Comets are cosmic snowballs of frozen gases, rock and dust roughly the size of a small town. When a comet’s orbit brings it close to the sun, it heats up and spews dust and gases into a giant glowing head larger than most planets. The dust and gases form a tail that stretches away from the sun for millions of kilometers.


10 Need-to-Know Things About Comets:

  1. If the sun were as tall as a typical front door, Earth would be the size of a nickel, dwarf planet Pluto would be the size of a head of a pin and the largest Kuiper Belt comet (about 100 km across, which is about one twentieth the size of Pluto) would only be about the size of a grain of dust.
  2. 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.
  3. Days on comets vary. One day on comet Halley varies between 2.2 to 7.4 Earth days (the time it takes for comet Halley to rotate or spin once). Comet Halley makes a complete orbit around the sun (a year in this comet’s time) in 76 Earth years.
  4. Comets are cosmic snowballs of frozen gases, rock and dust.
  5. A comet warms up as it nears the sun and develops an atmosphere, or coma. The coma may be hundreds of thousands of kilometers in diameter.
  6. Comets do not have moons.
  7. Comets do not have rings.
  8. More than 20 missions have explored comets from a variety of viewpoints.
  9. Comets may not be able to support life themselves, but they may have brought water and organic compounds — the building blocks of life — through collisions with Earth and other bodies in our solar system.
  10. Comet Halley makes an appearance in the Bayeux Tapestry from the year 1066, which chronicles the overthrow of King Harold by William the Conqueror at the Battle of Hastings.

SnowstormA surprising view of active jets on the unusual comet Hartley 2.

In the distant past, people were both awed and alarmed by comets, perceiving them as long-haired stars that appeared in the sky unannounced and unpredictably. Chinese astronomers kept extensive records for centuries, including illustrations of characteristic types of comet tails, times of cometary appearances and disappearances, and celestial positions. These historic comet annals have proven to be a valuable resource for later astronomers.

We now know that comets are leftovers from the dawn of our solar system around 4.6 billion years ago, and consist mostly of ice coated with dark organic material. They have been referred to as “dirty snowballs.” They may yield important clues about the formation of our solar system. Comets may have brought water and organic compounds, the building blocks of life, to the early Earth and other parts of the solar system.

As theorized by astronomer Gerard Kuiper in 1951, a disc-like belt of icy bodies exists beyond Neptune, where a population of dark comets orbits the sun in the realm of Pluto. These icy objects, occasionally pushed by gravity into orbits bringing them closer to the sun, become the so-called short-period comets. Taking less than 200 years to orbit the sun, in many cases their appearance is predictable because they have passed by before. Less predictable are long-period comets, many of which arrive from a region called the Oort Cloud about 100,000 astronomical units (that is,?100,000 times the distance between Earth and the Sun) from the Sun. These Oort Cloud comets can take as long as 30 million years to complete one trip around the Sun.

Comet McNaught setting over the Pacific Ocean.

Each comet has a tiny frozen part, called a nucleus, often no larger than a few kilometers across. The nucleus contains icy chunks, frozen gases with bits of embedded dust. A comet warms up as it nears the Sun and develops an atmosphere, or coma. The sun’s heat causes the comet’s ices to change to gases so the coma gets larger. The coma may extend hundreds of thousands of kilometers. The pressure of sunlight and high-speed solar particles (solar wind) can blow the coma dust and gas away from the sun, sometimes forming a long, bright tail. Comets actually have two tails – a dust tail and an ion (gas) tail.

Most comets travel a safe distance from the sun – comet Halley comes no closer than 89 million kilometers (55 million miles). However, some comets, called sungrazers, crash straight into the Sun or get so close that they break up and evaporate.

Scientists have long wanted to study comets in some detail, tantalized by the few 1986 images of comet Halley’s nucleus. NASA’s Deep Space 1 spacecraft flew by comet Borrelly in 2001 and photographed its nucleus, which is about 8 kilometers (5 miles) long.

deep_impactDetails of comet Tempel 1.

NASA’s Stardust mission successfully flew within 236 kilometers (147 miles) of the nucleus of Comet Wild 2 in January 2004, collecting cometary particles and interstellar dust for a sample return to Earth in 2006. The photographs taken during this close flyby of a comet nucleus show jets of dust and a rugged, textured surface. Analysis of the Stardust samples suggests that comets may be more complex than originally thought. Minerals formed near the Sun or other stars were found in the samples, suggesting that materials from the inner regions of the solar system traveled to the outer regions where comets formed.

Another NASA mission, Deep Impact, consisted of a flyby spacecraft and an impactor. In July 2005, the impactor was released into the path of the nucleus of comet Tempel 1 in a planned collision, which vaporized the impactor and ejected massive amounts of fine, powdery material from beneath the comet’s surface. En route to impact, the impactor camera imaged the comet in increasing detail. Two cameras and a spectrometer on the flyby spacecraft recorded the dramatic excavation that helped determine the interior composition and structure of the nucleus.

After their successful primary missions, the Deep Impact spacecraft and the Stardust spacecraft were still healthy and were retargeted for additional cometary flybys. Deep Impact’s mission, EPOXI (Extrasolar Planet Observation and Deep Impact Extended Investigation), comprised two projects: the Deep Impact Extended Investigation (DIXI), which encountered comet Hartley 2 in November 2010, and the Extrasolar Planet Observation and Characterization (EPOCh) investigation, which searched for Earth-size planets around other stars on route to Hartley 2. NASA returned to comet Tempel 1 in 2011, when the Stardust New Exploration of Tempel 1 (NExT) mission observed changes in the nucleus since Deep Impact’s 2005 encounter.

halley_giotto_bigComet Halley in 1986.

How Comets Get Their Names
Comet naming can be complicated. Comets are generally 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.

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.