Halley's comet is probably the best known of the periodically returning comets. It is one of the largest known to man.
Edmond Halley (1656-1742), the English mathematician and astronomer, was the first to calculate the orbital path of a comet. He discovered that the comet, which appeared in 1682, followed an eccentric path that would bring it back to Earth at regular intervals.
This observation correlated with the position of comets sighted in the years 1456, 1531 and 1607. Halley was the first to believe that this was the same comet. Under consideration of the gravitational influences of Jupiter and Saturn, which had been described by Isaac Newton, Halley correctly predicted the next appearance of the comet for 1758.
Today we know that Halley's comet reliably returns about every 76 years. We know of thirty of its appearances between 239 BC and 1986. It is expected to appear again in 2061 when it once again approaches close to the Sun.
Some of its appearances received special attention, as the exact descriptions from the years 1531 and 1607 show. The appearance of the comet in 1066 is recorded in the famous Bayeux Tapestry, a wall hanging narrating events leading up to the invasion of England by William the Conqueror. At the time, the comet was seen as a good omen for William and as a warning of the defeat of Harold II in the battle of Hastings.
Although the visibility of Halley's comet was poor during its last approach in 1986, probes were sent out to examine it more closely. The Giotto probe of the European space program came to within 600 kilometers of the comet, revealing interesting details.
The comet is much bigger than expected. It is 8 kilometers in width and height and 15 kilometers long. The nucleus is irregular in shape and has empty cavities. It contains water ice with dust particles and has a dark crust that prevents the evaporation of the ice.
The shape is long and pinched in the middle like a peanut. The surface has craters of a depth of up to 200 meters and 500-meter high protrusions. Areas of greater activity were found on the side facing the Sun. Here, gas and dust explode outwards from the surface at high speeds (1km/sec).
