On October 19 of this year the comet C/2013 A1 (more commonly known as Siding Spring) will make a very close approach to Mars. At its closest point, the comet could be only 120,000 km from the surface of Mars. There’s no chance it will hit the planet, but such a near miss could have an effect on both the planet and the probes orbiting it.
During this close approach, Mars will pass through the coma of dust particles surrounding the comet. Depending on the size and composition of this dust, there could be meteor showers on Mars at that time. Smaller particles could also interact with the upper atmosphere of Mars, causing it to heat and expand slightly. Such an effect likely wouldn’t have a serious effect on Martian weather, but it could be enough to be measured by orbiting probes.
One big question is whether Siding Spring will pose a risk to the probes orbiting Mars. Currently there are three active Martian orbiters (the Mars Reconnaissance Orbiter, Mars Odyssey, and Mars Express). Two more probes (the Mars Atmosphere and Volatile Evolution (MAVEN) and India’s Mars Orbiter Mission) will enter Martian orbit a few weeks before Siding Spring arrives. There are also two rovers (Curiosity and Opportunity) active on Mars, but they are well protected by Mars’ atmosphere and are at no risk.
The orbiting probes are designed to withstand some low-level impacts from dust, but it isn’t clear yet whether the coma of Siding Spring will exceed that level. Much of it will depend on just how active the comet becomes as it moves closer to the Sun. Recently Siding Spring has grown more active, even showing signs of cometary jets. If it continues to become more active, then there could pose a risk to the orbiters.
But the orbiters are not completely helpless. It is possible for their orbital trajectories to be modified so that they are on the opposite side of Mars when the comet passes by. They can also be oriented so that their vulnerable detectors face away from any incoming debris. Though it is possible for something to occur to one of the orbiters, it doesn’t seem to be a high risk.
What makes this event particularly exciting is that we will be able to observe the close approach of a comet on another world. The rovers may be able to observe meteor showers, the orbiters will catch a good view of the comet’s nucleus, and we will be able to see what effects a comet’s coma has on a planet’s atmosphere. It is an unanticipated benefit of sending probes on a journey to Mars.