COMETS

COMETS

We distinguish between 2 principal types of Comets

Orbital Characteristics of Selected Comets

Comet	Distance	from Sun	Last Orbital	Period
	Closest (AU)	Farthest (AU)	(Year)	(y)
Long	Period	Comets
Donati	0.58	313	1858	2000
Huamson	2.13	400	1962	290
Morehouse	0.95	Large	1908	Long
Burnham	0.50	Large	1960	Long
Kohoutek	0.14	Large	1973	Long
Short	Period	Comets
Encke	0.34	4.2	1987	3.3
Tempel II	1.4	4.8	1988	5.3
Swift-Tuttle	0.96	47	1982	120
Halley	0.59	28.4	1986	76

Comets originate mainly in the Oort cloud.

Comets are small bodies composed basically of ice and dust.

Comets appear to be remnants of the planetesimals that accreted to form the outer planets and the satellites. Unaffected by the processes of planetary differentiation, comets may tell us about the nature of the solids that condensed in the outer reaches of the ancient solar nebula.

Periodically, some comets are gravitationally forced into shorter elliptical orbits that take them into the inner solar system.

Once inside Mars orbit, these icy snowballs become active; their icy nuclei partially vaporize when they come close to the sun, forming large tails of gas and dust.

They develop into these distinct parts:

nucleus: relatively solid and stable, mostly ice and gas with a small amount of dust and other solids

coma: dense cloud of water, carbon dioxide and other neutral gases sublimed from the nucleus

hydrogen cloud: huge (millions of km in diameter) but very sparse envelope of neutral hydrogen

dust tail: up to 10 million km long composed of smoke-sized dust particles driven off the nucleus by escaping gases; this is the most prominent part of a comet to the unaided eye

ion tail: as much as several hundred million km long composed of plasma and laced with rays and streamers caused by interactions with the solar wind

Comet Halley Facts

Historical bits: Chinese records of Comet Halley go back

to at least 240 BC. In 1066 AD it was seen right before the Battle of Hastings. 1705 Edmond Halley predicted that the comet seen in 1531, 1607, and 1682 would return in 1758. It did, unfortunately Halley had died. The comet was later named in his honor.

5 spacecraft (USSR, Japan, EU) visited Comet Halley in 1986.

Giotto came to within 600km of nucleus.

The nucleus of Comet Halley is approximately 16x8x8 km.

Halley's nucleus is darker than coal.

The density of Halley's nucleus is very low: 0.1-0.4 gm/cm³ indicating that it is probably porous, perhaps because it is largely dust remaining after the ices have sublimed away.

Comet Shoemaker-Levy 9

In 1992, SL 9 passed by Jupiter within the Roche limit. It was broken into at least 21 separate fragments which were dispersed several million kilometers along its orbit. Between 16 July 1994 and 22 July 1994 the fragments impacted the upper atmosphere of Jupiter.

Fragment A struck Jupiter with its kinetic energy equivalent to about 225,000 megatons of TNT creating plume which rose about 1000 km above the Jovian cloudtops

Fragment G which struck Jupiter with an estimated energy equivalent to 6,000,000 megatons of TNT (about 600 times the estimated arsenal of the world). The fireball from fragment G rose about 3000 km above the Jovian cloudtops.

The dark semicircles south of the impact points are probably an ejecta blanket composed of fine material condensed from the plume. This material is either from the comet itself or from Jupiter and is suspended in the upper atmosphere. Some scientists refer to it as soot.