Comet ISON made its closest approach to Mars this week, and scientists in charge of the sharp-eyed camera aboard NASA's Mars Reconnaissance Orbiter say the long-awaited comet did not live up to expectations.

The HiRISE camera on NASA's Mars Reconnaissance Orbiter recored this series of images of Comet ISON. Credit: NASA/JPL/University of Arizona
 
It's too early to say what that means for ISON's swing by the sun in November, when observers on Earth are expected to get their best chance to see the comet in the sky.

MRO's High-Resolution Imaging Science Experiment, or HiRISE camera, took some time off its usual targets - the craters, dunes and rust-colored landforms of Mars - and pointed toward space as ISON sailed 8 million miles away from the red planet.

"Based on preliminary analysis of the data, the comet appears to be at the low end of the range of brightness predictions for the observation," HiRISE scientists Alan Delamere and Alfred McEwen wrote on the instrument's website. "As a result, the image isn't visually pleasing but low coma activity is best for constraining the size of the nucleus."

HiRISE was not designed to spot comets, and this week's observations were planned as an opportunity to collect bonus science.

Ball Aerospace and Technologies Corp. built the camera ahead of its launch aboard MRO in 2005. Scientists at the University of Arizona operate the camera, which is not affected by the ongoing partial government shutdown.

When used for its primary purpose of imaging the Martian surface, HiRISE can resolve objects as small as a dinner plate. MRO's view of ISON provides the closest view yet of the comet, which is speeding toward a grazing encounter with the sun on Nov. 28.

In this Hubble Space Telescope composite image taken in April 2013, the sun-approaching Comet ISON floats against a seemingly infinite backdrop of numerous galaxies and a handful of foreground stars. The icy visitor, with its long gossamer tail, appears to be swimming like a tadpole through a deep pond of celestial wonders. Credit: NASA/ESA
 
The comet's nucleus does not fill a single pixel in HiRISE's snapshots, but scientists can estimate ISON's size by observing the brightness of its coma, or atmosphere, composed of dust and water vapor streaming away from its rock-ice core.

"HiRISE saw a small spot at the position of ISON that is relatively bright, like a star, but moving relative to actual stars," Delamere and McEwen wrote. "The comet's coma is apparently very faint, so these data provide useful constraints on the size of the comet nucleus and its overall brightness, key measurements to understand its behavior and useful knowledge to subsequent observers."

Researchers say the coma will become more pronounced as ISON approaches the sun.

The best viewing from Earth will be in late November and early December, and although ISON's showing has not been as bright as first predicted, most astronomers expect the comet to be visible with the naked eye before and after it swings by the sun.

ISON will reach its closest point to Earth on Dec. 26 at a range of 40 million miles, six times farther than its flyby of Mars this week, according to the European Space Agency.