It may not look like much but it’s actually quite a lot: that bright smudge is Comet C/2013 A1 (Siding Spring) as it approached Mars to make its historic and much-anticipated close pass on Sunday, Oct. 19! The mountain-sized comet shot past Mars at an estimated distance of 88,000 miles traveling about 35 miles a second… that’s 20 times faster than a bullet fired from a 9mm handgun.
While the comet didn’t put on a big show in our sky here on Earth (although some photographers did capture it quite nicely in telescopes) the rovers on Mars and spacecraft in Martian orbit were keeping their electronic eyes on it… and NASA’s Opportunity rover, now nearly 11 years on Mars, was the first to send back a confirmed image!
At 18:27 UTC (2:27 p.m. EDT) on Oct. 19 the comet passed by Mars closer than any comet has come to either Mars or Earth in recorded history*. This is also Siding Spring’s first trip into the inner solar system. It will make its closest approach to the Sun (perihelion) on Oct. 25, coming within 1.39 AU before heading back out into the depths of the solar system. (See its orbital parameters here.)
We likely won’t see it around these parts again for another million years or so.
Researchers used the panoramic camera (Pancam) on NASA’s Mars Exploration Rover Opportunity to capture this 10-second-exposure view. This image was taken about two-and-a-half hours before the closest approach. The sky was still relatively dark, before Martian dawn. At the time of closest approach the morning sky was too bright for observations of the comet.
“It’s excitingly fortunate that this comet came so close to Mars to give us a chance to study it with the instruments we’re using to study Mars. The views from Mars rovers, in particular, give us a human perspective, because they are about as sensitive to light as our eyes would be.”
– Mark Lemmon, Opportunity science team member, Texas A&M University
Some stars and cosmic ray hits were also captured in the long-duration exposure. For a labeled version of the original image click here.
C/2013 A1 (Siding Spring) was discovered on January 3, 2013 by Robert H. McNaught at Siding Spring Observatory. At the time of discovery the comet was 7.2 AU from the Sun.
*The closest a comet has come to Earth in recorded history was on July 1, 1770 when D/1770 L1 (Lexell) passed at about 6 lunar distances (0.0146 AU). Siding Spring passed Mars at just over 1/3 a lunar distance, or .0008 AU.