Comet Siding Spring, discovered in January 2013, is less than a kilometre across and will pass Mars at 56 km/s, closing to within 139 500 km at 18:27 GMT (20:27 CEST) on 19 October.
Initially, the comet and its envelope of gas and dust were predicted to pass much closer to Mars, posing a serious risk to the fleet of orbiting spacecraft. Later observations confirmed that the miss distance will, in fact, be more comfortable, around 85,000 miles.
ESA’s teams flying Mars Express have spent months preparing for the encounter.
“In 2013, we had very little information about the comet, which was still very far and faint. In the worst case, we expected the pass to be much closer, and the comet to be much more active,” says Spacecraft Operations Manager Michel Denis.
“We designed a special mode for Mars Express that would minimise any risk due to impacts with cometary particles.
“This included turning off all instruments and non-essential onboard systems, and turning the spacecraft so as to use the large high-gain antenna as a shield.”
ESA's Mars Express Orbiter
currently orbiting Mars.
This image shows just how many satellites and probes humanity has sent to Mars.
Some more successful than others, and we still have much to learn about our near neighbour.