Recent comet sightings and the fiery path blazed across Russian skies by a large meteor have people pondering the possibility of a collision between Earth and some other heavenly body. Lost in the discussion is news from NASA that Mars is on schedule for a close encounter of its own in 2014, and the visiting comet may actually strike the red planet. Comet C/2013 A1 (Siding Spring) will be rendezvousing with Mars in October 2014, most likely passing by the planet at roughly the height of an earthly communication satellite. Estimates of the minimum distance between planet and comet range from about 100,000 km and 0, meaning a collision. If the comet does collide with Mars it is estimated the blast will be equivalent to that of a billion megatons of TNT. It would be an event of the same magnitude as the impact that killed off the dinosaurs 65 million years ago.
The recent meteor break-up over Russia, and the close pass of asteroid 2012 DA14, serve to reminded people that all objects in the solar system are subject to the occasional collision. A look at the surface of Earth's Moon, or the surface of Mars, gives ample evidence of collisions past. Lest we think it can't happen to us, Earth's surface also bares the scars of bombardment from space.
For example, the Manicouagan Crater, located in Canada, is one of the oldest known impact craters on Earth (see the image below). It is thought to have been caused by the impact of a 5 km (3 mi) diameter asteroid about 215.5 million years ago, during the Triassic Period. The crater is a multiple-ring structure about 100 km (60 mi) across, with its 70 km (40 mi) diameter inner ring its most prominent feature.
Over the years, the spacefaring nations of Earth have sent dozens of probes and rovers to explore Mars. Today there are three active satellites circling the red planet while two rovers, Opportunity and Curiosity, wheel across the red sands below. Mars is dry, barren, and apparently lifeless.
Soon, those assets could find themselves exploring a very different kind of world.
"There is a small but non-negligible chance that Comet 2013 A1 will strike Mars next year in October of 2014," says Don Yeomans of NASA's Near-Earth Object Program at JPL. "Current solutions put the odds of impact at 1 in 2000."
The nucleus of the comet is probably 1 to 3 km in diameter, and it is coming in fast, around 56 km/s (125,000 mph). "It if does hit Mars, it would deliver as much energy as 35 million megatons of TNT," estimates Yeomans.