A naked-eye comet - one visible to the unaided eye without telescope or binoculars - is an enjoyable sight, particularly for the brighter comets. On average, a naked-eye comet graces our skies about once every two years. However, most remain fairly faint or appear close to the Sun as seen from Earth, such that even experienced observers may require binoculars to spot them. Only rarely do two relatively bright naked-eye comets appear simultaneously. Such an event will take place in April and May of 2004, when skygazers will feast their eyes upon both Comets C/2001 Q4 (NEAT) and C/2002 T7 (LINEAR).

On February 9, the MicroObservatory telescope in Amado, Arizona took this photograph of Comet C/2002 T7 (LINEAR). Comet LINEAR is one of two comets expected to brighten to naked-eye visibility by May. The tail appears a fifth of a degree long in this image, which would translate into a real length in space of over 1.4 million miles (or 2.3 million km). Credit: Christine Pulliam, Harvard-Smithsonian Center for Astrophysics

Astronomer Dan Green (Harvard-Smithsonian Center for Astrophysics), Director of the Central Bureau for Astronomical Telegrams (CBAT), said, "As the clearinghouse for comet discoveries, CBAT has known of these comets for a long time. We have monitored them, collecting observations from around the world. If they brighten as predicted, then both may be visible to the naked eye in late April and part of May. If you haven't seen a comet, this is a great opportunity to go out and look at one."

Dirty snowballs
Historically, bright comets were interpreted as portents of doom, as in 1066 when the appearance of a comet, later known as Comet Halley, was blamed for the defeat of the Saxons at the Battle of Hastings. Comets were thought to be vaporous "exhalations" of the Earth, merely atmospheric phenomena. Only as science advanced in the 16th and 17th centuries were they recognized as true denizens of the solar system.

The object most people visualize when they hear the word "comet" actually has three components - a small, irregular nucleus; a spherical, gaseous coma surrounding it; and a broad, sweeping tail. The cometary nucleus is the source of the gas and dust that create a comet's dramatic appearance.

In 1950, Harvard astronomer Fred Whipple coined the term "dirty snowball" to describe a comet's nucleus. He began studying comets at a time when little was known about them, but he said, "It turns out they were simple." The nucleus is a chunk of ice and rock ranging in size from 100 yards or less up to several miles in diameter. Frozen gases (ices) of water, carbon dioxide, and methane are mixed with dust and rock into a conglomeration much like chocolate chip ice cream. As the comet nears the Sun, the Sun's heat vaporizes those ices, puffing off clouds of gas and dust that surround the nucleus to form a glowing coma. Radiation pressure from the Sun, combined with the solar wind, then sweep material from the coma outward to form a tail that can stretch across millions of miles of space.

Scientists are interested in comets for a number of reasons. "Comets are thought to have formed in the outer reaches of the solar system, and may thus contain rock and ices that date back billions of years. Also, comet tails are indicators of the solar wind and have helped us learn about the inner solar system. And not least, comets are known to hit planets from time to time, including Earth, so we need to keep an eye out for potential impactors," said Green.

One intriguing possibility directly links humanity to these visitors from the outer solar system. While Carl Sagan once said that we are star stuff, Fred Whipple would add that we are comet stuff. "Part of the water in our bodies comes from comets. That's because some proportion of the Earth's water comes from comets," said Whipple.

To a layperson, the appeal of a comet may reside less in its scientific value than in its dramatic display of cosmic splendor. Comets C/2001 Q4 (NEAT) and C/2002 T7 (LINEAR) both have the potential to provide pleasant sights when they swing through the inner solar system this spring.

Two bright comets for 2004
Comet NEAT is not especially keen, nor does Comet LINEAR travel a particularly straight line. Instead, both are named for the robotic telescope survey programs that discovered them. The programs locate comets so prolifically that many comets have shared the same names, including some reasonably bright comets, hence the importance of using the comets' full designations.

On August 28, 2001, the Central Bureau for Astronomical Telegrams at CfA announced the discovery of Comet C/2001 Q4 by the Near Earth Asteroid Tracking (NEAT) program, for which the comet was named.

Astronomers describe the brightness of celestial objects using a magnitude scale: the higher the magnitude number, the fainter the object. When found, Comet NEAT glowed at only 20th magnitude, about 400,000 times fainter than the faintest star visible to the unaided eye. Yet predictions indicate that Comet NEAT may brighten to 1st or 2nd magnitude in late April and remain that bright through mid-May, making it visible to skygazers, possibly even with light-polluted city skies. From non-light-polluted, clear skies, Comet NEAT may be visible to the unaided eye from early April through late June.

The second bright comet now approaching the Sun, Comet C/2002 T7 (LINEAR), was found by the Lincoln Laboratory Near Earth Asteroid Research (LINEAR) program and announced on October 29, 2002. To an experienced observer, Comet LINEAR may become visible to the naked eye in mid-March, when it is expected to brighten to 6th magnitude. (Stars as bright as 7th magnitude are visible to the unaided eye in dark skies, if they are directly overhead. A comet is harder to spot since its light is spread out rather than concentrated in a star-like point, and it may be located close to the horizon where extinction dims it further.) However, the position of Comet LINEAR in the constellation Pisces places it very close to the Sun in our sky, so observers will have to wait several weeks for their first good viewing opportunity.

Both comets are likely on their first trip through the inner solar system after having been nudged out of the Oort Cloud, the spherical reservoir of comets that surrounds our Sun far beyond the orbit of Pluto. As a result, peak brightness estimates are uncertain. "Comets do a lot of things that are unpredictable," said Green. If a comet should break apart, as happened with Comet C/1999 S4 (also called LINEAR) in the year 2000, it would never become bright in our skies. On the other hand, if a comet should undergo a sudden outburst, it could brighten substantially above predicted levels. Both professional and amateur astronomers currently are monitoring and will continue to monitor the comets.

Viewing tips
Both comets will present viewing challenges, since at their brightest they will also be relatively close to the Sun as seen from the Earth. As a result, they will appear either in the western sky shortly after sunset, or in the eastern sky shortly before sunrise. The glow of twilight may interfere with viewing.

Spotting either comet will be easier if observers first locate their target through binoculars, searching the relevant area of the sky for the fuzzy round glow of the coma, with a tail pointing up from the horizon. Once you know where to look, try lowering your binoculars and looking with the eyes alone.

Green recommended, "You'll need a good observing site with a low horizon, few city lights, and clear skies. Neither comet will be particularly easy to pick out, especially in the light-polluted skies that most people face. Your best bet is to attend a public open night at your local observatory or astronomy club, where experienced observers can help you."

The Southern Hemisphere will enjoy the best views of Comet C/2001 Q4 (NEAT), which moves from the constellation Tucana through Hydrus and into Dorado during April, passing by both the Small and Large Magellanic Clouds. It will be visible in the evening sky between sunset and about 11:00 PM local time. During May, Comet NEAT moves northward, becoming visible to observers at mid-northern latitudes early in the month. Comet NEAT appears in the evening sky after sunset as it slides from Canis Major through Cancer and into Ursa Major by month's end. Comet NEAT passes closest to Earth on May 7th at a distance of around 30 million miles (48 million kilometers). (For comparison, the Earth is at a distance of 93 million miles, or 150 million km, from the Sun.)

The most eye-catching views of Comet NEAT in the Northern Hemisphere are likely to occur during May 12-16, when the western horizon after sunset shows the comet and four bright planets (Venus, Mars, Saturn, and Jupiter) all in a line. The dramatic lineup not only offers wonderful photographic opportunities, but also graphically demonstrates the contrast between the clockwork regularity of orbiting planets and the irregular serendipity of visiting comets.

Comet C/2002 T7 (LINEAR) may be visible in the morning twilight just before sunrise in late April and early May to Northern-Hemisphere observers with a flat eastern horizon. The comet then disappears into the solar glare again as it moves from Pisces through Cetus, Eridanus, and Lepus to Canis Major. Comet LINEAR finally reappears in the twilit evening sky in late May, but will be fading, so observers should use binoculars to locate it before attempting to view it with the naked eye.

Southern Hemisphere observers will enjoy better views of Comet LINEAR, which will be visible in the early morning sky to the east from mid-April through early May. In mid-May, Comet LINEAR begins to swing into the evening sky, and for a few days may be visible both immediately after sunset in the west and immediately before sunrise in the east. Around May 20th, the comet becomes an evening object visible only in the western sky after sunset. Comet LINEAR passes closest to Earth on May 19th, at a distance of about 25 million miles (40 million km).

As June opens, both comets will fade as they speed ever farther from both the Sun and the Earth. Yet if current predictions hold, the brief but enjoyable appearances of Comet NEAT and Comet LINEAR will be remembered for years to come!

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.