India’s Mars Orbiter Mission and four other spacecraft gathered around the Red Planet are preparing to greet an emissary from the far reaches of the solar system, Comet Siding Spring.
Around midnight this Sunday (Oct. 19), the comet — named after an Australian observatory from where it was discovered in January last year — will hurtle past Mars, coming to within about 139,500 km of the planet. That is almost one-third of the average distance between Earth and Moon, and much closer than any comet is known to have passed by Earth.
The Indian Orbiter as well as three American and one European spacecraft now around Mars will be training their instruments on the comet. Two U.S. rovers trundling about on the Martian surface will also peer up with their cameras.
These will be the first spacecraft observations made from close proximity of a comet that originated in the Oort Cloud, said Asoke K. Sen of the Department of Physics at Assam University in Silchar, who studies comets. (The Oort Cloud is an extremely distant shell of icy bodies encircling the solar system.)
As the comet is coming into the inner solar system for the first time, it “will carry the signature of the pristine material out of which the sun and the planets were born some 4.6 billion years back,” he pointed out. Observations of the comet will therefore aid in understanding the origin and evolution of the solar system.
Previously, deep space missions have studied ‘Jupiter Family Comets’ in orbits with a periodicity of less than 20 years, he said. The Oort Cloud comets, on the other hand, take thousands to millions of years to return and some do not come back.
Probes have also visited Halley’s Comet, which is thought to have once originated in the Oort Cloud but then settled down in the solar system and lost its pristine nature after circling the sun many times, he added.
Last year, observatories on the ground as well as several spacecraft watched as Comet ISON, another visitor from the Oort Cloud, broke apart as it passed closed to the sun.
The spacecraft around Mars are well placed to capture images of Comet Siding Spring’s solid core, its nucleus.
With the mass of a small mountain, the comet is made up of rocky dust and various volatile ices, noted Carey Lisse, an astrophysicist at the Johns Hopkins University Applied Physics Laboratory in the U.S.
For the first time, spacecraft are in position that has the capability to “actually image and resolve an Oort Cloud comet's nucleus,” he said at a press briefing held last week by the American space agency, NASA.
Observations of Comet ISON last year showed that it was carbon-rich, maybe in the form of organic molecules. “So we are guessing that Siding Spring should show us an awful lot of organic-carbon-enriched material,” he added.
The comet has been leaving behind it a trail of dust and gas. How this cometary debris interacts with the Martian atmosphere will also be carefully watched.
As the dust from the comet will be moving at over 200,000 km per hour with respect to Mars, even tiny particles can significantly damage a spacecraft.
The orbits of all the Martian probes have been adjusted so that they will be behind the planet and shielded by it during the period of greatest risk.
Apart from spacecraft in and around Mars, space-based telescopes, such as the Hubble Space Telescope, other probes, ground observatories, and amateur astronomers will be following Comet Siding Spring as it passes close to Mars and then swings past the sun five days later.
