C/2013 A1 (Siding Spring) is an Oort cloud comet discovered on 3 January 2013 by Robert H. McNaught at Siding Spring Observatory using the Uppsala Southern Schmidt Telescope.
At the time of discovery it was 7.2 AU from the Sun and located in the constellation Lepus. Comet C/2013 A1 probably took millions of years to come from the Oort cloud. After leaving the planetary region of the Solar System, the post-perihelion orbital period (epoch 2050) is estimated to be roughly 1 million years.
C/2013 A1 passed the planet Mars very closely on 19 October 2014, at a distance of . After its discovery, there was thought to be a chance of a collision with Mars, but this possibility was excluded when its orbit was determined with about a 200 day observation arc.
All NASA Mars orbiters—including 2001 Mars Odyssey, Mars Reconnaissance Orbiter and MAVEN—as well as ESA's orbiter, Mars Express, and ISRO's orbiter, the Mars Orbiter Mission, reported a healthy status after the comet flyby on 19 October 2014. During the flyby, orbiters around Mars detected thousands of kilograms per hour of comet dust composed of magnesium, iron, sodium, potassium, manganese, nickel, chromium and zinc. In addition, the comet nucleus was determined to be between , much smaller than originally assumed. The nucleus rotates once every eight hours.
Discovery
The comet was discovered on 3 January 2013 by professional astronomer Robert McNaught at the Siding Spring Observatory at Coonabarabran NSW Australia and received the official designation C/2013 A1. It was named Siding Spring based on a tradition to identify the observatory that discovered it. Three images were obtained through the use of a CCD camera mounted on the Uppsala Southern Schmidt Telescope with a spherical mirror of 0.5 meters in diameter. Comet Siding Spring had an apparent magnitude of 18.4 to 18.6. At the time of its discovery, it was from the Sun.
Precovery images by the Catalina Sky Survey from 8 December 2012 were found quickly and announced with the discovery giving Comet Siding Spring a 29-day observation arc. On 3 March 2013, Pan-STARRS precovery images from 4 October 2012 were announced that extended the observation arc to 148 days.
Encounter with Mars
Comet Siding Spring passed extremely close to Mars on 19 October 2014 at 18:28 ± 0:01 UTC. Initial observations by Leonid Elenin on 27 February 2013, suggested that it might pass from the center of Mars. With an observation arc of 733 days, the nominal pass is from the center-point of Mars and the uncertainty region shows that it would not come closer than .
For comparison, Mars's outer moon Deimos orbits it at a distance of . Due to the uncertainty region, there was the possibility that it could pass Mars as far away as . It actually passed by at a distance of , at a relative velocity of . As seen from Mars, C/2013 A1 peaked at approximately apparent magnitude −6.
Predicted effects
The main body of the comet's tail was projected to miss Mars by some 10 Mars diameters. As a result, only higher-than-average-velocity meteoroid dust, ejected earlier in the approach of the comet, allow for impacts on Mars, its moons, and orbiting spacecraft. Dust particles ejected from the nucleus of the comet, at more than double the expected velocity when the comet was 3 AU from the Sun, could reach Mars approximately 43 to 130 min after the closest approach of the comet. There is a possibility for millimeter- to centimeter-size particles released more than 13 AU from the Sun, however, this is considered unlikely, although massive ejections from farther out have been deduced.
In 2013 it was thought possible that Comet Siding Spring would create a meteor shower on Mars or be a threat to the spacecraft in Mars orbit. Studies in 2014 showed the threat to orbiting spacecraft to be minimal. The greatest threat would be about 100 minutes after closest approach. Mars passed about from the comet's orbit around 20:10 UT.