CometsComets

Death by Photoshop

From archives.org

Comets’ AppearanceComets’ Appearance

• Dirty snowballs

•Do not streak across the sky!

• Move slowly from night to night (fast enough to have to track a long camera exposure such as Comet West, 1976, shown here)

• Named after discoverer(s)

Comet OrbitsComet Orbits

• Orbits highly eccentric—some parabolic or hyperbolic

• Period years to millions of years

• By Kepler’s 2nd law, spend most of their time far from the Sun

• Also range in inclination—the tilt of the orbit with respect to the ecliptic

Fuzzy DiscoveriesFuzzy Discoveries• Slow background motion

• Charles Messier made his list of objects (M-numbers…) that are not comets

ComponentsComponents

• solid ice/rock nucleus

• as approaches sun, heats up and gaseous coma expands

• solar wind blows it into tail …

A Tale of Two Tails…A Tale of Two Tails…

A A dust taildust tail arced due to arced due to motion of comet (like motion of comet (like water from a hose), and water from a hose), and shines by reflection.shines by reflection.

IonIon or or gasgas tail points tail points away from Sun, and away from Sun, and glows by line emission. glows by line emission. We went through the tail We went through the tail of Halley’s comet in 1910.of Halley’s comet in 1910.

Hale-Bopp 1995

Tail stays pointed away from the SunTail stays pointed away from the Sun

Ice SublimatesIce Sublimates

A 1-km nucleus loses ~3 meters of depth per passage by the sun

Sun-grazing comets Sun-grazing comets

Most famous: Halley’s CometMost famous: Halley’s Comet

Seen since 684 AD, earlier by ChineseSeen since 684 AD, earlier by Chinese

Bayeux TapestryBayeux Tapestry

Tells of William of Normand (France) , who defeated the Saxon king Harold II of England in 1066 (Battle of Hastings)

Dates to at least 1476

The Adoration of the Magi, Giotto, 1305The Adoration of the Magi, Giotto, 1305

Hevelius,Hevelius, c. 1661 c. 1661

Edmund HalleyEdmund Halley

Predicted the return of the comet – did return after his death.

Post-Halley painting

Still feared!Still feared!

1857 woodcut

Still feared?Still feared?

Last passage: recovered in ‘82Last passage: recovered in ‘82

Spacecraft encountersSpacecraft encounters

Lick Obs. January 1986Lick Obs. January 1986

January 1986 Disconnection EventJanuary 1986 Disconnection Event

(When solar wind reverses polarity)

Soviet Vega-2 Image and a ModelSoviet Vega-2 Image and a Model

Why not round??

Comet Hyakutake (1996)Comet Hyakutake (1996)

Many comets discovered every year, as I have told my students in the past…

Shoemaker-LevyShoemaker-Levy

Eugene and Carolyn Shoemaker, David Levy discovered a comet orbiting Jupiter that would hit it in 1994

String of Pearls…String of Pearls…

Impact!Impact!

Deep Impact! – NASA probeDeep Impact! – NASA probe Launched an impact probeLaunched an impact probe Impacted Comet Temple 1 on July 2005Impacted Comet Temple 1 on July 2005

800 lbs of mostly copper, hit at 10 km/s (Cu to control 800 lbs of mostly copper, hit at 10 km/s (Cu to control emission spectrum)emission spectrum)

AsteroidsAsteroids

•Also called “minor planets”

•Discovered accidentally or with surveys

• Few thousand have determined orbits

Gaspra a representativeGaspra a representative

•Asteroids a few to a few hundred kilometers in size (Gaspra 12x20x11)

• Note the craters

Ida and its ‘moon’ DactylIda and its ‘moon’ Dactyl

Dactyl 1.5 km diameter; image by Galileo probeDactyl 1.5 km diameter; image by Galileo probe

Dactyl

Asteroid beltAsteroid belt• Belt asteroids

•Trojans-a solution to the “restricted 3-body problem” in celestial mechanics….

… … Bode’s LawBode’s Law

Take the series 0.0, 0.3, 0.6, 1.2 … and add 0.4 to each to reproduce the spacing of the planets

Missing: 2.8

Just a fluke?

NEO’sNEO’s

Earth-crossing “Near Earth Objects” are of great concern

Their sizes…

NEO SizesNEO Sizes

Near Earth Asteroid Rendezvous Near Earth Asteroid Rendezvous (NEAR-Shoemaker)(NEAR-Shoemaker)

MathildeMathilde

Eros in stereo…Eros in stereo…

Actual dates later…

Eros Orbit and Eros Orbit and Final DescentFinal Descent

Descent/impact was on February 12, 2001