New Mexico desert, a chunk of solid aluminum gleams in the flickering fluorescent light. Roughly the size and shape of the New York City telephone book, the aluminum could get shot all day with a high-powered rifle and give the marksman nothing to show for his work but a sore trigger finger. Yet this chunk has been violated. A plastic pellet, smaller than a walnut, has smashed a ragged five-inch hole right through the aluminum's center. It is a sobering sight and an object lesson for NASA's orbital debris program: The pellet, accelerated to 15,000 miles per hour by a powerful gun here at the White Sands Test Facility near Las Cruces, shows the violence a piece of orbiting space junk can wreak on a spacecraft. "In space, you can get relative velocities of more than 30,000 miles per hour," says Don Henderson, project leader of the Hypervelocity Impact Test Facility. By comparison, a high-powered rifle bullet goes about 1,500 mph. "A fatality is definitely possible," says test facility manager Justin Kerr. "That's what we are trying to prevent." It's a big job. And we brought it on ourselves. Since the Soviet Union launched Sputnik 1 in 1957, the spacefaring nations have filled both near- and far-Earth orbits with stuff: satellites that conk out after a few years, exploded rocket stages, nuts and bolts, trash bags, even human waste from astronauts. A 1999 study estimated that 1,800 metric tons of debris whirl in low-Earth orbit (under 800 miles) alone.
Once a day, on average, a softball-or-larger-sized object tumbles back into Earth's atmosphere, but the hazard to ground dwellers is slight. Most pieces burn up from atmospheric friction; most of the remainder falls in the ocean or in desolate regions such as the Australian outback (where chunks of the U.S. space station Skylab famously fell in 1979, scattering debris that ranged from less than an ounce to more than a ton). "In 40 years, we have no recorded incidence of someone on Earth being hurt," says Nicholas Johnson, NASA's chief orbital debris scientist, but he concedes such an event could happen. This past January, a 150-pound titanium motor casing smacked into the Saudi Arabian desert. "Obviously," Johnson says, "had you been standing under that, you would have had a bad day." Still, the real hazards are in space. Although debris in lower orbits tends to fall and burn up in a few months or years, "once you get past 1,000 kilometers, lifetimes are on the order of a thousand years or more," Johnson says. Because so much of what is sent into space stays up there so long, and more is being added all the time, the density of debris in low-Earth orbit—where the shuttle and the space station fly—doubled between 1960 and 2000. The danger would be minimal if the debris all traveled in one clump in the same direction and at the same speed. But the world's space programs have used every orbit imaginable—prograde, retrograde, polar, elliptical, and variants thereof—and the speed of the debris increases as it comes closer to Earth and its gravitational pull. The result: A computer model of space junk resembles a swarm of bees, with objects whizzing at every velocity, altitude, inclination, and eccentricity. In July of 1996, the French military satellite CERISE collided with a briefcase-sized piece of debris from the Ariane 1, a French rocket launched 10 years earlier. ("The fact that they were both French is pure coincidence," Johnson says.) The impact—which occurred at an astounding 31,000 mph—vaporized the central section of CERISE's 20-foot-long stabilization boom, setting the satellite tumbling crazily in its polar orbit. If the satellite had been a manned spacecraft, the result might have been disastrous. Instead, altitude-control software allowed ground engineers to reorient CERISE and continue its mission. The sheared-off boom joined the growing crowd of space junk and, like all the other big pieces, was duly assigned a catalog number by NASA: 21995-033E. Space debris is much on the minds of NASA scientists these days, as the agency assembles its largest target yet: the 500-ton International Space Station, scheduled for completion by 2006. Designed for research—its six laboratories will study low-gravity biology, Earth ecology, and materials science—the station will float 250 miles above Earth, a region crisscrossed by cosmic junk. "Over 10 years," Johnson says, "there's a one in 20 chance of a critical component loss that could lead to loss of life or injury." To lessen the risk, NASA will use a simple game plan: Dodge the big stuff, shield against the little stuff, and add as little new junk as possible. When collisions happen anyway, crew members will patch the smaller holes and cracks with high-impact plastic disks, stuck on from the outside of the craft with toggle bolts and adhesive. Astronauts will handle larger ones by sealing off hatches, much as crews do in leaking submarines. Meanwhile, on Earth, NASA will be working overtime to steer the station clear of danger. The Space Surveillance Network is headquartered near Colorado Springs, about a mile inside the granite of Cheyenne Mountain. Here, in a humble 30-by-30-foot room, a half-dozen technicians monitor some 80,000 radar and optical observations from all over the world every day. NASA also tracks some 9,000 orbital objects larger than four inches across—about 600 are working satellites, the rest is junk—and uses radar to survey another 100,000 pieces between one and four inches across. "As you might guess, assembling it all is a Herculean task," Johnson says. So far, the Ariane 1 collision has been the only one of its scale. But every craft sent into orbit gets whacked repeatedly, most often by debris or micrometeorites no larger than a grain of sand. When American astronaut David Wolf was on board Mir in 1997, the space station lost power for a time, and he could hear micrometeorites skittering against the outer walls like hail. Debris creates an average of 32 tiny pits in the space shuttle's windows on each mission; NASA has had to replace some 120 shuttle windows because of the damage. By analyzing the material clinging to the windows with an electron microscope, researchers have found that 35 percent of the debris involved in the collisions is aluminum, 17 percent is paint, 11 percent is steel, and 4 percent is copper. Micrometeorites account for about 33 percent of the collisions, but they're less than half as dense as aluminum and so are relatively harmless.
Data from the Space Surveillance Network's computers go to space agencies the world over, each of which has its own collision-avoidance plan. The American program is "ultraconservative," Kerr says. If there is a one in 100,000 chance that the shuttle will collide with a large object, NASA will move the shuttle—and has done so six times in the program's 20 years of flights. The trickiest debris to assess are those between one and four inches in diameter—big enough to be dangerous, but too small to be worth tracking and avoiding. So the technicians at Cheyenne Mountain have no choice but to calculate the odds and roll the dice. "It's pure risk management," Johnson says. For example, to keep the shuttle from presenting too big a target, Kerr's team pesters mission controllers into orienting the rocket plane tail-first into the debris. "Sometimes they don't like to do it, because it complicates the mission," he says. "But on mission 73, we had them fly in a different position and told them to close the payload door. Debris hit that closed door." Had they not flown as suggested, Kerr says, the debris might have punctured a compressed gas tank, thereby terminating the mission. The White Sands Test Facility is where NASA sweats the small stuff. The vast majority of space debris is under an inch in diameter and therefore too small to be tracked by radar. Because there are countless millions of such pieces, the only possible defense is shielding. But that presents a classic engineering dilemma: Too little shielding and astronauts might die, but every extra pound launched into orbit on the space shuttle costs up to $10,000. How much is just enough? To find out, NASA runs the most powerful shooting gallery in the world. In building 272, a corrugated steel shed flanked by the Organ and Dona Ana mountains, four huge guns, called two-stage light-gas launchers, fire small aluminum balls at speeds approximating those of the blistering pace of space junk. It is patient, meticulous, important work, but it also taps the universal male urge to shoot and blow up stuff. "It's a pretty neat job," Henderson says, grinning broadly. A sign on the one-inch bore gun reads "Ol' Blue." Behind it, another reads "Say it, do it, shoot it." A display case reveals the damage that tiny objects in space can do: Kevlar-wrapped oxygen bottles ripped in half, steel cables shredded in two, and other, similarly violated bits of spacecraft paraphernalia. Most of the damage was done with aluminum balls smaller than peas. Today Ol' Blue will fire on a hull sample from a Russian-made docking module—the part of the International Space Station to which the Soyuzcapsule will attach. "The Russians have been using this kind of shielding for a long time, and they don't want to change it until they are convinced it's necessary, " Kerr says. He cradles the one-foot-square section. It consists of a layer of glass-fabric insulation, an aluminum sheet, a one-inch air space, another aluminum sheet, another air space, and a final, inner pressure wall of glass fabric and aluminum. Kerr thinks the section needs some synthetic-fiber stuffing in the empty spaces, which would help absorb even more kinetic energy.
Technicians bolt the section into the gun's target tank, then pump out the tank's air to replicate the vacuum of space. The gun—a 40-yard-long steel tube that terminates at the tank—is simple but ingenious. Nearly four pounds of exploding gunpowder drive a polyethylene piston down a compression chamber, squeezing the hydrogen gas inside to more than 100,000 pounds per square inch. The tremendous pressure hurtles forth an aluminum sphere cradled in a plastic case. Then a cone-shaped steel stripper peels the case away from the ball, and the ball whizzes into the target tank and into the target. As the ball hits, a digital camera whirring at 100 million frames per second captures the nanosecond of impact. The target tank releases X rays, and the equipment could theoretically fracture and fling shrapnel across the room, so Kerr and the technicians wait in a concrete-lined basement bunker. From there, the gunpowder explosion barely registers as a muffled whump. Today's test yields good news. Propelled at 15,000 mph, an 11/64-inch aluminum sphere—roughly the size of a corn kernel—ripped through the module's outer and middle layers but failed to penetrate the pressure wall. "That's good—maintaining the integrity of the pressure wall is what matters," Kerr says, eyeing the dented aluminum. But he will still press for extra stuffing: It's a relatively cheap and lightweight way to add resistance, and tests at other velocities and angles have persuaded him that at least some stuffing should be added. To the casual observer it seems odd that this apparently wispy triple wall of aluminum and fabric could stop a ball hurtling at such speeds, but Kerr says smart engineering creates a kind of physics jujitsu—the particle's tremendous velocity is used against it. "The particle expends its energy and essentially vaporizes and spreads as it passes through the outer skin," he says. "So it hits the pressure wall as a mist." The tendency to vaporize means that "sometimes the faster the projectile travels, the better the pressure wall holds up." These kinds of tests lead to changes in spacecraft design. The puncture risk for one of the space shuttle's ammonia-filled radiator panels, which are used to bleed excess heat from equipment and the crew compartment, used to be one mission in 62—poor odds, given that a single puncture can terminate a mission. Adding just one layer of 1/50-inch-thick aluminum strips boosted the puncture odds to one in 1,000. "We can definitely live with that," Kerr says. Dodging, shielding, and calculating risks have helped NASA avert a serious space-junk disaster so far. But nothing will solve the problem if we keep filling the heavens with trash. The worst-case scenario is a "cascade," in which the debris becomes so plentiful that it collides with itself, breaking into so many pieces that orbital space becomes as dangerous—and uninhabitable—as a sniper range. But that will happen only if the spacefaring nations do nothing. Prodded by NASA, they are, in fact, doing a great deal. More than 100 scientists and engineers now work full- or part-time on orbital debris problems, and NASA engineers believe disaster can be forestalled indefinitely. "I have heard people engage in a sky-is-falling point of view, who say the risks are becoming unacceptable," Kerr says, standing next to Ol' Blue. "Frankly, we are the ones who are operating spacecraft in orbit. We don't have our heads in the sand. We are managing the problem." It's only appropriate, some would add, that NASA takes the lead: The United States and the former Soviet Union are far and away the greatest space polluters. Of the 8,831 large objects tracked as of April 2001, 44 percent are American and 44 percent are Soviet, most of them pieces of exploded rockets. Some scientists have proposed zapping debris with Earth-based lasers, sweeping the most polluted orbits with open-umbrella-shaped grabbers, or even sending up a half-mile-wide "Nerf ball" to absorb debris. But Kerr says you can't make a good economic case for any cleaning gadget. So far the only workable solution has been to slow the rate of space littering. And the best way to start, NASA has reasoned, is to keep old rockets from exploding in the first place. As a rocket struggles against Earth's gravity, it has to shed stages to trim its mass and allow it to climb farther. As long as the chunks stay intact, they aren't much of a problem: They're huge, easy to track, and easy to avoid. But NASA discovered in the late 1970s that spent stages have a tendency to explode. As they drift in space, solar radiation brings the oxidizer in their fuel tanks to a boil. If, as often happens, a tank's relief valve fails, or the wall between the oxidizer and the fuel ruptures, the resulting explosion could blow the whole stage apart. NASA engineers soon learned to jigger propellant systems so that they burn or vent until they're empty. But the Russians were slower to solve the problem. In 1992, Johnson had to go to Russia himself to persuade scientists to follow NASA's lead. "In everything they have launched since 1996, no more problem," he says. The world's other space agencies also have adopted U.S. protocols enthusiastically, and even tougher United Nations guidelines will be voted on in 2004. But space is rapidly going private. Sea Launch, an entirely commercial partnership between Boeing and several international aerospace companies, has launched six successful missions from its floating equatorial platform since 1999, and more private space initiatives are in the works. Yet so far the major aerospace companies have agreed to follow NASA's standards, Johnson says. "They recognize that it is simply in everyone's best interest to maintain the safety of orbital space." At White Sands, the hour grows late. The reddening desert sun angles through the open door of the hypervelocity test center and glows through the big hole in the aluminum slab. Looking at this piece of accidental modern art, it's hard not to think of the relative weakness of human flesh, of how one discarded tube of peanut butter from mission A could punch a similar hole through an astronaut on mission R. Kerr thinks about it, too, but he insists that he does not worry. "There will always be hazards in space, and orbital debris is just one part of it," he says. "From now on, for at least the next 15 years, there will always be human beings in space. We're not going to get to the point where they are as safe as the average person walking down the street, but they can be as safe as a test pilot. We are making a shield for them. I would personally be willing to stand behind that shield." To Shield a Ship Shielding a spacecraft against space debris is, quite literally, rocket science—a daunting engineering task with a variety of possible solutions.
The Whipple shield, invented by astronomer Fred Whipple, was introduced in the 1940s and is still used today. It consists of a .08-inch-thick sacrificial bumper, usually made of aluminum, and a pressure wall between four and eight inches behind it. Debris that pierces the bumper breaks into a fine mist, dispersing so much kinetic energy that it sprays relatively harmlessly against the inner pressure wall. A "stuffed" Whipple shield also has layers of Kevlar, a synthetic material used in bulletproof vests, in the space between the bumper and the wall. The Mars module shield, a prototype developed for a future manned trip to that planet, consists of layers of Mylar, Nextel (a woven ceramic fabric), Kevlar, and foam. The foam is designed to compress, because the whole Mars "Transhab" spacecraft must fit within the space shuttle's cargo bay. The most critical areas are guarded by multi-shock shields: staggered layers of Nextel at varying distances from one another. The layers repeatedly slow and pulverize entering debris so that it sprays harmlessly on the pressure wall—if it reaches the wall at all. — B.L. The Orbital Debris Quarterly News, an online NASA newsletter, has updated information on current research relating to space debris, including the latest statistics on how much junk is out there: www.orbitaldebris.jsc.nasa.gov/newsletter/ news_index.html. For additional info on the White Sands Test Facility, visit www.wstf.nasa.gov. NASA's Hypervelocity Impact Technology Facility site addresses the problem of orbital debris: hitf.jsc.nasa.gov/ hitfpub/main/index.html. "Orbital Debris, A Technical Assessment," by the National Research Council's Committee on Space Debris, is available on the National Academy Press Web site: www.nap.edu/catalog/4765.html.
