Earth's Moon, our nearest neighbour.

Scientists believe that billions of years ago a Mars-sized object struck the Earth, from all the debris the Moon was formed. It revolves around the Earth every 27.32 days, rotating around its axis at the same speed as the Earth and consequently, its far side is permanentiy turned away from us and cannot be seen from the Earth's surface.

Statistics:

Orbital Period: 27.322 days

Lunar Month: 29.53 days

Mean Distance from the Earth: 384,400km

At Perigree (closest aproach): 356,410km

At Apogee (greatest distance): 406,697km

Diameter: 3476km, just over one quarter of the Earth's size.

Mean Orbital Velocity: 1.02km/second



The Moons of Mars, the Red Planet

Mars has two small Moons , Phobos and Deimos, both discovered in 1877 by Asaph Hall.

Phobos is the largest and orbits just 6,000km above the surface of Mars, the closest orbit of any moon to its home world. Phobos is slowly creaping toward Mars @ a rate of 1.8 meters every 100 years and will eventualy crash into it. Phobos was named after a messenger of the Roman god of war.

Deimos is the smallest moon in the solar system, measuring just 15x12x11km in size and orbits Mars every 30 hours, it is named after the Roman god of dread. Both moons are irregular in shape and heavily cratered.

Jupiter's Moons

Galileo Galilei discovered four moons orbiting Jupiter in 1610 The moons are Io, Europa, Ganymede and Callisto collectively known as the Galilean satellites.

Io similar in size to the Earth's moon, orbiting 422,000 miles from Jupiter is the most volcanically active body in the Solar System, spewing sulphurous gases and other material 100's of kilometers above its surface. Io is subject to imense tidal forces caused by Jupiter pulling it in one direction and sister moons Europa and Ganymede pulling it in the other, this causes the surface to bulge in and out as much as 100 meters, the pressure is then released by its volcanoes.

Europa a strange-looking moon with virtually no craters and a very flat surface, orbiting about 670,000 miles from Jupiter. Scientists beleive that below its thin cracked surface is a deep liquid ocean, which might have allowed life to arise.

Ganymede with a diameter of 5262 km is so large that if it orbited the Sun instead of Jupiter it would be called a planet. It orbits 1.07 million kilometers from Jupiter and has surface features, including mountians, valleys, craters and lava channels.

Callisto orbits about 1.88 million kilometers from Jupiter and is the third largest moon in the solar system, it is also the most cratered moon, with its crust dating back 4 billion years. Callisto is a half-ice, half-rock moon with a thin carbon dioxide atmosphere.

Other moons were found orbiting Jupiter by the two Voyager spacecrafts in 1979 and 1980, these are: Metis, Adrastea, Amalthea and Thebe. Recent improvment in detection techniques have now brought the total number of Jovian moons to 28.



Moons of Saturn

There are at least 24 and possibly as many as 30 moons orbiting Saturn.

Pan, Atlas, Prometheusand Pandora are four small shepherd moons that help Saturn maintain its elaborate ring system.

Titan is the largest at 5,150 km in diameter, the second largest moon in the solar system and also one of the most interesting. The moon orbits 1.22 million km from Saturn with an atmosphere made up of mainly nitrogen and other hydrocarbons - the key elements for amino acid proteins, the building blocks of life. Scientists believe Titan may resemble a primordial Earth and the European Space Agency is planing to send the Huygens probe, part of the Cassini mission to Saturn, to penetrate its atmosphere in January 2005.

Mimashas a crater about one-third the size of the moon itself. The crater is 10 km deep, 130 km wide and with peaks rising 6 km above the crater floor. The impact would have came close to destroying the moon.

Enceladus, Saturn's eighth moon orbits 238,000 km above the planets surface, it is a bright moon with a smooth surface and no large craters.

Tethys, discovered in 1684 by Giovanni Cassini, is a large moon 295,000 km from Saturn. Tethys is accompanied by two Trojan moons Telesto and Calypso, they orbit 60 degrees in front and 60 degrees behind Tethys respectively. Tethys is made up of mainly ice with its surface cratered and cracked, one trench is 65 km wide and covers three-fourths of its surface.

Dione was also discovered by Cassini in 1684, it orbits 377,400 km above Saturn and has one small Trojan moon called Helene, which flies 60 degrees ahead of Dione. Scientists believe that Dione may have been turned around - perhaps many times - by large impacts and this is how its heavily cratered surface came to be turned away from Saturn, rather than towards it, as some cratering models predict. Its surface has several craters larger than 35 km, the size scientists believe it would take to spin the moon around.



Uranus

Uranus has 21 moons 18 of them have names

Cordelia and Ophelia, the two innermost moons shepherd the planets thin Epsilon ring, they orbit 49,770 km and 53,790 km above their home world.

Biance, Cressida, Desdemona, Juliet, Portia, Rosalind, Belinda and Puck make up the other small inner moons. They were all discovered by Voyager 2 during its 1986 flyby, scientists know very little about these very dark satellites.

Miranda is a very odd looking small moon, orbiting @ 129,000 km and is 470 km in diameter, it appears to have been shattered and reformed up to 5 times in its past. Its surface is a jumbled mixture of features with 20 km deep canyons scaring its face.

Titania orbits Uranus @ 436,000 km and is the largest satellite @ 1,578 km in diameter.

Ariel orbits Uranus @ 191,000 km, is the brightest moon of Uranus and is 1,183 km in diameter.

Umbriel is a very dark moon and orbits Uranus @ 265,980 km.

Oberon, the 2nd largest moon of Uranus and orbits @ 583,420 km.



Neptune

Neptune has 8 moons, its innermost satellite Naiad is 54 km in dia. and orbits 23,200 km above it. Naiad circles close to Neptune's equator and completes one orbit every 7 hours 6 minutes.

Thalassa is 50 km in dia and orbits 25,200 km above Neptune.

Despina is 150 km in dia. and orbits 27,700 km above Neptune.

Galatea is 180 km in dia. and orbits 37,200 km above Neptune.

Larissa is 190 km in dia. and orbits 48,800 km above Neptune.

Proteus is 400 km in dia. and orbits 92,800 km above Neptune.

Triton is the largest of Neptune's moons, discovered in 1846 by William Lassell less than one month after the planet was discovered. It orbits 354,700 km from its home planet and measures 2,700 km in diameter. Triton is the only moon in the solar system to have a retrograde (in the opposite direction of Neptune's rotation) orbit, it is also the coldest known body with temperatures as low as minus 235 degrees Celsius, making its mainly nitrogen atmosphere condense into frost. Triton is gradually getting closer to Neptune and will eventually, in the next 10 to 100 million years, collide with it.

Nereid was discovered in 1949 by Gerard Kuiper, it has the distinction of the moon with the most eccentric orbit, its distance from Neptune varies from about 1.35 million km to more than 9.62 million km. It takes Nereid 360 days to complete just one orbit of Neptune.



Pluto and moon

Charon is the only known moon of Pluto and is about half the size of its mother planet. The pair are sometimes refereed to as the double planet because of their size comparison, compared to other moons and planets. Due to their comparative size they exert about the same gravitational hold on each other as they fly through space and are the only planet-moon duo in the solar system that keep the same face towards each other.