Blogging the Periodic Table
Sometimes it's clumsiness—astronauts have fumbled away pliers, spatulas, and gloves while on spacewalks. Sometimes it's laziness—jettisoning bags of garbage or frozen urine saves astronauts the hassle of dealing with it more responsibly. Sometimes it's bravado—China blew up a defunct satellite in 2007 to show off its anti-satellite missiles, sending shrapnel hurtling through space. Regardless of the source of the junk, we tend to treat space nowadays like a highway median, and the celestial equivalents of pop cans and exploded tires have become an increasing worry for scientists.
Some 19,000 pieces of space junk wider than four inches in diameter orbit Earth nowadays, any of which could wallop a satellite and knock it out of commission. But in some ways, larger objects pose less danger, since scientists can monitor them and (usually) steer clear. What's really worrisome are the smaller bits that circle Earth swiftly (every 90 minutes) and remain too small to track. At such speeds even paint flakes are like rocks bouncing up from the road, and tiny scraps have already dinged the windshield of a space shuttle and gouged the Hubble Space Telescope. Hundreds of thousands of other fragments remain undetected.
Obviously we need to clean up after ourselves, but most ideas to clear space of debris—zapping it with ground-based lasers, for example—seem unworkable. Now, though, a team led by Naval Research Laboratory scientist Gurudas Ganguli thinks it has a solution. Counterintuitively, the trio of scientists proposes sending more junk into space, but a special kind of junk—an 18-mile-thick cloud of tungsten dust.
The density of space junk peaks around 620 miles up, in the middle of so-called low-Earth orbit. That's bad, because many weather, scientific, and reconnaissance satellites circle in various low-Earth orbits. But that height also offers an opportunity. Below about 560 miles, small objects start to feel a significant drag from the Earth's upper atmosphere. This drag causes them to slowly spiral toward Earth, and they eventually burn up in the atmosphere. The tungsten cloud could theoretically provide extra drag on objects orbiting above the 600-mile mark, slowing the itty-bitty debris down enough to fall below the 560-mile threshold. Tungsten wouldn't clear up space instantly—objects at 560 miles can still circle for decades. But that's vastly better than the centuries-long orbits of fast-moving objects even a little higher.
The advantage of this plan is its simplicity: After a rocket carries the tungsten dust into space and sprinkles it around, Isaac Newton takes care of the rest. It's fairly cheap as well, because we wouldn't need much tungsten—maybe 20 tons. The naval scientists picked tungsten because it's tough and dense (70 percent denser than lead). Yet it's abundant enough that mining 20 tons wouldn't cost too much (unlike mining the densest elements, iridium or osmium, two of the rarest metals in existence). And again, tungsten dust seems more practical than the other solutions that scientists and enthusiastic amateurs have proposed—lasers, ion cannons, huge butterfly nets, "a giant claw machine," interstellar Roombas with "Kevlar bags," giant magnets, expanding blobs of insulation foam that would swallow the junk, and so on.
That said, there could be a downside to sending 20 tons of heavy metal dust aloft. Eventually, the tungsten cloud would itself fall toward Earth. Tungsten isn't acutely toxic, and Ganguli and friends argue that, spread over many years, all that dust would not amount to much, especially compared with the hundreds of tons of micro-meteors and other space dust that already flits down onto Earth each day. But their five-page paper outlining the tungsten cloud devotes 54 words to the potential environmental impact, hardly an exhaustive look. Astronomers might also object, because the dust could interfere to an unknown degree with light streaming toward Earth from space. Fighting through swarms of microscopic dust could give satellites fits, too, though again, the naval scientists argue the impact would be negligible. (Most satellites point their instruments either straight down toward Earth or straight out into space, and therefore away from what would be mostly horizontal streams of tungsten dust.)
These scenarios all assume, though, that the tungsten dust will behave, and that the ionosphere or solar wind or whatever else won't interact with it in funny ways. For instance, what if the tungsten doesn't disperse in nice soft poofs but clumps together? Something similar happened with Project West Ford, a Cold War operation in the early 1960s to improve the reliability of radio communication (in case the Soviets sabotaged our undersea cables) by giving transmitters something solid to bounce signals off in space. To howls worldwide, the U.S. injected 480 million inch-long copper needles into orbit, clusters of which still circle Earth. Or, some observers have suggested that the dust could swell outward—perhaps even form a Saturn-like ring of Element 74 around Earth.
So, yeah, the idea still needs polishing. But if the growing amount of space junk wipes out a few billion-dollar satellites soon, a silvery tungsten cloud could be the least of many evils.