At http://phys.org/print318845868.html … we have some interesting possibilities as Comet Siding Spring comes close to Mars in October. . The May 2014 issue of Icarus wonders how the atmosphere of Mars will react. However, Mars has a thin atmosphere and the comet will be 138,000 km away at its closest point – so we should not expect too much. It is not the nucleus but the tail and the coma of the comets that may prove more interesting, sweeping across Mars. Will the coma cause a reaction, especially if the atmosphere is breached. There are lots of telescopes around the world set to swing towards Mars in October. These include NEOWISE and Hubble – and the Mars Odyssey (among others).
Meanwhile, at http://phys.org/print318845782.html … an asteroid has turned into a comet and is now bright enough to see in a 12 inch telescope – and will probably get brighter still. It will come within 29 million miles of the Earth in July and is desribed as very dark, but fairly large, up to 7 miles across – not to be sniffed at.