NASA / ESA / J.-Y. Li (Planetary Science Institute), and the Hubble Comet ISON Imaging Science Team

This NASA Hubble Space Telescope image of Comet C/2012 S1 (ISON) was photographed on April 10, when the comet was slightly closer than Jupiter's orbit at a distance of 386 million miles from the sun (394 million miles from Earth).

By Irene Klotz
Discovery News

A small but incredibly bright comet heading toward the sun could do more than dazzle Earth’s skies when it arrives later this year. Scientists say Comet ISON, already shedding dust at the prodigious rate of about 112,000 pounds per minute, could spark an unusual meteor shower.

Computer simulations predicting the location and movement of the comet’s dust trail show Earth will be passing through the fine-grained stream around Jan. 12, 2014.

Some of the particles, which are smaller in diameter than a red blood cell, should be pushed back by the pressure of sunlight, allowing them to be captured by Earth’s gravity when the planet plows through the largely invisible stream.

PHOTOS: Close Encounters with Comets

“As the comet passes Earth’s orbit going into the sun, you’ll have particles trailing behind it. But since it’s passing so close to the sun, you’re also going to have particles pushed away by the pressure of the sunlight. That means we’ll have particles coming outward and also falling inward. We don’t often deal with particles that come both directions,” said Bill Cooke, lead scientist at NASA's Meteoroid Environment Office at the Marshall Space Flight Center in Huntsville, Ala.

The particles are so small that even though they will hit the atmosphere at about 125,000 mph, instead of burning up, triggering so-called “shooting stars,” they will be stopped entirely, predicts astronomer Paul Wiegert, with the University of Western Ontario in Canada.

The only visible and detectable sign of the comet dust might be a proliferation of bright blue clouds at the edge of space. Scientists suspect these so-called noctilucent, or “night-shining” clouds are be seeded by dust in the upper atmosphere.

PHOTOS: Russian Meteor Strike Aftermath

Eventually, the trapped comet dust will make its way -- silently and invisibly -- to the planet’s surface.

Comet ISON, which was discovered in September 2012 by amateur astronomers in Russia, is believed to be making its first swing into the inner solar system, so unlike repeat fliers, it hasn’t laid down a rich dust trail from previous orbits for Earth to fly through.

ISON is an acronym for the telescope the astronomers were using, the International Scientific Optical Network.

ANALYSIS: Awesome Mars-Comet Impact Less Likely

If the comet survives -- and that’s a big if -- the comet will about 700,000 miles above the surface of the sun when it makes its closest approach on Nov. 28. The closest it will come to Earth will be about 40 million miles on Dec. 26.

A comet in the 1970s passed 10 times farther away from the sun than ISON's orbit and partially disintegrated, noted Cooke.

“ISON may very well not survive. I guess we won’t know for sure until we look for it to come out from behind the sun,” Cooke told Discovery News.

Currently the comet is about 280 million miles away from Earth and approaching the outer part of the asteroid belt.

Kim Poor

This illustration of a bright comet over Mars was created by artist Kim Poor.

By Leonard David
Space.com

A close encounter between Mars and Comet C/2013 A1 (Siding Spring) in 2014 is creating both opportunity and anxiety in scientific circles. Scientists are in the early stages of assembling a comet-watching campaign that uses a spacecraft currently orbiting the Red Planet, as well as rovers on the Martian surface.

Scientists are also investigating what techniques could be used to prevent cometary debris from hitting Mars-orbiting spacecraft as the comet and planet converge.

The Mars-bound comet was discovered by Rob McNaught on Jan. 3 at Siding Spring Observatory in Australia. Scientists estimate that this comet arrived from our solar system’s distant Oort cloud and has been on a more than 1-million-year journey. The comet could contain volatile gases that short-period comets often lack due to their frequent returns to the sun’s neighborhood.

Scientists expect the comet’s closest approach to Mars to occur on Oct. 19, 2014, at about 11:45 a.m. PDT.

At that time, the comet will be on the sunward side of Mars. The comet and its tail should be a stunning sight in the predawn Martian sky just before the closest approach, as well as in the post-dusk sky just after the closest approach. [Photos: Amazing Comets of 2013]

NASA / JPL

Comet C/2013 A1 (Siding Spring) will pass extremely close to Mars on Oct. 19, 2014. There is even a small possibility that it could impact the planet, although new tracking data has minimized this prospect.

Will the comet hit Mars?
The close encounter will give scientists the opportunity to make observations, said Richard Zurek, chief scientist for the Jet Propulsion Laboratory's (JPL) Mars Program Office and project scientist for NASA's Mars Reconnaissance Orbiter (MRO).

"Given the present uncertainties of the comet's path and size, it is difficult to do any detailed planning now, but that will change by the end of the year, with continued observations of the comet," Zurek told Space.com. 

New observations of Siding Spring have allowed NASA’s Near-Earth Object (NEO) Office at the JPL to refine the comet’s orbit.

The latest orbital plot places the comet’s closest approach to Mars a little farther out than previously estimated — at about 73,000 miles (117,000 kilometers) from the surface of the Red Planet.

Therefore, the chance that the comet will hit Mars has diminished to about 1 in 8,000, JPL NEO experts said.

Future observations of the comet are expected to refine the orbit  predictions further.

Comet excitement
Zurek said the comet’s close encounter with Mars is an exciting prospect for researchers. For example, MRO’s High Resolution Imaging Science Experiment — or HiRISE camera — should get several pixels across the comet nucleus.

"So, there should be some fascinating science that could result from observations by various instruments at Mars of the nucleus, coma and, of course, the tail," Zurek said.

NASA's  Curiosity and Opportunity are located near Mars'equator, so their best views of the comet from the Martian surface may come when the comet is visible during the predawn hours, low over the southeast horizon.

However, "More information is needed before we can understand what they might see," Zurek said.

Safety measures on Mars
There is a slight possibility that the comet could graze, or even hit, Mars, Zurek said. "If that is not ruled out by future observations of the comet, we will have to start thinking about what precautions we should take," he said.

Those safety measures would include positioning the orbiters so that they are on the other side of Mars at the time of comet impact. 

"We are not worrying about that right now, since the probability is very low and likely to be ruled out in the next few months by continued monitoring of the comet's progress," Zurek said.

Even tiny comet dust particles traveling with a relative velocity of 56 km per second (just over 125,200 mph, or 201,600 km/h) could sandblast Mars-orbiting spacecraft. The satellites could potentially be ordered to turn away or feather their solar panels to mitigate the possible dust impacts. 

Comet ISON campaign: a rehearsal
Siding Spring isn’t the only comet that will approach Mars within the next two years. Comet ISON, discovered in September 2012, will streak through the inner solar system this December and could be one of the brightest comets ever seen. [Photos of Comet ISON]

However, according to Zurek, the distance between Siding Spring and Mars will be 100 times smaller than the distance between Comet ISON and Mars.

"That's close enough that the orbiters — Mars Odyssey, MRO, and the European Space Agency's Mars Express — at least could see structure in the coma, and tail and make estimates perhaps of particle size, etc.," Zurek said. "ISON observations also would help us practice for Siding Spring's much closer passage a year later.”

"We are certainly expecting to observe Siding Spring with HiRISE and other MRO instruments," added Alfred McEwen, director of the Planetary Image Research Lab at the University of Arizona in Tucson and the principal investigator for MRO's HiRISE.

"Potential hazards to the spacecraft and instruments will be analyzed,” McEwen told Space.com. But the probability that this will be a major hazard seems low — at least to me — but the trajectory and comet properties remain poorly known. We have plenty of time to study this and get ready."

Leonard David has been reporting on the space industry for more than five decades. He is former director of research for the National Commission on Space and co-author of Buzz Aldrin's new book, "Mission to Mars – My Vision for Space Exploration," out in May from National Geographic. Follow us @Spacedotcom, Facebook and Google+. Original article on Space.com.

• Photos: Spectacular Comet Views from Earth and Space
• Comet Gives Mars Close Shave, NASA Calculations Show | Video
• Comet of the Century? Sun-Grazing Comet ISON Explained (Infographic)

Copyright 2013 Space.com, a TechMediaNetwork company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

NASA / JPL-Caltech / UMD (Tony Farnham)

The comet that may put on a spectacular light show during a November date with the sun, was observed by the Deep Impact mission. The spacecraft has also had close fly-bys of comet's Tempel 1 and Hartley 2 and scientific observations of Garradd.

By Joe Rao
Space.com

The promising Comet ISON continues on its way in toward a late November rendezvous with the sun, cosmic close encounter that will bring the celestial object to within 800,000 miles (1.2 million km) of the sun's surface.

Many have already christened ISON as the "Comet of the Century," but this may be premature, since the comet’s performance will hinge chiefly on whether it can survive its extremely close approach to the sun on Nov. 28. During that encounter, the comet will approach close to the sun's surface — called the photosphere — while also plunging through its intensely hot corona whose temperature exceeds 1 million degrees Fahrenheit (555,000 degrees Celsius).   

A comet that performs well en route toward the sun — that is, steadily brightens as it comes closer – would seem more likely to survive as opposed to an object that brightens more slowly, or fails to brighten much at all. In the latter case, perhaps the volatile material which boiled off the comet’s core (called the nucleus) and initially made the comet look unusually bright becomes exhausted while the comet is still far out in space.  

The result is a comet that is nothing more than a small and dark solid lump that fails to get very bright at all or perhaps even fragments or disintegrates as it comes to within a hairbreadth of the sun. [Photos of Comet ISON]

NASA / JPL-Caltech

This is the orbital trajectory of comet C/2012 S1 (ISON). The comet was just inside the orbit of Jupiter. In November, ISON will pass less than 1.1 million miles (1.8 million kilometers) from the sun's surface. The fierce heating it experiences during this close approach to the sun could turn the comet into a bright naked-eye object.

Has ISON's brightness has slowed?
This pessimistic scenario might be one that may be ruminating through the minds of some comet observers now concerning Comet ISON.

Astronomers measure the brightness of objects in the night sky on a scale of magnitude on which smaller numbers represent brighter objects, with negative numbers denoting exceptionally bright objects. Since Comet ISON's discovery in September 2012, the comet has brightened only a little, from magnitude +17.3 on Oct. 15 to magnitude +15.5 as of a couple of weeks ago. 

To get an idea of just how faint this is, the comet is currently visible to the eye only under dark, pristine skies and by using telescopes with very large apertures of around 30 inches. Most of the observations of the comet so far have been made through the use of long exposure photographs or image sensors such as a charge-coupled device (CCD).

Still too early to tell
Comet ISON is still very far from the Earth (386 million miles or 621 million kilometers), as well as the sun (403 million miles or 648 million km). The comet was discovered in September 2012 by Russian amateur astronomers Vitali Nevski and Artyom Novichonok using the International Scientific Optical Network. Comet ISON's official designation is C/2012 S1 (ISON).

On March 6, John Bortle, a highly regarded amateur observer who has viewed hundreds of comets spanning over five decades, wrote this assessment of Comet ISON: 

"The much hyped Comet ISON is not evolving in the fashion we had earlier anticipated. Rather than slowly but steadily gaining in brightness it has stagnated at basically near 16th magnitude for a couple of months now. After experiencing an interval where the coma's degree of condensation grew quite strong, the object threw out an unexpected strong but short tail that has persists right down to today. However, following this episode the coma subsequently faded, became less condensed and smaller, all bad signs regarding the 'health' of the comet's nucleus. Whether ISON becomes a Great Comet next fall, or just another in a long string of Great Flops, is now much more a question than ever before."

Space.com will continue to monitor the progress of Comet ISON in the weeks and months to come.  We plan to post another update in late April.   

Meanwhile, another comet is currently wowing stargazers with an unexpectedly long-lived display.

The Comet Pan-STARRS has dominated the attention of comet observers in the Northern Hemisphere throughout March and still remains an exciting target for stargazers. The comet made its closest approach to the sun on March 10, and became visible to the unaided eye for some stargazers soon afterward. It was appearing low on the western horizon just after sunset, which made the comet hard to spot for some due to the bright twilight.

You find out how to see Comet Pan-STARRS here.

Editor's note: If you have an amazing picture of Comet ISON, Pan-STARRS or any other night sky view that you'd like to share for a possible story or image gallery, send photos, comments and your name and location to Managing Editor Tariq Malik at spacephotos@space.com.

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, N.Y. Follow us @Spacedotcom, Facebook or Google+. Originally published on SPACE.com.

• Amazing Comet Photos of 2013 by Stargazers
• Will Comet ISON be Comet of the Century? | Video
• Comets: Formation, Discovery and Exploration

NASA / JPL-Caltech / UMD (Tony Farnham)

The comet that may put on a spectacular light show during a November date with the sun was observed by the Deep Impact mission. The spacecraft has also had close fly-bys of comet's Tempel 1 and Hartley 2 and scientific observations of Garradd.

By Mike Wall
Space.com

Astronomers are already getting set for the arrival of Comet ISON, which may become one of the brightest comets ever seen when it cruises through the inner solar system this fall.

NASA has brought together a small team of experts to organize an observing campaign for Comet ISON, which could potentially shine as brightly as the moon when it makes its closest pass by the sun in late November if the most optimistic scenarios play out.

Coordinating the efforts of observatories on the ground and in space should help wring as much quality science as possible out of the comet's solar flyby, Comet ISON Observing Campaign (CIOC) officials said.

"It's a rare opportunity that we've got such a long heads-up time, so we actually have time to organize a campaign like this," said Karl Battams, a scientist at the U.S. Naval Research Lab in Washington and a member of the eight-person CIOC Team. "There's a lot of new science that we could get from this." [Photos of Comet ISON in Night Sky]

Bringing everyone together
Russian amateur astronomers Vitali Nevski and Artyom Novichonok discovered Comet ISON in September 2012 after poring over photographs taken by a telescope run by the International Scientific Optical Network. Hence the comet's official name, which is C/2012 S1 (ISON).

NASA / JPL-Caltech

This is the orbital trajectory of comet C/2012 S1 (ISON). The comet is currently located just inside the orbit of Jupiter. In November 2013, ISON will pass less than 1.1 million miles (1.8 million kilometers) from the sun's surface. The fierce heating it experiences during this close approach to the sun could turn the comet into a bright naked-eye object.

ISON is a sungrazing comet, meaning its long, looping path through space takes it extremely close to our star. Indeed, ISON is forecast to zoom just 680,000 miles (1.1 million kilometers) above the solar surface on its closest approach, which will take place on Nov. 28.

Comet ISON could put on a spectacular show around this time, experts say, potentially glowing so brightly that it's visible in the daytime sky. (The comet poses no impact threat to Earth.)

Skywatchers aren't the only people looking forward to the icy wanderer's solar encounter. Astronomers are excited as well, for ISON's flyby could give them a rare window into comet composition.

"Sungrazers experience the most intense thermal and gravitational stresses of any comet. There's a lot of sublimation of material that doesn't normally sublimate," Battams told Space.com. "Once (ISON) gets really close in to the sun, then we could start to see some of the composition that you wouldn't normally get."

So Battams and his colleagues have contacted major ground-based observatories, raising awareness of the comet's flyby and encouraging them to solicit ISON-observing proposals. The response has been warm, with telescope operators typically voicing enthusiasm about the campaign, Battams said.

The CIOC crew has also reached out to some spacecraft mission teams, asking them to consider taking a look at the comet at some point. And a number of them are already on board.

"Observing campaigns are planned by the SOHO, STEREO and SDO solar missions; by Spitzer, Chandra and Hubble space telescopes; and by the Deep Impact, JUNO, Mercury MESSENGER, Mars Odyssey and Mars Reconnaissance Orbiter missions," the CIOC website states. "Other missions at or on Mars are looking into observing ISON, as are a handful of other NASA Planetary missions. We welcome and encourage our international partners to contact us and join in the fun!"

In fact, Deep Impact has already begun its ISON work. The NASA probe, which has also studied several other comets in deep space, snapped its first photos of Comet ISON in January.

Sizzle or fizzle?
There's no guarantee that ISON will live up to the hype; it may fall apart before even making its closest solar approach. Comets are notoriously unpredictable and sometimes fizzle out despite great expectations, as Comet Kahoutek did in 1973.

The wild-card factor with ISON is especially high, Battams said, since this is apparently the comet's first trip through the inner solar system from the distant, icy Oort cloud. Astronomers and skywatchers probably won't get a real sense of how the comet is going to behave until early August, when water ice should start sublimating in earnest, he added.

But the CIOC Team is proceeding as if Comet ISON will put on a dazzling show, because that's the scientifically prudent thing to do.

"We have to go forward with optimism and prepare for something really special, something really spectacular, and hope that it happens," Battams said. "If it does, we're absolutely going to be in place to take an unprecedented and comprehensive set of observations of the comet."

Follow Mike Wall on Twitter @michaeldwall. Follow us @Spacedotcom, Facebook or Google+. Originally published on Space.com.

• Comet of the Century? Sun-Grazing Comet ISON Explained (Infographic)
• Amazing Comet Photos of 2013 by Stargazers
• Comets: Frozen Seeds Of Life From Beyond The Solar System | Video

Copyright 2013 Space.com, a TechMediaNetwork company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

NASA

This still from a NASA video identifies comet C/2012 S1 (ISON), better known as Comet ISON, in a telescope image.

By SPACE.com

The newly discovered Comet ISON is just a faint speck in the sky right now, but later this year, when it flies closer to the sun, it could shine as bright as the moon and may even be visible in broad daylight, scientists say.

"Comet ISON is a sungrazer," comet-tracker Karl Battams of the U.S. Naval Research Lab said in a NASA statement. "The orbit of the comet will bring it very close to the sun, which we know can be a spectacular thing."

That apparition, if Comet ISON delivers on its promise, is expected to peak in late November. While some astronomers a have touted ISON as a potential "Comet of the Century," there is still some uncertainty on how bright the object will actually become, according to scientists with NASA and other observatories.

Officially known as C/2012 S1 (ISON), Comet ISON is currently near the orbit of Jupiter, though astronomers say it is already quite bright for an object so far away. Its exceptional glow suggests that it has a fairly large nucleus in the 0.6- to 6-mile (1- to 10-kilometer) range, estimated astronomer Matthew Knight of the Lowell Observatory, according to NASA. [Photos of Comet ISON in Night Sky]

Russian astronomers Vitali Nevski and Artyom Novichonok discovered the comet in September 2012 using a 15.7-inch (0.4-meter) reflecting telescope of the International Scientific Optical Network (ISON), near Kislovodsk. They predicted that at its closest point to the sun, which occurs on Nov. 28, 2013, the comet will come within 800,000 miles (1.2 million km) of the sun's surface and could transform into a dazzling night sky object with a long, dusty tail.

But that's only if the Comet ISON manages to survive its fiery flyby — it could be destroyed by tidal forces or solar radiation as it approaches the sun, scientists say.

"I'm old enough to remember the last 'Comet of the Century,'" said Don Yeomans of NASA Near-Earth Object Program. Yeomans was referring to a distant comet named Kohoutek that promised to put on such a show in 1973, but turned out to be a bust. "It fizzled. Comets are notoriously unpredictable."

Knight, however, noted that an object smaller than C/2012 S1, Comet Lovejoy, put on a stunning display after it soared through the sun's atmosphere in 2011 and emerged intact on the other side.

"Comet ISON is probably at least twice as big as Comet Lovejoy and will pass a bit farther from the sun's surface" Knight said. "This would seem to favor Comet ISON surviving and ultimately putting on a good show."

Another possibility, astronomers said, is that Comet ISON could break apart, which could give it an appearance akin to a cosmic string of pearls when viewed through a telescope. Yeomans assured that such a scenario would not pose a threat to Earth.

"Comet ISON is not on a collision course," he said. "If it breaks up, the fragments would continue along the same safe trajectory as the original comet."

Editor's note: If you have an amazing picture of Comet ISON or any other night sky view that you'd like to share for a possible story or image gallery, send photos, comments and your name and location to managing editor Tariq Malik at spacephotos@space.com.

Follow SPACE.com on Twitter@Spacedotcom. We're also onFacebookandGoogle+.

• Photos: Spectacular Comet Views from Earth and Space
• Astronaut's Spectacular View of Comet Lovejoy From Space
• Comets: Formation, Discovery and Exploration

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