26P/Grigg–Skjellerup

Comet Grigg–Skjellerup (formally designated 26P/Grigg–Skjellerup) is a periodic comet. It was visited by the Giotto probe in July 1992.<ref name=giotto>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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.<ref name=giotto2>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The comet last came to perihelion (closest approach to the Sun) on 25 December 2023,<ref name=MPC/><ref name="Horizons2023"/><ref name="Kinoshita"/> but was 1.8 AU from Earth and only 31 degrees from the Sun.<ref name="Horizons2023"/>

Observational history

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 Ľubor 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.Template:R

In 1972 the comet was discovered to produce a meteor shower, the Pi Puppids, and its current orbit makes them peak around April 23, 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.Template:R

The apparitions of 1997 and 2002 were very unfavorable due to solar conjunctions, thus no observations were conducted at those times.Template:R

Orbit

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.

Physical characteristics

The comet nucleus is estimated to be Template:Cvt in diameter.Template:R Light-curve analysis from the Giotto flyby in 1992 revealed that Grigg–Skjellerup is surprisingly an old comet compared to 1P/Halley, suggesting that 26P is estimated to be around 89 comet-years in age.Template:R Ground-based photometry of the comet reveal a non-spherical nucleus with a rotation period longer than 12 hours.Template:R

The comet is a type locality for the mineral brownleeite.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Exploration

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} In 1987, Grigg–Skjellerup 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.Template:R

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.Template:R Giotto reached Grigg–Skjellerup on 10 July 1992 at a distance of Template:Cvt, 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.Template:R

Giotto was deactivated just 13 days after its flyby of Grigg–Skjellerup on 23 July 1992.

Cancelled proposals

Template:Distinguish In 1972, a NASA spacecraft mission based from the Explorer 47/50 satellite called Cometary Explorer was proposed to intercept Grigg–Skjellerup at a distance of Template:Cvt by April 1977,Template:R with an option to flyby 21P/Giacobini–Zinner on a potential mission extension in 1979.Template:R This would serve as a precursor mission for an eventual mission to Halley in 1986,Template:R however it was rejected due to budget cuts.Template:R

Popular culture

• In Neal Stephenson's science fiction novel Seveneves, 26P/Grigg–Skjellerup serves as a potential source of water and rocket propellant for the "Cloud Ark" survivors, and is frequently referred to by the nickname "Greg's Skeleton" by way of homophonic transformation.Template:R

See also

• List of minor planets and comets visited by spacecraft

External links

• Template:JPL Small Body
• 26P/Grigg–Skjellerup at Gary W. Kronk's Cometography
• 26P/Grigg–Skjellerup at Seiichi Yoshida's website
• ESA website about 26P/Grigg–Skjellerup
• Recovery of comet 26P/Grigg–Skjellerup (Remanzacco Observatory : December 26, 2012)