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.