Space needs a litter law. Even a nylon fiber can dent a spacecraft
JOSEPH Loftus held a piece of aluminum plate in one hand, and a centimeter-long nylon fiber in the other. The fiber had rammed the plate at orbital speed, blasting out a walnut-sized cavity. ``That's what we mean by hypervelocity damage,'' Loftus said. ``The only way to keep space clean is to have a litter law and enforce it.''
As assistant to the director for planning at Johnson Space Center in Houston, Loftus is future-oriented. But he doesn't need prophetic skill to foresee the need for a ``space junk'' treaty. Space-faring nations habitually use the sky for a litter bin. The advent of permanently-occupied space stations will make that sloppy habit intolerable.
The President's National Commission on Space (NCS) notes that thousands of objects with millions of pounds of collective mass have been tossed into orbit. Some satellites and larger debris eventually reenter the atmosphere and burn up. But launches continue. According to the recent NCS report, ``the space debris population remains constant at approximately 5,000 pieces large enough to be tracked from Earth.'' Thousands of smaller pieces drift around our planet undetected. When astronauts repaired the Solar Maximum Mission satellite, they brought back some heat control louvers that showed over 1,000 impacts. Many of those would be due to unnatural micrometeors.
These ubiquitous bits often come from collisions between larger objects. Such collisions are not like cars banging together on Earth. Objects orbit at typical speeds of 7,000 to 17,000 mph. When they collide, the speed of the shock waves generated in the objects can exceed the speed of sound. Those objects then behave more like water than solids: Even aluminum and steel fly apart into millions of pieces. (Tests of antisatellite weapons make an awful mess.)
Really small particles -- a millionth of a meter or so across -- aren't a problem. But even soft material a millimeter or centimeter in length is about as gentle as a high-speed bullet. Permanent space stations will have to have meteor bumpers of sacrificial material to protect them.
It would be better to avoid the problem as much as possible by cleaning up the sky. Space planners have talked about this for decades. Now it's becoming a matter of practical concern.
The key issues are to identify what debris is important and how best to deal with it. Except for the brief transits of interplanetary vehicles, both manned and unmanned spacecraft stay largely within certain orbital zones. Low earth orbits range over a few hundred miles from our planet's surface. The so-called geosynchronous orbit, where a satellite moves at the same speed as the Earth turns, is 22,300 miles high. It's also called the Clarke orbit, after science writer Arthur C. Clarke, father of the communications satellite.
Most satellites orbit within one or the other of these zones. They tend to avoid the space in between, where Earth's radiation belts endanger electronic components and living organisms. These preferred zones are where debris tends to accumulate. And, of course, they're the zones we would most like to keep clean.
There's not much we can do about the really small stuff. The orbital vacuum cleaners and meteor scoops of science fiction aren't practical -- at least, not yet. But larger objects can be captured. A strategy to prevent spacecraft and spent rocket casings from becoming derelict and breaking up into debris would be best of all.
Loftus explains that sending old spacecraft back down to burn up in the atmosphere may not be the best idea. It takes energy to de-orbit a spacecraft, as it does to boost it into orbit in the first place. And energy costs money. It may be better to park old satellites in a holding area until they can be collected all at once, if that becomes necessary.
For example, he notes that there are a couple of zones along the Clarke orbit where satellites are unusually stable, and don't tend to drift off-station. This orbit is popular and crowded. Many weather and communications satellites operate there. So getting rid of old satellites is a continuing concern. Now, they are sometimes boosted out of the way into a higher orbit. Loftus suggests herding them into the two stable zones. We'd know where they were and could get them when we wanted them.
Obviously, these and any other arrangements for dealing with space litter are an international matter. That's why a space junk treaty seems desirable. Loftus says it's already a staple item on the menu of international technical conferences that deal with space. So formal diplomatic moves toward a treaty might come by the early 1990s.
Meanwhile, we might think about a mascot for the orbital clean-up brigade. It can't be Ranger Rick the raccoon or Smokey the Bear. Maybe we should co-opt E.T.
Robert C. Cowen is the Monitor's natural science editor.