A Moon made in a collision
Most scientists think the MOON formed about 4.5 billion years ago, when a body about the size of MARS crashed into the young Earth. The collision flung out a vast cloud of molten rock, which gathered into a ball in ORBIT around us. Rocks brought back by the Apollo astronauts are strikingly similar to rocks on Earth, which fits the idea.
The Moon is still slowly drifting away, by about 3.8 centimetres a year, roughly the speed your fingernails grow. We know because the Apollo crews left mirrors on the surface, and scientists fire lasers at them and time the reflection. The same side of the Moon always faces us, because it spins exactly once for every trip around the Earth. Its GRAVITY is about a sixth of ours, and its pull raises the tides in the sea.
Planets that break the rules
SATURN is so light for its size that, if you could find a bath big enough, it would float. VENUS spins so slowly that its day is longer than its year, and it spins backwards compared with most planets, so the Sun rises in the west. Venus is also the hottest planet, at around 465°C, even though MERCURY is closer to the Sun; its thick cloudy atmosphere traps the heat like a greenhouse.
JUPITER, the biggest planet, has a storm called the Great Red Spot that has been raging for at least 190 years and is wider than the Earth, although it has been shrinking. NEPTUNE was the first planet found by maths rather than by looking. In 1846 a French astronomer, Urbain Le Verrier, worked out from wobbles in the orbit of Uranus where an unseen planet must be, and an astronomer in Berlin pointed a TELESCOPE there and found it within an hour.
Shooting stars are comet dust
A METEOR, or shooting star, is a grain of space dust, often no bigger than a grain of sand, burning up as it hits the air at tens of kilometres a second. Meteor showers happen every year on the same dates, when the Earth passes through the trail of dust left behind by a COMET. The Perseids, which peak in mid-August, come from a comet called Swift–Tuttle. A piece that survives to reach the ground is called a meteorite, and a big one can leave a CRATER.
An ASTEROID is a lump of rock or metal, most of them orbiting between Mars and Jupiter. A total ECLIPSE of the Sun happens because of a lucky coincidence: the Sun is about 400 times wider than the Moon, but also about 400 times further away, so from Earth they look almost exactly the same size.
A universe of galaxies
The light from the nearest STAR after the Sun, Proxima Centauri, takes over four years to reach us. Our Sun is one of perhaps a few hundred billion stars in the Milky Way GALAXY, and there are likely hundreds of billions of galaxies in the observable COSMOS. A NEBULA is a cloud of gas and dust, and some are nurseries where new stars are forming.
People have been exploring it all in person for only a few decades. The ROCKET, the SATELLITE, the ASTRONAUT and the first SPACEWALK came in a rush in the 1950s and 1960s, as our space race page tells, and our astronauts page covers the people who flew.