This Sunday, a comet will come closer to Mars than any other comet has ever been seen to approach a planet without actually hitting it, sending our assorted spacecraft orbiting the red planet running for cover. Comet C/2013 A1 (Siding Spring) comes within a hair’s breadth of Mars on 19 October 2014, making its closest approach at 18:28 UT at a distance of merely 132,000 kilometres (82,000 miles). Only Shoemaker–Levy 9 has been seen to come closer to a planet, when it hit Jupiter in 1994.
The comet’s proximity to Mars means that this is going to be a big science event as NASA’s spacecraft such as the Mars Reconnaissance Orbiter, MAVEN and Mars Odyssey, the European Space Agency’s Mars Express and India’s Mars Orbiter Mission, will get the chance to study the comet and how it interacts with Mars’ atmosphere. However, it will also be a dangerous time for the spacecraft as the red planet will pass through the edge of the comet’s tail. Comet Siding Spring is shedding 100 kilograms of dust every second as it nears the Sun, spraying dust particles away at 56 kilometres per second. Should even a millimetre-sized dust particle strike a spacecraft, it could rip through it like a bullet, and a cloud of high velocity dust could smash a spacecraft to shreds. Yet while the risk is not as high as had been first thought, says Bruce Jakosky, the Principal Investigator on the MAVEN mission, precautions will still need to be taken.
“For a long time we thought that Siding Spring would be a tremendous risk to MAVEN,” he says. “Over the last six months a couple of different groups have done elaborate, extensive modelling of how dust is released from a comet and what its trajectories would be, and we now believe that the risk is much, much lower than originally thought. If we do get hit, the number of particles that hit us should be low enough that we don’t expect there to be much damage. But we’re not stupid – we’re going to take precautions just in case.”
To be on the safe side, all the spacecraft orbiting Mars have been shifting their orbits over the past few weeks so that the spacecraft will be able to take shelter on the far side of the planet. Even though they will only be shielded for 20 minutes before making their way back around the planet again and exposing themselves to the comet, that 20 minutes will be during Mars’ passage through the densest part of the comet’s tail, which is about 100 minutes after the comet’s closest approach.
“In addition we are also going to turn the spacecraft so we present the smallest cross-sectional area to the dust to minimise our chances of getting hit,” says Jakosky. “We’re going to turn off the high-voltage on all of the instruments because again that minimises the damage if we do get hit by dust. Some of the instruments will continue to collect data at that time, but any with high voltage will be turned off.”