Comet C/2013 A1 (Siding Spring) will pass within 139,500 km (86,000 miles) of the surface of Mars at 5:51 tomorrow morning (Monday 20 Oct, Sydney time – or 19 October at 18:51 UTC). This means Comet Siding Spring will pass 10 times closer to Mars than any (recorded) comet has flown by Earth! But it will be traveling at 50 km/s which is too fast to be captured by Mars’s gravitational pull.
Comet Siding Spring, Mars, and Milky Way dust (Photo by Steve Lee (AAO))