The comet known as Siding Spring brushed past Mars on Sunday, passing 140,000km from the 'Red Planet', less than half the distance between Earth and the Moon and 10 times closer than any known comet has passed by Earth.

There had been concerns that with the comet coming so close to Mars, gas and dust in its tail could change the planet's atmosphere and damage a number of spacecraft observing the event.
 
Named after the observatory in Australia which first spotted it last year, Siding Spring has come from the far reaches of our Solar System.

Its journey from the Oort cloud towards the Sun, at a speed of 56km a second, took more than a million years.

During this time it has slowly warmed, and gradually developed a tail full of gas and dust.

Initially scientists thought the comet would collide with Mars, but after tracking its approach they calculated that it would miss the 'Red Planet,' but only just.
 
"At the time of the close encounter with Mars the comet will almost be the closest to the Sun," said Francisco Diego, a Senior Research Fellow at the Department of Physics and Astronomy, University College London.

"When they do that they release a lot of material, dust, solid particles and gases, water and hydrogen.

"All these particles, all these ejecta are going to interact with the atmosphere of Mars and that will be the interesting thing," he told Al Jazeera.

External forces

The interaction of the comet's gas and dust in the Martian atmosphere is being recorded by no fewer than five spacecraft.

Francisco Diego, from University College London, talks about the Siding Spring's close encounter with Mars

NASA has two rovers on the planet's surface - Opportunity and Curiosity.

Last month, India's Mangalyaan obiter arrived at Mars, joining two NASA probes orbiting the planet, part of a mission to understand how external forces affect the Martian atmosphere. One of these is the Mars Reconnaissance Orbiter.
 
"It's going to look at shape and rotation and the brightness of the nucleus - or the darkness of nucleus," said Kelly Fast, Planetary Scientist at NASA.

"It's going to study the coma composition and it's also going to look at the atmosphere of Mars and see if it can detect any changes from the interaction between the comet and Mars."
 
Billions of years ago, the Martian atmosphere was thick and supported water, but this changed over time, leaving just a thin coat of carbon dioxide.

It is believed the atmosphere was gradually depleted by solar winds and energy from the Sun.

The scientists say the comet's interaction with the atmosphere could shed light on how this occurred.

Spacecraft threatened
 
The comet could pose a threat to the spacecraft, as they are battered by the gas and dust in its tail.
 
"The dust from the comet may be a hazard to our spacecraft," says Jim Green, a scientist at NASA.

"We've studied and modelled it extensively. And when Mars gets very close to the dust tail, all our spacecraft will be on the opposite side of the planet, so the planet will provide the additional protection we believe we need to be able to make these observations safely from our Mars spacecraft."
 
Scientists believe the dust and gas from the comet could cause auroras in the Martian atmosphere.

These were first observed in 2004, but this time they are expected to be brighter, more numerous and be visible to some of the cameras on the spacecraft.

The scientists will also be tracking the exact motion of the comet, information they say will help us better prepare for the next time a comet comes calling.

Source: Al Jazeera