Comet Siding Spring as seen by Hubble on 11 March 2014.
|Discovered by||Siding Spring Observatory
0.5-m Schmidt (E12)
|Discovery date||3 January 2013|
|Orbital characteristics A|
|Perihelion||1.39875 AU (q)|
|Orbital period||several million years inbound (Barycentric solution for epoch 1950)
~1 million years outbound
(Barycentric solution for epoch 2050)
|Next perihelion||25 October 2014|
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 0.5-meter (20 in) 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. Precovery images by the Catalina Sky Survey from 8 December 2012 were quickly found. On 3 March 2013, Pan-STARRS precovery images from 4 October 2012 were announced that extended the observation arc to 148 days.
The comet was discovered on 3 January 2013 by amateur astronomer Robert McNaught and received the official designation of C/2013 A1 once it was discovered by the Siding Spring Observatory, in Australia. Three images were obtained through the use of CCD cameras mounted on an Uppsala Southern Schmidt Telescope with a parabolic mirror of 0.5 meters in diameter. The comet appeared as an object of magnitude 18.4 to 18.6. Before McNaught, the object had been detected on 8 December 2012 by the telescope of the Catalina Sky Survey observatory of the University of Arizona, although at that time its orbit could not be determined. At the time of discovery, the comet was at 7.2 AU from the Sun.
Precovery images acquired by the Catalina Sky Survey, taken on 8 December 2012 with a Schmidt telescope equipped with a parabolic mirror of 68 cm diameter, were then examined. Subsequently, two other images taken on 4 October 2012 with a 1.8-meter Pan-STARRS telescope with a Ritchey-Chrétien optical configuration, mounted on the summit of Haleakalā, on the island of Maui, in Hawaii, were examined when the comet was estimated as an object ranging from magnitude 19.7 to 20.
Encounter with Mars
Comet Siding Spring will pass extremely close to Mars on 19 October 2014 at 18:33 UTC. Initial observations by Leonid Elenin on 27 February 2013, suggested that it might pass 0.000276 AU (41,300 km; 25,700 mi) from the center of Mars. With an observation arc of 662 days, the nominal pass is 0.00088 AU (132,000 km; 82,000 mi) from the center-point of Mars and the uncertainty region shows that it will not come closer than 0.00086 AU (129,000 km; 80,000 mi). For comparison, Mars's outer moon Deimos orbits 0.00016 AU (24,000 km; 15,000 mi) from the planet. Due to the uncertainty region, there is a small possibility that it will pass Mars as far away as 0.00090 AU (135,000 km; 84,000 mi). It will pass Mars at a relative velocity of 56 km/s (35 mi/s). As seen from Mars the comet will reach about apparent magnitude −6.
The main body of the comet's tail is 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, would allow the particles to reach Mars about 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, though massive ejections from farther out has 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. More recent studies in 2014 show the threat to orbiting spacecraft to be minimal. The biggest threat will be about 100 minutes after closest approach. Mars will pass about 27,000 km (17,000 mi) from the comets orbit around 20:10 UT.
The coma of the comet is projected to affect the high Martian atmosphere by more than doubling the amount of hydrogen in the high atmosphere for a period of several tens of hours and slightly warming it by about 30 K for a few hrs - the combination increasing the effect of atmospheric drag on Mars Reconnaisance Orbiter and the yet to arrive MAVEN spacecrafts causing a measurable increase in orbital decay because of atmospheric ram pressure. These spacecraft will be approaching Mars to minimum altitudes of 250 km and 150 km and orbital periods of 3 and 4 hours, respectively. The amount of drag cannot be narrowed down greatly until the production rate of the comet is known but it could be from 1.6 to 40 times normal drag. MAVEN, in particular, also has instruments to observe the gas composition of the atmosphere and observe the changes. Mars' closest orbiting moon, Phobos, orbits far higher at a minimum distance of 9234.42 km more than 10 times the height of Mars' atmosphere.
|3 km (1.9 mi)||5.3 million Mt||45 km (28 mi)|
|5 km (3.1 mi)||24.5 million Mt||71 km (44 mi)|
|8 km (5.0 mi)||100 million Mt||108 km (67 mi)|
|15 km (9.3 mi)||660 million Mt||188 km (117 mi)|
|20 km (12 mi)||1.57 billion Mt||242 km (150 mi)|
|50 km (31 mi)||24 billion Mt||544 km (338 mi)|
Estimates for the diameter of the nucleus have varied from 1 to 50 km (0.62 to 31.07 mi). But the nucleus is now known to be only about 700 meters (0.4 mi) in diameter (roughly twice the diameter of asteroid 2010 XG11 that approached Mars on 29 July 2014.) Based on early upper-limit size estimates, the resulting upper-limit energy of a hypothetical impact with Mars was 20 billion megatons. The diameter of such a hypothetical impact crater would be roughly ten times the diameter of the comet's nucleus. A 700 meter impactor would create around a 7–10 km crater.
The odds of an impact with Mars were 1 in 1250 in March 2013, 1 in 2000 in late March 2013, 1 in 8000 by April 2013, and 1 in 120,000 by 8 April 2013. The 8 April 2013 JPL Small-Body Database 3-sigma solution was the first estimate to show that the minimum approach by Comet Siding Spring would miss Mars. There is no risk of the comet nucleus impacting into Mars.
As seen from Earth, on 19 October 2014 Mars will be in the constellation Ophiuchus, and will be 60 degrees from the Sun. Mars and Comet Siding Spring will also be visible to the STEREO-A spacecraft during the 2014 encounter. The spacecraft MAVEN and Mars Orbiter Mission will arrive at Mars one month before Comet Siding Spring's closest approach. Already in orbit around Mars are Mars Reconnaissance Orbiter, Mars Express, and 2001 Mars Odyssey; all these artificial satellites may be exposed to potentially damaging particles. The level of risk will not be known for months, but NASA is already evaluating "possible precautionary measures" as it prepares for studying the comet. Two key strategies to lessen risk are to get orbiters behind Mars during the minutes of highest risk and to orient orbiters so that their most vulnerable parts are not in the line of impact. On the ground are the Curiosity and Opportunity rovers.
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