Comets appear irregularly in the sky—some are faint, others spectacular, some are periodic (e.g. Haley’s famous comet), others are long-period comets, whose periods are usually too long to be derived.

Many of the most spectacular comets are long-period comets.

The Sun evaporates material from the comet and a glowing envelope or “coma” (“hair”) forms (comet means “hairy one”). Near the Sun the pressure of sunlight and of the solar wind “blow” the outermost part of the coma into a glowing tail, which–since the force extending it comes from the Sun–generally extends outwards from the Sun, whether the comet is approaching the Sun or receding from it. Very few comets actually hit the Sun–in fact only after satellite images became available have such impacts been observed.

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Observations of comet tails often show two tails with slightly different directions and slightly different colors, too. The (usually) brighter tail points consists of dust pushed by the pressure of sunlight. Its glow is scattered sunlight and its spectrum resembles the Sun’s. The other tail is driven by the solar wind, which is driven about 400 km/sec (average) by the million-degree heat of the solar corona, the outermost layer of the Sun. As its light shows, such a tail consists of ions, atoms of the coma which were stripped by sunlight of one or more electrons. The solar wind consists of ions too, and in fact, the existence of an ion tail was the first strong evidence for the existence of a “solar corpuscular radiation” as the solar wind was then called.

The solar wind and the ion tail are both so rarefied that collisions between their particles hardly ever occur. However, the solar wind arises among the Sun’s magnetic field lines, and the interaction between moving plasma and a magnetic field creates electric fields and currents. These can modify the flow or else, deform the field lines. The point to note is that, when the electric field caused by that interaction of flowing plasma and magnetic field lines reaches ions blown off the comet, it picks them up and carries them along with the solar wind.

The solar wind flows away from the Sun at something like 400 km/sec, but meanwhile the source, the comet itself, may be moving at 20-50 km/sec. The chain of particles emitted into the ion tail during a certain second then extends 400 km radially from the Sun, but since those particles had started at points 20-50 km apart on the comet’s trajectory, the line they form is not quite radial, not necessarily the same as the direction of the tail pushed by sunlight.

Fred Whipple called comets dirty snowballs. Recent observations of comets by spacecraft suggest they should instead be called snowy dirt balls. Sunlight erodes comets constantly, helped by the solar wind. On a near-solar orbit they are not expected to last very long. Gradually all volatile material such as ice evaporates, the showy tail thins out and disappears, and all that’s left of the comet are grains of rocky stuff which gradually disperse along its orbit. When Earth crosses such an orbit, many such grains hit the atmosphere and because of their high velocity, are vaporized by air resistance, emitting a brief bright glow. These are meteors. Meteorites are the objects which hit the atmosphere and sometimes even reach ground, while meteors are the flashes or streaks in the sky. Several “meteorite showers” recur annually, and are believed to mark the crossing of the orbits of former comets.

Type of Comets

• Short-period comets (comets that orbit the sun in less than 200 years) reside in the icy region known as the Kuiper Belt beyond the orbit of Neptune from about 30 to 55 AU.
• Long-period comets (comets with long, unpredictable orbits) originate in the far-off reaches of the Oort Cloud, which is five thousand to 100 thousand AUs from the sun.
• Sungrazing comets are comets that passes very close to the Sun at perihelion. Sungrazers usually evaporate during their closest approach to the sun but larger ones can survive. These larger sungrazers may still break up due to the huge tidal forces created by the sun’s gravity.

Up to 1979, only 9 sungrazers had been seen from the ground. Beginning in 1979 sungrazers began to be observed from space. But up until the SOHO spacecraft launched in 1995 only a few tens of such observations were made. Now as of October 2012 SOHO has observed 2,574 sungrazer comets. About 90% of these comets are members of the Kreutz group and the rest are from 4 other groups, the Kracht, Kracht 2a, Marsden, and Meyer groups.

Named after a 19th-century German astronomer who studied them in detail, Kreutz sungrazers are fragments from the breakup of a giant comet at least 2000 years ago. Several of these fragments are thought to pass by the sun and disintegrate every day. Most are too small to see, but occasionally a big fragment attracts attention.

Insights on Comet Tails Are Blowing in the Solar Wind

Oliver Price, a planetary science PhD student at University College London’s Mullard Space Science Laboratory in the United Kingdom, has developed a new image-processing technique to mine through the wealth of data about comet tails. Price’s findings offer the first observations of striations forming in the tails, and an unexpected revelation about the Sun’s effect on comet dust.

How Comets Get Their Names

The naming convention for comets is not particularly straightforward. Comets are named for their discoverer — either a person or a spacecraft. This International Astronomical Union guideline was developed only in the last century. For example, comet Shoemaker-Levy 9 was so named because it was the ninth short-periodic comet discovered by Eugene and Carolyn Shoemaker and David Levy. Since spacecraft are very effective at spotting comets many comets have LINEAR, SOHO or WISE in their names. Comets also can be named for a observing network such as Comet ISON, which is named for the International Scientific Observing Network. The complete designation for Comet ISON is C/2012 S1 ISON.

Significant Dates

• 1070-1080: The comet later designated Halley’s Comet is pictured in the Bayeux Tapestry, a chronicle of the Battle of Hastings of 1066.
• 1449-1450: Astronomers make one of the first known efforts to record the paths of comets across the night sky.
• 1705: Edmond Halley publishes that the comets of 1531, 1607, and 1682 are the same object and predicts its return in
• 1758. The comet arrives on schedule and is later named Halley’s Comet.
• 1986: An international fleet of five spacecraft converges on comet Halley as it makes its regular (about every 76 years) pass through the inner solar system.
• 1994: In the first observed planetary impact by a comet, awed scientists watch as fragments of comet Shoemaker-Levy 9 smash into Jupiter’s atmosphere.
• 2001: Deep Space 1 flies by and photographs comet Borrelly.
• 2004: NASA’s Stardust spacecraft collects dust samples from comet Wild 2 and images the nucleus.
• 2005: The Deep Impact impactor collides with comet Tempel 1 to enable scientists to study the interior of the nucleus.
• 2006: The Stardust sample return capsule lands in Utah carrying cometary particles and interstellar dust.
• 2009: Scientists announce that the amino acid glycine, a building block of life, was collected by the Stardust spacecraft from comet Wild 2.
• 2010: The Deep Impact spacecraft studies its second cometary target, Hartley 2, a small, hyperactive comet.
• 2011: The Stardust spacecraft encounters Tempel 1 and captures views of the Deep Impact impact site, the opposite side of the nucleus, and evolution on the comet’s surface.

Famous Comets