The Discovery: Edmond Halley was the first person to recognise that this comet was periodic. This discovery was made in 1705 after he had computed parabolic orbits for 24 comets observed from 1337 to 1698. His analysis of the list revealed the comets of 1531, 1607, and 1682 moved in almost identical orbits and were separated by intervals of roughly 75 years. From this information, he predicted the comet would next appear in 1758. Halley died in 1742. The comet that now bears his name was recovered on 1758 December 25, by Johann Georg Palitzsch (Prohlis, Germany), a German farmer and amateur astronomer.
Closest Approach to Earth: The comet and Earth experienced their closest approach to one another on 837 April 10, when their separating distance equalled 0.0342 AU (3.2 million miles). It was seen worldwide for a few nights either side of this date, with a maximum tail length of 60 degrees. Overall, the comet was seen from March 21 to April 28 of that year.
Most Observed Apparition: The comet's recent return during the 1980s was the most observed return of any comet in history. The comet was first seen on 1982 October 16 and was last detected on 1994 January 11. The comet passed perihelion on 1986 February 9.46.Final Observation of Halley's Comet on 1994 January 11, when 18.82 AU from the sun, or near the orbit of Uranus. This was not one of the more favorable appearances, as the maximum magnitude only reached 2.6 in early March 1986, while the tail grew to about 15 degrees by mid-March. The highlight of this apparition was the armada of space probes sent to visit the comet during February and March 1986. The European Space Agency's Galileo probe flew close enough to obtain an excellent image of the comet's nucleus.
The comet produces two meteor showers every year. The Eta Aquarids produce a maximum of about 20 meteors per hour for Northern Hemisphere observers and 50 meteor per hour for Southern Hemisphere observers on May 4/5. The Orionids produce a maximum of about 20 meteors per hour on October 20/21.
The spacecraft encountered Halley on March 13, 1986, at a distance of 0.89 AU from the sun and 0.98 AU from the Earth and an angle of 107 degrees from the comet-sun line. The goal was to come within 500 km of Halley's comet at closest encounter. The actual closest approach was measured at 596 km.
During the Giotto extended mission, the spacecraft successfully encountered Comet P/Grigg-Skjellerup on July 10, 1992. The closest approach was approximately 200 km. The heliocentric distance of the spacecraft was 1.01 AU, and the geocentric distance, 1.43 AU at the time of the encounter. The payload was switched-on in the evening of July 9. Eight experiments were operated and provided a surprising wealth of exciting data. The Johnstone Plasma Analyser detected the first presence of cometary ions 600,000 km from the nucleus at 12 hours before the closest approach. The Dust Impact Detectors reported the first impact of a fairly large particle at 15:30:56. Bow shocks/waves and acceleration regions were also detected. After the P/Grigg-Skjellerup encounter, the spacecraft was retargeted for a possible 1999 encounter pending the existence of sufficient fuel and funding for ground operations support.