Halley's comet was seen with the naked eye in October and November 1985 and in early spring 1986: perihelion was on Feb. 9, 1986 and it passed through its ascending and descending nodes on Nov. 27, 1985 and Apr. 11, 1986. Detailed observations were made using instruments on Earth and in five spacecraft: ESA's Giotto, the Soviet-led missions Vega 1 and Vega 2, and the Japanese craft Sakigake and Suisei. Pictures were obtained by Giotto of the dark potato-shaped nucleus (16 × 8 km) and of structure and activity near the nucleus. The comet's development as it rounded the Sun was recorded and studies were made of cometary composition, of processes producing and occurring in the coma and tails, and the interaction between comet and solar wind. Halley's comet will return in 2061.
a bright comet, the first for which an elliptic orbit was calculated, thereby proving the periodicity of its return to the sun. The English astronomer E. Halley, who compiled the first catalog of the elements of the orbits of comets that appeared between 1337 and 1698, noted the coincidence of the paths of the comets of 1531, 1607, and 1682 and proposed that they corresponded to one and the same comet that revolved around the sun with a period of 75-76 years. In 1705, Halley predicted the return of the comet in 1758. In 1758 the French scientist A. Clairaut developed a method of accounting for the perturbations of the comet’s motion caused by the attraction of the planets Jupiter and Saturn and refined the estimate of the date of the comet’s passage through the perihelion. It occurred on Mar. 12, 1759, within the probable period of time indicated by Clairaut. This was a brilliant confirmation of Newton’s mechanics. The perihelion of the orbit of Halley’s comet is 0.587 astronomical unit and the aphelion, more than 35 astronomical units. The next passage of the comet was in 1835. By this time, the perturbations in the comet’s motion by the planet Uranus, which was recently discovered by the English astronomer W. Herschel were also taken into account. The comet passed through the perihelion on November 16, three days later than calculated. The study of Halley’s comet by the German astronomer F. W. Bessel marked the beginning of the mechanical theory of comet forms, later continued by the Russian astronomer F. A. Bredikhin. Investigations of Halley’s comet during its last appearance (perihelion May 19, 1910), based on numerous observations, made it possible to obtain the first information on the physical nature of comets and stimulated P. Cowell to develop a more complete method of calculating perturbations by planets. Together with A. Crommelin, he traced the future motion of Halley’s comet as well as its past motion. It has turned out that up to 1909, Halley’s comet was observed 29 times, for the first time in A.D. 446. The next passage of Halley’s comet through the perihelion will occur in January 1986. This will afford one of the most convenient opportunities in the second half of the 20th century for sending a probe-rocket to a comet with the aim of directly determining the composition of comets and the state of matter within them.
O. V. DOBROVOL’SKII