I don’t think I’m spoiling too many surprises when I reveal that the plot of the film Gravity, a low-orbit spectacular starring Sandra Bullock and George Clooney, involves spacecraft getting hit by space debris. It’s a less fanciful premise than it might seem: in 2009, two unmanned satellites hit each other without warning, nearly 800km above Siberia.
That collision heralded a serious problem, first flagged in 1978 by Donald Kessler, then an astrophysicist at Nasa. The concern isn’t that space debris will rain down on us here on Earth: it’s that it will stay up there in space.
The two satellites that collided, Cosmos-2251 and Iridium-33, weighed almost a ton and a half altogether. The result was at least a thousand fist-sized chunks of metal, any one of which could destroy a further satellite, and produce hundreds of further chunks. It takes time for these chunks to fall out of orbit.
What worried Kessler – and still does – was the prospect of a chain reaction. Too much debris in orbit would make it impossible to launch the satellites that have become an indispensable part of life back on Earth.
Nasa is tracking 21,000 pieces of junk 10cm across or bigger – like small cannonballs. In low Earth orbits, they are travelling at about 7km a second (25,200km/h). But space hasn’t been made impassable by debris just yet. There’s quite a lot of room up there, after all. Low Earth orbits are common but include a variety of altitudes, so objects have plenty of ways to fail to hit each other. Geosynchronous orbits, popular with communications satellites, must be exactly 42,164km from the centre of the Earth. But satellites that far out share more than 22bn sq km of space.
Still, some orbits are more crowded than others; more collisions are surely just a matter of time. That was the opinion of a 2011 report from the National Academy of Sciences, “Limiting Future Collision Risk to Spacecraft”, which argued that there is already enough junk crashing into other junk that the problem will worsen even if there are no further launches.
Deliberately moving the debris somewhere safer seems possible, but pricey. It’s expensive to tidy up a satellite – or to design one that tidies itself up – and while the benefits of doing so are widely shared, the costs are not. So the clean-up doesn’t happen.
The regulation of satellites is no simple matter: Cosmos-2251 was launched by the Russian military; Iridium-33 by a US corporation. The single largest space-junk incident was in 2007, when the Chinese military blew up a satellite just to show that it could. The regulatory authority capable of dictating to all three of those parties does not exist. (The United Nations did issue voluntary guidelines in 2010.)
Economists such as Molly Macauley of Resources for the Future, a think-tank, have been pondering this problem for some time. The obvious economic solution, recently revived by three researchers, Nodir Adilov, Peter Alexander and Brendan Cunningham, is a tax on new satellite launches. Macauley has proposed linking the level of this tax to the design of the satellite – safer designs would attract a lower charge. Another possibility is that satellite operators would put down a deposit, to be refunded once the obsolete satellite had been pushed into a safer orbit.
This is one of those all-too-common situations when it is easier for economists to announce the optimal policy than it is for politicians to implement it. As with climate change, there’s a burden to be shared here, a threat of uncertain magnitude, and plenty of opportunity for free riding.
Yet this is a far cheaper problem than climate change, with a smaller number of decision makers. It should be easier to reach an agreement on space junk than on greenhouse gases. Alas, that is a not a very encouraging comparison.
‘The Undercover Economist Strikes Back’, by Tim Harford, is published by Little, Brown