Feb. 5, 2013 at 7:56 PM ET
Comet ISON is still too dim to be seen with the naked eye, but a fresh series of images from NASA's Deep Impact spacecraft tracks what skywatchers hope will become the "Comet of the Century" nine months from now.
The pictures were taken by Deep Impact's Medium-Resolution Imager over a 36-hour period on Jan. 17 and 18, when the spacecraft was 493 million miles (793 million kilometers) from the comet, and roughly 185 million miles (300 million kilometers, or 2 AU) from Earth.
"This is the fourth comet on which we have performed science observations, and the farthest point from Earth from which we've tried to transmit data on a comet," Tim Larson, project manager for the Deep Impact spacecraft at NASA's Jet Propulsion Laboratory, said today in a news release. "The distance limits our bandwidth, so it's a little like communicating through a modem after being used to DSL. But we're going to coordinate our science collection and playback so we maximize our return on this potentially spectacular comet."
Deep Impact began its comet-watching mission with a bang — by shooting a projectile into Comet Tempel 1 in 2005 and watching the resulting blast. The camera-equipped mothership went on to study Comet Hartley 2, and then Comet Garradd, and now Comet ISON.
The new comet on the block, whose formal name is C/2012 S1, was discovered last September by two Russian astronomers using the International Scientific Optical Network's 16-inch (40-centimeter) telescope near Kislodovsk. It's coming in from the icy outskirts of the solar system, known as the Oort Cloud, and is still more than 474 million miles (763 million kilometers) from the sun. Nevertheless, the sun's warmth has already kicked up a cometary tail, extending more than 40,000 miles (64,000 kilometers) from ISON's nucleus.
ISON will stay far away from Earth: JPL projects that its closest approach will come on Dec. 26, at a distance of 40 million miles (64 million kilometers), or nearly half the distance between our planet and the sun. But it's expected to pass within 1.1 million miles (1.8 million kilometers) of the sun's surface a month earlier. If Comet ISON survives, that sun-grazing episode could make the dirty snowball's tail glow brighter than 2011's Comet Lovejoy or 2007's Comet McNaught. Some even think ISON's coma could shine brighter than the full moon — on a par with, say, the Great Comet of 1680.
"We see sungrazers all the time, but most are only seen as they flare up very close to the sun," University of Maryland astronomer Tony Farnham, a member of the Deep Impact science team, said in a news release. "With this comet, we are able to study it from where it is currently, farther from the sun than Jupiter and about five times farther from the sun than Earth, until its closest approach to the sun, called its perihelion, on Nov. 28.”
It's too early to tell whether Comet ISON will live up to the big buildup. Heck, it's even too early to tell how bright another eagerly awaited starry messenger, Comet PANSTARRS, will get next month when it swings through the inner solar system.
"One website says that Comet PANSTARRS has fizzled, and might not even be visible to the naked eye at its best, while another says it is back on track to be almost as bright as Hale-Bopp was in 1997," skywatcher Stuart Atkinson notes on his "Waiting for ISON" blog.
Whether ISON or PANSTARRS are hits or misses, it's worth monitoring all the cometary buzz over the months ahead. To follow ISON's progress, keep an eye on Atkinson's blog as well as the Comets-ML mailing list and SpaceWeather.com's comet gallery. Right now, the hottest topic isn't necessarily Comet ISON or PANSTARRS: It's Comet Lemmon, which is coming into its prime time after months of anticipation.
Correction for 10:30 p.m. ET: I originally wrote that the tail could shine brighter than the full moon, but in a Twitter message, German science writer Daniel Fischer points out that the speculation about brightness has focused on the coma. I've corrected that reference. This Flash interactive demonstrates the difference.
More about comets:
Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.