The end of the dinosaurs?
About 65 million years ago, something strange happened to the Earth. A 'missile' 10 km (6 miles) wide, through to be an asteroid from outer space, hit the Yucatan peninsular in Mexico. It dug a crater 12 km (7 miles) deep and 100 km (60 miles) wide. The force of the impact blasted molten droplets of rock across the Americans and set of huge firestorms. Tidal waves 1 km (1.2 mile) high crashed along the shores, and a dust cloud blotted out the Sun. The dinosaurs may have died out as a result.

Diamond wall
The stone walls of the old church in millions of tiny diamonds, the size of specks of dust. These were formed from molten rock, blasted from the Earth's surface when a meteorite crashed onto it 35 million years ago.

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What happens when a meteor hits the Earth?
About 50 tons of meteors enter the Earth's atmosphere every day, mostly dust-sized particles, but sometimes rocks several metres wide. Most of this material is burnt up as it hurtles through the atmosphere at speed of between 32 and 95 km/second (20-60 miles/sec), the blaze can be seen from the Earth as 'shooting' or 'falling' stars. Around 500 meteors a year are big enough, more than a centimetre or so wide, to survive the passage through the atmosphere and do it the Earth. At Meteor Crater in Arizona, United State, a hole 1.2 km (3/4 mile) wide and 170 m (600 ft) deep was blasted out by one such meteor about 20 000 years ago.

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The travel of comets

1. High speed from space : A comet speeds through the Earth's atmosphere at up to 20 km/second (12 miles/sec), causing the atmosphere to heat up quickly.

2. Explosion in the sky : The comet explodes 9 km (6 miles) above the Earth's surface. The heat vaporises rocks, which rain back down on Earth.

3. Shock effects : Shock waves from explosion travel outwards from the centre of the blast. They flatten the trees and other structures. The shock waves also trigger earthquakes and set off fires.

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How often do comets come near the Earth?
Comets are like dirty snowballs, lumps of rock, ice and gas, that travel around the Solar System, guided in their orbit by the gravity of the Sun. They reappear in the night sky at regular intervals, at anything between every six years for some and every 2000 years for others, depending on the size of their orbits. Some comets shine spectacularly as they approach the Sun and ice burns off as gas vapour, forming a blazing tail up to 300 million km(200 million miles) long. Halley's comet, which reappears every 76 years, was first spotted in 239 BC, it was first illustrated in the Bayeux tapestry, which commemorated the Norman invasion of England, and was photographed most recently on March 13, 1986, by the space probe Giotto.

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What are asteroids?
Tens of thousands of rocks, ranging in size from 1.6 km (1 mile) to 1000 km (600 miles) across, are flying about the Solar System. They are called asteroids, and are mostly found in a belt which lies far beyond the Earth, between the orbits of Mars and Jupiter. The Earth (along with the three other inner planets of the Solar System, Mercury, Venus and Mars) is struck by an asteroid every 200 000 years, or so scientists believe. Our Moon, which does not have an atmosphere to protect it from flying objects, is marked with thousands of impact craters.

Space rock
On its way to Jupiter, the space probe Galileo photographed a stony asteroid called Ida, which is 58 km (36 miles) long and 21 km (13 miles) wide.