Halley’s Comet
*Halley’s Comet is a comet that appears in the sky about every 76 years
*Halley’s Comet has become the best-known comet because it returns more often than most comets and usually be seen without the aid of a telescope
*Halley’s Comet orbits the Sun in a long oval path that brings it close to the Sun about every 76 years
*Halley’s Comet was named for the English astronomer Edmond Halley
*Astronomers expect the Halley’s Comet next return in 2061
When the comet goes back in space
*as a comet gets farther away from the Sun it gets colder
*when it gets colder not as much ice melts and not as much gas and dust is released then the head and tail start to shrink
*After a while the comet freezes until it gets closer to the Sun
Ancient times
*Comets are ancient remains of ice and dust left over after the planets formed
*in ancient times they believed a comet was a bad sign and that disease, wars and other disasters would follow
*Ancient Britons believed they had a good reason to fear the comet that appeared in 1066
*in 1066 Britain was invaded by the French, the French killed the British ruler King Harold
*People in ancient times believed that comets were “long-haired” stars
How comets change when close to Sun
*1st the Sun’s heat warms the outer surface of the comet and melts some of its ice
*2nd the ice changes into gas and forms a cloud around the nucleus, at the same time dust that was frozen in the ice is released in the gas
*3rd the cloud of gas and dust begins to reflect sunlight
*4th as the comet comes closer to the Sun the cloud around the nucleus grows larger and shines more brightly in the sunlight
The comet’s main parts
*a comet has 3 main parts the nucleus, the head, and the tail
*the brightest part of a comet, the comet’s head, is made up of the nucleus and the coma
*deep in the core of the coma is the only solid part of the comet, the nucleus
*the gas and dust form an outer layer called the coma
*the two visible parts of a comet are the head and the tail
*the nucleus is small, sometimes measuring less than 10 miles across
*a comet’s coma can be larger than the Sun which is 864,000 miles wide
*a comet’s solid core is often only several tens of miles in diameter, smaller than most bodies in the solar system
A comet’s tail
*A comet’s tail can stretch as long as 100 million miles
*the dust tail is up to 10 million km long composed of smoke-sized dust particles driven off the nucleus by escaping gases
*the tail of a comet is longest when it is close to the Sun, and it always points away from the sun so it follows behind
*the Sun’s radiation and the solar wind push against the particles of gas and dust around a comet’s nucleus, forming the comet’s tail
*if the tail is straight and blue it is made mostly of gas
*if it is curved and yellow it is made mostly of dust
*well-developed comets have 2 tails
*many comets grow 2 tails, one straight and the outer curved
*a comet’s tail can be hundreds of millions of kilometers long and stretch across most of the sky
* As a comet nears the Sun it sends off strong jets of gas and dust, mostly from the coma
*when a comet gets close enough to the Sun the energy in the sunlight turns the ice into gas, releasing dust
Comet’s orbits
*since their orbits are so elliptical few of them pass near Earth
*comets are chunks of ice and dust whose orbits are usually very long narrow ellipses
*comets travel through space in an orbit that takes them around the Sun
*a comet whose orbit takes it near the Sun is also likely to either impact one of the planets or the Sun or to be ejected out of the Solar System by a closer encounter
Where Comet’s came from
*Astronomers think that most comets are collected in a “cloud” at the outer reaches of the Solar System called the Oort cloud
*other comets came from an area called the Kuiper Belt that is just beyond Neptune’s orbit, near Pluto
*the Oort cloud is 3 trillion miles away from the Sun
Additional Information
*since comets are brightest when near the Sun, they are usually visible only at sunrise or sunset
*a few comets actually hit the Sun and are destroyed instantly
*the closer a comet gets to the Sun the faster it gets because of the Sun’s gravitational pull
*when the gravity between the Sun and the Oort cloud causes dust and ice to come together a comet is formed *”long-period” comets travel much farther and may take thousands of years to return to the Sun
*the point in a comet’s orbit when the comet is nearest to the Sun is called its “perihelion”
*when a comet is farthest from the Sun it is at its “aphelion” point
*a comet usually is brightest when it is at its perihelion
*most comets are named after the person or persons who discovered them
*Comets that take less than 200 years or less to complete their orbit around the Sun are called short-period comets
*after 500 or so passes near the Sun off most of a comet’s ice and gas is lost leaving a rocky object