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The Kreutz group of sungrazing cometsSungrazing comets have been observed for many hundreds of years. In the late 1880's and early 1890's, Heinrich Kreutz studied the possible sungrazing comets which had been observed until then and determined that some were sungrazers and some were not. He also found that those which were indeed sungrazers all followed the same orbit. That is, they were all fragments of a single comet which had broken up. It is probable that the original comet, and its fragments, have broken up repeatedly as they orbit the sun with a period of about 800 years. In honor of his work, this group of comets is named the Kreutz sungrazers.
As far as I know, there is no definition of a sungrazing comet. Also,
no comets have ever been seen to hit the solar surface called the
photosphere. No asteroids have been seen to hit the sun either. The
Kreutz sungrazers come within about 50,000 km (perihelion distance of
0.005 AU) of the solar surface. Thus, sungrazing comets just pass
through the solar atmosphere. For that matter, all known comets pass
through the sun's atmosphere, because the sun's atmosphere -- the corona --
extends way, way beyond Pluto. (We don't yet know where the corona
ends, but we may actually know in just a few years. One of the Voyager
spacecraft will get to the boundary eventually!) We only know about a
comet when it becomes active. The reason a comet becomes active is
because it is heated by the Sun. The distinct tails form because of
the influence of the Sun, by radiation and by the solar wind.
Sungrazing comets have been observed possibly as far back as the year -371. Brian Marsden has speculated that a comet seen by Aristotle and Ephorus may be a Kreutz sungrazer. Cool, huh?! However, until 1979, only about 9 sungrazing comets had been seen, all from the ground.
Then, beginning in 1979, space based observatories began to detect sungrazing comets. These observatories are instruments known as coronagraphs. A coronagraph is designed to look at the solar atmosphere. In order to do that, an artificial eclipse is created by occulting (blocking out) the bright disk of the sun.
The SOLWIND instrument on the P78-1 satellite discovered 6 sungrazers between 1979 and 1984. You may remember the P78-1 satellite because it was actually destroyed, on purpose, from the ground in a US military exercise. It was a satellite which had lived out its scientifically useful life anyway.
The CP coronagraph on the Solar Maximum Mission (SMM) discovered 10 sungrazing comets between 1987 and 1989. SMM also observed the comet discovered by SOLWIND in 1984. You may remember the SMM satellite because it was retrieved by the Space Shuttle in 1984, repaired in space, and returned to orbit. That was the first time the shuttle had ever retrieved an object in space, so it was also the first time a satellite had been repaired in space.
SOHO's cometsThere is a new coronagraph (actually 3 in 1) instrument up in space right now. It is the Large Angle Spectrometric Coronagraph (LASCO) on the Solar and Heliospheric Observatory (SOHO) satellite. Since the turn on of the LASCO instrument on December 30, 1995 we have discovered over 1100 new comets, of which over 900 belong to the Kreutz sungrazing group. The majority of the remaining comets belong to four new groups -- the Meyer, Marsden, and Kracht I and II groups -- that were declared based solely on LASCO comet observations. (The KrachtII group is only tentatively declared a separate group, as it currently contains just three members.) Kreutz comets approach the Sun 10x closer than comets from the four new groups. You can find some images and movies of SOHO comets here.