A comet hurtling into our solar system from deep space could next year score a direct and cataclysmic impact on Mars, astronomers say.

According to current calculations, comet C/2013 A1 (Siding Spring) is set for a near miss that will bring it within 23,000 miles of the surface of the Red Planet.

But the unpredictable nature of comet orbits, which can change as jet-like geysers of steam erupt from their surfaces as they near the Sun, means it could pass further away, or veer into a direct collision course.

Respected astronomer Phil Plait, author of Slate’s Bad Astronomy blog, has calculated that even if the comet is just nine miles across – a low estimate – an impact with Mars would cause a one billion megaton explosion.

That, he says, is 25million times larger than the largest nuclear weapon ever tested on Earth.

Much more likely, however, is that the comet will just miss Mars, which will nevertheless mean that the planet will pass through the cloud of sublimating gas spewed from the visiting rock as it draws closer to the Sun.

Like asteroids, comets are large chunks of space rock that orbit around our solar system. Unlike asteroids, however, comets are packed with ice.

This ice is not necessarily just water, but also things like carbon dioxide and carbon monoxide that on Earth we generally think of as gases, which in the chilly reaches of space are frozen inside the nucleus of the comet.

As the comet hurtles along its orbit closer to the Sun, these substances turn directly from solids to gases, often erupting from the comet through geyser-like vents.

At the moment, C/2013 A1 is over a billion kilometres from the Sun, somewhere past Jupiter, which means it is still very cold.

However, as it draws closer it will began to vent more gas, changing its path and surrounding the nucleus with a large fuzzy cloud known as a coma – which can be up to several hundred thousand kilometres across.

That means that the comet’s coma – which is also filled with grit-like rubble from the comet itself – could be bigger than the distance the nucleus is predicted to miss Mars.

‘If that does happen, it’ll be the gods’ own meteor shower for the Red Planet,’ Plait writes.

It will also be interesting to see the effect of the grit shower on Mars’s two lumpy, potato-shaped moons, Phobos and Demos, which would be peppered with grit as the comet streaks past.

And if the nucleus does hit Mars, the effect would be apocalyptic for the planet.

Estimates for the size of the comet’s core range from nine to 30 miles across, and astronomers say it will be moving at a phenomenal speed of 120,000mph upon impact – giving it huge kinetic energy.

Such an impact would leave a scar on Mars hundreds of kilometres across, says Plait.

Even worse for Earth-based observers, it would almost certainly destroy all our probes in orbit around and on the surface of the planet.

‘The ejecta would come screaming off the planet and sent every which way in orbit around Mars,’ Plait writes. ‘It would be like orbiting into a shotgun blast.’

Comet C/2013 A1 was first spotted on January 3, making it the first comet to be discovered this year.

Extrapolations of its orbit predict it will make its closest approach to Mars in October next year.