Comets

• Some history: Easily visible, and known since antiquity; Often considered to be signs of important events, sometimes bad omens and carriers of disease in their tails; Up to Tycho Brahe, who measured the parallax of one in 1577, they were thought to be atmospheric phenomena.
• What are they? Dirty, dark iceballs in elongated orbits; The ones we see from Earth have a several-km wide dark nucleus, which usually develops a much larger halo ("coma") and one or two tails of dust and ions, many millions of km long (in addition to chunks they leave behind...), when approaching the Sun.
• Sizes: In the outer Solar System some are bigger than 100 km (there are perhaps 35,000 of these, and 1 billion small ones!) Size is difficult to determine from Earth, estimates depend on how much light they reflect (comet nuclei are very dark).

Special Comets and Exploration

• Halley's comet: The most famous one; Has a very elongated orbit; Seen by the Chinese in 240 BC, comes back every 76 yrs, the last time was 1986.
• Shoemaker-Levy 9: Broke apart and smashed into Jupiter in 1994.
• Recently: Comet Hale-Bopp in 1997, Ikeya-Zhang in 2002, then C/2002 V1 (NEAT), C/2004 F4 (Bradfield), and other ones that we saw breaking apart; Comets McNaught in early 2007 and Holmes in late 2007; Several new ones seen every year by amateurs; More than 1000 discovered by SOHO.
• Effects on Earth: Direct hits can be very destructive, but past comet impacts were probably responsible for bringing to Earth most of the water we see on it today; What if we pass through a comet's tail?
• Exploration: Very interesting because of what they tells us about the origin of the Solar System; The first missions were to Halley [Giotto flyby in 1986] and Borrelly [Deep Space 1 in 2001]
• Recent missions: The sample return mission Stardust flew by Wild 2 in Jan 2004, collected dust samples and returned them to Earth in 2006; Deep Impact blasted a hole in Tempel-1 in 2005; ESA's Rosetta, launched in 2004, will meet its target comet in 2014.

  Origin and Evolution

• Short period comets: Those with periods up to a few hundred years have orbits close to the ecliptic and come from the Kuiper belt, 30–50 AU from the Sun, where Pluto and other distant, frozen leftover planetesimals are located.
• Long period comets: Comets with long periods are more icy, have randomly oriented orbits, and come from the Oort cloud, up to 50,000–100,000 AU; Probably formed from objects ejected in random directions by close encounters with solar system planets, and ground up into small pieces by collisions. There may be a trillion of them, but they probably only make up at most a few tens of Earth masses.
• Effect of the Sun: Each passage near the Sun makes an ice layer evaporate away (especially the Sun-grazing ones), and radiation damage causes them to become dark; After 500 or so passes they may fall apart (we saw several that did in recent years).
• Other changes: Even far from the Sun, they are affected by radiation, collisions with other objects and sandblasting from dust grains in the interstellar medium (plus heat from passing stars and supernovae...), so even "ancient" ones are often not preserved in their original form; Some could be captured by planets (Saturn's moon Phoebe?).

Beyond the Solar System

• Heliopause: The place where the Sun's influence ends; The solar wind meets the external interstellar wind and its bow shock, and real interstellar space begins.
• The galactic neighborhood: Patterns of interstellar gas and molecular clouds, and stars from 4 light years away.