Jim Green, director of NASA's planetary science division said to Space.com "On October 19, we're going to observe an event that happens maybe once every million years,"
Comet Siding Spring, also known as C/2013 A1, was discovered in 2013 by astronomer Rob McNaught using Australia's Siding Spring Observatory. The comet is making its first trip through the inner solar system from the frigid, faraway Oort Cloud, which lies about 50,000 astronomical units from the sun.
Because Siding Spring has never been "heat-treated" before, the incoming comet likely remains largely unchanged since its formation 4.6 billion years ago, researchers said.
Furthermore, NASA says that a direct hit by Comet Siding Spring could make Mars hospitable for life.
A direct hit on Mars' backside wouldn't tear the planet apart, but it would produce an explosion at somewhere around a billion megatons of TNT. That would create a huge crater, blast tons of debris into space and perhaps set off a great flood.
We could have liquid water flooding the planet and possibly a denser atmosphere as the impact heats up dry ice in the soil.
“There is a small but non-negligible chance that Comet 2013 A1 will strike Mars in October 19,” said Don Yeomans of NASA’s Near-Earth Object Program at NASA Jet Propulsion Laboratory in Pasadena, Calif. “Current solutions put the odds of impact at 1 in 2000.”
According to NASA, the comet is likely 1-50 kilometers (0.6-1.30 miles) wide and traveling at 56 kilometers per second (125,000 mph). “It if does hit Mars, it would deliver as much energy as 35 million megatons of TNT,” added Yeomans. That would be a violent impact with global effects."
“I think of it as a giant climate experiment,” said Michael Meyer, lead scientist for the Mars Exploration Program at NASA headquarters. “An impact would loft a lot of stuff into the Martian atmosphere — dust, sand, water and other debris. The result could be a warmer, wetter Mars than we’re accustomed to today.”
Anyway, NASA gears up for the epic encounter and their extensive fleet of science assets will observe Comet Siding Spring before and after it makes its closest approach to Mars on Oct. 19, 2014.