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 Comet Siding Spring passed just 87,000 miles (140,000 km) from
Mars, less than half the distance between Earth and the moon and 10
times closer than any known comet has passed by Earth, NASA said.
The comet, named for the Australian observatory that discovered it
last year, is believed to be a first-time visitor to the inner solar
system, having departed the Oort Cloud, located beyond Neptune’s
orbit, more than a million years ago.
Comets are believed to be frozen remnants left over from the
formation of the solar system some 4.6 billion years ago.
“This comet is on its way plunging in toward the sun, growing a
tail,” astronomer David Grinspoon, with the Planetary Science
Institute in Tucson, Arizona, said during a live webcast of the
comet’s flyby on Slooh.com.
The comet made its closest approach to Mars at 2:27 pm EDT, hurling
past at about 126,000 mph (203,000 kph).
NASA’s three Mars orbiters and two rovers, as well as orbiters owned
by the European Space Agency and India were expected to monitor the
comet’s approach and fly-by, which may have left Mars engulfed in a
cloud of comet dust.
“The comet has never ever been closer to the sun that 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 during
a press conference last week.
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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 that 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.
Mars’ atmosphere, though much thinner than Earth’s, will shield
NASA’s Opportunity and Curiosity rovers from comet dust, which may
trigger meteor showers.
Mars also will pass directly through the comet’s coma, which is an
envelope of gas and dust surrounding the comet’s nucleus, providing
an unprecedented opportunity for study, Grinspoon said. “This is a
really rare event.”
(Editing by Eric Walsh)
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