The first comet discovered this year, Comet C/2013 A1, is currently projected to pass within about 23,000 miles (37,000 km) of the surface of Mars late in 2014. While this event in itself promises spectacular views for astronomers, the uncertainty of the comet’s orbit includes a significant chance of an impact on Mars. If this happens, the impact would be hundreds of times more powerful than the asteroid impact that wiped out the dinosaurs on Earth.
C/2013 A1 was discovered on January 3, 2013, by experienced comet hunter Robert McNaught, who has discovered 74 comets and 467 asteroids. The discovery was made using a 0.5 m (20-inch) Schmidt telescope at Siding Spring Observatory in New South Wales, Australia. After his discovery, astrographs showing the comet on December 8 were found from the Catalina Sky Survey, and used to help establish the orbit.
At discovery, the new comet was well beyond the orbit of Jupiter. It is approaching the inner solar system from south of the ecliptic on a hyperbolic orbit, and is presently moving sunward at about 35 km/s (22 m/s). Based on its brightness and distance, the size of the new comet is estimated at roughly 20-40 kilometers (12.5-25 miles). For comparison, asteroid DA14, which recently passed within 27,700 km (17,200 miles) of Earth’s surface, was only about 30 m (100 ft) in diameter.
What is not typical is that, based on the latest observations, the new comet’s orbit passes Mars on October 19, 2014 at a distance of less than 37,000 km (23,200 miles). The comet’s velocity relative to Mars at that point will be about 56 km/s (35 m/s). The uncertainty in the orbit indicates a collision with Mars is possible.
This would not be a minor collision. Rather, it would be a once in a billion years collision, generating a crater roughly 500 km (300 mi) in diameter. The equivalent explosive force would be in the range of five to twenty billion megatons of TNT. I’m not sure that any number can really let us understand the magnitude of the collision – does half a trillion Hiroshima bombs convey a more comprehensible image? Another comparison is that the impact is about equal to a tenth of a second of the Sun’s total energy output focused on one spot on the Martian surface. Not a pleasant experience for any planet.