Mars fleet set to observe Comet Sliding Spring flyby

NASA says it has secured its Mars probes in preparation for a rare, extremely close pass by a comet from the outer edge of the solar system.

Discovered in January 2013, Comet Siding Spring is due to fly just 140,000 kilometres from Mars less than half the distance between Earth and the moon on Oct. 19. That is 10 times closer than any known comet has flown by Earth, NASA scientists told reporters on a conference call Thursday.

The comet is a rare visitor from the distant Oort Cloud, a collection of frozen remnants from the formation of the solar system. The spherically shaped cloud is located about 50 times as far from the sun as Earth.

"Oort Cloud comets, it's hard to plan missions to them because you don't know where they're going to come from and how they are going to behave," said Padma Yanamandra-Fisher, senior research scientist at the Space Science Institute in Rancho Cucamonga, California.

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Comet Siding Spring is believed to be a first-time visitor to the inner solar system, gravitationally elbowed out of the solar system's backyard by a passing star about a million years ago.

"The comet has never ever been closer to the sun than we think maybe Jupiter, Saturn, Uranus or Neptune's distance. This is its first passage into what we call the 'water-ice line,' where it's really starting to blow its water off," astrophysicist Carey Lisse, with Johns Hopkins University in Laurel, Maryland, told reporters.

Orbiters shielded behind Mars

Initially, NASA was concerned the comet's dusty tail could pose a threat to orbiting spacecraft as it brushes past Mars. Later assessments somewhat allayed those concerns, but NASA still opted to tweak its satellites' orbits so they would be behind the planet during the most risky part of the flyby.

"Mars will be right at the edge of the debris cloud, so it might encounter some of the particles or it might not," NASA Mars scientist Rich Zurek, with the Jet Propulsion Laboratory in Pasadena, California, said in a statement.

Although much thinner than on Earth, the atmosphere on Mars will shield NASA's Opportunity and Curiosity rovers from comet dust, which may trigger meteor showers.

NASA is also watching for auroras that could result from the comet's debris interacting with the atmosphere of Mars.

In addition to its two rovers on the surface, NASA has three operational orbiters circling Mars. The European Space Agency and India also have spacecraft in orbit.

NASA says its orbiter Odyssey, as well as the Mars Reconnaissance Orbiter will manoeuvre to put the Red Planet between themselves and the comet's debris during the dustiest part of the encounter as will the Maven orbiter, which arrived Sept. 21 to study the Martian atmosphere.

All the probes are expected to be part of a grand science campaign to study the comet during its Martian rendezvous, looking to see how Siding Spring is impacted by the planet's gravity and how the comet affects the atmosphere on Mars.

An armada of ground- and space-based telescopes, including NASA's Hubble observatory, already are observing the comet. Comet Siding Spring's closest approach to Mars is expected at 2:27 p.m. ET on Oct. 19. It will soar past the Red Planet at about 203,000 km/h.

During the encounter, scientists hope to learn more about water, carbon and other materials that existed during the formation of the solar system some 4.6 billion years ago.

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