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Kamikaze Comets

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H-alpha image of a bright solar flareFebruary 11, 2000 -- In just four years of operation, the Solar and Heliospheric Observatory (SOHO) spacecraft has found 102 comets, making it by far the most successful comet-hunter in history. Most of this amazing number are suicidal comets that vaporize as they plunge into the solar atmosphere.

One hundred years ago Heinrich Kreutz in Kiel, Germany, realized that several comets seen buzzing the Sun seemed to have a common origin, because they came from the same direction among the stars. These comets are now called the Kreutz sungrazers. A whopping 92 of SOHO's 102 comet discoveries belong to that class.

Right: This Kreutz sungrazing comet from April 30, 1998 was observed in the LASCO C2 telescope. The comet fades rapidly as it approaches the Sun over a period of just 5 hours. Click on the image for a more detailed animation.

Nearly all of SOHO's comet discoveries have showed up in images from the Large Angle and Spectrometric Coronagraph (LASCO) instrument. This is a set of coronagraphs that view the space around the Sun out to 12.5 million miles, while blotting out the bright solar disk with masks. LASCO watches for ejections of electrically charged gas from the Sun that threaten to disturb the Earth's space environment. As a bonus of unanticipated size, it also proved ideal for capturing objects falling to the Sun.

"SOHO is seeing fragments from the gradual breakup of a great comet, perhaps the one that the Greek astronomer Ephorus saw in 372 BC," said Dr. Brian Marsden of the Center for Astrophysics in Cambridge, MA. "Ephorus reported that the comet split in two. This fits with my calculation that two comets on similar orbits revisited the Sun around AD 1100. They split again and again, producing the sungrazer family, all still coming from the same direction."

Their ancestor must have been enormous by cometary standards. "The rate at which we've discovered comets with LASCO is beyond anything we ever expected," said Biesecker. "We've increased the number of known sungrazing comets by a factor of four. This implies that there could be as many as 20,000 fragments."

Hubble images of Shoemaker-Levy 9

Above: Comet Shoemaker-Levy 9, named after its co-discoverers, was often referred to as the "string of pearls" comet. It is famous for its suggestive appearance as well as its collision with the planet Jupiter! The comet's original single nucleus was torn to pieces by Jupiter's strong gravity during a close encounter with the solar system's largest planet in 1992. The pieces are seen in this composite of Hubble Space Telescope images to be "pearls" strung out along the comet's orbital path. [more information]

Life is perilous for a sungrazer. The mixture of ice and dust that makes up a comet's nucleus is heated like the proverbial snowball in hell, and it can survive its visit to the Sun only if it is quite large. What's more, the strong tidal effect of the Sun's gravity can tear the loosely glued nucleus apart. The disruption that created the many SOHO sungrazers was similar to the fate of Comet Shoemaker-Levy 9, which went too close to Jupiter and broke up into many pieces that eventually fell into the massive planet in 1994.

The history of splitting gives clues to the strength of comets, which will be of practical importance if ever a comet seems likely to hit the Earth. Also, the fragments seen as SOHO comets reveal the internal composition of comets, freshly exposed, in contrast to the much-altered surfaces of objects like Halley's Comet that have visited the Sun many times.

Below: Comet SOHO #100 was spotted by Kazimieris Cernis of Vilnius, Lithuania, on 4 February, 2000 in this LASCO coronagraph image. The shaded disk is a mask in the instrument that blots out direct sunlight. The white circle added within the disk shows the size and position of the visible Sun. Calculations show that the latest comets discovered with SOHO (numbers 100, 101, and 102) are previously unknown (undiscovered) comets. The 102nd comet was caught by Dr. Douglas Biesecker, a member of the SOHO team personally responsible for 45 of the discoveries.

SOHO 100The count of SOHO's comet discoveries would be one fewer without a late bonus from SOHO's Solar Wind Anisotropies (SWAN) instrument, which looks away from the Sun to survey atomic hydrogen in the Solar System. In December 1999, the International Astronomical Union retrospectively credited SOHO with finding Comet 1997 K2 (SOHO # 93) in SWAN full-sky images from May to July 1997. It remained outside the orbit of the Earth even at its closest approach to the Sun, and thus did not vaporize entirely.

SOHO (the Solar and Heliospheric Observatory) is a mission of international cooperation between NASA and the European Space Agency. It is managed by the Goddard Space Flight Center for the NASA HQ office of Space Science.Web Links

Solar Cinema - January 20, 2000 NASA Science News. Cool movies of a recent solar prominence.

Solar Smoke Rings - February 3, 2000 NASA Science News. As solar maximum approaches, the Sun is belching billions of tons of gas into interplanetary space.

SpaceWeather.com -follow the latest events on the Sun

SOHO home page -real-time images, screen savers, and more