A comet's close shave with Mars this weekend could reveal some key insights about the Red Planet and the solar system's early days, researchers say.
Comet Siding Spring will zoom within 87,000 miles (139,500 kilometers) of Mars at 2:27 p.m. ET Sunday. Scientists will observe the flyby using the fleet of spacecraft at Mars, studying the comet and any effects its particles have on the planet's thin atmosphere.
"We're going to observe an event that happens maybe once every million years," Jim Green, director of NASA's planetary science division, said earlier this month. "This is an absolutely spectacular event." [See photos of Comet Siding Spring]
Siding Spring, whose core is 0.5 to 5 miles (0.8 to 8 kilometers) wide, probably formed somewhere between Jupiter and Neptune about 4.6 billion years ago — just a few million years after the solar system began coming together.
Many of the objects in the region where the comet was born were incorporated into newly forming planets, but Siding Spring apparently had a close encounter with one of these planets and was booted out into the Oort Cloud, a frigid comet repository at the very outer reaches of the solar system. A million years ago or so, a star passing by the Oort Cloud is thought to have jolted the comet's orbit again, sending it on its first-ever trip into the inner solar system.
The comet's history helps explain why scientists are so excited about its current journey: Since Siding Spring has never been "heat-treated" by the sun before, it's a pristine object that looks much the same today as it did 4.6 billion years ago.
"If we study the comet — its composition, its structure — it will tell us a lot about how we think maybe the planets were formed," said Carey Lisse, a senior astrophysicist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland.
The interaction between Mars' upper atmosphere and the particles shed by Siding Spring should also reveal insights about the Red Planet's air, researchers said.
All of the operational spacecraft at Mars will attempt to observe Sunday's flyby. NASA's Mars Odyssey, Mars Reconnaissance Orbiter and the newly arrived MAVEN probe will watch from safe vantage points in Martian orbit, as will India's Mars Orbiter Mission and Europe's Mars Express. NASA's Curiosity and Opportunity rovers will point their cameras skyward from the Red Planet's surface.
— Mike Wall, Space.com
It may be possible to spot Comet Siding Spring from Earth, using a moderate-sized telescope — say, an 8-inch telescope with magnification in the range of 200x to 400x. Check out this Space.com guide for observing tips. The Slooh virtual observatory will be covering the event with online streaming video from observatories in South Africa and the Canary Islands, plus audio commentary, starting at 2:15 p.m. ET Sunday. Slooh will air a follow-up program with pictures from an observatory in Chile at 8:30 p.m. ET Sunday.
This is a condensed version of a report from Space.com. Read the full report. Follow Mike Wall on Twitter and Google+. Follow Space.com on Twitter, Facebook or Google+.