CometsComets

Comets

Hale-Bopp and the Andromeda galaxy

Comet miscellaneousComet miscellaneous

Discovered as faint, fuzzy patchesDiscovered as faint, fuzzy patches Travel in highly elliptical orbitsTravel in highly elliptical orbits Brightens and develops a tail as it Brightens and develops a tail as it

nears the Sunnears the Sun Emit no visible light of their ownEmit no visible light of their own A few dozen are discovered each A few dozen are discovered each

year, more go by unnoticedyear, more go by unnoticed

Comet miscellaneousComet miscellaneous

They are essentially ice left over They are essentially ice left over from the formation of the gas from the formation of the gas giantsgiants

Are named after their discoverers Are named after their discoverers – the first 2 people to report it– the first 2 people to report it

About half of all discoveries are by About half of all discoveries are by amateur astronomersamateur astronomers

The name comet comes from the The name comet comes from the Greek word Greek word kometeskometes, meaning , meaning “wearing long hair”“wearing long hair”

This comet appeared to pass in front of the Ring Nebula

An earlier view

Parts of a CometParts of a Comet

Nucleus: main solid bodyNucleus: main solid body– Typically only a few kilometers wideTypically only a few kilometers wide– The only part that remains when the The only part that remains when the

comet is far away from the suncomet is far away from the sun Coma: the halo around the nucleusComa: the halo around the nucleus

– Forms and becomes larger as the Forms and becomes larger as the comet comes toward the suncomet comes toward the sun

– Can become almost as large as Can become almost as large as Jupiter or SaturnJupiter or Saturn

Parts of a CometParts of a Comet

Hydrogen envelope: Surrounds Hydrogen envelope: Surrounds the cometthe comet– Invisible, stretches millions of km in Invisible, stretches millions of km in

spacespace Ion tail: straight, makes glowing Ion tail: straight, makes glowing

linear streamerslinear streamers Dust tail: broad and gently curvedDust tail: broad and gently curved

– Rich in dust that reflects sunlightRich in dust that reflects sunlight

Comets tails, cont.Comets tails, cont.

Always point away from the sun, Always point away from the sun, due to solar wind due to solar wind

Lies outside the comets orbitLies outside the comets orbit Is in front of the comet, once it Is in front of the comet, once it

goes around the sungoes around the sun

This picture of Hale-Bopp shows the white dust tail and the blue ion tail

Dust and ion tails of Comet NEAT

Physical PropertiesPhysical Properties

Its mass decreases with time (it Its mass decreases with time (it loses material as it goes around loses material as it goes around the sun)the sun)

Made of dust particles, methane, Made of dust particles, methane, ammonia, carbon monoxide, and ammonia, carbon monoxide, and water icewater ice

Long Period CometsLong Period Comets

Comets may go as far as 50,000 Comets may go as far as 50,000 A.U. from the sunA.U. from the sun

With very elliptical orbits, they slow With very elliptical orbits, they slow down, according to Kepler’s 2down, according to Kepler’s 2ndnd law law

Most take hundreds of thousands Most take hundreds of thousands (maybe millions) of years to (maybe millions) of years to complete an orbit. complete an orbit.

May be part of a huge cloud (Oort May be part of a huge cloud (Oort Cloud) orbiting far from the sunCloud) orbiting far from the sun

How many comets do you see?

Comet Bradford is to the left and Comet LINEAR to the right.

Short Period CometsShort Period Comets

Fewer in numberFewer in number Have orbital periods of less than Have orbital periods of less than

200 years200 years Do not venture far beyond the Do not venture far beyond the

orbit of Plutoorbit of Pluto Originate in the Kuiper beltOriginate in the Kuiper belt

Short v’s Long Period Short v’s Long Period CometsComets Short period comets orbit in Short period comets orbit in

roughly the same plane as the roughly the same plane as the planets (ecliptic)planets (ecliptic)

Long period comets orbit in all Long period comets orbit in all directionsdirections

Comet Encke returns every 3.3 years. Its orbit goes just outside of Mars and back!

Halley’s CometHalley’s Comet

British astronomer Edmund Halley British astronomer Edmund Halley realized that a comet seen in 1682 realized that a comet seen in 1682 was not a one time event.was not a one time event.

He calculated its path and found He calculated its path and found that it had an orbital period of 76 that it had an orbital period of 76 yearsyears

He predicted that it would He predicted that it would reappear in 1758reappear in 1758

He did not live to see it, but he He did not live to see it, but he was correctwas correct

Halley’s CometHalley’s Comet

Ancient cultures record Halley’s Ancient cultures record Halley’s comet in every pass since 240 B.C.comet in every pass since 240 B.C.

Was photographed in 1910 and Was photographed in 1910 and again in 1986again in 1986

In 1986, a Soviet craft, Vega 2, In 1986, a Soviet craft, Vega 2, traveled through its coma and was traveled through its coma and was only 8000 km away from its nucleusonly 8000 km away from its nucleus

Halley’s CometHalley’s Comet

A European spacecraft, Giotto, A European spacecraft, Giotto, came within 600 km of its nucleus came within 600 km of its nucleus to take picturesto take pictures

Found that the nucleus was Found that the nucleus was potato shaped (15 km x 10 km) potato shaped (15 km x 10 km) and is almost blackand is almost black

Will next be near us in 2061Will next be near us in 2061 When it passed in 837 AD, it was When it passed in 837 AD, it was

as bright as the crescent moonas bright as the crescent moon

Comet Halley’s nucleus

Hale-BoppHale-Bopp

Biggest comet to visit the solar Biggest comet to visit the solar system in more than 200 yearssystem in more than 200 years

Was visible to the naked eye for Was visible to the naked eye for more than a yearmore than a year

Will return…in 2,380 years…Will return…in 2,380 years…sorry!sorry!

Comet Hale-Bopp early after its discovery

Comet Shoemaker-Comet Shoemaker-Levy 9Levy 9 Discovered in 1993 orbiting Discovered in 1993 orbiting

JupiterJupiter Jupiter’s gravity ripped it into 21 Jupiter’s gravity ripped it into 21

piecespieces These pieces crashed into Jupiter These pieces crashed into Jupiter

a year latera year later

Shoemaker-Levy 9 impacting Jupiter

Comet Ikeya-Shang and the Northern Lights!

Comet Hyakutake and a cactus

Comet McNaught – 2007

Comet Holmes – Fall 2008

2010 – Comet Hartley2 and the Perseus Double Star Cluster