FROM THE UNIVERSITY OF TEXAS McDONALD OBSERVATORY, AS HEARD ON KCBS RADIO WEEKDAYS @ 9:52 A.M., 7:35 P.M. & 2:52 A.M.

Stardate 10/17/14: Mars is about to dodge a big snowball — a comet that will swing just 82,000 miles above the planet’s surface on Sunday. That’s just a third of the distance between Earth and the Moon, so it’s quite a close call. The comet would be a spectacular sight under the dark Martian sky. And it could even spawn a short but brilliant meteor shower.

Comet Siding Spring is making its first trip through the inner solar system. It’s spent almost its entire life in the Oort Cloud — a vast shell of icy bodies far from the Sun. Perhaps nudged out of its orbit by the gravity of a passing star, it’s been streaking toward the Sun for the last million years or so.

Such comets are like icy time capsules — they preserve a record of the early solar system. Siding Spring probably formed fairly close to the Sun, from the same disk of debris that gave birth to Earth and the other planets. The gravity of one of the giant outer planets hurled it into the Oort Cloud.

In the deep freeze of interstellar space, the comet has changed little since its formation. So as it zips toward the Sun, it gives scientists a chance to see a relic from the distant past — a snowball that preserves the same ingredients that gave birth to our own world.

Spacecraft in orbit around Mars will watch the comet — but not all the time. Tiny bits of dust around the comet could be hazardous, so the craft will hide behind Mars for a while. We’ll have more about that tomorrow.

Copyright ©2014 The University of Texas McDonald Observatory

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