Age of Comets Article, Orbit Information, Solar System Facts -- National Geographic No telescope was needed to see comet Hyakutake above the Palomar Observatory in San Diego County, California, as it came within 9.3 million miles (15 million kilometers) of Earth in 1996. Photograph by Don Bartletti Written by William Newcott Republished from the pages of National Geographic magazine Whistling and moaning, a 50-mile-an-hour (80-kilometer-an-hour) wind whipped among the telescope domes atop Kitt Peak. Just a few feet below, turning gray in the dusk, slid a river of clouds that had been rising and dropping all day. And high above, comet Hale-Bopp hung suspended like a feathery fishing lure, its tail curving off a bit, as if blown to the side by the punishing wind. One by one, stars winked on in a darkening sky. In each of the telescope domes, teams of astronomers prayed that the wind would drop below 40 miles per hour (64 kilometers an hour), the point at which they'd be able to open the sliding doors and get back to work. The sky turned indigo. Then black.

Viewed from the summit, 6,873 feet (2,095 meters) above Arizona's Sonoran Desert, Hale-Bopp's bright dust tail, along with a dimmer, all but transparent blue one, seemed to grow by degrees. Among the brightest comets ever seen, Hale-Bopp had been visible for months from midtown Manhattan, of all places. But here, on a moonless night in the mountains in the desert, the length of Hale-Bopp's tail became visible—a wispy, delicate veil. Along with eclipses, comets have been the most feared and admired sky spectacles of all. But while astronomers have been able to predict eclipses for thousands of years, only in the 1700s was a comet's return correctly predicted, by Edmond Halley. Some comets swing around the sun every few years. Others, like Hale-Bopp, may take thousands of years. Most can be seen only with a telescope. But every once in a while—a few times a century, perhaps—an impressive one is visible to the naked eye. And in the past two years the world has witnessed not one but two of them.

Hyakutake in 1996 had one of the longest tails on record, stretching more than halfway across the sky; Hale-Bopp in 1997 had one of the most brilliant heads, nearly as bright as the star Sirius. Add the Jupiter crash of comet Shoemaker-Levy in 1994, Halley's most recent visit in 1986, vivid comet West in 1976, and the scientifically signifiant—if visually disappointing—Kohoutek in 1973-74, and you could say that we are indeed living in the age of comets. Hovering in the most fragile of gravitational balances, a fleet of dirty, lumpy snowballs numbering in the trillions is barely held in orbit by the pull of the sun. They are stored in the Oort cloud, a huge, diffuse sphere of cometary nuclei in the far reaches of the solar system. Close to the sun, yet still beyond Neptune, circle what may well be their brethren, in a great disk called the Kuiper belt. Comets are leftovers, scraps of material that didn't make it to planethood in the events creating our solar system.

Once, many astronomers believe, the solar system was full of comet nuclei, chunks of ice and dust left over from the formation of the sun. Most clumped together to form planets, leaving a relative handful—averaging perhaps a few miles wide, with temperatures as low as minus 400 degrees Fahrenheit (minus 240 degrees Celsius)—as time capsules of the early solar system. They orbit in a perpetual deep freeze until some subtle gravitational nudge upsets the delicate balance. Then the great fall begins. Imperceptibly at first, a snowball drifts toward the sun and steadily accelerates. As solar radiation heats the comet, the ice within sublimates, escaping as gas from vents at the surface. Sometimes jets of sublimating ice whirl off the rotating comet nucleus like a fireworks pinwheel. Dust trapped in the ice breaks free. Pushed back by the pressure of the sun's radiation, the dust streams out behind the comet in what appears as a fiery tail. Now the comet is among the fastest things in the solar system. It whizzes past the inner planets at around 100,000 miles an hour (160,000 kilometers an hour).

The tail can stretch a hundred million miles (160 million kilometers). A quick whip around the sun, a long journey back into night, and the show's over. Until gravity calls it back. With each visit to the sun the comet loses more of its ice. Eventually it may become a rocky ghost, its glory days gone for good. Kitt Peak's solar telescope is bored right into the mountain, with mirrors on top that can be oriented to follow the sun's path across the sky. Solar studies are one of astronomy's few day jobs, and in 1973 Frank Scherb and Fred Roesler, physicists at the University of Wisconsin, got the bright idea of using the telescope at night to track comet Kohoutek. Scherb has been back for every major comet since then, except for Hyakutake in 1996. "That one just came up too fast," he said, showing me through the cramped corner of the telescope facility that has been made available to him. A few computers whirred in a row, their terminals displaying Hale-Bopp images taken the night before. He punched up a series taken a few minutes apart through filters that brought out various parts of the light spectrum.

With each image, the tail changed shape a bit, indicating the presence of different molecules. "The most important new result we've got here is that we're detecting an unusual kind of atomic carbon," Scherb told me. "It's being thrown off by the many types of carbon compounds in the comet." Besides the bountiful hydrogen and oxygen found in cometary ice, astronomers knew that comets contained a lot of carbon monoxide. But radio, infrared, and ultraviolet measurements have revealed a whole zoo of carbon compounds in comets, some of which are essential building blocks in all known life-forms. Two dozen carbon molecules have turned up in Hale-Bopp, along with trace amounts of nitrogen, sodium, and sulfur. The growing list of compounds found in Hale-Bopp supports the theory that the ingredients for life were delivered to Earth by comets. Scherb's carbon atom measurements will help determine how much of each compound is present in the average comet and whether or not there is enough to keep the theory alive. I asked Scherb about Hale-Bopp's transparent blue tail, which gave the comet the appearance of a celestial bird-of-paradise flower. "That's the ion tail," he explained.

"As water sublimates off the comet, the sun's radiation strips an electron from the water molecules, creating charged particles. Those ions get picked up by the solar wind, and they drive back to form the ion tail." Ron Oliversen, a NASA scientist working with Scherb, hopes to use the ion tails of comets to get the "weather reports" from the distant solar system. "Ion tails are a kid of wind sock for the solar win," he said. "If we can follow comets far enough, we can get some idea of what the solar wind is doing out there." Alan Hale was on the phone. Again. Giving driving directions so that a TV crew could find his house. Again. The co-discoverer of comet Hale-Bopp was riding the wave of media frenzy on this, the day his comet would make its closest approach to the sun. His four-year-old son, Tyler, had greeted me at the door of their home and then ushered me into the cluttered living room. As dad intoned some well-worn answers to an interviewer, Tyler proudly showed me his Thomas the Tank Engine toy.

"My daddy," he confided, "is a star guy." Much has been made of the fact that Hale found the comet while looking through a telescope in his driveway. Few accounts note that to reach his driveway, you must follow 9 miles (14.5 kilometers) of unpaved road in the Sacramento mountains of southern New Mexico. Far from city lights, it's the perfect place for comet hunting. The sky was absolutely clear the night Hale made history. It was just after midnight on July 23, 1995. "Sagittarius was high in the south. I was looking at a star cluster there, and I noticed a fuzzy object. I checked a star atlas, and it showed nothing there. So I knew it was a possible comet." As a trained astronomer who has seen some 200 comets, Hale knew exactly what to do. He headed inside and sent an e-mail to the Central Bureau for Astronomical Telegrams in Cambridge, Massachusetts, to register his findings. A few hours later he got a response: His new comet was officially designated C/1995 o1. His name would also be attached. But he was not alone.

That same night, about 400 miles (644 kilometers) away, Tom Bopp was out with some pals looking through telescopes in the desert outside Phoenix, Arizona. He, too, noticed something fuzzy in Sagittarius and pointed it out to the friend who owned the telescope he was using. "You might have something there, Tom," the friend said. He was right. Bopp, a manager at a construction materials factory, had never seen a comet. He missed Kohoutek. He even managed not to see Halley. "I knew I had to contact the Central Bureau, but I didn't have the address with me," Bopp told me. "I had to drive all the way back home and get it." In the wee hours he roused Western Union and fired off a telegram to Cambridge—where its arrival was greeted with the same bemusement that might accompany a horse and buggy at a Porsche service garage. "Nobody sends telegrams anymore," laughed Brian Marsden, who has run the bureau since 1968.

"I mean, by the time that telegram got here, Alan Hale had already e-mailed us three times with updated coordinates." A jovial Brit with a shock of gray hair, Marsden is the Man Who Names the Comets, the leading voice on a committee that has the final say. "These things can get quite contentious," he added. "Some people try to cheat. They'll see a comet and call a friend, who will then also report it. Usually we can weed them out." Marsden's brow furrowed—the first time in a half hour the smile left his face. "But I'll tell you, there's at least one comet where I'm sure one of the names was faked." Marsden was seething. Tom Bopp and I were sitting at a restaurant near his home in Glendale, a Phoenix suburb. Neither of us thought of it at the time, but the restaurant was part of the same chain where, one week before, 39 members of a California cult had reportedly eaten their last meal before ritually committing mass suicide. They claimed they were departing on a spaceship that was trailing comet Hale-Bopp. Weird stuff seems to go with major comets. When a comet appeared in A.D.

60, the people of Rome assumed it meant the impending death of their still new emperor, Nero. He responded by exiling a potential rival. When another comet turned up just four years later, ancient historians say he ordered the execution of hundreds of nobles. It is said that Moctezuma II saw a comet in 1517 that foreshadowed the downfall of the Aztec empire. In 1910, a wave of hysteria swept over the United States amid reports that Earth was about to pass through Halley's tail. For Tom Bopp, the comet did indeed portend heartbreak. Three days before we met, as his comet reached its most spectacular point, his brother and sister-in-law were killed in a late night car wreck. They had been out photographing the comet. "This," he told me quietly, "has been the best week of my life. And the worst." David Levy's alarm goes off at 3 a.m. Trying not to disturb his wife, he leaves the house and trudges to a homemade observatory behind his home in Vail, Arizona, near Tucson.

Levy and his friends Gene and Carolyn Shoemaker composed a remarkably prolific comet-hunting group before Gene was killed in a car accident in Australia last July. Working individually and together, they discovered 40 comets. "On average a dozen comets are found each year," said Levy. "It is unpredictable." Even a comet's periodic return can vary. Hale-Bopp had a 4,200-year period, but thanks to a gravity assist from Jupiter it will be back in 2,4000 years. Sometimes a comet such as Shoemaker Levy 9 will go into orbit around Jupiter and act as a moon. Others leave Jupiter's orbit and go back around the sun. Comets are like cats; they both have tails, and they both do precisely what they want to do." One thing comets do is crash into things. Imagine yourself atop the Empire State Building with the world's most powerful bowling-ball cannon, capable of reaching the general vicinity of Washington, D.C. You are randomly firing bowling balls as a lone Volkswagen bug circles the Washington Beltway. It's not likely that you will hit that Beetle anytime soon, but one of these days, one of these millennia, you will inevitably crush one vintage compact car.

The last bowling ball to devastate earth—either a comet or an asteroid—hit about 65 million years ago. It blasted a crater more than a hundred miles (160 kilometers) wide under today's Yucatan peninsula and threw up a global cloud of dust and gas. The sun was all but blocked for years, dropping average temperatures to near freezing and killing off more than half of the plant and animal species on Earth. The dinosaurs died. On the other hand, many mammal species survived. "If you want some idea of the devastation that's possible from a comet," said David Levy, "take a look at what just one of the fragments of Shoemaker-Levy 9 did to Jupiter. Here's something as long as two or three football fields, and it left a mark bigger than Earth." In May of 1983 the comet IRAS-Araki-Alcock got within 3 million miles (4.8 million kilometers) of Earth. At its closest Hale-Bopp was 122 million miles (196 million kilometers) away.

Had Hale-Bopp arrived just four months earlier, it would have come within 11 million miles (17.7 million kilometers) of Earth, shone 50 times brighter—and been visible in daytime for about a week. Comets can bring more than destruction when they slam into a planet. There's evidence that a good storm of comets in a planet's formative stages could provide an ocean or two. We may well be swimming in comet melt-off. "It could take as few as five Hale-Bopps to fill the Great Lakes," says astronomer Michael U. Mumma of NASA's Goddard Space Flight Center. One of the most popular theories for the origin of Earth's oceans has been that the water was delivered via comets. The theory has problems though. Studies of Hale-Bopp—and of Hyakutake and Halley before it—show that cometary water contains twice as much deuterium as seawater. If the oceans came primarily from comets, the proportion ought to be closer. While most scientists were envisioning such big splash models, one man was thinking about a cosmic drizzle.

In 1986 Louis Frank, a physicist from the University of Iowa, suggested that each year Earth is bombarded by ten million house-size mini-comets weighting up to 40 tons (36 metric tons) each. Vaporizing high above Earth's atmosphere, these cometlike objects would deliver, in 20,000 years of so, enough moisture to cover the planet with an inch (2.5 centimeters) of water. Frank's credentials are impeccable. Since 1958 he has worked on scientific payloads for 40 U.S. spacecraft. But his theory was soundly rejected by astronomers. Then in September 1996 NASA's Polar satellite, using cameras and filters designed by Frank, took images of what he says are cosmic snowballs evaporating in long, cloudlike streaks. "These snowballs are cometlike, but they are just about all water. No dirt," said Frank. "They're very, very fragile. And between Earth and Jupiter there are probably enough of them to fill all of our oceans with water." After a brief period of uncertainty Frank's critics still don't buy it.

"Right now there are a half-dozen papers either published or in preparation that disagree with Lou's interpretation," said NASA's Mumma. "For one thing, we don't know how far those streaks are from the camera. Some suggest they might be ice flakes coming from the craft itself. This has been a raging controversy for years, and Lou's images have not made it go away." As Hale-Bopp passed the sun and headed back toward the deep-space icebox, the biggest news seemed to be the discovery, with the Isaac Newton Group of telescopes in the Canary Islands, of a third tail. Visible only with a sodium light filter, the tail was 400,000 miles (644,000 kilometers) wide, 30 million miles (48 million kilometers) long, and made of sodium atoms. David Levy remains more enamored of a 1986 comet Halley finding that bore both scientific and philosophical significance. "Halley showed carbon, hydrogen, oxygen, and nitrogen in similar proportions as found in the human body," he said.

"I think it just goes to show that we are, as Gene Shoemaker used to say, the progeny of comets." The winds had died down when I left Kitt Peak an hour after sunset, and Hale-Bopp was already low in the sky. I drove down the winding road and pulled over at the mountain's base. The starts shone like, well, like only stars can shine. To the west, above where the sun had set, a dim, misty light climbed into the darkness. It was the zodiacal light, the reflection of sunlight off the disk of dust that floats between the planets—the scattered remains of comet tails, slowly drifting toward incineration by the sun. The sound of crunching rocks startled me. A few feet away three wild horses, black on black against the nightscape, wandered past. They never glanced skyward at the gossamer swath of Hale-Bopp nor at the wondrous spectacle that is the night sky on a clear night, comet or no. It felt good to be human.

28 The sky, for the most part, is predictable; the Sun rises and sets, the Moon goes through its phases, and the constellations appear pretty much the same on the day that you are born as the day you die. The appearance of a great comet, however, offers a wonderful exception; though many are on periodic orbits, many others enter our inner solar system unannounced, putting on dazzling displays. A bright comet may even be visible in the daytime. The recent discovery of comet C/2012 S1 (ISON) has raised some speculation that we may just be in for such an event in late 2013, with some scientists predicting that comet could be as bright as the full Moon. Here’s a compilation of 10 remarkable comets from the past few hundred years. 10. Comet Lovejoy NASA astronaut Dan Burbank captured this image of Comet Lovejoy visible near Earth’s horizon on Dec. 22, 2011. In November 2011, Australian observer Terry Lovejoy discovered a faint +13th magnitude object. Comet C/2011 W3 Lovejoy was a Kreutz sun grazer, one of a handful seen each year.

It surprised astronomers, however, when it survived its fiery plunge just 87,000 miles above the Sun’s surface (about a third the distance between the Earth and Moon) to become a fine morning object for Southern Hemisphere observers, similar to Ikeya-Seki in the 1960s. Comet Lovejoy was visible in the daytime, and astronauts even photographed it from the International Space Station. (A quick primer: the brightness of an astronomical object is described as its magnitude. The lower the number, the brighter the object, and one step in magnitude is equal to 2.512 times in brightness.) 9. Comet McNaught Comet McNaught, over Wellington, New Zealand; Chris Wall The first bright comet of the 21st century, Comet C/2006 P1 McNaught reached a brilliance of several times that of Venus at magnitude -5.5 and was termed the Great Comet of 2007. One of the brightest comets in the past century, McNaught sported a brilliant fan-shaped tail and was seen by millions worldwide.

If you missed it on the first pass, you’re out of luck — McNaught is on an extremely long period orbit, and won’t return to the inner solar system for another 92,600 years. 8. Comets Hale-Bopp and Hyakutake Comet Hale-Bopp; Philipp Salzgeber The 1990s were a great decade for comets. In 1994, the astronomical world watched in awe as fragments of Comet ShoemakerLevy-9 impacted Jupiter, causing tremendous damage to the giant planet. Then in 1995, observers Alan Hale and Thomas Bopp independently discovered what was to become one of the most observed comets of the 20th century. What immediately excited astronomers was its relatively large discovery distance; beyond the orbit of Jupiter, Comet C/1995 O1 Hale-Bopp had to be an intrinsically large and bright object. Like Halley’s Comet in 1910, Hale-Bopp was “scooped” during its anticipated 1997 passage by the sudden appearance of Comet C/1996 B2 Hyakutake in early 1996.

Both served as fine studies in the appearance of bright comets; one small and close up, and one large and bright, but farther away. Hale-Bopp displayed a fine twin ion and dust tail; Hyakutake passed just over 9 million miles from Earth and sported a tail that stretched halfway to the zenith. Both became circumpolar objects in the night sky and were above the horizon for observers based in mid-to-high northern latitudes for the entire night. 7. Lexell’s Comet Comet IRAS-Araki-Alcock, viewed in infrared light; NASA Infrared Processing and Analysis Center In 1770, Comet D/1770 L1 Lexell made the closest recorded passage by a comet to our planet of all time, passing only 1.4 million miles from Earth. This is less than six times the Earth-Moon distance, and the comet sported a coma over four times the size of a full Moon as seen from Earth. The closest approach to Earth by a comet in the 20th century was Comet C/1983 H1 IRAS-Araki-Alcock, which passed 2.9 million miles from us in 1983. 6.

The Eclipse Comet of 1948 Comet McNaughton photographed from Cape Town, South Africa; Steve Crane A surprise greeted observers during the total solar eclipse of Nov. 1, 1948, when a bright comet was sighted next to the Sun. Formally titled C/1948 V1, the Eclipse Comet of 1948 was one of the last to be discovered in this accidental fashion. The eclipse comet was briefly visible to the naked eye for the remainder of the year. 5. The Great January Comet of 1910 Many who anticipated the passage of Halley’s Comet actually saw this comet (officially designated C/1910 A1) several months earlier. First noticed by South African diamond miners in the early morning of Jan. 12, 1910, this comet went on to make a fine display for Northern Hemisphere observers, and boost the “Halley hype” in the minds of many, including the media. 4. The Great March Comet of 1843 Painting of the Great Comet of 1843.

The 19th century was a great era for comets, and one of the most memorable was comet C/1843 D1, dubbed the Great March Comet. Another Kreutz group sun grazer, the Great March Comet displayed a tremendous fan-shaped tail over two astronomical units in length (1 AU equals the distance between the Earth and Sun). By the way, the dust tails of comets are swept back via pressure from the solar wind and actually precede comets as they head back out of the solar system. 3. The Great September Comet of 1882 Great Comet of 1882 photographed from South Africa. A bright daytime comet, Comet C/1882 R1 was first spotted by several witnesses in the Southern Hemisphere and went on to become one of the great comets of the 19th century. The comet was also notable for producing a bright anti-tail, a sharp, spikey-looking feature that appears to be pointing sunward. Interestingly, this comet transited the Sun on Sept. 17, 1882, and astronomer W.H. Finlay famously tracked the comet almost right up to the solar disk — a feat we don’t recommend trying to duplicate. 2.

The Great Comet of 1680 Painting of the Great Comet of 1680 over the Netherlands. One of the brightest comets of the 17th century, the Great Comet of 1680 is also sometimes referred to as Kirch’s Comet. The subject of many paintings of the era, Isaac Newton used the orbit of the comet as a study in the verification of Kepler’s laws of motion. There is some thought that the aforementioned Comet ISON, which is set to visit the inner solar system in late 2013, may be related to the Great Comet of 1680. 1. Halley’s Comet The nucleus of Halley’s Comet, photographed in 1986 by European spacecraft Giotto; Halley Multicolor Camera Team, Giotto Project, ESA The best known of all periodic comets, Halley’s Comet visits the inner solar system every 75-76 years. The comet was the first to have its orbit calculated by Sir Edmond Halley, who predicted the return of the great comet in 1759.

Famously, American author Mark Twain was born in 1835 during the passage of Halley’s Comet and predicted he would “expect to go out with it,” which he promptly did in 1910. A fleet of international space probes visited the comet in 1986, and it will next grace Earth’s skies in 2061. One More: Comet Kohoutek Lubos Kohoutek briefs the news media on the comet bearing his name, 1973; University of Arizona Comets can be fickle beasts, often defying predictions and failing to perform. A fine cautionary tale is that of C/1973 E1 Comet Kohoutek, discovered in 1973. Astronomers projected it would become the “Comet of the Century” and the media trumpeted those claims. Kohoutek partially disintegrated and failed to live up to expectations. Ironically, the experience from Kohoutek caused the media to largely ignore a fine comet less than two years later, Comet C/1975 V1 West. Written by Dave Dickinson David Dickinson is a backyard astronomer, science educator and retired military veteran.

He lives in Hudson, Fla., with his wife, Myscha, and their dog, Maggie. He blogs about astronomy, science and science fiction at Share: Stunning Comet's Size Shocks Scientists Stunning Comet's Size Shocks Scientists By SPACE.com Staff | April 12, 2010 07:03pm ET MORE Comet McNaught over the Pacific Ocean. Image taken from Paranal Observatory in January 2007. Credit: S. Deiries/ESO. Comet McNaught, the so-called Great Comet of 2007, has been identified as the biggest comet measured to date, according to scientists, whose calculations were based on the comet's overall influence in space. Instead of using the length of the comet's tail to measure the scale of the comet, astronomers used data from the ESA/NASA Ulysses spacecraft to determine the size of the region of space disturbed by the comet's presence ? a cosmic wake across the solar system. Through analysis of magnetometer data, scientists found evidence of a decayed shockwave surrounding the comet, which was created when ionized gas emitted from the comet's nucleus joined the fast-flowing particles of the solar wind.

That, in turn, caused the solar wind around the comet to abruptly slow down. Great Comet McNaught Comet C/2006 P1 McNaught was discovered by astronomer Robert H. McNaught in August 2006. In January and February 2007, Comet McNaught became the brightest comet visible from Earth for the past 40 years. [Photos of Comet McNaught.] Serendipitously, the robotic Ulysses spacecraft made an unexpected crossing of Comet McNaught's tail in 2007, which allowed astronomers to collect valuable and pertinent data. Ulysses encountered McNaught's tail of ionized gas downstream of the comet's nucleus, at a distance of more than 1.5 times the distance between the Earth and the sun. That?s about 139 million miles (224 million km). The average distance between the Earth and sun, called 1 astronomical unit, is about 93 million miles (150 million km). This was far longer than the spectacular dust tail that was visible from Earth in 2007.

Ulysses' 2007 encounter with McNaught was one of three unplanned encounters that Ulysses made with comet tails during its 19-year mission, which ended in 2009. The other encounters included Comet Hyakutake in 1996. Hyakutake is the current record-holder for the comet with the longest measured tail. Measuring a comet But the data collected from Ulysses' encounter with McNaught allowed scientists to gauge the size of that comet. "It was very difficult to observe Comet McNaught's plasma tail remotely in comparison with the bright dust tail, so we can't really estimate how long it might be," said Dr. Geraint Jones of University College London's Mullard Space Science Laboratory, who will present the findings at the Royal Astronomical Society National Astronomy Meeting in Glasgow, Scotland, on April 13. "What we can say is that Ulysses took just 2.5 days to traverse the shocked solar wind surrounding Comet Hyakutake, compared to an incredible 18 days in shocked wind surrounding Comet McNaught," Jones said.

"This shows that the comet was not only spectacular from the ground, it was a truly immense obstacle to the solar wind." Scientists also compared the crossing times for other comet encounters, which further revealed the huge scale of Comet McNaught. The Giotto spacecraft's encounter with Comet Grigg-Skjellerup in 1992, for example, took less than an hour from one shock crossing to another. Similarly, it took only a few hours to cross the shocked region of the famed Halley's Comet. In 2007, the nucleus of Comet McNaught was estimated to be less than 15 miles (25 km), but Jones used a different measure to calculate the comet's scale. "The scale of an active comet depends on the level of outgassing rather than the size of the nucleus," Jones explained.

"Comet nuclei aren't necessarily active over their entire surfaces; what we can say is that McNaught's level of gas production was clearly much higher than that of Hyakutake." The Stunning Comet McNaught: Part 1, Part 2 The Best Comets of All Time NASA's Stardust Journey to a Comet Subscribe To SPACE.com Comets: Facts About The 'Dirty Snowballs' of Space Comets: Facts About The 'Dirty Snowballs' of Space By Charles Q. Choi, Space.com Contributor | November 15, 2014 12:46am ET MORE Halley's Comet as photographed May 8, 1910, by Dr. G.W. Ritchey using the 60-inch (1.5-meter) telescope at Mount Wilson Observatory, Calif., during the comet's last appearance. The head of the comet and the beginning of its long tail are shown. Short, straight streaks are background stars. Credit: NASA/JPL A comet is an icy body that releases gas or dust. They are often compared to dirty snowballs, though recent research has led some scientists to call them snowy dirtballs.

Comets contain dust, ice, carbon dioxide, ammonia, methane and more. Astronomers think comets are leftovers from the gas, dust, ice and rocks that initially formed the solar system about 4.6 billion years ago. Some researchers think comets might have originally brought some of the water and organic molecules to Earth that now make up life here. To research this hypothesis, the Rosetta mission , which landed a probe on a comet on Nov. 12, 2014, is studying its nucleus and environment, observing how it changes as it approaches the sun. Comets orbit the sun, but most are believed to inhabit in an area known as the Oort Cloud , far beyond the orbit of Pluto. Occasionally a comet streaks through the inner solar system; some do so regularly, some only once every few centuries. Many people have never seen a comet, but those who have won't easily forget the celestial show. Physical characteristics The solid nucleus or core of a comet consists mostly of ice and dust coated with dark organic material , according to NASA, with the ice composed mainly of frozen water but perhaps other frozen substances as well, such as ammonia, carbon dioxide, carbon monoxide and methane.

The nucleus may have a small rocky core. As a comet gets closer to the sun, the ice on the surface of the nucleus begins turning into gas, forming a cloud known as the coma. Radiation from the sun pushes dust particles away from the coma, forming a dust tail, while charged particles from the sun convert some of the comet's gases into ions, forming an ion tail. Since comet tails are shaped by sunlight and the solar wind, they always point away from the sun. At first glance, comets and asteroids may appear very similar. The difference lies in the presence of the coma and tail. Sometimes, a comet may be misidentified as an asteroid before follow-up observations reveal the presence of either or both of these features. The nuclei of most comets are thought to measure 10 miles (16 kilometers) or less. Some comets have comas that can reach nearly 1 million miles (1.6 million km) wide, and some have tails reaching 100 million miles (160 million km) long. We can see a number of comets with the naked eye when they pass close to the sun because their comas and tails reflect sunlight or even glow because of energy they absorb from the sun.

However, most comets are too small or too faint to be seen without a telescope. Comets leave a trail of debris behind them that can lead to meteor showers on Earth. For instance, the Perseid meteor shower occurs every year between August 9 and 13 when Earth passes through the orbit of the Swift-Tuttle comet. Orbital characteristics Astronomers classify comets based on the durations of their orbits around the sun. Short-period comets need roughly 200 years or less to complete one orbit, long-period comets take more than 200 years, and single-apparition comets are not bound to the sun , on orbits that take them out of the solar system, according to NASA. Recently, scientists have also discovered comets in the main asteroid belt — these main-belt comets might be a key source of water for the inner terrestrial planets. Scientists think short-period comets, also known as periodic comets , originate from a disk-shaped band of icy objects known as the Kuiper Belt beyond Neptune's orbit, with gravitational interactions with the outer planets dragging these bodies inward, where they become active comets.

Long-period comets are thought to come from the nearly spherical Oort Cloud even further out, which get slung inward by the gravitational pull of passing stars. Some comets, called sun-grazers , smash right into the sun or get so close that they break up and evaporate . Comet McNaught C/2009 R1 was visible on June 6, 2010. Credit: Michael Jäger Naming In general, comets are named after their discoverer . For example, comet Shoemaker-Levy 9 got its name because it was the ninth short-periodic comet discovered by Eugene and Carolyn Shoemaker and David Levy. Spacecraft have proven very effective at spotting comets as well, so the names of many comets incorporate the names of missions such as SOHO or WISE. History In antiquity, comets inspired both awe and alarm , " hairy stars " resembling fiery swords that appeared unpredictably in the sky.

Often, comets seemed to be omens of doom — the most ancient known mythology, the Babylonian "Epic of Gilgamesh," described fire, brimstone, and flood with the arrival of a comet, and Emperor Nero of Rome saved himself from the "curse of the comet" by having all possible successors to his throne executed. This fear was not just limited to the distant past — in 1910, people in Chicago sealed their windows to protect themselves from what they thought was the comet’s poisonous tail. For centuries, scientists thought comets traveled in the Earth's atmosphere , but in 1577, observations made by Danish astronomer Tycho Brahe revealed they actually traveled far beyond the moon. Isaac Newton later discovered that comets move in elliptical, oval-shaped orbits around the Sun, and correctly predicted that they could return again and again. Hubble Space Telescope observations of Comet 103P/Hartley 2, taken on Sept. 25, 2010, are helping in the planning for a Nov. 4 flyby of the comet by the Deep Impact eXtended Investigation (DIXI) on NASA's EPOXI spacecraft.

Credit: NASA, ESA, H. Weaver (The Johns Hopkins University/Applied Physics Lab) Chinese astronomers kept extensive records on comets for centuries, including observations of Halley's Comet going back to at least 240 B.C., historic annals that have proven valuable resources for later astronomers. A number of recent missions have ventured to comets. NASA's Deep Impact collided an impactor into Comet Tempel 1 in 2005 and recorded the dramatic explosion that revealed the interior composition and structure of the nucleus. In 2009, NASA announced samples the Stardust mission returned from Comet Wild 2 revealed a building block of life . In 2014, the European Space Agency's Rosetta spacecraft entered orbit around Comet 67P/Churyumov-Gerasimenko. The Philae lander touched down on Nov 12, 2014. Famous comets Halley's Comet is likely the most famous comet in the world, even depicted in the Bayeux Tapestry that chronicled the Battle of Hastings of 1066. It becomes visible to the naked eye every 76 years when it nears the sun.

When Halley's Comet zoomed near Earth in 1986, five spacecraft flew past it and gathered unprecedented details, coming close enough to study its nucleus, which is normally concealed by the comet's coma. The roughly potato-shaped, 9-mile-long (15 km) comet contains equal parts ice and dust, with some 80 percent of the ice made of water and about 15 percent of it consisting of frozen carbon monoxide. Researchers believe other comets are chemically similar to Halley's Comet. The nucleus of Halley's Comet was unexpectedly extremely dark black — its surface, and perhaps those of most others, is apparently covered with a black crust of dust over most of the ice, and it only releases gas when holes in this crust expose ice to the sun. The comet Shoemaker-Levy 9 collided spectacularly with Jupiter in 1994, with the giant planet's gravitational pull ripping the comet apart for at least 21 visible impacts .

The largest collision created a fireball that rose about 1,800 miles (3,000 km) above the Jovian cloud tops as well as a giant dark spot more than 7,460 miles (12,000 km) across — about the size of the Earth — and was estimated to have exploded with the force of 6,000 gigatons of TNT. A recent, highly visible comet was Hale-Bopp , which came within 122 million miles (197 million km) of Earth in 1997. Its unusually large nucleus gave off a great deal of dust and gas — estimated at roughly 18 to 25 miles (30 to 40 km) across — appeared bright to the naked eye. Comet ISON was expected to give a spectacular show in 2013. However, the sun-grazer did not survive its close encounter with the sun and was destroyed in December. Additional reporting by Nola Taylor Redd, Space.com Contributor Explore the solar system "Biggest" Comet Measured "Biggest" Comet Measured Comet McNaught is "one of the giants," astronomers say.

View Images Comet McNaught seen over the Pacific Ocean in 2007. Photograph courtesy S. Deiries The Great Comet of 2007 made an even bigger impression on the solar system than anyone realized, according to a new study that measured the size of the comet 's wake. Related Content Long-Lost Comet Lander Finally Found In January 2007 people around the world watched comet McNaught streak across the sky. It was the brightest comet seen since 1965 and, in some places, was visible to the naked eye in broad daylight. (See a picture of comet McNaught over New Zealand .) The visible tail was about 35 degrees long, or roughly the same apparent size as 70 full moons lined up in the night sky. By chance, the European Space Agency's Ulysses spacecraft plowed through the width of the comet's tail in February 2007. Designed to study the sun's atmosphere, the probe was able to record information on how the comet's passage affected the solar wind, which is actually charged particles constantly streaming from the sun.

(Related: "Sun's Mysterious Waves Found; May Be Solar Wind Source." ) When a comet nears the sun, heat vaporizes the space rock's ices, releasing dust, which forms the comet's shining tail. But comets also have invisible tails of charged gases, or plasma, that interact with the solar wind. Like a boat stirring up a wake, a comet's plasma tail creates "bow waves" in the solar wind on either side of the comet. Biggest Comet's "Shocking" Tail In the case of comet McNaught, Ulysses found that the plasma tail slowed the solar wind inside the bow wave to half its normal speed. The solar wind usually travels at about 435 miles (700 kilometers) a second at that distance from the sun. But inside the comet's tail, the wind was moving at less than 250 miles (400 kilometers) a second. It took Ulysses a record 18 days to pass through the region of solar wind "shocked" by the comet's tail.

By contrast, in 1996 Ulysses had zipped through the wake of comet Hyakutake in just 2.5 days. Another ESA spacecraft, Giotto, had crossed the tail of Halley's comet in just a few hours. "This shows that McNaught really presented an enormous obstacle to the solar wind," said Geraint Jones of University College London , who presented the findings today at the 2010 Royal Astronomical Society National Astronomy Meeting in Glasgow, Scotland. Compared with Halley's comet, McNaught also flung out more than three times as much charged gas, the Ulysses data show. Comet McNaught "One of the Giants" In terms of the comet's physical size, astronomers think McNaught's core is less than 15 miles (25 kilometers) wide. The new data don't give an exact length for the comet's full plasma tail, although Jones and colleagues note that Ulysses was about 139 million miles (224 million kilometers) away from the comet's core when it crossed the tail.

Jones thinks the width of the shocked region around the tail is a better measure of a comet's size, since it's a sign of how much of an impact the object makes on its surroundings. By that standard, the new data would make comet McNaught the biggest comet yet recorded. Still, only a handful of comet tails have been measured by spacecraft so far, making it likely that McNaught will lose its crown at some point in the future. (Read about a green, two-tailed comet that recently swung past Earth .) "There are probably larger ones around," Jones said. "But McNaught is definitely one of the giants." Comment on This Story

Question: Which is the brightest comet in the solar system?
Answer: Halley's comet