Comet Siding Spring C/2013 A1 – a relatively rare “Oort Cloud” comet – will pass within 88,000 miles of our red planetary neighbor Mars on Sunday, Oct. 19 at about 2:28 p.m. EDT. That distance is one-third the mileage from Earth to the moon. In the cosmic scheme, that’s a close shave. The atmosphere of the comet could interact with the atmosphere of Mars, possibly leading to Martian auroras.