For comparison, Mars's outer moon Deimos orbits it at a distance of 0.00016 AU (24,000 km; 15,000 mi). Due to the uncertainty region, there was the possibility that it could pass Mars as far away as 0.000934 AU (139,700 km; 86,800 mi). It passed Mars at a relative velocity of 56 km/s (35 mi/s). As seen from Mars, C/2013 A1 peaked at approximately 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, 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 27,000 km (17,000 mi) from the comet's orbit around 20:10 UT.
The coma of the comet is projected to more than double the amount of hydrogen in the high atmosphere for a period of several tens of hours and to warm it by about 30 K for a few hours—the combination increasing the effect of atmospheric drag on the Mars Reconnaisance Orbiter and MAVEN spacecraft 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 any changes to the gas composition of the atmosphere. The closest orbiting moon of Mars, Phobos, orbits far higher, at a minimum distance of 9,234.42 km (5,738.00 mi), more than 10 times the height of Mars's atmosphere.
Estimates for the diameter of the nucleus have varied from 1 to 50 km (1 to 31 mi), but now the nucleus is known to be only approximately 400–700 meters (0.2–0.4 mi) in diameter, roughly 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 24 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 (4–6 mi) 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.
Maven detected an intense meteor shower. Comet Siding Spring has a rotation period of approximately 8 hours. Debris from Comet Siding Spring added a temporary, but strong layer of ions to Mars's ionosphere (the first time such a phenomenon has been observed on any planet), and a few tons of cometary dust were vaporized high in Mars's atmosphere. Magnesium, iron, and other metals were observed to have had been deposited. An observer on the surface would have seen a few tens of meteors during the plane crossing.
During the flyby of Mars at a proximity of 140,000 km, Comet Siding Spring's magnetic field, generated by its interaction with the solar wind, caused a violent turmoil that lasted for several hours, long after its flyby. Its coma washed over Mars with the dense inner coma, reaching or almost reaching the planet's surface. The cometary magnetic field temporarily merged with and overwhelmed Mars's weak magnetic field.