Download Comets - Helios

Comets
Astronomy 311
Professor Lee Carkner
Lecture 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 inclined
c) It has several moons
d) HST has resolved craters on its
surface
e) It crosses the orbit of Neptune
Which correctly lists TNO regions
from closest to the sun to furthest
away?
a)
b)
c)
d)
e)
Centaurs, Kuiper belt, Scattered disk
Scattered disk, Kuiper belt, Centaurs
Kuiper belt, Scattered disk, Centaurs
Kuiper belt, Centaurs, Scattered disk
Scattered disk, Centaurs, Kuiper belt
How are TNOs discovered?
a) By looking for the spectrum of ice
b) By surveys conducted with X-ray telescopes
c) 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)
b)
c)
d)
e)
Centaurs
Resonant objects
Kuiper belt
Plutinos
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 (½q) = (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 SwiftTuttle
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