J.-Y. Li (PSI) / NASA / ESA

Comet C/2012 S1 (ISON) takes on a fuzzy glow in an April 10 image from the Hubble Space Telescope. The blue tint has been added to the black-and-white imagery.

Comet ISON, the long-traveling iceball that skywatchers hope will turn into the "Comet of the Century," takes on a fuzzy glow in an image captured two weeks ago by the Hubble Space Telescope and unveiled on Tuesday.

The picture was taken on April 10, using Hubble's Wide Field Camera 3, when the comet was 386 million miles (621 million kilometers) from the sun and 394 million miles (634 million kilometers) from Earth. That's just inside the orbit of Jupiter. Right now, the comet's brightness is roughly magnitude-16, which means it can only be seen with telescopes. But comet-watchers are hoping that ISON will get dramatically brighter as it swings around the sun in late November. Some have said the comet could match the brightness of Venus or even the full moon.

The reason for those hopes — and the reason for all the "coulds" and "mights" — is that ISON appears to be a long-period comet, coming in from the far reaches of the solar system for the first time in living memory. Such comets are unpredictable: Will they shed lots of dust and glowing gas, or will they turn out to be duds? ISON's orbit is due to bring it within 700,000 miles (1.2 million kilometers) of the sun's surface. That could cause ISON to crumble like Comet Elenin did in 2011, or it could spark a flare-up of Comet Lovejoy proportions.

Unlike Comet Lovejoy, which lit up the Southern Hemisphere's skies during 2011's holiday season, Comet ISON should be visible from the Northern Hemisphere — which means Americans might get an eyeful during this year's winter holidays. (There's that pesky "might"!)

The picture from Hubble helps astronomers get a better fix on the current state of Comet ISON: The nucleus appears to be no larger than three or four miles (five to seven kilometers) across. In Tuesday's image release, the Hubble team says that's "remarkably small, considering the high level of activity observed in the comet so far." The comet's fuzzy head, known as the coma, measures about 3,100 miles (5,000 kilometers) across, or a little less than the distance from New York to Dublin. ISON's tail extends more than 57,000 miles, far beyond Hubble's field of view.

Detailed readings from Hubble could unlock the secrets of ISON's origins, University of Maryland astronomer Michael A'Hearn said in a news release. "We want to look for the ratio of the three dominant ices, water, frozen carbon monoxide, and frozen carbon dioxide, or dry ice," A'Hearn said. "That can tell us the temperature at which the comet formed, and with that temperature, we can then say where in the solar system it formed."  

Comet ISON was discovered last September and is formally known as C/2012 S1 (ISON). It takes its name from the International Scientific Optical Network, a group of observatories in 10 countries managed by Russia's Keldysh Institute of Applied Mathematics.

J.-Y. Li (PSI) / NASA / ESA

This contrast-enhanced image was produced from Hubble's view of Comet ISON to reveal the subtle structure in the inner coma of the comet. Such enhancements help astronomers determine the comet's shape and evolution, plus the spin of its solid nucleus.

More about comets:

• Moon and Comet PanSTARRS pair up
• Flash interactive: Inside a comet
• Cosmic Log archive on comets

The image was created from Hubble data from proposal 13198: J.-Y. Li (Planetary Science Institute), P. Lamy (Laboratoire d'Astrophysique de Marseille), H. Weaver (JHU/APL), M. A'Hearn and M. Kelley (University of Maryland), M. Knight (Lowell Observatory), T. Farnham (University of Maryland), and I. Toth (Hungarian Academy of Sciences).

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.

Minoru Yoneto

Minoru Yoneto captured this picture of Comet PanSTARRS shining over Queenstown, New Zealand, on March 2. The comet's tail has two components, consisting of glowing gas and shining dust. Yoneto told SpaceWeather.com that "it's a splendid appearance."

Observers in the Southern Hemisphere have been watching Comet PanSTARRS for weeks, but the Northern Hemisphere is due to get its first looks at one of the year's most eagerly anticipated sky extravaganzas this week. And there's good news for northerners: The up-and-down expectations for the cometary show are trending upward again.

"In the Southern Hemisphere we have a few days to enjoy it," Argentine photographer Victor Gabriel Bibe, one of many observers who have been tracking the comet's brightening glow, said in an email.

On Tuesday, PanSTARRS makes its closest approach to Earth. On Thursday or so, it should start becoming visible to Northern Hemisphere observers in the western sky, low to the horizon just after sunset. The best photo ops will come March 12 and 13, when PanSTARRS pairs up with the crescent moon.

The brightness of an astronomical object is measured by magnitude, with lower numbers denoting brighter objects. Magnitude +6 is about the limit for naked-eye observations under prime conditions. Magnitude +2 is equivalent to the brightness of Polaris, the North Star. Early on, astronomers said PanSTARRS could get to magnitude zero, putting it in a league with some of the brightest stars in the sky. More recently, they noted that the comet wasn't brightening as quickly as they originally thought and revised the forecast to around +2. Now, the consensus is that it could get to +1 or brighter — maybe not dazzling, but definitely not bad.

"As long as it continues its behavior for a few days, it looks like the Northern Hemisphere — even us city-dwellers — might get a pretty good view of this thing," said Karl Battams of the Naval Research Laboratory in Washington.

The comet was discovered in June 2011 by the Panoramic Survey Telescope and Rapid Response System, or Pan-STARRS, in Hawaii — which helps explain the genesis of its official name, Comet C/2011 L4 (PanSTARRS). The "C" means that the comet is considered a non-periodic newbie, coming in for the first time from the Oort Cloud on the solar system's edge. The "L4" means it was the fourth comet discovered during the first half of June.

Victor Gabriel Bibe

Comet PanSTARRS shines above a mountain range in Tierra del Fuego, Argentina. The picture was taken by Ushuaia photographer Victor Gabriel Bibe. For more of Bibe's pictures, check out El Cielo de Tierra del Fuego.

Dieter Willasch

Dieter Willasch snapped this picture of Comet PanSTARRS on March 2 from Somerset West in South Africa. Visit Astro-Cabinet for more images by Willasch.

NASA

This chart shows Comet PanSTARRS' location in the Northern Hemisphere evening sky after sunset for several dates during prime time. The position of the crescent moon is shown for March 12. Watch a NASA video about Comet PanSTARRS.

Even though PanSTARRS is still a few days away from Northern Hemisphere visibility, you can take advantage of these tips to maximize your comet-viewing experience:

• Scope out a spot with good western exposure and a minimum of trees, buildings or hills to spoil the view. PanSTARRS won't get very high in the sky, so you'll want to scan the horizon as soon as the sun goes down. But not before! It'd be a tragedy to damage your eyes for the sake of a comet.
• The farther you are from city lights and cloudy weather, the better you'll be able to see the comet. "I was very lucky to observe the comet, because in the area where I live, the weather is very unstable and the sky is always cloudy. Every time the sky clears, I attempt to observe," Bibe said.
• Although the comet is visible to the unaided eye, binoculars will enhance the view — particularly when it comes to seeing the tail. "To the naked eye, this comet has a stellar aspect. But with 10x50 binoculars you can see the tail clearly," Bibe said.
• Once it's visible in the Northern Hemisphere, the comet will rise higher in the sky on each succeeding night. If PanSTARRS follows the expected trend, the viewing should be best between March 10, when it makes its closest approach to the sun, and March 13 or so. After about that time, the glare of the waxing moon could take some of the shine off the comet. PanSTARRS is projected to fade from naked-eye visibility in April.

After PanSTARRS: ISON
PanSTARRS is the first of two comets expected to take the spotlight this year. The other one is Comet ISON, which has the potential to get much, much brighter than PanSTARRS in November. Some experts are hoping it will equal the brightness of the full moon, although Battams says it's way too early to make firm predictions.

To prepare for ISON's arrival, Battams and other researchers are helping NASA organize a comet observing campaign. "We just want to make sure that all the major observatories are aware of this," Battams said.

ISON is expected to pass as close to the sun's surface as 684,000 miles (1.1 million kilometers), which could produce a dramatic brightening of the comet when it swings back out of the inner solar system. It could produce scientific insights as well.

"Sungrazing comets are unique objects that experience the most extreme thermal and gravitation forces our solar system has to offer them," the campaign's Web page says. "However, rarely do we get to see these objects more than a few hours before their demise. Comet ISON offers us the rare opportunity to study a sungrazer in great detail, for an extended period, and place it in the context of other comets."

For updates on the year of the comets, check in with the "Waiting for ISON" blog as well as SpaceWeather.com and the Remanzacco Observatory website, and follow @ISONUpdates on Twitter.

Got comet pictures? Share them via NBC News' FirstPerson photo upload site and we'll pass them along in a future posting.

More about comets:

• NASA probe tracks 'Comet of the Century'
• Comet shows and other sky highlights for 2013
• Flash interactive: Inside a comet
• EarthSky preview for PanSTARRS
• Space.com preview for PanSTARRS
• Sky & Telescope preview for PanSTARRS

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.

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.

Rolando Ligustri / CARA

A false-color, long-exposure image highlights Comet ISON and its faint but brightening tail in the center, surrounded by the trails of background stars. The image was captured by Rolando Ligustri of CAST Italia, using a remote-controlled telescope in New Mexico. The brightness of the comet is about magnitude 15.6.

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:

• Video: A comet to watch in 2013
• Flash interactive: Inside a comet
• 'Comet of the Century' and other highlights

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.

David Lillo / AFP - Getty Images

Comet McNaught shines above Chile in 2007. Will Comet ISON be as bright in 2013?

Next year's most eagerly awaited shows in the skies above might not happen — but that's exactly what makes them so eagerly awaited. There's nothing like uncertainty to build up the drama, and right now, Comet PANSTARRS and Comet ISON are surrounded by bright haloes of uncertainty.

The picture should be getting clearer in the weeks ahead for the comet formally known as C/2011 L4 PANSTARRS, which was discovered in 2011. It'll take a few more months to get a fix on C/2012 S1 ISON, which was first spotted this September. All we can say right now is, if the comets live up to their current high expectations, PANSTARRS could blaze as bright as Venus in March — and then, in November and December, ISON could outshine the moon to the "Comet of the Century."

"If Comet ISON can survive perihelion passage ... then we are almost surely in for a striking display in the morning sky as Comet ISON recedes from the Sun next December," veteran observer John Bortle said this month on the Comets Mailing List. "Its immense tail, partly the result of our extremely favorable viewing circumstances in this case and just as with the Great Comet of 1680, could well result in a tail of amazing length and surface brightness, even if tipped by only tiny, relatively insignificant head."

The best part is that these comets will be visible in the Northern Hemisphere, unlike the spectacles created by Comet McNaught in 2007 and by Comet Lovejoy a year ago. Why let the Southern Hemisphere have all the fun?

PANSTARRS and ISON are just two of the highlights coming up for skywatchers next year. Here's my top-10 list for 2013, plus some bonus picks from Space.com skywatching columnist Joe Rao:

Jan. 2-3 for Quadrantid meteors: If the weather's clear, the Quadrantid meteors should put on serviceable show this year. The Quadrantids are sparked by debris from asteroid 2003 EH1, and appear to emanate from an area of the sky known as Quadrans Muralis, around the northern tip of the constellation Bootes. The peak rate is expected to reach 80 meteors per hour, but the glare of a waning gibbous moon could interfere somewhat. "Unlike the more famous Perseid and Geminid meteor showers, the Quadrantids only last a few hours, so it's the morning of Jan. 3 or nothing," NASA says. Check out NASA's Quadrantids website for a video feed on the nights of Jan. 2-4.

April 25 for partial lunar eclipse: Three eclipses of the moon are coming during 2013 — and although none of them will be spectacular, they're worth keeping an eye on if you're in the right place. The April 25 partial eclipse will be visible from Europe, Africa, Asia and the Pacific. The other two lunar eclipses are a nearly imperceptible hint of a penumbral eclipse on May 25, and a somewhat deeper penumbral eclipse on Oct. 18-19 (visible, at least in part, from the Americas, Europe, Africa and Asia). Even if you miss seeing these eclipses with your own eyes, there'll be plenty of photo galleries showing the moon in its best light.

March for Comet PANSTARRS: The comet is due to streak past Earth on March 5 and make its turn around the sun, known as perihelion, on March 9-10. The prime time for observers at mid-northern latitudes will come after perihelion, when PANSTARRS will be visible in the evening sky. On March 12, the comet is expected to share the sunset's afterglow with a beautiful crescent moon.

Issei Kato / Reuters file

Clouds cast a pall over an annular solar eclipse as seen from Hirai Daini Elementary School in Tokyo on May 21, 2012. An annular eclipse is due to occur on May 10, 2013, and in November there'll be a hybrid eclipse that morphs from annularity to totality.

May 9-10 for annular solar eclipse: A "Ring of Fire" eclipse will roll across Australia, Papua New Guinea and the South Pacific, with a partial solar eclipse visible from a wider swath of the Pacific. If past history is any guide, some of us in North America will be watching the event unfold on the evening of the 9th, via webcasts from the scene. 

May 24-28 for planetary party: Mercury, Venus and Jupiter mix it up in western skies over a series of nights in May, with Saturn and the moon adding their shine. The main event may well be the Venus-Jupiter conjunction on May 28 — but it won't be as spectacular as the double-planet feature we saw in February, because this one will take place so soon after sunset. 

June 23 for Supermoon: The moon goes full just after this year's closest approach to Earth, meaning that it'll look 14 percent bigger and 30 percent brighter than it does at maximum distance. Last May's Supermoon made such a splash that I suspect this could turn into an annual moon-watching event. 

Aug. 11-13 for Perseid meteors: Annual meteor showers have their ups and downs, and the advance word is that 2013 will be an "up" year for the Perseids. The moon will be a mere crescent in the morning sky, cutting down on the glare. The flux of shooting stars is expected to be normal, peaking at around 100 meteors per hour.

Oct. 12 for moon observation: International Observe the Moon Night provides an opportunity for veteran skywatchers to show you the moon at its best — no, not during the full moon, but during the first-quarter phase. That's when you can get a good look at the moon's craters and shadowy mountains. Check in with the InOMN website for updates.

Nov. 3 for hybrid solar eclipse: This hybrid is a strange one, starting out as an annular "Ring of Fire" eclipse and turning into a total eclipse as the moon's shadow races across the planet. The track of annularity-totality runs across the Atlantic, goes through the middle of Africa and ends up in Somalia. If you can't afford a cruise or an expedition, keep a watch on the webcasts.

November-December for Comet ISON: Will ISON shine "brighter even than the full moon" a year from now? That seems hard to believe right now, but by next autumn, astronomers should have a good idea just how much of a phenomenon the comet could turn into. NASA's Curiosity rover may be able to snap a picture when ISON passes by Mars in September, and it could become visible to the naked eye in October. It's due to come well within a million miles of the sun at perihelion on Nov. 28 — and that will be the most dramatic moment for skywatchers. Some comets, like last year's Comet Elenin, break up when they slingshot around the sun. Others, like Comet Lovejoy, survive the encounter spectacularly. If ISON lucks out, we could well be raving about the Great Christmas Comet of 2013 by this time next year. (Just don't believe anyone who tells you it's a doomsday comet.)

Bonus round: Over at Space.com, Joe Rao's "13 must-see stargazing events for 2013" also include a close conjunction of the moon and Jupiter on Jan. 21, great evening views of Mercury from Feb. 2 to 23, and a holiday show featuring Venus in December. And don't forget the northern lights: Although auroral displays are hard to predict, the height of the sun's 11-year activity cycle should bring some great light shows to Earth's higher latitudes in 2013. 

Update for 8:50 p.m. ET: British educator-astronomer Stuart Atkinson has set up a blog titled "Waiting for ISON" to monitor the comet countdown. Atkinson is also in charge of "The Road to Endeavour" blog about the Opportunity rover on Mars; and The Gale Gazette, which keeps tabs on NASA's Mars Curiosity mission.

More about the coming comets:

• Video: A comet to watch in 2013
• Flash interactive: Inside a comet
• Comet PANSTARRS could be a stunner (or not)
• Comet ISON just might show up in daytime skies

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.