26P/Grigg–Skjellerup

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.

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.

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 2.6 km in diameter. 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. 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.

Exploration

Main article: Giotto (spacecraft)

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.

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 Grigg–Skjellerup on 10 July 1992 at a distance of 200 km, 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 Grigg–Skjellerup on 23 July 1992.

Cancelled proposals

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 1000 km by April 1977, with an option to flyby 21P/Giacobini–Zinner 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.

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.

References

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|archive-url=https://web.archive.org/web/20230216094336/https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%2790000375%27&START_TIME=%272023-Dec-25%27&STOP_TIME=%272023-Dec-26%27&STEP_SIZE=%271%20hour%27&QUANTITIES=%2719%27 |archive-date=2023-02-16 |url-status=live

|archive-url=https://web.archive.org/web/20110520032039/http://jcometobs.web.fc2.com/pcmtn/0026p.htm |archive-date=2011-05-20 |url-status=live |access-date=2023-07-24}}

References

1. (11 March 2011). "Giotto, ESA's first deep-space mission: 25 years ago".
2. (10 March 2006). "Giotto's second comet encounter".
3. "Comet 26P/Grigg-Skjellerup, Outer Space".
4. (12 June 2008). "NASA Finds New Type of Comet Dust Mineral".