Announcements

Announcements

• Announcements

• HW6 available today, due in a week
• Use Kevin as the TA for this one
• 2 In-class assignments left in 3 lectures

Observations of comets

• Observations of comets

• What are comets?

• Composition and structure

• Cometary tails

• Ion and dust tails

• Where do comets come from?

• Orbits of comets
• Oort cloud
• Scattered Kuiper Belt

Comets have been known from ancient times

• Comets have been known from ancient times

• Thought to foreshadow disasters and major battles

• Pre-telescopes the known solar system was a pretty empty place

• Moon and the Sun
• Mercury, Venus, Mars, Jupiter, Saturn
• And COMETS
• No Uranus
• No Neptune
• No planetary Moons (except ours)
• No Asteroids
• No Kuiper Belt Objects

People have recorded comet sightings for millennia

• People have recorded comet sightings for millennia

Ancient Greeks thought comets were atmospheric phenomena

• Ancient Greeks thought comets were atmospheric phenomena

• In the west this went unchallenged until telescopes came along
• Tycho Brahe’s parallax measurements proved this wrong
• Comets were much further away than the Moon

• Renaissance astronomers thought comets moved in straight lines through the solar system

• Even Kepler argued they shouldn’t follow elliptical orbits like the planets

• In the 1680s astronomers tracked a comet and showed it had an elliptical orbit

• Comets were solar system objects – just like planets

Newton finally settled this in his ‘Principia Mathematica’ (1687)

• Newton finally settled this in his ‘Principia Mathematica’ (1687)

• Showed that comets moved in parabolic or elliptical orbits by the Sun’s gravity

If they have orbits… then they’re periodic

• If they have orbits… then they’re periodic

• The same comet should come back

• In 1705 Edmund Halley connected the dots…

• Used Newton’s laws to figure out the orbit of many comets
• Comets seen in 1531, 1607, and 1682 were the same object
• Predicted a return in 1759

• Halley’s comet has been seen ~30 times

Many telescopic observations of comets (including Halley’s comet)

• Many telescopic observations of comets (including Halley’s comet)

• Even a few spacecraft missions

• Giotto & Vega
• Comet Halley
• Deep-space 1
• Comet Borrelly
• Stardust
• Comet Wild 2
• Deep Impact
• Comet Tempel 1

Comets have several parts

• Comets have several parts

• Nucleus
• ~10 km
• Coma
• ~1,000,000 km
• Almost as big as the sun!
• 1,400,000 km
• Hydrogen envelope
• ~10,000,000 km
• Tail
• Ion tail
• Dust tail
• ~100,000,000 km
• About 2/3 of 1AU!

Cometary nuclei are usually invisible from the Earth

• Cometary nuclei are usually invisible from the Earth

• Hidden by the coma
• Spacecraft missions can visit far from the Sun when the coma is inactive

Comet Nuclei are ‘dirty snowballs’

• Comet Nuclei are ‘dirty snowballs’

• Random mixtures of ices and dark stuff
• Ices
• Mostly water ice
• A little CH4, CO, CO2 etc
• Dark ‘stuff’
• Organic compounds (H,C,O)
• Rock-like material
• Like asteroids
• Very small objects
• Not enough self-gravity for
• a round shape

When comets are close to the sun

• When comets are close to the sun

• Surface heats up
• Ice sublimates (turns to vapor)
• Dark organic stuff gets concentrated on the surface

Comets are ice rich

• Comets are ice rich

• …but among the darkest objects in the solar system

• Albedo of 2-4%
• Like tar
• Comet nuclei are very hard to see without their comas

This thick crust builds up over many orbits

• This thick crust builds up over many orbits

• Sublimating ice comes out in jets
• Collapse pits form on the surface from removal of sub-surface ice
• Jets act like rocket engines – can alter the orbits of comets

Comet nuclei are typically small < 40km

• Comet nuclei are typically small < 40km

• Mass estimates come from spacecraft flybys

• Comets are very low density

• Contain significant internal voids

Sublimation jets produce cometary atmosphere

• Sublimation jets produce cometary atmosphere

• Mostly water ice crystals – some dust
• Comet’s gravity can’t hold onto this material

• Occasionally a big piece of the comets surface will break off exposing fresh ice

• Comet Holmes brightened by a factor of 1 million within a few days

What happens to the water ice crystals?

• What happens to the water ice crystals?

• UV solar radiation breaks up the water molecules

Comets have two tails

• Comets have two tails

• Ion tail of OH- and H+
• Ions are swept up by the solar wind
• Ion tails point away from the Sun
• Blue-ish in color

Dust tails

• Dust tails

• Also swept by the solar wind but less efficiently
• Dust tail is brighter and whiter
• Tail direction affected by the comets motion and is curved

Comets can appear to have a tail and an anti-tail

• Comets can appear to have a tail and an anti-tail

Cometary orbits are very different from asteroids

• Cometary orbits are very different from asteroids

• Comets have very elliptical orbits
• Comets have randomly inclined orbits
• Comets have very large orbits

Divided into short period (<200 years) and long period (>200 years)

• Divided into short period (<200 years) and long period (>200 years)

• Short period comets

• Jupiter family comets (Periods <20 years)
• Orbits controlled by Jupiter
• All low inclination
• Halley family comets (Periods 20-200 years)
• Come from the Kuiper Belt
• Spread in inclinations
• Eventually transition to Jupiter family comets

Long-period comets

• Long-period comets

• Have totally random inclinations
• Have very long periods/large orbits
• Many of these appear to be on their first pass through the inner solar system
• A body with a semi-major axis of 10,000 AU will orbit once every million years

The Oort cloud

• The Oort cloud

• A spherical cloud of billions of comets far from the sun
• Explains the random inclinations of the long-period comets

Comets form closer to the giant planets

• Comets form closer to the giant planets

• Gravitational encounters

• Fling them into very distant orbits
• Allow the giant planet to migrate

• Passing stars randomize the orbital inclinations

• Less so for objects closer to the sun

• Only a small fraction of the original objects survive

• Sharp outer edge of the Kuiper belt is not continuous with the Oort cloud

What knocks these comets into the inner solar system?

• What knocks these comets into the inner solar system?

• Planets have no influence here

• Passing stars?

• Nearest star ~4 light years away
• ~250,000 AU
• Twice the Oort cloud distance

• Galactic tides?

• As the sun orbits the galactic center
• Takes ~250 million years

Why do all the short-period comets have low inclinations?

• Why do all the short-period comets have low inclinations?

• They come from a disk not a spherical cloud
• This is why the Kuiper Belt was postulated

Scattered disk objects encounter Neptune

• Scattered disk objects encounter Neptune

• Are perturbed into smaller orbits
• Wander among the gas giants as Centaurs
• (half KBO, half comet)
• About 1/3 make it to the inner solar system
• Become Jupiter family comets
• Other 2/3 are swept up by one of the giant planets
• Takes 1-10 million years

Comets lose more ice on each pass close to the sun

• Comets lose more ice on each pass close to the sun

• Eventually the thick outer cover seals off the ice
• No more cometary activity

• Some asteroid-like objects are in comet-like orbits

• Some asteroid-like objects suddenly develop comas

• Impacts disturb surface cover
• or
• Move closer to the sun
• Chiron developed coma and tail
• People were puzzled as this was before KBOs were known

Another common fate of weak cometary bodies is to break up

• Another common fate of weak cometary bodies is to break up

• Tidal forces from close approaches to planets

What are comets?

• What are comets?

• Dirty snowballs – removal of ice leave dirt on the surface
• Ice sublimates in jets through a debris cover and produces a coma

• Cometary tails

• Ions tails are bluish and point away from the sun
• Dust tails move slower and so are curved due to comet’s motion

• Where do comets come from?

• Short-period comets are dominated by Jupiter
• Low inclination orbits means resupply from a disk – the Kuiper belt
• Long period comets have random inclinations
• Resupply from a distant spherical reservoir – the Oort cloud