Current solutions put the odds of impact at 1 in 2000.
The nucleus of the comet is probably 1 to 3 km in diameter, and it is coming in fast, around 56 km/s (125,000 mph).
If it does hit Mars, it would deliver as much energy as 35 million megatons of TNT.
An impact wouldn't necessarily mean the end of NASA's Mars program. But it would transform the program- along with Mars itself.