Extract from ABC News
Updated
A comet the size of a small mountain has whizzed past Mars, wowing space enthusiasts with the once-in-a-million-years encounter.
The
comet, known as Siding Spring, passed just 140,000 kilometres 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 made its closest approach to Mars at 5:27am (AEDT), hurling past at about 203,000 kilometres per hour.
NASA's fleet of Mars-orbiting satellites and robots on the planet's surface were primed for the fly-by of the comet, hoping to photograph the rare event.
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 Ort 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 4.6 billion years ago.
The comet is over one kilometre wide and is only about as solid as a pile of talcum powder.
"This comet is on its way plunging in toward the Sun, growing a tail," astronomer David Grinspoon from the Planetary Science Institute in Tucson, Arizona, said.
"This is its first passage into what we call the water-ice line, where it's really starting to blow its water off."
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.
Initially, NASA was concerned the comet's dusty tail could pose a threat to orbiting spacecraft as it passed.
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 fly-by.
"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 said in a statement.
Mars's 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.
Reuters/AFP
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