If you knew what was going on above your head, you probably wouldn't sleep at night. It was 54 years ago this week that the first satellite — Russia's little Sputnik — went into orbit, and in the half-century since, we've made travel to near-earth space largely routine. That, by almost any measure, is a good thing, but like so many good things human beings build or do or achieve, a very bad thing comes along with it: junk.
For every spacecraft that goes into orbit, all kinds of rubbish — bolts, boosters, adapter rings, insulation, even paint chips — are also released. Five decades of that kind of littering has created a very big mess: according to tallies from NASA, NORAD (North American Aerospace Defense Command), the Federal Communications Commission and other domestic and international agencies, there are currently 17,000 objects measuring 4 in. (10 cm) or greater circling the earth. There are a whopping 200,000 in the 1-in. to 3-in. (2.5 cm to 7.5 cm) range, and millions smaller than an inch.
Many of those objects in the largest category are much, much larger than a mere 4 in. (10 cm). Indeed, they are entire spacecraft themselves. Aged satellites — some the size of a school bus and weighing on the order of tons — that have outlived their usefulness often wind up simply drifting through space like an abandoned barge. In late September, a 6-ton NASA research satellite, launched in 1991, came plummeting home, falling safely into the south Pacific, but not before causing a lot of scientists a lot of jitters as they watched the roulette wheel of the satellite's orbit, trying to calculate just when and where the ship would lose enough energy to drop from the sky. No sooner had that scare ended than the German space agency announced that a 2.5-ton space telescope called ROSAT would be tumbling home sometime in late October or early November — landing spot still unknown. In August, problems with Russia's Soyuz booster, which is the only space taxi American astronauts currently have, triggered a lot of talk about whether the International Space Station (ISS) would have to be abandoned, causing the entire 357-ft. (109 m) facility to plunge back to earth.
The ISS is not going anywhere soon, thanks to onboard boosters that can stabilize and raise its orbit, but the rest of the junk will ultimately go down, if it doesn't collide with another spacecraft first — a whole other category of possible calamity. Just such a cosmic crack-up occurred in 2009 between decommissioned American and Russian satellites, an accident that would have been instantly fatal if one or both of the vehicles had been inhabited.
The very enormity of the debris belt would seem to make many more such accidents inevitable, but as after the 2009 collision, the risk is minimized by the fact that all objects orbiting at the same altitude also move at the same speed — 17,500 m.p.h. (28,200 km/h). That means they're essentially standing still relative to one another. To close the gap between two objects, as, say, a Soyuz spacecraft does when approaching the ISS, it's necessary to change that relative speed — in this case, by the Soyuz tweaking its thrusters to accelerate ever so slightly. Relative to the station, it's moving at a very safe crawl, just enough to nose into a docking port.
The problem comes when two objects in effect change lanes — descending to a lower orbit (which requires slowing down) or climbing to a higher one (which calls for speeding up). That's the equivalent of cars merging badly on a freeway — with similarly messy results. Even worse is when two vehicles flying in different orbital planes — or inclinations to the equator — collide. The U.S. launches most of its spacecraft at a 28.5-degree inclination in order to ensure that the ships fly over the U.S. land mass; the Russians fly mostly at 51.6 degrees so that they cover their own home turf the same way. The 2009 accident was a demonstration of what happens when the two planes cross.
While it's relatively easy to track big pieces of debris, small ones — as small as a grain of sand — can do their own kind of damage, since the speed at which they travel increases the energy they pack all out of proportion to their size. The ISS was built of different airtight compartments mostly because that was the easiest way to get all of the components to orbit, but the various nodes do serve as individual panic rooms into which astronauts could retreat if a high-speed space grain penetrated the hull of one of the other compartments, causing it to depressurize.
No one on the ground need to worry about the small stuff — or even the medium or relatively big stuff — since the earth's atmosphere sees to it that most debris is incinerated well before it reaches the surface. But if the re-entering vehicle is too big, the atmosphere can't vaporize it all — as happened when chunks of NASA's Skylab crashed into the Australian outback in July of 1979. What's more, some parts of some ships are resistant to the flames of re-entry — one of the concerns about the faltering German satellite, which includes heavy, heat-resistant glass and ceramic that were used, in part, to construct its mirror. About 30 such pieces of varying sizes are predicted to make it to the ground.
The odds of any one person getting clobber ed by the ROSAT ship are considered relatively steep — about 1 in 2,000, compared with 1 in 3,200 for the American satellite that hit the Pacific. Still, that's hardly off the statistical charts. The remedies for all of this are few. Designers can work to build their ships with fewer breakaway parts, which most indeed try to do. Good tracking makes it possible to predict more accurately when a spent satellite will re-enter, and as long as there's some fuel and power aboard (which there often isn't) a safe landing spot somewhere in the ocean can be chosen and targeted. The launch of any spacecraft is a splendid and fiery thing; the end is usually far more ignominious. The trick, at the very least, is to keep it safe.