A sungrazing comet is a comet that passes extremely close to the Sun at perihelion – sometimes within a few thousand kilometres from the Sun's surface. Although small sungrazers can completely evaporate during such a close approach to the Sun, larger sungrazers can survive many perihelion passages. However, the strong evaporation and tidal forces they experience often lead to their fragmentation.
Up until the 1880s, it was thought that all bright comets near the Sun were the repeated return of a single sungrazing comet. Then German astronomer Heinrich Kreutz and American astronomer Daniel Kirkwood determined that, instead of the return of the same comet, each appearance was a different comet, but each were related to a group of comets that had separated from each other at an earlier passage near the Sun (at perihelion). Very little was known about the population of sungrazing comets until 1979, when coronagraphic observations allowed the detection of sungrazers. As of October 21, 2017, there are 1495 known comets that come within ~12 solar radii (~0.055 AU). This accounts for nearly one third of all comets. Most of these objects vaporize during their close approach, but a comet with a nucleus radius larger than 2–3 km is likely to survive the perihelion passage with a final radius of ~1 km.