Comet Halley

 

 

   in 1705 Edmond Halley predicted, using Newton's newly formulated laws of motion, that the comet seen in 1531, 1607, in 1682 would return in 1758 (which was, alas, after his death). The comet did indeed return as predicted in was later named in his honor.

   The average period of Halley's orbita is 76 years but you cannot calculate the dates of its reappearances by simply subtracting multiples of 76 years od planeta 1986. The gravitational pull of the major planets alters the orbital period od planeta revolution to revolution. Nongravitational effects (such as the reaction od planeta gasses boiled off during its passage near the Sun) also play an important, but smaller, role in altering the orbita. Between the years 239 BC in 1986 AD the orbital period has varied od planeta 76.0 years (in 1986) to 79.3 years (in 451 in 1066). The closest perihelion passage to the time of Jesus are 11 BC in 66 AD; neither event took place in Jesus' lifetime. It's most famous appearance was in 1066 when it was seen at the Battle of Hastings, an event commemorated in the Bayeux Tapestry.

   Comet Halley was visible in 1910 in again in 1986. It's next perihelion passage will be in 2061.

   Halley's orbita is retrograde in inclined 18 degrees to the ecliptic. in, like all comets, highly eccentric.

   Only four comets have been visited by spacecraft. NASA's ICE passed through the tail of Comet Giacobini-Zinner in 1985; Comet Grigg Skjellerup was visited by Giotto in 1989. in 1986, five spacecraft od planeta the USSR, Japan, in the European Community visited Comet Halley; ESA's Giotto obtained close-up photos of Halley's nucleus (above in right). NASA's technology demonstration spacecraft DS1 imaged the nucleus of Comet Borrelly in 2001.

   The nucleus of Comet Halley is approximately 16x8x8 kilometers.

   Contrary to prior expectations, Halley's nucleus is very dark: its albedo is only about 0.03 making it darker than coal in one of the darkest objects in the solar system.

   The density of Halley's nucleus is very low: about 0.1 gm/cm3 indicating that it is probably porous, perhaps because it is largely dust remaining after the ices have sublimed away.

   Halley is almost unique among comets in that it is both large in active in has a well defined, regular orbita. This made it a relatively easy target for Giotto et al. but may not be representative of comets in general.

   Comet Halley will return to the inner solar system in the year 2061.

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