Pluto Diameter 0.18DE Rotation Period 6 days 9 hours Orbital Period 250 years Distance from Sun 39.44AU Orbit Eccentricity 0.25 Tilt 118 degrees Temperature possibly –228C to -238C Atmosphere probably N, CO and methane – when not frozen Gravity 0.069g Moons 1 Visits none Pluto is a bit of a mystery. We have never visited the planet, and even HST is only capable of producing the image above. It is the smallest of the planets by far, and has a very large moon called Charon. The combination of its size, very elliptical orbit, very large companion, and the fact that it doesn’t orbit in the ecliptic plane like the rest of the planets, has made many astronomers claim that Pluto is not really a planet at all, just a very large member of the Kuiper belt. The Kuiper belt is a band of rocky objects that orbit just outside Pluto. They are generally smaller than Pluto, but it may be that Pluto is simply a large member of this belt. Pluto may have a very thin nitrogen, carbon monoxide and methane atmosphere, but it is so cold out there that this probably freezes solid onto the surface when Pluto is not at its closest to the Sun. A mission was being planned by NASA, called the Pluto-Kuiper Express, but budget cuts have put the project back so far that it now looks doubtful that it will ever be launched. Pluto’s orbit is so elliptical that up until February 1999 it was closer to the Sun than Neptune.

Pluto and Charon Pluto's eccentric orbit intersects Neptune's orbit, so sometimes is closer to the sun than Neptune Pluto has an atmosphere when it is close to the sun, which then freezes as the planet moves away from the sun Charon discovered in 1978 (diameter 1200 km, about half the size of Pluto) Pluto is in a 3:2 orbit resonance with Neptune This is the best image of Pluto and Charon, and concludes the tour at the edge of the SS. For more information about the planets, plus pretty pictures, there is a superb website by Bill Arnett, called The Nine Planets. I would recommend this as the first (and usually only) port of call for anyone wanting to know anything about the SS. You can find it online at www.seds.org/nineplanets/nineplanets/nineplanets.html. Or search the web using the keywords “Arnett nine planets”.

The Outer Solar System – The Kuiper belt An asteroid belt beyond the orbit of Neptune between 30-100 AU, consisting of icey bodies more like comets than asteroids Also the Centaur group (Chiron etc) of asteroids that lie beyond the orbit of Saturn may also be Kuiper belt objects Red Orbits = Plutinos Blue Orbits = Classical Kuiper Belt Objects Black Orbits = SKBOs

Quaoar – The Largest Kuiper Belt Object Quaoar – pronounced Kwah-o-ar. Discovered June 4 2002. Mike Brown and Chadwick Trujillo. 4bn miles from earth, orbits in 288 days. Larger than Charon, and more massive than all of the asteroid belt. 1 bn miles beyond pluto. Discovered June 4 2002, by Michael Brown and Chadwick Trujillo (California Institute of Technology)

Quaoar – pronounced Kwah-o-ar. Discovered June 4 2002 Quaoar – pronounced Kwah-o-ar. Discovered June 4 2002. Mike Brown and Chadwick Trujillo. 4bn miles from earth, orbits in 288 days. Larger than Charon, and more massive than all of the asteroid belt. 1 bn miles beyond pluto.

Comets Comets are essentially dirty lumps of ice that melt as their highly eccentric orbits take them closer to the sun. The melting process creates geysers of carbon dioxide and water vapour that forms a kind of atmosphere called a coma around the comet's nucleus Two tails trail beyond the comet in its orbit – one tail consists of dust and points away from the comet's direction of motion the other tail consists of ionised plasma that points away from the sun

The Giotto probe Launched in 1985 by ESA to study Comet Halley on its last close encounter with Earth (next time will be 2061)

(above) Nucleus of Comet Wild 2 as observed by the Stardust probe (right) Nucleus of Comet Halley as observed by the Giotto probe

There are two types of Comets Long period comets have aphelia of 50,000 AU and are thought to originate in the theorised Oort Cloud of comets – they have only ever been observed once (e.g. Hyakutake). High and random orbital inclinations suggest the Oort cloud is a large shell rather than a disc. Periodic comets reach perihelion roughly every 100 years, and are thought to originate in the Kuiper belt disc (e.g. Halle-Bopp, Halley's Comet)

Meteor Showers Meteor showers are regular annual events caused by the Earth passes through a patch of its orbit where a periodic comet has left behind a dust trail These dust particles burn up in the Earth's atmosphere leaving behind bright trails that light up the sky

All meteor trails in the sky appear to originate from a single radiant.

The largest meteoroids will survive the passage through the Earth's atmosphere and land on the surface as a meteorite. Like asteroids, some meteorites are Iron based, others are just rocks