NASA has released a dramatic and scientifically accurate artist impression of what it would look like early morning next Monday week (October 20) when comet Siding Spring sweeps past Mars in a near head-on collision with the red planet. It’s the first time this pristine, Australian-discovered comet has fallen into the gravitational grip of the inner solar system, and at 203,000 km/h, or 56km per second, it will get as close as 139,500 kilometres to the Martian surface, or about one-third the average earth-moon distance. Comet Siding Spring is at least 4 billion years old, and its trajectory makes it a first time visitor to the neighbourhood, from its origins in the so-named, highly tenuous Oort Cloud that envelopes the solar system at great distance. (The Oort cloud is said to be the long-lasting remnant of the original rotating mass of dusts and gases from which the planets formed around the Sun, where the primordial comets like Siding Spring reside.) When a passing star, or other gravitational event, dislodged Siding Spring, the slightest drift one way or another could have caused it to miss the inner solar system altogether, or caused it to plough with devastating force right into Mars, or our moon, or catastrophically, into our planet. Famous Australian comet hunter Rob McNaught discovered it from an observatory on the central-western New South Wales Siding Spring Mountain, for which the comet was named. The discovery occurred on January 3, 2013 -- far too late for technology to be developed or deployed to prevent this estimated 500- to 5000-metre-wide cometary nucleus hitting Earth, had that been the roll of the cosmic dice. McNaught’s funding for comet hunting ran out soon after his discovery, and 10 days later, the massive Siding Spring Mountain bushfire spared his telescope but destroyed his nearby home. He is among the most illustrious of Australia’s many distinguished astronomers, a modest and dedicated scientist whose place in history is assured, even if his professional income isn’t. At an American press conference Jim Green, director of NASA's planetary science division, said: “We're going to observe an event that happens maybe once every million years. We're getting ready for a spectacular set of observations.” These will be the first close-up observations of a comet from another planet in history. NASA (and India) has Mars satellites in orbits performing research that never envisaged a sudden close shave from a comet, as well as two active surface Mars Rovers, Opportunity and Curiosity. (Comet Shoemaker-Levy 9 broke into fragments which then collided with Jupiter in 1994, causing huge blemishes on the gas giant that were observed from earth orbit by the Hubble Space Telescope.) The red planet orbiters have all had their orbits shifted so that they can observe the comet without excessive risk from being hit by the dust grains and gases spewing off its nucleus, which at 56km per second, could cripple or destroy them. Mars is easily visible to the naked eye in the western sky for several hours after sunset from southern Australia, in the same area as red star Antares, in the constellation of Scorpio. While skywatchers with very good telescopes and tracking drives may just be able to pick up comet Siding Spring before and after its closest Mars approach, the actual event occurs when Mars is below the horizon from Australia, early on the morning of October 20 near to sunrise (local time).