Comet siding spring
An artist's impression of comet Siding Spring.Wiki Commons

A comet the size of a small mountain will fly past Mars on Sunday at a speed of 126,000mph, in an event that happens once in a million years.

The Slooh Community Observatory will broadcast the Comet Siding Spring passing by the Red Planet on 19 October, which you can watch here.

The Virtual Telescope Project will also host a webcast, starting at 17.45 (BST).

Officially called C/2013 A1 (Siding Spring), the celestial body is an Oort cloud comet discovered on 3 January 2013.

Astronomers have estimated that it has taken millions of years for the comet to travel from the Oort Cloud – which extends nearly one light year from the Sun.

Nasa has five robotic explorers on Mars, which will be witnessing the once-in-a-lifetime event, as the comet makes its first appearance into the inner solar system. A European and an Indian spacecraft will also be circling the planet to capture data on the comet.

Nasa's Opportunity and Curiosity rovers will be shielded from potentially dangerous debris from the comet by the Martian atmosphere.

Comet Siding Spring will pass the planet within 87,000 miles. The nucleus of the ice body is thought to be at least half a mile in diameter.

According to astronomers, the comet formed during the first couple of million years of the solar system's birth, around 4.6 billion years ago. It was discovered last year by Robert H McNaught at Siding Spring Observatory in New South Wales, Australia.

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

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

The best views of the comet from Earth with a telescope will be from Southern Hemisphere countries such as Australia and South Africa.

Originally, it was believed there was a small chance it would hit Mars, but with more research and observation it was discovered that the comet will fly by the planet.