NASA Science Briefing: Comet Siding Spring at Mars

NASA’s Curiosity Mars rover has started a summer road trip, skirting dangerous sand as it heads for its next target.

Amateur astronomers around the globe have a ringside seat as Comet 2013 A1 (Siding Spring) hurtles through space en route to an incredibly close encounter with the planet Mars on Sunday, 19 October.

Where did the two natural satellites of Mars, Phobos and Deimos, come from? For a long time, their shape suggested that they were captured asteroids. However, the shape and course of their orbits contradict this hypothesis. Two independent and complementary studies now provide an answer: these satellites formed from the debris of a gigantic collision between Mars and a protoplanet one-third its size.