About Comet Grigg-Skjellerup
Most periodic comets stay much closer to the Sun and have much shorter orbital periods - less than 20 years - than Comet Halley. For example, Giotto's second target, Comet 26P/Grigg-Skjellerup, completes one circuit of the Sun every 5.1 years. The comet travels around the Sun in the same direction as the planets and its orbit ranges from just inside the orbit of Jupiter to the orbit of the Earth.
It is named after John Grigg, a New Zealand teacher of singing, who first saw it on 23 July 1902, and James Francis Skjellerup, an Australian telegraphist working in South Africa, who rediscovered the comet on 17 May 1922. By using computers to analyse its orbit, astronomers eventually realised in 1987 that it was the same object as Comet 1808 III - originally discovered by Jean-Louis Pons as long ago as 6 February 1808!
During this century, Grigg-Skjellerup's orbit has been significantly altered by close approaches to Jupiter. The most significant alteration came on 17 March 1964 when the comet gained a significant amount of energy as it passed about 50 million kilometres from the giant planet. This close encounter changed the comet's orbital inclination (tilt) and lengthened the orbit so that its perihelion (the point in the orbit when the comet is nearest the Sun) was moved outwards and now almost intersects the Earth's orbit. This made it a relatively easy object to reach for Giotto. The orbits of the spacecraft and Grigg-Skjellerup eventually crossed on 10 July 1992, just 12 days before the comet reached perihelion.
As its nucleus has been warmed by the Sun many times during each close pass, it has lost a large amount of gas and dust over its lifetime. Today, Grigg-Skjellerup is much smaller and less active than Halley's Comet. During peak activity near the Sun, it releases less than 1% of the gas and dust ejected by Halley (i.e. less than 300 kg per second compared with three tonnes per second).