The first images of Comet Siding Spring, taken by the High Resolution Imaging Science Experiment (HiRISE) camera aboard NASA's Mars Reconnaissance Orbiter, suggest it's smaller than previously believed.
The images, taken from a miniumum distance of about 86,000 miles, are at a scale of "about 150 yards (138 metres) per pixel," according to a NASA press release.
"Telescopic observers had modeled the size of the nucleus as about half a mile, or one kilometer, wide," NASA said in its post. "However, the best HiRISE images show only two to three pixels across the brightest feature, probably the nucleus, suggesting a size less than half that estimate."
The image is actually to compounded images, taken nine minutes apart. The top row of pictures show what NASA describes as "full dynamic range, showing the nucleus and bright coma near the nucleus." The bottom row shows "... versions where the fainter outer coma is brightened, saturating the inner region," according to the release.
The real comet is the eight-bit smudge at the top and what you're seeing is most likely about 450m across.
Comet Siding Spring soared past Mars at around 33 miles per second, so had it hit anything it would have caused serious damage.
These images were made possible due to precise pointing and slewing of the Mars Reconnaissance Orbiter by engineers at Lockheed Martin in Denver, based on comet position calculations by engineers at NASA's Jet Propulsion Laboratory, Pasadena, California.
HiRISE was able to take three images 12 days before closest approach, when the comet was barely detectable above the "noise level" of the images. These early images showed that the comet wasn't where researchers thought it would be, according to NASA.
This new angle on the comet was used to update its predicted location and timing at closest approach. Without the update, there is a good chance the comet could have been outside the HiRISE image area in the best images.
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