They’ve done it again. NASA engineers and scientists successfully slewed the Mars Reconnaissance Orbiter into position to get pictures of comet C/2013 Siding Spring during its close flyby on October 19. I think all of us were waiting for pictures more like this one which show more than a bit of fuzz. Not to disrespect fuzz. Fuzz or comet dust seeded the early Earth with important organic compounds and still makes for awesome meteor showers right up to the present day.
The top set of photos uses the full dynamic range of the camera to accurately depict brightness and detail in the nuclear region and inner coma. Prior to its arrival near Mars astronomers estimated the diameter of the nucleus or comet’s core at around 0.6 mile or 1 kilometer. But based on these images taken at much closer range, its true size is less than 1/3 mile or 0.5 km across. The bottom photos overexpose the nuclear region but reveal an extended coma and a short tail extending to the right.
Comet Siding Spring is a new visitor to the inner solar system, hailing from the distant repository of comets called the Oort Cloud far past Neptune and the icy asteroids that populate the Kuiper Belt.
It slid sunward on its cigar-shaped orbit for millions of years as the planets wheeled around the Sun like balls in a roulette wheel. By pure chance, Mars happened to lie only 87,000 miles from the comet on its journey toward the Sun.
Photographing a fast-moving target from orbit is no easy trick. You have to pan the MRO’s camera at the precise rate needed to shoot a time exposure without blurring the image. Engineers at Lockheed-Martin in Denver did exactly that based on comet position calculations by engineers at the Jet Propulsion Lab. To make sure they knew exactly where the comet was, the team photographed the comet 12 days in advance. To their surprise, the orbital calculations were just a bit off. Using the new positions, MRO succeeded in locking onto the comet during the flyby. Without this earlier check, cameras may have missed seeing Siding Spring altogether!
I’ve also added a new, annotated version of the photo taken by the Opportunity Rover and used in the blog earlier today. From the rover’s point of view, the comet buzzed across the constellation Cetus at the time, while here on Earth we see it in the summertime constellation Ophiuchus.
NASA deserves a pat on the back for their great work in acquiring these images and getting them to us within 24 hours. There will be much more on the observational side (and hopefully more photos!) in the weeks and months to come.