GEM,GIOTTO EXTENDED MISSION Present status ESA's Space Science Committee, meeting in Paris on September 18th, 1989, decided that GIOTTO's new target would become Grigg Skjel- lerup in July 1992, provide that the check-out is successful. GIOTTO survived after its highly successful mission, when it hurtled 605 km from the nucleus of Halley's comet, in the night 13 and 14 March 1986. The spacecraft was protected by a special shield when it passed through the coma at a speed of 65 km/s and was bombarded with impacting dust. The coma is the dust and gas envelope that is seen to develop around the nucleus of a comet as it approaches the Sun. The first European spacecraft to escape from the control of the Earth's gravity, GIOTTO is now in a state of hibernation, orbiting the Sun every ten months between 100 and 200 million km away from Earth. The decision to reactivate the spacecraft was taken in June 1988. Scientists hope it will have withstood its four years of forced hibernation. The check-out phase will start on February 19th, 1990, when contact can be reestablished with the spacecraft via the 10 m diameter dish antennas, of NASA's Deep Space Network. All operations for the check out will be conducted from ESOC, ESA's Space Operations Centre in Darmstadt, FRG. Experts expect the camera to work properly but they will also have to check the other instruments on board. In comparison, Comet Halley can be regarded as relatively young as it spends only a small fraction of its orbital period of ap- proximately 75 year in the inner solar system. The mission On 2 July 1990, GIOTTO will pass within 22,000 km of our planet and it will be redirected towards its new target, by a manoeuvre using the Earth's gravitational force to accelerate the spacecraft. This gravity assist manoeuvre will be performed by experts at ESOC. The spacecraft gets a boost of energy at the expense of the gravita- tional and orbital energy of the planet - the more massive, the bigger the boost. This gravitational sling-shot technique is a good example of how the theory of astrodynamics is used to enable deep.space probes to go further and be more effective as remote gatherers of information. Scientific goals The new target, Comet Grigg-Skjellerup ( named after the astrono- mers who discovered it), is not as active as Halley's Comet in producing dust and gas. It is favored by scientists both in terms of time opportunity and cometary science. It is an "old", relative- ly dust-poor comet; therefore its non-gravitational accelerations are small and perturb its motion only slightly, and the orbit can be predicted fairly accurately. The expression "old", in connection with Grigg-Skjellerup, is used in the sense that the comet with its short orbital period of 5.1 years has spent a considerable fraction of time in the inner solar system and has received a large radiation input from Sun causing the loss of most of the volatile material. Comets are of particular interest to astronomers since it is thought that the materials are made of are primeval samples of the pre-solar nebula from which the planets were formed four and a half thousand million years ago. GIOTTO showed that the nucleus of comet Halley is a rigid, cohesive body, conforming in principle the "dirty snowball" theory of veteran cometologist Dr Fred Whipple. However this theory needs to be double-checked as the nucleus seems to be incredibly loosely constructed, almost like cigarette ash. GIOTTO also revealed that only 10 to 20 % of the surface is active, shooting up geysers of gas and dust to feed the comet's coma and tail. But after all the exiting results the question remains, do the snapshots from Giotto show phenomena common to other comets? Comet Grigg-Skjellerup On 22 July 1902, a New Zealander, John Gripp, discovered a 9th- magnitude comet in the evening sky. He continued observing the comet for the following two weeks, but, due to the late report of his discovery, no other individual viewed the comet. Although, at that time, the comet's orbital period was less than five years, the comet was not seen again until May 1922 by a South African, J.F.Skjellerup. It was the British astronomer Gerald Merton who linked the observations from two apparitions and showed they were one and the same comet. Since 1922, the comet has been observed at every one of the subsequent 12 apparitions. Consequently, its orbit is rather well-known. Comet Grigg-Skjellerup has an aphelion (the point in its solar orbit when it is furthest from the Sun) distance of 4.93 AU, ( the Astronomical Unit is used for measuring distances within the Solar System, and is defined as the mean distance between the Centre of the Earth and the Centre of the Sun and is closely equal to 150 million km) and thus belongs to the Jupiter class of comets. Three times this century its motion has been significantly perturbed by close approaches to the giant planet. The last and closest encounter on 17 March 1964 when the comet passed only 0.33 AU from Jupiter. The accumulative effect of these perturbations has been to increase the orbital period from 4,8 to 5,1 years. The inclina- tion relative to the ecliptic plane has increased from 8 deg. to 21 deg. and the perihelion ( the point when it is closest to the Sun) distance has increased from 0.75 AU to 0,99 AU. Perihelion, therefore, presently lies just inside the Earth's orbit and in fact the orbits of the two bodies very nearly intersect.