NASA just released an image that shows four vague blobs of light. Normally, such an image wouldn't create much excitement, but this particular image is pretty special. Comet C/2013 A1 Siding Spring buzzed Mars over the weekend, putting NASA's three Mars orbiter spacecraft on high alert. The orbiters had to hide behind the planet to protect themselves from dust released by the comet, but they also managed to gather data and images of the comet.
The Mars Reconnaissance Orbiter used its High Resolution Imaging Science Experiment (HiRISE) camera to capture images of Siding Spring from about 86,000 miles away. What researchers discovered after studying the images is that the nucleus of the comet is rather smaller than originally expected, coming in at less than a quarter-mile across.
A composite released by NASA shows two versions of the HiRISE images. The top blobs show the nucleus and coma (the hazy-looking envelope around the nucleus). The bottom images have the faint outer coma brightened. "The images are the highest-resolution views ever acquired of a comet coming from the Oort Cloud at the fringes of the solar system. Other spacecraft have approached and studied comets with shorter orbits," NASA notes.
The orbiters weren't the only human-made devices keeping an eye on the comet. The Opportunity rover looked up from the surface of Mars and captured images of the comet as it passed by. From Opportunity's viewpoint, the comet appeared as a bright object in the sky, not too much larger than a star.
"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," said Mark Lemmon, an Opportunity science team member. "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."