Once-in-a-million-years comet Siding Spring to zoom past Mars from furthest corner of solar system

The comet will be the first to ever be studied in this detail

A blazing comet the size of a small mountain will pass close to planet Mars on Sunday at a speed of 126,000 mph during an event that happens once in a million years.

Nasa's five robotic explorers on the planet will be given the job of witnessing the comet named Siding Spring, which is making its first known venture into the inner solar system.

Spacecraft from Europe and India will attempt to watch the incoming iceball, which will pass the planet within 87,000 miles, and then hide behind the planet for protection against potentially-destructive debris from the comet tail.

"We certainly have fingers crossed for the first images of a comet from the surface of another world," said Nasa programme scientist Kelly Fast.

Spacecraft including the Hubble Space Telescope and ground observatories will also be keeping a lookout for the first comet to have ever come so close to Earth in recorded history.

"We're getting ready for a spectacular set of observations," said Jim Green, head of Nasa's planetary science division.

The comet hails from the Oort Cloud on the outer perimeter of the solar system and it has a nucleus estimated to be at least a half mile in diameter.

It formed during the first million or two years of the solar system's birth 4.6 billion years ago and, until now, ventured no closer to the sun than perhaps the orbits of Jupiter, Saturn, Uranus or Neptune. It comes around every one or more million years.

With advanced technology to hand, it will be the first Oort Cloud comet to be studied up close in detail.

"We can't get to an Oort Cloud comet with our current rockets ... so this comet is coming to us," said Carey Lisse, senior astrophysicist at Johns Hopkins University's applied physics laboratory.