ON APRIL 13th 2015 the Minor Planet Centre (MPC), an office with a staff of six which looks after such matters for the International Astronomical Union, recorded hundreds of newly discovered asteroids—a typical daily haul. The one to which it assigned the name 2015 PDC, though, stood out.
When asteroids are discovered in orbits that come close to the Earth’s, as this one did, the MPC makes various calculations to see if they pose a threat. Because observations of small, distant rocks cannot be made with perfect accuracy, those calculations define a “corridor” where the asteroid might be. The calculations for 2015 PDC showed that on September 3rd 2022 the Earth would cross the corridor where the asteroid might be. The two might collide…
…As 2015 PDC’s orbit became better known the corridor in which it might be on that fateful day in 2022 shortened. But it still contained the Earth. By June the probability of an impact had risen to 1%, making it the most threatening asteroid astronomers had ever seen. By September governments in America, Europe, Russia and China had started work on space missions aimed at changing the asteroid’s orbit by ramming into it. Even at a speed of more than 10km (6 miles) a second, hitting a billion-tonne asteroid with a few tonnes of spacecraft will make only a minute difference to the asteroid’s orbit.
But a minute difference, made early enough, can provide the margin between a near miss and a hit that is palpable on a planetary scale. And 2015 PDC looked like providing such a hit. Early estimates put its diameter between 140 and 400 metres. Even if it were at the small end of that range, though, when it hit the Earth at 11km a second it would release as much energy as hundreds of large nuclear warheads set off simultaneously. At the large end the hundreds would become thousands.