On October 1, the comet ISON will pass near Mars, where monitoring equipment can gain more information on what could be the comet of the century. As long as the coincidental government shutdown causing the furlough of nearly the entire workforce of NASA doesn't interrupt these observations, that is.
One of the key points everyone is hoping to determine is the exact size of the comet: its dust cloud has obscured that fact so far. The hope is that the comet is over half a kilometer wide (that's about 1600 feet, for the more American of us). If that is the case, when the comet brushes right up against the sun this November, it will probably survive the trip.
If it survives, it is set to come out of the Sun's glare this Thanksgiving day, and offer up an unforgettable view for anyone with a telescope and an absence of light pollution in the Northern hemisphere. If it is anything like comet Lovejoy for the Southern Hemisphere in 2011, this comet will even be easily viewable with the naked eye.
(A view of comet Lovejoy from Australia last year.)
Comets are unpredictable, and it can be difficult to tell for certain what is going to happen. This is especially true because this is ISON's first visit to the inner solar system. It is very possible that ISON is going to fizzle out on its approach to the Sun, or never emerge from behind the Sun's glare. Or, it could simply not be as dazzling is it is being hyped up to be.
Either way, October 1's brush with the comet is very important for scientists. It will be a sort of practice run for future comet observations from Mars. Comet Siding Spring, for example, will pass very close to Mars next year. The science value of observing comets is unknown, since it has never happened this way before, but that is part of why it is so exciting.
While, again, no word seems to be coming from NASA about how the furloughs will affect the ISON observation (possibly because their entire public relations department has the day off), an unprecedented 16 NASA satellites were slated to take part in the observations. The scale of the operation hints at how seriously NASA has been taking the event.