Astronomy Picture of the Day
APOD: 1996 December 10 - Comet Halley's Nucleus
Explanation: Here is what a comet nucleus really looks like. For all active comets except Halley, it was only possible to see the surrounding opaque gas cloud called the coma. During Comet Halley's most recent pass through the inner Solar System in 1986, however, spacecraft Giotto was able to go right up to the comet and photograph its nucleus. The above image is a composite of hundreds of these photographs. Although the most famous comet, Halley achieved in 1986 only 1/10th the brightness that Comet Hyakutake did last year, and a similar comparison is likely with next year's pass of Comet Hale-Bopp. Every 76 years Comet Halley comes around again, and each time the nucleus sheds about 6 metres of ice and rock into space. This debris composes Halley's tails and leaves an orbiting trail that, when falling to Earth, are called the Orionids Meteor Shower.
APOD: 1997 October 3 - Comet Halley and the Milky Way
Explanation: Comet Halley was photographed superposed in front of the disk of our Milky Way Galaxy in 1986 by the Kuiper Airborne Observatory. Comet Halley is the bright white streak near this photograph's centre. Comet Halley is the most famous comet in history, and returns to the inner Solar System every 76 years. Stars visible in our Milky Way Galaxy typically lie millions of times further in the distance and orbit the Galactic centre every 250 million years. Billions of comets are thought to orbit our Sun but most do not get close enough for us to see. Similarly, billions of stars orbit our Milky Way's centre but do not get close enough for us to see.
APOD: 1996 July 6 - Edmond Halley's Greatest Discoveries
Explanation: Sir Edmond Halley was quite a discoverer. Born in 1656, he computed in 1705 that a bright comet was periodic and would make another appearance in 1758. The comet appeared as predicted and is now known as Comet Halley. Unfortunately, Halley died in 1742 and never saw his prediction come true. In 1716 Halley proposed two types of diving bells that would enable people to explore the deep sea. Halley pioneered our understanding of trade winds, tides, cartography, naval navigation, mortality tables, and stellar proper motions. Halley (incorrectly) proposed that the Earth was made of concentric spheres the size of the inner planets each of which might contain life. Perhaps Halley's greatest discovery, however, was that his contemporary Isaac Newton had discovered a powerful mathematical formulation of gravity.
Authors & editors:
& Jerry Bonnell (USRA)
NASA Technical Rep.: Jay Norris. Specific rights apply.
A service of: LHEA at NASA/ GSFC
& Michigan Tech. U.