Sen— Comet ISON's long-anticipated pass by Mars showed that the comet is on the low end of brightness predictions.
The Mars Reconnaissance Orbiter's (MRO) High Resolution Imaging Science Experiment (HiRISE) camera snapped several photos of the comet on September 29 as it ventures towards the inner Solar System.
Astronomers worldwide are divided about just how bright Comet ISON will appear when it gets to its closest point to Earth in late December. Some have said it's possible, given the comet's size and distance to Earth at that time, that it will be among the brightest naked-eye comets ever seen, and visible even in daylight.
Others, watching ISON's brightness against the predictions, have said it's possible the comet will not be all that bright -- providing it survives that long.
"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," the HiRISE team wrote in an update.
"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. As a result, the image isn't visually pleasing, but low coma activity is best for constraining the size of the nucleus."
The Mars Reconnaissance Orbiter spacecraft spotted Comet ISON on Sept. 29, using its High Resolution Imaging Science Experiment (HiRISE). Credit: NASA/JPL/University of Arizona
The comet was about 13 million kilometres (eight million miles) from MRO when the spacecraft snapped these pictures. Three more observations were expected to take place in early October, but the comet is not expected to show up as brightly in these observations.
The HiRISE team did not rule out the comet increasing in brightness as it approaches the Sun.
"The comet, like Mars, is currently 241 million kilometers from the sun," the team added. "As the comet gets closer to the sun, its brightness will increase to Earth-based observers and the comet may also become intrinsically brighter as the stronger sunlight volatilizes the comet's ices."
Comet ISON's most crucial moment will be when it swings behind the Sun (from Earth's perspective) on November 28. Comet ISON will come within only 1.2 million kilometres (730,000 miles) of the Sun's surface, which will subject it to intense gravitational forces and heating. Some scientists suggest the comet may break up during this time.
Providing it survives, it's difficult to predict how bright the comet will appear when it emerges and swings closest to Earth at a distance of some 64 million kilometres (40 million miles). Since the comet's composition is not exactly known, this makes it hard to determine by how much the comet will brighten as its ices melt.
The last spectacular naked-eye comets in the northern hemisphere came in 1996 (Comet Hyakutake) and 1997 (Comet Hale-Bopp), making skywatchers eager to see another one now that digital cameras are more popular. Comet ISON itself was discovered on Sept. 21, 2012 by Vitali Nevski and Artyom Novichonok using the International Scientific Optical Network (ISON) near Kislovodsk, Russia.