Comets…another major class of solar system objects

Comets appear at irregular intervals and are not confined to the ecliptic

Observed features noted in “De Cometis” by Seneca, ~ A.D. 62 “Ratio terrorem prudentibus

excutit”

Principal Observational Features

• Coma• Dust Tail• Ion tail

Ion tail glows with light of molecular ions, CO+, N2

+, CO2+

Question for the august assembly: Why would you

expect the ion tail to move on a different trajectory than the dust

tail????

Tail of a comet can stretch a substantial fraction of an a.u.

Point to keep in mind: Comets obey Kepler’s Laws: they move

on elliptical orbits around the Sun. They can be perturbed by gravitational force from planet.

The Structure of a Comet

• Deduced about 40 years ago

• Nucleus of solid matter in center of coma

Ice in nucleus sublimes to form coma

A remarkable thing about comets is that a prominent structure

extending over 1 astronomical unit has its origin in a solid

object which can be from 1 – 10 km in diameter. Even a very

large one would have a diameter of 50 kilometers.

Nucleus forms coma and tails by sublimating ice gas.

Cometary nucleus must be ices such as water, CO2, and CH4. Ions are produced by photoionization of molecules by UV light

Until 20 years ago these were (well established) theoretical predictions. Starting in 1986, there have been a

number of spacecraft visits to comets, and images have been made of the cometary nuclei. See p171 of text

Comet Halley (about 15 km long)

Comet Wild 2 (visited by Stardust about one year ago)

Orbits of Comets

Recall properties of the orbits of the major planets

• Semimajor axis a: 0.4- 30 au.

• Eccentricity: 0.00 – 0.090 – 0.20

A cometary orbit (Halley’s Comet)

“I’m a believer…” (in ellipses)

Properties of cometary orbits (see Table 7-1, p156)

• Table gives a, eccentricity, and q (perihelion distance) for a whole bunch of comets

• Short Period Comets: a ~ few au, ecc ~ 0.39 – 0.96 (highly eccentric compared with major planets)

• Long Period Comets: a~ 160, 200, or greater, ecc ~ 0.996, 0.990, 1.0

Question: why do these classes have the names they do?

The Oort Cloud: vast repository of comets