Phil Plait Talk

Comets Consists of a nucleus (Ice-Rock Densities g/cc) containing dust & frozen gas Tail appears when nucleus heats from Sun – gases are released. Objects originate from Kuiper belt and Oort Cloud

Comet nuclei

Comets: Coma Appears when comet about 5AU from the sun – Comet warms up releasing dust and gas

Comet Dust Tail Tails form about 1AU from Sun. Released dust is pushed by radiation pressure from sun, creating a dust trail. Dust Tail

Plasma (ion) tail Released gas is stripped of electrons from Sun’s UV light. Ions (in this case atoms lacking electrons) are pushed by ions from the sun called Solar Wind creating the plasma tail Plasma Tail

Plasma tail always points away from sun. Tail is usually about Million Km long (.01AU) Size of nucleus <10Km Key Points

Rosetta Mission: Did water on Earth come from comets?

Comet 67P/Churyumov-Gerasimenko

Rosetta Results: Deuterium to water ratio is much larger on comet than on Earth so probably not – at least from comets originating from 67P distance. Also….organic compounds are present!

Meteor Showers Larger dust particles released from comet aren’t easily “pushed” by radiation pressure from Sun Dust particles are left in the inner solar system and create Meteor showers The most meteors seen are at predawn as Earth moves into the dust. Radiant

Evidence of comet material on Earth Tutankhamun’s Brooch: At center of yellow scarab is a piece of comet!

Comet Trajectories

Comet Trajectories: Halley nears Earth’s orbit twice

Halley’s Comet verified Newton’s theory of Gravity Halley 1680/82 saw famous comet Paid Newton to publish orbital mechanics theories (Principia) in order to understand motion Using old observational data (1531, 1607) determined this was the same object (Was noted as early as 1 st century.) Made prediction of return in 1758 and it did.

Origins of Comets I Kuiper belt: about AU away (Neptune and beyond) Most of these objects have stable orbits; move in same direction as planets until... …Gravity from a larger Kuiper belt object pushes smaller objects off orbits – pulled in by Sun’s gravity.

Origins of Comets II Oort cloud surrounds the solar system 1000 – 100,000 AU in distance trillions of objects Orbits have random direction, tilts to ecliptic and eccentricities

The planets take up a very small area of solar system. Kuiper belt objects have ice-rock densities. It is assumed that Oort Cloud Objects have similar densities as well.

Oort cloud distance scale