Comets

Astronomy 311Professor Lee

CarknerLecture 22

Which of the following is not true about Pluto ?

a) Its composition is different from that of any other planet

b) Its orbit is highly inclinedc) It has several moonsd) HST has resolved craters on

its surfacee) It crosses the orbit of Neptune

Which correctly lists TNO regions from closest to the sun to furthest away?a) Centaurs, Kuiper belt, Scattered

diskb) Scattered disk, Kuiper belt, Centaursc) Kuiper belt, Scattered disk, Centaursd) Kuiper belt, Centaurs, Scattered diske) Scattered disk, Centaurs, Kuiper belt

How are TNOs discovered?

a) By looking for the spectrum of iceb) By surveys conducted with X-ray

telescopesc) By looking for objects that change

position between telescope exposures

d) By the way their gravity alters the orbits of the planets

e) By looking for objects similar to Neptune

Which kind of TNOs have had the least orbital alteration?

a) Centaursb) Resonant objectsc) Kuiper beltd) Plutinose) Scattered disk

Upcoming

Quiz #3 on Monday, Oct 27 Covers Gas Giants through The Sun

Final exam Monday Nov 3, 3pm Covers entire course

Observing project due next Friday, Oct 31

Comets Throughout History

People throughout history have observed the passing of comets

“When beggars die, there are no comets seen;The heavens themselves blaze forth the death of princes.”

--Julius Caesar, Act II, Scene II

Comet Halley Around 1700 Edmund

Halley was studying the records of a comet that seemed to reappear at regular intervals

In 1758 the comet returned just as Halley predicted

Comet Halley will return again in 2061

Finding Comets

Large observatories do not have time to spend looking for them

If you see a faint fuzzy patch in the sky with your telescope, that is a good candidate for a new comet

Comets are generally named after their discoverers, e. g. Comet Hale-Bopp More and more comets are being found by automated observatories

Observing a Comet

When we look at a comet with our eyes (or a small telescope) we see:

Coma:

Tail: Long streamer of gas and particles that can be more than 100 million km long

Structure of a Comet

Comet Tails The tail is the most visible and most dramatic part of a comet

Ion Tail (blue)

Always points away from the Sun Dust Tail (yellow)

Points roughly away from the Sun, but is curved back towards the Sun by gravity

The Two Tails of a Comet

Angular Size

Often given in arcminutes 60 arcminutes = 1 degree

tan (½) = (½s)/d Where s and d have the same units

The Heart of the Comet

This is what the comet looks like far from the Sun and is the source of the tail and the coma

Composed of rock and ice

Comet Jets

The heat from the Sun boils off material The material of the comet is well mixed

These jets can change a comet’s orbit Comet orbits cannot be strictly predicted by

Newton’s laws

Comet Composition A comet is a mixture of ice and rock

Comets are composed of:

Water (ice) Ammonia

Comets contain many carbon compounds including C2, CH, CN (cyanogen)

Comet Orbits Comets have highly elliptical orbits that

bring them close to the Sun and then back to the Kuiper Belt or Oort Cloud

Short period comet orbits are constantly being altered by Jupiter and Saturn

Meteor Showers

As the comet circles the Sun its orbit fills up with lost material

When the Earth passes through this material we get a meteor shower Meteors are small dust particles and thus

burn up before they reach the ground

Anatomy of a Meteor Shower

The Perseid Meteor Shower

Occur every year around August 12

Get about 50 meteors per hour One of the best meteor showers

The Perseids are debris from Comet Swift-Tuttle

Comet Deliveries

Comets could be a source of volatiles, including water

Could the Earth’s water have come from comets?

Death of a Comet At each passage, the comet loses material

Eventually all the volatiles will boil off

Comets can also hit a planet or be ejected from the solar system in a close encounter

Spacecraft Studying Comets Imaging

Gathering Stardust (1999) -- gathered (Jan 2004) and

returned (2006) a sample of the coma of Comet Wild 2

Impacting

Landing Rosetta (2004) -- will land a probe on the

nucleus of Comet 67P/Churyumov-Gerasimenko (2014)

Next Time

Read Chapter 16

Summary Comets are small (10 km) bodies

that have highly elliptical orbits that originate in the Kuiper belt or Oort cloud

The Sun boils off material making the comet visible

Comets can produce meteor showers and large impacts

Summary: Comet Structure

Nucleus: small (10km) core that is the source of the comet material

Coma: large (~1 million km) cloud of gas around the nucleus

Tail: comets have two tails, both pointing away from the Sun: Ion -- pushed by solar wind Dust -- pushed by solar light pressure

Jets: gas expelled from the nucleus under pressure