What makes comets active

Most comets have highly eccentric orbits which take them far beyond the orbit of Pluto; these are seen once and then disappear for millennia. Only the short- and intermediate-period comets like Comet Halley , stay within the orbit of Pluto for a significant fraction of their orbits. After or so passes near the Sun off most of a comet's ice and gas is lost leaving a rocky object very much like an asteroid in appearance. Perhaps half of the near-Earth asteroids may be "dead" comets. 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 close encounter esp.

By far the most famous comet is Comet Halley but SL 9 was a "big hit" for a week in the summer of Meteor shower sometimes occur when the Earth passes thru the orbit of a comet. Some occur with great regularity: the Perseid meteor shower occurs every year between August 9 and 13 when the Earth passes thru the orbit of Comet Swift-Tuttle. Comet Halley is the source of the Orionid shower in October. Many comets are first discovered by amateur astronomers.

Since comets are brightest when near the Sun, they are usually visible only at sunrise or sunset. Charts showing the positions in the sky of some comets can be created with a planetarium program. JPL's lucky peanuts are an unofficial tradition at big mission events.

It's suspected that about 5, years ago a comet swept within 23 million miles of the Sun, closer than the innermost planet Mercury. Models and lab tests suggest the asteroid could be venting sodium vapor as it orbits close to the Sun, explaining its increase in brightness. A one-time visitor to our inner solar system is helping explain more about our own origins. A wayward young comet-like object orbiting among the giant planets has found a temporary parking place along the way.

As Chile and Argentina witnessed the total solar eclipse on Dec. When scientists downlinked data from Parker Solar Probe's sixth orbit, there was a surprise waiting for them: a sungrazing comet. Two Views of a Sungrazing Comet. The new mission will intercept an as-yet-undiscovered comet as it enters the inner solar system. The mission consists of three spacecraft that will capture snapshots of the comet from different angles, creating a 3D profile of the object and characterizing its surface, composition, shape and structure.

Comet interceptor is due to launch in according to ESA. Halley's Comet is likely the most famous comet in the world, even depicted in the Bayeux Tapestry that chronicled the Battle of Hastings of It becomes visible to the naked eye about every 75 years when it nears the sun.

When Halley's Comet zoomed near Earth in , five spacecraft flew past it and gathered unprecedented details, coming close enough to study its nucleus, which is normally concealed by the comet's coma. Researchers believe other comets are chemically similar to Halley's Comet. The nucleus of Halley's Comet was unexpectedly extremely dark black — its surface, and perhaps those of most others, is apparently covered with a black crust of dust over most of the ice, and it only releases gas when holes in this crust expose ice to the sun.

The comet Shoemaker-Levy 9 collided spectacularly with Jupiter in , with the giant planet's gravitational pull ripping the comet apart for at least 21 visible impacts. The largest collision created a fireball that rose about 1, miles 3, km above the Jovian cloud tops as well as a giant dark spot more than 7, miles 12, km across— about the size of the Earth — and was estimated to have exploded with the force of 6, gigatons of TNT. A relatively recent, highly visible comet was Hale-Bopp , which came within million miles million km of Earth in Its unusually large nucleus gave off a great deal of dust and gas — estimated at roughly 18 to 25 miles 30 to 40 km across — appeared bright to the naked eye.

Comet ISON was expected to give a spectacular show in However, the sun-grazer did not survive its close encounter with the sun and was destroyed in December that year. In , scientists found what could be the largest comet ever seen.

Astronomers estimate this icy body has a diameter of 62 miles to miles km to km , making it about 10 times wider than a typical comet. The comet will make its closest approach to our planet in but will remain at quite a distance even then. Most comets remain undetected frozen balls of ice in the outer reaches of our solar system. The few that occasionally venture inward warm up as they get closer to the Sun and grow the long tails that we characteristically see as they flash through the sky.

Comet Hyakutake, seen right, came as close as 9. Comets are completely frozen when they are far from the Sun, and most are just a few kilometers across. A comet's appearance changes as it approaches the Sun. Solar heat warms the comet, giving it the following anatomy:. As a comet accelerates towards the Sun, its surface temperature increases, and ices begin to sublimate into gaseous form. By the time the comet comes within about 5 AU of the Sun, sublimation has formed a noticeable atmosphere that easily escapes the comet's weak gravity.

The coma forms as the escaping atmosphere drags away dust particles that have been mixed with the sublimating ice. More ice turns into gas as the comet approaches the Sun.

Comet tails are expansions of the coma.