A comet from the outer reaches of the solar system on Sunday made a rare, close pass by Mars while robotic science probes and telescopes watched. 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 4.6 billion years ago.
"This comet is on its way plunging in toward the sun, growing a tail," astronomer David Grinspoon of the Planetary Science Institute said during a live webcast of the flyby on Slooh.com.
The comet made its closest approach to Mars at 2:27 p.m. EDT, hurling past at about 126,000 mph (203,000 kilometers per hour). NASA’s three Mars orbiters and two rovers, as well as orbiters operated by the European Space Agency and India, monitored the comet’s approach and flyby. Telescopes on Earth and in space watched as well.
Initially, NASA was concerned the comet’s dusty tail could have posed a threat to orbiting spacecraft. Later assessments allayed those concerns, but NASA still opted to tweak its satellites’ orbits so they would be behind the planet during the riskiest part of the flyby.
Mars was due to pass directly through the comet’s coma, which is the envelope of gas and dust surrounding the comet’s nucleus. That should providing an unprecedented opportunity for observations, Grinspoon said: "This is a really rare event."