A PAIR of middle-aged tourists (see previous post) are not the only thing headed for Mars. Comet C/2013 A1 (Siding Spring) is also on its way. Discovered on January 3rd, some calculations of its orbit, according to Phil Plait, the rather good “Bad Astronomer”, have it passing 37,000km above the surface of the planet in October 2014—roughly the height at which communication satellites orbit Earth, and a remarkably close shave by cosmic standards. An official NASA website puts the most likely “close-approach” distance between the comet and Mars at something more like 100,000km.
But the minimum close-approach distance is zero. Comets do not move smoothly on their tracks like ball bearings or planets. The gases that blow off their surfaces as the sun warms them up push them hither and yon, changing their trajectories. So, though the odds are strongly against it (how strongly no one can yet say) the comet has a real if small chance of actually hitting the planet.
Which would be cool, in all sorts of ways. For a start, it should be simply spectacular. Given the unusual speed of the comet (which is moving so fast that it may well be coming from outside the solar system—cool upon cool) and the fact that it is travelling the wrong way round the sun, from a planet’s point of view, Mr Plait estimates that its impact should yield a blast equivalent to that of a billion megatons of TNT. It would be an event on the same sort of scale as the impact that drove the dinosaurs extinct 65m years ago. If it really is that big, and if the comet were to hit the side of Mars facing Earth (it seems that it might do, but it might also hit the far side), then the blast could well be visible to the naked eye, even in daylight.