Spacefiles

Of the nine planets, Mercury orbits on the inside track - the closest planet to the Sun. Baked and irradiated, Mercury is a cratered little world - a pristine record of the impactors that rained from space during the early Solar System. Mercury is weird. It has double sunrises and a day twice as long as its year.

Venus is Earth gone wrong - a lifeless planet with a dense and choking atmosphere and temperatures to melt lead. Constantly shrouded in cloud, Venus could once have been Earth's twin with oceans and continents, even simple life. But there is a theory that as the Sun matured and its luminosity increased, Venus became hell.

We live on the largest inner planet, third from the Sun and the first with a moon. Earth is lucky - at just the right distance from the Sun for life to evolve in the oceans, for green plants to produce breathable air and for humankind to develop agriculture and civilization. But from space there is no sign of the six billion people on Earth.

The Moon was probably formed when a body the size of Mars twice hit early Earth. The first collision was a glancing blow. The second, two days later, was a major impact that threw enough material into orbit to form the Moon. Since then, the Moon has been steadily receding.

A total eclipse of the Sun is the greatest spectacular in the Solar System. It happens when the Moon, which is 400 times smaller than the Sun, completely obscures the Sun, which is 400 times farther from us than the Moon - an astonishing celestial coincidence.

Mars, the red planet, is the world on which next we will walk. The Martian day is a comfortable 24-and-a-half hours. But the rest is strange - planet-wide dust storms, temperatures overnight of minus 100 degrees and a daytime high just above freezing. Mars has the biggest volcano in the Solar System, Olympus Mons, and the largest geological fault, Mariner Valley.

Between the orbits of Mars and Jupiter is the asteroid belt, a swarm of rocky bodies. Every so often, perturbed perhaps by the gravitational tug of Jupiter, asteroids collide and tumble towards the Sun. If they pass Mars, they can threaten Earth. 65 million years ago an asteroid impact most probably killed the dinosaurs.

Jupiter, bigger than all the other planets combined, could swallow Earth 1,323 times. Yet this giant ball of gas has no solid surface. Comprising 90 percent hydrogen and ten percent helium and orbited by more than 60 moons, Jupiter is a mini solar system. Four of them are big enough to make small planets. The moon Europa has an icy crust that may conceal an ocean.

Saturn, second largest of the giant gas planets, rules a dazzling domain. Forming a halo that would stretch from Earth to the Moon, the rings of Saturn are billions of moonlets - from grains of dust to rocks the size of tanks. The planet is so light it would float in water. Titan, Saturn's greatest moon, is bigger than the planet Mercury.

In the cold, dark outer reaches, orbit the giant gas planets of Uranus and Neptune. Uranus is twice as far from the Sun as Saturn. Neptune is so distant it takes 165 years to orbit the Sun. Uranus rotates on its side - possibly knocked over in a collision that may also have shattered and re-assembled the craggy moon Miranda. Neptune is a very windy planet.

Farthest planet from the Sun, Pluto is so remote and small, it was not found until 1930. The discovery was made by a 24-year-old farmer's son, Clyde Tombaugh. At the Lowell Observatory, in Flagstaff, Arizona, USA, Tombaugh photographed 45-million objects in the cosmos before spotting Pluto. And it was not until 1978 that Pluto's huge moon Charon was discovered by Jim Christie.

The nomads of the Solar System, comets swing through the planets on wild eccentric orbits. Comets plunge into the Sun, some just graze it. Others, like Halley's Comet every 76 years, make regular periodic appearance in our skies. Comets are from the Kuiper Belt and the Oort Cloud, a vast halo of debris that stretches a third of the way to the nearest star.

Navigating the stars and the 88 constellations of the night sky. A beginner's guide to using vivid and easily recognizable patterns - like Orion, the Hunter, and Ursa Major, the Great Bear - as signposts in the heavens. How to find Sirius, the brightest star; Polaris, the north pole star, and the southern celestial pole, where there is no marker star.

The history of astronomy. From the Babylonians and ancient Chinese, the first astronomers, to the Egyptians, who created our calendar of 365 days, and the Greeks who discovered Earth is round. Copernicus, in the 16th century, worked out that the planets orbit the Sun. Galileo first used a telescope on the sky. Isaac Newton improved it - and described the effects of gravity.

The story of manned spaceflight. With the German Werner von Braun launching American rocketry and Sergei Korolev masterminding the Soviet space program, the Russians put the first cosmonaut in space and the Americans the first astronaut on the Moon. Then came America's Space Shuttle and the development of orbiting space stations, largely by the Russians.

Monitoring Earth from space. A constellation of artificial satellites keep an eye on our planet. They help predict our weather and the threat of tornadoes, floods and drought. Telecommunications depend on satellites. Shipping and road traffic navigate by satellite. Spies in the sky monitor farm crops, pests, forest fires, volcanoes and oil slicks.

Space stations. A review of ISS, the International Space Station, currently being assembled, and its forerunners. Most famous was Mir, run by the Russians for 14 years. ISS has now replaced Mir. But ISS is hugely over budget and way behind schedule due to problems with its two major partners.

The search for extra-terrestrial life is on. Earth is perfect for life. Our temperate planet is neither too near nor too far from the Sun. With Venus too close and Mars too distant, if they once had life, they most likely don't have now. The only other possibility in the Solar System is Europa, a moon of Jupiter.

The theory of the Big Bang - how the Universe exploded from an infinitesimal speck to create matter, radiation, time and space. Within the first trillion-trillion-trillionth of a second, the cosmos grew a hundred million times to less than the size of an atom. Then, in another instant, the Universe was the size of a galaxy.