Kepler’s discovery of the elliptical nature of orbits helped pave the way for a better understanding of cometary motion. Most significant historical episode was Halley’s prediction of the return of the Great Comet in 1757. Halley's comet has been followed down through the ages and made a foreboding appearance during the Battle of Hastings in 1066 and appears in the Bayeaux Tapestry. Every passage of this comet since 240 BC has been noted. Improvements in mathematical techniques during the early 19th century made the prediction of orbits more reliable and many comets were observed and catalogued. Historical
Comets consist largely of compounds of carbon, hydrogen, oxygen, and nitrogen, i.e. so-called “CHON” compounds. These compounds include ices of water, ammonia, methane, carbon monoxide, and smaller amounts of other, more complex compounds. Comets are also made of dust containing silicate minerals found in the crusts of the terrestrial planets, and a mix of silicate and “CHON” similar to that found in carbonaceous chondrites. When far from the sun the comet is a cold, dark "ice ball" only a few km across. As it nears the sun it begins to "melt" and forms nucleus. Eventually a coma extending as much as 100 000 km from the nucleus forms. The comet becomes surrounded by a sheath of hydrogen gas that is easily excited to glow. The pale white light that we see is the result of fluorescence. Cometary tails can reach tens of millions of km in length! What is a Comet?
The Orbits of Comets Comets are classified as either short-period or long- period objects: Short Period: (such as Halley's) with periods from years to decades, low orbital inclination and prograde motion. Long Period: centuries to thousands of years (Hyakutake: Period 18 400 a), any inclination and just as likely retrograde as prograde
A comet in a long, elliptical orbit becomes visible when the sun's heat vaporizes its ices and pushes the gas and dust away in a tail.
The orbit of the Leonids' parent body, comet Tempel-Tuttle, projected onto the ecliptic plane (i.e., the plane in which the Earth orbits the Sun). The indicated positions of the planets and the comet are those of February 28, 1998, the date of the comet's closest approach to the Sun.
Kuiper Belt Disk-shaped region beyond the orbit of Neptune that contains countless icy objects; the source of short-period comets such as Halley.
Oort Cloud A roughly spherical volume, extending more than 100,000 AU from the Sun, in which up to a trillion small icy bodies are thought to reside; the source region for “new” and long-period comets.
Passing stars can perturb Oort Cloud send comets “falling into the solar system Origin of periodic mass extinctions?
Impacting comets may have brought water to the early Earth
Several spacecraft encountered Halley's comet in 1986, but the most significant was the European Space Agency's Giotto. Launched on July 2nd 1985, it encountered Halley on March 13th 1986, approaching to within 600km of the comet's nucleus. Giotto carried many scientific instruments including cameras and dust detectors. Giotto found that the nucleus of the comet measured around 16 by 8 by 7.5 km, and that the comet was indeed the "dirty snowball" which Fred Whipple had suggested years previously. The nucleus showed hills and "valleys", although the shape was being altered continuously by the jets of gas coming through the surface and forming the tail and coma. Giotto Mission to Halley’s Comet
Stardust Discovery Mission 1999-2006 Stardust is the first U.S. space mission dedicated solely to the exploration of a comet, and the first robotic mission designed to return extraterrestrial material from outside the orbit of the Moon. The Stardust spacecraft will fly between about 63 to 93 miles (100 to 148 km) in front of the nucleus, and through the halo of gases and dust at the head of comet Wild 2. During this passage the spacecraft will collect dust and volatiles. A volatile is material difficult to capture or hold permanently. The comet samples are expected to be made up of ancient pre-solar interstellar grains and nebular condensates that were incorporated into comets at the birth of the solar system.
Comet Nucleus Mission (ESA) Rosetta's eleven-year mission began with a launch on March 2, 2004 from Kourou, French Guiana. Flying first out to Mars and then back to Earth, Rosetta will use the gravitational momentum from both planets to slingshot it farther into space. It will then pass by asteroid Steins in Sept. 2008 and complete another Earth gravity assist in November 2009. Rosetta will fly by asteroid Lutetia in July 2010 and finally reach comet Churyumov-Gerasimenko in May 2014.