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Mars satellite spots Comet Siding Spring's nucleus, and it's surprisingly tiny

Credit: NASA/JPL-Caltech/University of Arizona

Scott Sutherland
Meteorologist/Science Writer

Wednesday, October 22, 2014, 10:54 AM - On Sunday, October 19, the robotic orbiters and rovers we have investigating the planet Mars had a new target to examine - Comet Siding Spring as it passed just 140,000 kilometres away - and we now have the first images from this amazing encounter.

The composite above contains images taken by the HiRISE camera, on board the Mars Reconnaissance Orbiter, showing two different views of the comet. The top two images show everything dimmed-down except what should be the bright nucleus. The bottom two images ramp up the brightness again, to reveal the comet's coma (the 'atmosphere' of gases that surround it due to being heated by the Sun).

In its normal operation, pointed down at the surface of Mars from over 24,000 km up, HiRISE produces incredible high-resolution images where each pixel covers about 30 centimetres across. Aimed at Comet Siding Spring on Sunday, over five times that distance away, each pixel of its images covered just under 140 metres on a side. Originally, from telescope observations, scientists with the MRO mission expected to see a comet nucleus roughly a kilometre across. They were surprised to discover that Siding Spring only took up two or three pixels in HiRISE's view, at the most. That puts its maximum size at just over 400 metres, which is curious, given how bright the comet is.

Another view of the comet was captured from the surface of Mars, specifically by the PanCam of NASA's Opportunity rover.

Credit: NASA/JPL-Caltech/Cornell Univ./ASU/TAMU

According to the NASA JPL website:
"This image is from a 50-second exposure taken about two-and-a-half hours before the closest approach of the comet's nucleus to Mars. The sky was still relatively dark, before Martian dawn. At the time of closest approach, the morning sky was too bright for observation of the comet. The comet, some nearby stars, and some effects of cosmic rays hitting the camera's light detector are labeled."
"The image has been processed by removal of detector artifacts and slight twilight glow. The duration of the exposure resulted in a 12.5-pixel smear from rotation of Mars. The smear for the comet is at a slightly different angle from the others, due to the comet's own motion across the sky."

Two other NASA orbiters - Mars Odyssey and MAVEN - also imaged the comet as it passed, and Lauren Edgar, a scientist with the US Geological Survey's Astrogeology Science Center, reported that "Curiosity successfully observed the comet with Mastcam, Navcam and ChemCam RMI."

With no other images making it to the web yet, presumably the data from these observations are being held back as the mission teams pore over them and write up scientific papers. These will likely be presented at two upcoming conferences - the 46th Meeting of the Division for Planetary Sciences (American Astronomical Union) in Tucson, in mid-November, and the Fall Meeting of the American Geophysical Union, a month later in San Francisco. So, we will likely need to wait until then for more updates on this amazing encounter.

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