Close Encounters with Comets A small comet named Hartley 2 will soon make one of the closest approaches to Earth of all comets in recent centuries. Here is a portrait of Hartley 2, as well as a look at nine other comets that have made close, bright or famous appearances in recent years.

Hartley 2 (2010) This little comet will possibly become visible to the naked eye on Oct. 20 as it passes by Earth a mere 11 million miles away. Astronomers will also get a close-up look of the comet, with a fly-by of NASA's Deep Impact spacecraft scheduled to start on Nov. 4, 2010. Hartley 2 was discovered in 1986. Its orbital path was far from Earth until a few close approaches to Jupiter drew its orbit closer. Skywatchers will be able to find Hartley 2 just below the star Capella in the constellation Auriga on Oct. 20.

McNaught (2007) Comet McNaught is a one-hit wonder among comets. After lighting up the daytime sky in 2007, McNaught jetted and will likely never be seen again. The brightest comet in more than 40 years, Comet McNaught earned the title of Great Comet of 2007. Its spectacular tail was popular with camera-toting skygazers. It also shed light on the composition of comets when the Ulysses spacecraft flew through the tail in 2007 and discovered a decrease in solar wind. The great comet's discoverer, Australian astronomer Robert McNaught, went on to discover another bright comet, McNaught C/2009 R1, which became visible to the naked eye in June 2010.

Schwassman-Wachmann (2006) This comet surprised astronomers in 1995 by breaking apart into 3 mini-comets. It's been disintegrating ever since. By the time it made a close pass by Earth in 2006, the comet was already in more than 30 pieces. The pieces of the comet will make their closest approach to Earth in 2022.

Hale-Bopp (1997) Comet Hale-Bopp is commonly considered the most widely seen comet of the 20th century, as it was visible to the naked eye for a record-breaking 18 months in 1996 and 1997. The giant comet was first spotted out beyond the orbit of Jupiter. It was 1,000 times brighter than Halley's Comet at the same distance. NASA estimates its nucleus at 19 to 25 miles across, which puts its diameter at two to five times the size of the dinosaur-killing comet or asteroid that scientists believe struck the Earth 65 million years ago. Hale-Bopp won't be back in our neck of the universe until the year 4385.

Hyakutake (1996) "The Great Comet of 1996" flew close to Earth and was visible to the naked eye. It lit up the night sky with a bluish-green light, which scientists attributed to an emission of diatomic carbon. Comet Hyakutake was the first comet observed to emit X-rays.

Shoemaker-Levy 9 (1994) This comet's crash with Jupiter in 1994 was the first observed collision of two solar system bodies. The crash produced plumes thousands of miles high, gas bubbles and dark scars in the atmosphere. Shoemaker-Levy 9 was also the first comet to be seen orbiting a planet, instead of the sun. Jupiter most likely captured the comet into orbit in the 1960s or 1970s.

Swift-Tuttle (1992) Astronomers predicted Comet Swift-Tuttle could collide with the Earth in 2126, but quickly revised their calculations to forecast it passing by at a comfortable distance of 15 million miles. This comet is the source of the Perseids meteor showers, which are visible each year in the summer night sky.

Halley's Comet (1986) Possibly the most famous comet, Halley's Comet is visible from Earth every 75 or 76 years. This makes it the only comet visible to the naked eye that could appear twice in a human lifetime. There have been records of this comet being seen since ancient times, but it wasn't until 1705 that the English astronomer Edmond Halley determined that these appearances were all the same comet. Halley's Comet will make its next appearance near Earth in 2061.

IRAS-Araki-Alcock (1983) This small comet came closer to the Earth than any other comet in the last two centuries. It appeared as a blur in the sky about the size of the full moon at its closest: about 3 million miles away. A NASA satellite was able to show that this comet contained sulfur, the first discovery of molecular sulfur in a comet.

West (1976) This was one of the brightest comets of the century, but received little media attention at the time, partly because of a disappointing show by Comet Kohoutek three years earlier. For those who were watching, though, Comet West didn’t disappoint; it had a broad tail and was bright enough to be visible in daylight. With an estimated orbit of 558,000 years, Comet West won’t be back for a long time.