Pelted by Paint,Downed by Debris

Abstract

Missile defenses will put valuable satellites at even greater risk.

We think of space as empty, but the space near Earth is littered with debris. More than 9,000 objects larger than 10 centimeters in diameter, nearly all manmade, are currently being tracked, and there are probably more than 100,000 pieces of orbiting debris larger than a marble. 1 Yet the crowded near-Earth orbits inhabited by this debris are where the Bush administration wants to put certain parts of its proposed missile defense system—Space-Based Lasers and thousands of “Brilliant Pebbles” space-based interceptor missiles. These weapons were previously forbidden by the 1972 Anti-Ballistic Missile (ABM) Treaty, which the United States withdrew from in June.

Weaponization of space would make the debris problem much worse, and even one war in space could encase the entire planet in a shell of whizzing debris that would thereafter make space near the Earth highly hazardous for peaceful as well as military purposes.

The nickname “Star Wars” for missile defense all too accurately reflects the popular fantasy about how things work in space. In the Star Wars movies and in hundreds of other popular science fiction films, we see things blow up in space and the fragments quickly dissipate, leaving empty space behind.

But in reality, space does not clear after an explosion near our planet. The fragments continue circling the Earth, their orbits crossing those of other objects. Paint chips, lost bolts, pieces of exploded rockets—all have already become tiny satellites, traveling at about 27,000 kilometers per hour, 10 times faster than a high-powered rifle bullet. A marble traveling at such speed would hit with the energy of a one-ton safe dropped from a three-story building. Anything it strikes will be destroyed and only increase the debris.

With enough orbiting debris, pieces will begin to hit other pieces, fragmenting them into more pieces, which will in turn hit more pieces, setting off a chain reaction of destruction that will leave a lethal halo around the Earth. To operate a satellite within this cloud of millions of tiny missiles would be impossible: no more Hubble Space Telescopes or International Space Stations. Even communications and GPS satellites in higher orbits would be endangered. Every person who cares about the human future in space should also realize that weaponizing space will jeopardize the possibility of space exploration.

To a scientist whose research has benefited enormously from space observations, these prospects are horrifying. Many of the important astronomical satellites are in low Earth orbit (from the lowest practical orbits—about 300 kilometers—to about 2,000 kilometers above the Earth). The Cosmic Background Explorer, which operated from 1989 to 1994, is at 900 kilometers and the Hubble Space Telescope is at about 600 kilometers.

In addition, most Earth-observing satellites are also in low Earth orbit, both those that study changes in climate and vegetation and those for military surveillance. Low orbits permit the highest-resolution imaging, and are also easiest to reach with existing launch vehicles.

NASA's Landsat-7 is in a 705-kilometer orbit, for instance, and the European Space Agency's (ESA) ERS-2 is in a 780-kilometer orbit. NASA's new international Aqua satellite, which will collect information about the Earth's water cycle, was launched in May into a 705-kilometer orbit.

These satellites are already at increasing risk from space debris. At any moment, only about 200 kilograms of meteoroid mass are within 2,000 kilometers of the Earth's surface. But within this same altitude range are roughly 3 million kilograms of orbiting debris introduced by human activities, most from about 3,000 spent rocket stages and now-inactive satellites. Most of the approximately 4,000 additional objects several centimeters in size or larger resulted from the fragmentation of more than 120 satellites.

The main threat to satellites near Earth is from the 1,000 kilograms of 1-centimeter or smaller debris particles, especially the approximately 300 kilograms of debris smaller than a millimeter. A BB-size fragment has the same destructive energy as a bowling ball moving at 100 kilometers an hour.

An average small satellite in an 800-kilometer orbit currently has about a 1 percent chance per year of failure due to collision with a BB-size piece of debris. 2 The danger to a large satellite such as the Hubble Space Telescope or the International Space Station is even greater. 3

Meanwhile, the amount of small debris is increasing. Random collisions between man-made objects are still relatively rare, but the density of the objects may already be sufficiently great at the 900-1,000-kilometer and 1,500-1,700-kilometer altitudes that a chain reaction or cascade of collisions could be sustained. 4 Increases in the debris population will increase the threat at even lower orbital altitudes, creating an environment hostile to satellites.

Each dot represents a piece of space debris.

In a lecture in April, astronaut Sally Ride recalled a run-in with space debris on her first shuttle flight:

“About halfway through the flight there was a small pit in the window of the space shuttle and we didn't know what it was. An awful lot of analysis was done while we were in orbit to make sure that the strength of the window would sustain reentry. It did. We were all fine. But the analysis afterward showed that our window had been hit by an orbiting fleck of paint, and the relative velocities were enough that the paint actually made a small but visible gouge in the window. Well, a fleck of paint is not the same as a small piece of metal traveling at that same speed. So, as soon as you start increasing the amount of junk in a low Earth orbit, you have an unintended byproduct that starts putting some of your own quite valuable satellites at possible risk.

“What if anti-satellite testing proceeds and we start testing rockets that clobber satellites and explode them in space? What if enough of that goes on that there's the equivalent to a test range up in low Earth orbit?” 5

Offensive weapons fn space pose a great threat. Fortunately, offensive weapons have not yet been introduced into space—except for a series of Soviet anti-satellite weapons tests between 1968 and 1982, and the intentional destruction in 1985 of the still-operating Solwind satellite in a demonstration by the U.S. military. Many of these tests generated hundreds of pieces of trackable debris.

Kinetic-kill vehicles like Brilliant Pebbles would be sure to generate great quantities of space debris just during their initial deployment, and far more if they were ever used. Basing weapons in space would require hundreds of individual attack satellites in order that at least one be near its time-urgent target, such as a missile in boost phase, and each would have to circle the Earth every 90 minutes. 6

Any kind of space warfare would put all satellites at risk. The explosion of nuclear weapons in space (prohibited by the Outer Space Treaty, but routinely considered by military planners) would indiscriminately destroy unprotected satellites by electromagnetic pulse or nuclear radiation. 7

Perhaps worst of all would be the deliberate injection into low Earth orbit of large numbers of particles as a cheap but effective anti-satellite measure. Any country that felt threatened when the United States began to place lasers or other weapons into space would only have to launch the equivalent of gravel to destroy sophisticated weaponry.

Internationally agreed policies should be adopted now to help avert a tragedy of the commons in space. They include:

• • Pledging not to introduce attack weapons into space.

• • Avoiding fragmentation of satellites from explosions due to accidents and anti-satellite weapons tests, the main cause of space debris. Explosions of any kind in space should be prohibited.

• • Designing boost and deployment systems for satellites that minimize the creation of debris. All satellites in low Earth orbit should be required to carry a mechanism—rockets or inflatable devices that increase drag— causing them to burn up in the atmosphere within, say, 25 years after their useful life has ended.

• • Banning nuclear reactors in orbit, since they are an environmental threat, useful for military purposes only. 8

• • Minimizing light pollution from orbit.

The space age is only 45 years old, yet there are so many artificial objects in the near-Earth environment that random collisions between them threaten scientific and other uses of low Earth orbit. Fragmentation of debris could eventually produce enough dust to cause a lingering twilight as it is illuminated by sunlight, a new sort of light pollution. 9

Space agencies need to take active steps to prevent the buildup of debris, and it is an encouraging first step that NASA and ESA have succeeded in eliminating the Delta and Ariane upper stage explosions that were a major source of the problem. But much more effort is needed, and it may even be necessary to deploy special spacecraft to remove some of the larger pieces of debris at the altitudes where the critical density for a cascade may have already been reached. Designing these devices will be a useful exercise, not least because it will help to impress on public officials the cost of space debris. 10

National political leaders usually take a short-range view, hardly ever stretching past the next change of government. Astronomers measure time in millions and billions of years. The public needs to think with at least an intermediate perspective about the environmental degradation that our increasingly powerful technology is causing on and near our beautiful but fragile planet. •