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)Centaurs, Kuiper belt, Scattered disk b)Scattered disk, Kuiper belt, Centaurs c)Kuiper belt, Scattered disk, Centaurs d)Kuiper belt, Centaurs, Scattered disk e)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)Centaurs b)Resonant objects c)Kuiper belt d)Plutinos e)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 C 2, 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