Comets
EVSC 488/ ASTR 351
7 February 2005
- Recent Comets:
- Much maligned throughout history as harbingers of doom,
comets are "dirty-snowballs" (recipe here) in orbit around the
Sun. They become prominent when solar heat causes sublimation of trapped volatiles, producing the
coma and tail.
- Most are no larger than 10 km across.
- Unlike the planets, they tend to have highly elliptical
orbits.
- Halley's comet orbits the Sun once every 76 years.
- According to Kepler's 3rd
law (p2=a3), a=18 AU.
It's orbit carries it beyond Saturn.
- Most comets spend most of their time at vast distances
from the Sun.
- During their brief visits close to the Sun,
ices sublimate from their surface making them a spectacular sight.
- Because comets are most active when they are
close to the Sun they are usually best viewed in the morning and evening skies.
- Anatomy of a Comet
- Nucleus - a less than 10 kilometer diameter ``dirty
snowball'' consisting of water, carbon dioxide, methane, and ammonia ices and dust grains.
- Interior Models
- Mantle
- Nuclei compared:
- The nuclei are thought to be planetary "building blocks" preserved from the time of the origin of the Solar System.
- Encounters with the forming giant planets flung some of them halfway out to the nearest star.
- Coma -- cloud of gas and dust grains, often greenish in color, produced by the sublimation of the nucleus when warmed by the Sun. Diameter = 100,000 km.
- Submillimeter Observations
- Tail -- coma material pushed back by the ``solar wind" and light pressure from sunlight.
- Comet tails point away from the Sun and can be up to 1AU long.
- Since gas and dust behave differently under the
influence of sunlight pressure and the solar
wind comets usually have separate gas and dust tails.
- The gas tail, often called an ion tail, points
directly away from the Sun. It is produced when neutral gas molecules in the coma are
ionized by solar UV photons: CO + hv -> CO+.
- Dust particles are pushed out of the coma by radiation
pressure. They appear white or pink because they reflect incident sunlight
(more efficiently at longer wavelengths). Because the individual particles
are in orbit about the Sun, they produce a curved tail.
9 February 2005
- Origin of Comets
- Comets have long been considered icy planetesimals preserved from the
beginnings of the Solar System...
- Interaction with the forming planets ejected them into
the ``Oort cloud" -- a vast reservoir of comet nuclei surrounding
the solar system which extends nearly 1/2 way to the nearest
star (50,000 AU).
- The Oort Cloud may contain 1,000,000,000,000 (a trillion) cometary
nuclei, although the combined mass of these comets is less than
that of the Earth.
- Only a tiny fraction of these icy bodies are on highly elongated
orbits which will carry them close to the Sun.
- Long vs. Short Period comets:
- The Oort Cloud is the source for of long period (>200 yrs) comets.
They may be passing near the Sun for the first time or are on such
elliptical orbits that
they visit the neighborhood of the Sun only every few million years.
- Short period (<200 yrs) comets are former long period comets trapped
in the Solar System when their orbital path was modified by
gravitational attraction when passing close to a large planet.
The source of the short period comets is thought to be the Kuiper Belt
(30 - 50 AU).
- Examples of short period comets: Halley's Comet orbits the Sun once every 76 years.
Comet Encke orbits the Sun every 3.3 years.
- A comet loses about 1% of its
mass with each passage near the Sun. Short period comets
eventually become inactive and look more like asteroids
after several hundred orbits.
- Kuiper Belt objects (KBO's): (1000+ and counting)
- Comets are generally thought to be primitive bodies - icy planetesimals preserved from the origin of the Solar System..... or are they?
- Long period comets (Oort Cloud) are likely primordial.
- Short period comets are likely produced by collisions in the Kuiper Belt.
- Saturn's Moon Phoebe - Dead Comet or
Captured KBO?
- Albedo - dark 8%
- Size - relatively small, ~214 km, compared to other moons of Saturn, large for a comet nucleus!
- Orbit
- Retrograde - 548 days
- Distance from Saturn - 215 Saturn radii (Iapetus, the next closest to Saturn, is 59 Saturn radii)
- Rotation is NOT synchronous - 9.4 hours
- Meteor Showers and their relationship to Comets
A nice Comets and Meteor Showers site
- Missions
- 1986 GIOTTO, Vega 1 & 2 - Halley
- 2001 DEEP SPACE 1
- 2002 COmet Nucleus TOUR -> CONTOUR - Encke , Schwassmann-Wachmann 3, d'Arrest (or other newly discovered comet)
- 2004 STARDUST
- 2005 DEEP IMPACT 4 July 2005!
Announcements:
Cassini Spacecraft to fly 1200 km over Enceladus NEXT Thursday, 17 February!
Cassini Imaging Team Leader, Carolyn Porco, comes to UVA,
Monday, 4 April.
Astronomy Colloquium 4pm, 201 Astronomy Building.
