Product Code Database
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 evaporation during such a close approach to the Sun, larger sungrazers can survive many perihelion passages. However, the strong evaporation and 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 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). JPL Small-Body Database Search Engine. 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.
Sungrazer comets were some of the earliest observed comets because they can appear very bright. Some are even considered . The close passage of a comet to the Sun brightens the comet not only because of the reflection from the comet nucleus when it is closer to the Sun, but the Sun also vaporizes a large amount of gas from the comet, and the gas reflects more light. This extreme brightening might allow naked-eye observations from Earth, depending on how volatile the gases are and whether the comet is large enough to survive perihelion. These comets provide a useful tool for understanding the composition of comets from the observed outgassing activity. They also offer a way to probe the effects that solar radiation has on other Solar System bodies.
Comet C/1882 R1 appeared only two years after the previously observed sungrazer, so this convinced astronomers that these bright comets were not all the same object. Some astronomers theorized that the comet might pass through a resisting medium near the Sun, and that would shorten its period. When astronomers observed C/1882 R1, they measured the period before and after perihelion and saw no shortening in the period, which disproved the theory. After perihelion this object was also seen to split into several fragments, and therefore Kirkwood's theory of these comets coming from a parent body seemed like a good explanation.
In an attempt to link the 1843 and 1880 comets to the comet in 1106 and 371 BC, Kreutz measured the fragments of the 1882 comet and determined that it was likely a fragment of the 1106 comet. He then designated that all sungrazing comets with similar orbital characteristics as these few comets would be part of the Kreutz Group.
The 19th century also provided the first spectrum taken of a comet near the Sun, which was taken by Finlay and Elkin in 1882. Later the spectrum was analyzed and Iron and Nickel spectral lines were confirmed.
In 1987 and 1988 it was first observed by SMM that there could be pairs of sungrazing comets that can appear within very short time periods, ranging from a half of a day up to about two weeks. Calculations were made to determine that the pairs were part of the same parent body but broke apart at tens of AU from the Sun. The breakup velocities were only on the order of a few meters per second, which is comparable to the speed of rotation for these comets. This led to the conclusion that these comets break from and that comets C/1882 R1, C/1965 S1, and C/1963 R1 probably broke off from the Great Comet of 1106.
Coronagraphs allowed measuring the properties of the comet as it reached very close to the Sun. It was noted that sungrazing comets tend to peak in brightness at a distance of about 12.3 or 11.2 Solar radius. It is thought that this variation stems from a difference in dust composition. Another small peak in brightness has been found at about 7 solar radii from the Sun and is possibly due to a fragmentation of the comet nucleus. An alternative explanation is that the brightness peak at 12 solar radii comes from the sublimation of Amorphous solid , and the peak at 11.2 solar radii is from the sublimation of Crystallinity olivines. The peak at 7 solar radii could then be the sublimation of pyroxene.
The of 1843 and 1882, Comet Ikeya–Seki in 1965 and C/2011 W3 (Lovejoy) in 2011 were all fragments of the original comet. Each of these four was briefly bright enough to be visible in the daytime sky, next to the Sun, 1882's comet outshining even the full moon.
In 1979, C/1979 Q1 (Solwind) was the first sungrazer to be spotted by US satellite P78-1, in taken on 30 and 31 August 1979. C/1979 Q1 – SOLWIND 1, cometography.com.
Apart from Comet Lovejoy, none of the sungrazers seen by SOHO has survived its perihelion passage; some may have plunged into the Sun itself, but most are likely to have simply evaporated away completely.
|
|
