Comets Astronomy 311 Professor Lee Carkner Lecture 22

Upcoming  Quiz #3 on Monday, Nov 6  Covers Gas Giants through The Sun  Final exam Monday Nov 13, 3pm  Covers entire course  Observing project due next Friday, Nov 10

Comets Throughout History  People throughout history have observed the passing of comets  Comets have been thought to be bad omens and harbingers of doom “When beggars die, there are no comets seen; The heavens themselves blaze forth the death of princes.” --Julius Caesar, Act II, Scene II

Edmund Halley

Comet Halley  Around 1700 Edmund Halley was studying the records of a comet that seemed to reappear at regular intervals  He used Newton’s new laws to determine its orbit (P=76 years so A=18 AU)  In 1758 the comet returned just as Halley predicted  He was the first to realize that comets are solar system objects on highly elliptical orbits  Comet Halley will return again in 2062

Comet Halley in 1986

What is a Comet?  A comet is an icy planetesimal left over from the formation of the solar system  Comets have highly elliptical orbits which bring them into the inner solar system  The solar heat and the solar wind create a highly visible cloud and tail of material

Finding Comets  Comets are quite faint and hard to see when they are far from the Sun  Large observatories do not have time to spend looking for them  Comets are generally found by amateur astronomers  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

Comet Hale-Bopp in 1997

Comet Hale-Bopp

Comet C/1999 S4 (LINEAR) from Augustana

Observing a Comet  When we look at a comet with our eyes (or a small telescope) we see:  Coma: A sphere of glowing gas and particles about 1 million km in diameter  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  A comet generally has 2 tails:  Ion Tail (blue)  Composed of ions swept off the comet by the solar wind  Always points away from the Sun  Dust Tail (yellow)  Composed of dust particles swept up by light pressure  Points roughly away from the Sun, but is curved back towards the Sun by gravity

The 2 Tails of Comet Halley

The Two Tails of a Comet

The Heart of the Comet  At the center of the comet is the nucleus  This is what the comet looks like far from the Sun and is the source of the tail and the coma  Size: ~10 km  Composed of rock and ice

The Nucleus of Comet Halley

Comet Borrelly from DS1

Deep Impact Hits Temple

Stardust Fly-by of Wilt 2

Comet Jets  When the comet is far from the Sun it has no tail or coma  The heat from the Sun boils off material  The material of the comet is well mixed  Sometimes volatiles boil inside the comet and are released as a jet  These jets can change a comet’s orbit  Comet orbits cannot be strictly predicted by Newton’s laws

Outgassing From Borrelly

Jets on Hale-Bopp

Comet Composition  A comet is a mixture of ice and rock  “dirty snowball”  Comets are composed of:  Silicates (rock)  Water (ice)  Carbon Dioxide (“dry ice”)  Ammonia  Organic material  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  Comets get these orbits from a gravitational encounter with a planet or another comet  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  Mostly small dust and ice grains  When the Earth passes through this material we get a meteor shower  Meteor showers are annual events  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  It is warm out, so people are more likely to observe it  Get about 50 meteors per hour  One of the best meteor showers  Meteors appear to come from the direction of the constellation Perseus  The Perseids are debris from Comet Swift- Tuttle

Comet Impacts  Comet impacts were probably common in the early solar system and still happen today  Example: Comet Shoemaker-Levy 9 and Jupiter  Many of the craters that we see on the Moon and other bodies may have been made by comets  Impacting comets larger than about 1-2 km would cause global climate change and mass extinctions

Comet Impact on Ganymede

Comet Deliveries  Many comets have impacted the inner planets (especially during the heavy bombardment period)  Comets could be a source of volatiles, including water  Comets are a means of bringing volatiles from the outer to the inner solar system  Could the Earth’s water have come from comets?

Death of a Comet  At each passage, the comet loses material  The material in the coma and tails is swept away and lost  Eventually all the volatiles will boil off  After several more passes Comet Halley will become a piece of rock on a highly elliptical orbit and will not be visible from Earth  Comets can also hit a planet or be ejected from the solar system in a close encounter

Fragmentation of a Comet

Break-up of Comet LINEAR S4

Spacecraft Studying Comets  Imaging  Giotto (1985) -- took close-up pictures of Comet Halley in 1986  Gathering  Stardust (1999) -- gather (Jan 2004) and return (2006) a sample of the coma of Comet Wild 2  Impacting  Deep Impact (2004) – blasted a 25m deep crater into Comet Temple (2005)  Landing  Rosetta (2004) -- will land a probe on the nucleus of Comet 67P/Churyumov-Gerasimenko (2014)  It is hard to get close to a comet due to all the dust particles around it

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