While all the robots were practicing duck-and-cover to avoid being sandblasted by comet Siding Springs this weekend, they were also trying to catch a glimpse of the event. Here are the first images of comet Siding Springs grazing past Mars and what we've learned so far.
Comet Siding Spring as seen from the surface of Mars by the Opportunity rover during a 50-second exposure. While the star-trails reflect the planet's rotation, the comet's trail pulls in a different direction due to its own movement. Image credit: NASA/JPL-Caltech/Cornell Univ./ASU/TAMU
When comet Siding Spring slipped past Mars on Sunday, it marked the closest encounter of a comet with a planet we've ever observed that didn't result in a collision. It's also the first time we've had high-resolution cameras on satellites positioned to be able to directly image the nucleus of a long-period comet. While all the robots and satellites were hiding behind Mars and shifted into safety modes to reduce the risk of damage from the trail of dust, gas, and particles, they were also struggling to get a good look at this historic event. All the satellites emerged intact, and now we're starting to see what they saw.
Two-exposure blink image highlights the comet's movement on October 19, 2014. Image credit: NASA/JPL-Caltech/Cornell Univ./ASU/TAMU
The Opportunity rover imaged the comet using its panoramic camera. The image is from two and a half hours before closest approach. When the comet was even closer, the stars above Opportunity were lost to a brightening dawn sky. Opportunity science team member Mark Lemmon explained his delight at getting to see the comet as Opportunity saw it:
"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."
The satellites around Mars also got in on the action. The HiRISE camera on the Mars Reconnoissance Orbiter managed to take the first-ever resolved images of the nucleus of a long-period comet. While the fuzzy pixilated mess doesn't look as spectacular as the images we're used to getting from HiRISE, they're already teaching us something new.
The two best images (left, right) of Comet Siding Spring as seen by HiRISE on the Mars Reconnoissance Orbiter taken nine minutes apart. Top images are the comet nucleus and bright coma with full dynamic range; bottom images are the same shot brightened to make the faint outer coma visible and over-saturating the inner region. Image credit: NASA/JPL-Caltech/University of Arizona
It's going to take weeks and months to properly sort through all the data and learn everything that we can discover from the observations. Right now we can already see the cometary nucleus is smaller than expected. The previous estimates based on telescope observations for the comet's size were on the order of a kilometre in diameter. Now, given the resolving power of HiRISE of 138 meters per pixel at the comet's closest approach, it appears that the nucleus is but half that size, a wee chunk of dust and ice for causing so much fuss!