Comet GriggâÂÂSkjellerup (formally designated 26P/GriggâÂÂSkjellerup) is a periodic comet. It was visited by the Giotto probe in July 1992. The spacecraft came as close as 200 km, but could not take pictures because some instruments were damaged from its encounter with Halley's Comet. The comet last came to perihelion (closest approach to the Sun) on 25 December 2023, but was 1.8 AU from Earth and only 31 degrees from the Sun.
The comet was discovered in 1902 by John Grigg of New Zealand, and rediscovered in its next appearance in 1922 by John Francis Skjellerup, an Australian then living and working for about two decades in South Africa where he was a founder member of the Astronomical Society of Southern Africa. In 1987, it was belatedly discovered by ýubor Kresák that the comet had been observed in 1808 as well, by Jean-Louis Pons. Pons observed the comet on 6 and 9 February, which was insufficient to calculate an approximate orbit.
In 1972 the comet was discovered to produce a meteor shower, the Pi Puppids, and its current orbit makes them peak around 23 April, for observers in the southern hemisphere, best seen when the comet is near perihelion.
During the comet's 1982 approach it was detected using radar by the Arecibo Observatory.
The apparition of 2002 was very unfavorable due to solar conjunctions, thus no observations were conducted at that time.
The comet has often suffered the gravitational influence of Jupiter, which has altered its orbit considerably. For instance, its perihelion distance has changed from 0.77 AU in 1725 to 0.89 AU in 1922 to 0.99 AU in 1977 and to 1.12 AU in 1999.
The comet nucleus is estimated to be in diameter. Light-curve analysis from the Giotto flyby in 1992 revealed that GriggâÂÂSkjellerup is surprisingly an old comet compared to 1P/Halley, suggesting that 26P is estimated to be around 89 comet-years in age. Ground-based photometry of the comet reveal a non-spherical nucleus with a rotation period longer than 12 hours.
The comet is a type locality for the mineral brownleeite.
In 1987, GriggâÂÂSkjellerup was selected as the second comet targeted by the Giotto mission due to its perihelion in 1992 occurring very close to the Earth's orbit itself.
By February 1990, the mission control at the European Space Agency reactivated the spacecraft after four years of hibernation following the Halley mission, subsequently executing the first ever Earth flyby in space exploration history to reach 26P on July 1990. Giotto reached GriggâÂÂSkjellerup on 10 July 1992 at a distance of , much closer than its approach to Halley's Comet, but was unable to obtain images as its camera was destroyed during the Halley rendezvous in 1986. Despite this, the spacecraft was able to measure the interaction of the solar wind and how it affects the coma of this comet.
Giotto was deactivated just 13 days after its flyby of GriggâÂÂSkjellerup on 23 July 1992.
In 1972, a NASA spacecraft mission based from the Explorer 47/50 satellite called Cometary Explorer was proposed to intercept GriggâÂÂSkjellerup at a distance of by April 1977, with an option to flyby 21P/GiacobiniâÂÂZinner on a potential mission extension in 1979. This would serve as a precursor mission for an eventual mission to Halley in 1986, however it was rejected due to budget cuts.