Asteroids

• Appearance: They show up as much fainter objects than planets, and faster-moving; A few observations and Kepler's laws give their orbits. Most are in the Asteroid Belt, between the orbits of Mars and Jupiter (2.1–3.3 AU, with gaps due to resonances with Jupiter).
• What are they? Debris and fragments of would-be planets; Their orbits were disrupted by each other and by Jupiter, and by now too few remain to make up a new planet.
• Interest: Tracking Near Earth and Potentially Hazardous Objects; Information on material that was around in the early Solar System and what happened since; Possible mining?
• Exploration: Our best photographs from Earth were taken by the Hubble Space Telescope, several spacecraft have flown by and photographed asteroids; One landed on Eros in 2001, one landed on Itokawa and returned sample material in 2010.
• Size and shape: At least 200 are larger than 100 km (the largest is 950-km Ceres, the only roughly round one), many more larger than 1 km, from pictures or estimates of their albedo.
• Mass and composition: We can only find their mass if they have an orbiting object (natural or artificial). Many, like Vesta, have suffered collisions and heating to the point of melting; Others, like Ceres, have not been modified much since the beginning of the Solar System.
• Other examples: We know of several double asteroids, like Ida and Dactyl.

Meteoroids

• Incoming debris: About 50,000 tons of material per year, most in very small pieces (every night you're likely to find one dust particle from space on every surface the size of the hood of your car). Most come from asteroids and comets, a few from the Moon or Mars, some are original pieces of leftover solar system material, and a few are interstellar intruders!
• Meteors or "shooting stars": Small grains, at most pea-sized that crash into our atmosphere at 160,000 mph [much faster than a rifle bullet]; Millions burn up every day tens of miles up in our atmosphere. Meteor showers occur at fixed dates, when the Earth goes through the trail of a comet. Several fireballs, larger objects that burn longer and can produce sound, are seen every year, but most are not found on the ground.
• Meteorites: Every year thousands of football sized meteorites and some larger ones hit the ground, depositing hundreds of tons of material; Most go unnoticed (they are easier to spot and collect in arid or icy places). Nobody in recorded history is known to have been killed by a meteorite; but they have caused damage to buildings and property.
• Larger impacts: The 1908 Tunguska event in Siberia was probably caused by a 60-m wide low-density asteroid, the 50,000-yr old Barringer Meteor Crater in Arizona by a 40-m iron one; Such impacts can be very destructive but happen only every 100 years or so; At least 120 old craters of this type are known. Objects larger than 1 mile come once every 100,000 years or more, and are catastrophic. The biggest ones have caused mass extinctions, like the 5-6 mile Yucatán impact which probably led or contributed to the extinction of dinosaurs 65 Myr ago.

  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.
• Examples: The most famous one is Halley's comet; It has a very elongated orbit and comes back every 76 years. Shoemaker-Levy 9 broke apart and smashed into Jupiter in 1994. Several new ones are seen every year by amateurs.
• What are they? Dirty, dark iceballs from the outer Solar System; 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, millions of km long (in addition to the trail of icy chunks...), when approaching the Sun. In the outer Solar System there are thousands bigger than 100 km, and many more small ones. Size is difficult to determine from Earth, estimates depend on how much light they reflect.
• 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. In addition to flybys, Stardust collected dust samples from Wild 2 in 2004 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 comet in 2014.
• Origin and fate: 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. Comets with long periods are more icy, have randomly oriented orbits, and come from the Oort cloud. Each passage near the Sun makes an ice layer evaporate away (especially the Sun-grazing ones), and radiation damage can cause them to become very dark; After 500 or so passes they may fall apart. Others can end up captured by planets.

Pluto and its Moons

• Name: From the Roman god of the dead and the underworld.
• Discovery: Predicted in the 1800s (on the basis of inaccurate measurements of the position of Uranus and Neptune), then observed (by chance) with a telescope in 1930 close to the position predicted by Percival Lowell; Not visible with the naked eye.
• Orbit: Its average distance from the Sun is 40 AU, but sometimes it is closer than Neptune, due to the orbit's eccentricity; tilted 17.2°! Period 248 yr, locked in a 3:2 resonance with Neptune (that prevents them from colliding).
• Classification: Classified as a planet after its discovery, when no other Trans-Neptunian Object was known; The status grew increasingly uncertain after the 1992 discovery of other large objects in the Kuiper Belt, until an object larger than Pluto (2003 UB313, Eris) was discovered, and in 2006 the IAU voted to reclassify it as a "dwarf planet".
• Exploration: Not visited yet, but New Horizons, launched in 2006, should reach it by 2015.
• Appearance: Our best images are blurred ones from the HST. It looks similar to Triton, with a frosty surface and a thin atmosphere. It is 1/5 of the Earth's size, with a very tilted rotation axis.
• Moons: The largest moon by far is Charon, named after the boatman who ferries souls across the river Styx, discovered in 1978; It has 1/6 of Pluto's mass, 1/2 of Pluto's radius, and has active geysers on its surface. It is not clear whether the pair originated from an impact. There are at least four additional, recently discovered moons.

  The Kuiper Belt and Farther Out

• The Kuiper Belt: A swarm of icy/rocky comet-like objects beyond the orbit of Neptune, between 30 and 50 AU or so from the Sun, (Kuiper Belt Objects or Trans-Neptunian Objects); We now know more than 1000 objects, but the total number must be much larger. Probably formed from leftover material beyond the orbit of Neptune after planet formation.
• Examples: The largest known objects are Eris (2003 UB313 "Xena", the "10th planet", 2400 km across) with its own moon; Pluto; Sedna (2003 VB12, possibly 1500 km in diameter, on a larger, very eccentric orbit); and Quaoar (2002 LM60, about 1300 km in diameter, larger than Charon, in an almost circular orbit at 43 AU).
• 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.
• Oort Cloud: A region up to 50,000–100,000 AU from the Sun, with possibly a trillion icy objects (which probably only make up at most a few tens of Earth masses); Probably formed from objects ejected in random directions by close encounters with solar system planets, and ground up into small pieces by collisions.