PYTS/ASTR 206 – Comets 1 l Announcements n HW6 available today, due in a week wUse Kevin as the TA for this one n 2 In-class assignments left in 3 lectures

PYTS/ASTR 206 – Comets 2 PTYS/ASTR 206 – The Golden Age of Planetary Exploration Shane Byrne – Comets

PYTS/ASTR 206 – Comets 3 In this lecture… l Observations of comets l What are comets? n Composition and structure l Cometary tails n Ion and dust tails l Where do comets come from? n Orbits of comets n Oort cloud n Scattered Kuiper Belt

PYTS/ASTR 206 – Comets 4 l Comets have been known from ancient times n Thought to foreshadow disasters and major battles l Pre-telescopes the known solar system was a pretty empty place n Moon and the Sun n Mercury, Venus, Mars, Jupiter, Saturn n And COMETS n No Uranus n No Neptune n No planetary Moons (except ours) n No Asteroids n No Kuiper Belt Objects Observations of comets

PYTS/ASTR 206 – Comets 5 l People have recorded comet sightings for millennia 167 BC 687 AD 1986 AD

PYTS/ASTR 206 – Comets 6 l Ancient Greeks thought comets were atmospheric phenomena n In the west this went unchallenged until telescopes came along n Tycho Brahe’s parallax measurements proved this wrong wComets were much further away than the Moon l Renaissance astronomers thought comets moved in straight lines through the solar system n Even Kepler argued they shouldn’t follow elliptical orbits like the planets l In the 1680s astronomers tracked a comet and showed it had an elliptical orbit n Comets were solar system objects – just like planets ☄

PYTS/ASTR 206 – Comets 7 l Newton finally settled this in his ‘Principia Mathematica’ (1687) n Showed that comets moved in parabolic or elliptical orbits by the Sun’s gravity e = 1 A parabolic orbit

PYTS/ASTR 206 – Comets 8 l If they have orbits… then they’re periodic n The same comet should come back l In 1705 Edmund Halley connected the dots… n Used Newton’s laws to figure out the orbit of many comets n Comets seen in 1531, 1607, and 1682 were the same object n Predicted a return in 1759 l Halley’s comet has been seen ~30 times 687 AD 1986 AD 167 BC

PYTS/ASTR 206 – Comets 9 l Many telescopic observations of comets (including Halley’s comet) l Even a few spacecraft missions n Giotto & Vega wComet Halley n Deep-space 1 wComet Borrelly n Stardust wComet Wild 2 n Deep Impact wComet Tempel 1

PYTS/ASTR 206 – Comets 10 l Comets have several parts n Nucleus w~10 km n Coma w~1,000,000 km wAlmost as big as the sun! 1,400,000 km n Hydrogen envelope w~10,000,000 km n Tail wIon tail wDust tail w~100,000,000 km wAbout 2/3 of 1AU! What are comets?

PYTS/ASTR 206 – Comets 11 l Cometary nuclei are usually invisible from the Earth n Hidden by the coma n Spacecraft missions can visit far from the Sun when the coma is inactive Comet Tempel 1 Comet Holmes

PYTS/ASTR 206 – Comets 12 l Comet Nuclei are ‘dirty snowballs’ n Random mixtures of ices and dark stuff n Ices wMostly water ice wA little CH4, CO, CO2 etc n Dark ‘stuff’ wOrganic compounds (H,C,O) wRock-like material Like asteroids n Very small objects wNot enough self-gravity for a round shape

PYTS/ASTR 206 – Comets 13 l When comets are close to the sun n Surface heats up n Ice sublimates (turns to vapor) n Dark organic stuff gets concentrated on the surface

PYTS/ASTR 206 – Comets 14 l Comets are ice rich l …but among the darkest objects in the solar system n Albedo of 2-4% n Like tar n Comet nuclei are very hard to see without their comas

PYTS/ASTR 206 – Comets 15 l This thick crust builds up over many orbits n Sublimating ice comes out in jets n Collapse pits form on the surface from removal of sub-surface ice n Jets act like rocket engines – can alter the orbits of comets

PYTS/ASTR 206 – Comets 16 l Comet nuclei are typically small < 40km l Mass estimates come from spacecraft flybys l Comets are very low density n Contain significant internal voids From Wikipedia Water ice ~0.9 g/cm 3

PYTS/ASTR 206 – Comets 17 l The Deep Impact mission n 370-kg (815-lb) copper impactor n Analysis of vapor plume n Crater 100m wide, 30m deep l Composition was ‘dirtier’ than expected n Data analysis still in progress

PYTS/ASTR 206 – Comets 18 l Sublimation jets produce cometary atmosphere n Mostly water ice crystals – some dust n Comet’s gravity can’t hold onto this material l Occasionally a big piece of the comets surface will break off exposing fresh ice n Comet Holmes brightened by a factor of 1 million within a few days Coma of Comet Holmes

PYTS/ASTR 206 – Comets 19 l What happens to the water ice crystals? n UV solar radiation breaks up the water molecules H2OH2O OH - H+H+ We can see this using ultraviolet light

PYTS/ASTR 206 – Comets 20 l Comets have two tails n Ion tail of OH - and H + n Ions are swept up by the solar wind n Ion tails point away from the Sun n Blue-ish in color

PYTS/ASTR 206 – Comets 21 l Dust tails n Also swept by the solar wind but less efficiently n Dust tail is brighter and whiter n Tail direction affected by the comets motion and is curved Direction of comet’s motion

PYTS/ASTR 206 – Comets 22 l Comets can appear to have a tail and an anti-tail Earth This observer sees the this comet Ion TailDust Tail

PYTS/ASTR 206 – Comets 23

PYTS/ASTR 206 – Comets 24 l Cometary orbits are very different from asteroids n Comets have very elliptical orbits n Comets have randomly inclined orbits n Comets have very large orbits Where do comets come from?

PYTS/ASTR 206 – Comets 25 l Divided into short period ( 200 years) l Short period comets n Jupiter family comets (Periods <20 years) n Orbits controlled by Jupiter n All low inclination n Halley family comets (Periods years) n Come from the Kuiper Belt n Spread in inclinations n Eventually transition to Jupiter family comets

PYTS/ASTR 206 – Comets 26 l Long-period comets n Have totally random inclinations n Have very long periods/large orbits n Many of these appear to be on their first pass through the inner solar system n A body with a semi-major axis of 10,000 AU will orbit once every million years

PYTS/ASTR 206 – Comets 27 l The Oort cloud n A spherical cloud of billions of comets far from the sun n Explains the random inclinations of the long-period comets

PYTS/ASTR 206 – Comets 28 l Comets form closer to the giant planets l Gravitational encounters n Fling them into very distant orbits n Allow the giant planet to migrate l Passing stars randomize the orbital inclinations n Less so for objects closer to the sun l Only a small fraction of the original objects survive l Sharp outer edge of the Kuiper belt is not continuous with the Oort cloud

PYTS/ASTR 206 – Comets 29 l What knocks these comets into the inner solar system? n Planets have no influence here l Passing stars? n Nearest star ~4 light years away n ~250,000 AU n Twice the Oort cloud distance l Galactic tides? n As the sun orbits the galactic center n Takes ~250 million years Voyager Spacecraft around here

PYTS/ASTR 206 – Comets 30 l Why do all the short-period comets have low inclinations? n They come from a disk not a spherical cloud n This is why the Kuiper Belt was postulated Plutinos: All in the 3:2 resonance with Neptune Classical KBOs Called Cubewanos (after 1992 QB1) Scattered disk objects: Very eccentric orbits Comet source

PYTS/ASTR 206 – Comets 31 l Scattered disk objects encounter Neptune n Are perturbed into smaller orbits n Wander among the gas giants as Centaurs w(half KBO, half comet) n About 1/3 make it to the inner solar system wBecome Jupiter family comets wOther 2/3 are swept up by one of the giant planets wTakes 1-10 million years

PYTS/ASTR 206 – Comets 32 l Comets lose more ice on each pass close to the sun n Eventually the thick outer cover seals off the ice n No more cometary activity l Some asteroid-like objects are in comet-like orbits l Some asteroid-like objects suddenly develop comas n Impacts disturb surface cover or n Move closer to the sun wChiron developed coma and tail wPeople were puzzled as this was before KBOs were known End of comets

PYTS/ASTR 206 – Comets 33 l Another common fate of weak cometary bodies is to break up n Tidal forces from close approaches to planets l Old debris corridors cause meteor showers when the Earth passes though them

PYTS/ASTR 206 – Comets 34 In this lecture… Next: Formation of the Solar System l Reading l Chapter 15-7 to 15-9 to revise this lecture l Chapter 8 for next lecture l What are comets? n Dirty snowballs – removal of ice leave dirt on the surface n Ice sublimates in jets through a debris cover and produces a coma l Cometary tails n Ions tails are bluish and point away from the sun n Dust tails move slower and so are curved due to comet’s motion l Where do comets come from? n Short-period comets are dominated by Jupiter wLow inclination orbits means resupply from a disk – the Kuiper belt n Long period comets have random inclinations wResupply from a distant spherical reservoir – the Oort cloud