ASTEROIDS
![Picture](/uploads/3/7/8/1/37819847/asteroid-1_orig.jpg)
Asteroids are small, rocky bodies that have been left over from the formation of the planets 4.5 billion years ago. They are often known as 'minor planets'.
There are thought to be billions of these chunks of rock. Most of them lie in a doughnut-shaped 'main belt' between the orbits of Mars and Jupiter. On the inner edge of this main belt, asteroids take about three years to orbit the Sun. Those near the outer limit of the main belt take twice as long.
The word 'asteroid' means 'star-like'. They were given this name because they can only be seen as points of light in most telescopes. By the early 20th century, astronomers were calling them the 'vermin of the skies'. They were visible in such large numbers that they were spoiling photographs of distant galaxies and nebulas.
The largest of the asteroids, Ceres, was the first to be discovered. It was found by Giuseppi Piazzi from Palermo, Sicily, on 1 January 180
There are thought to be billions of these chunks of rock. Most of them lie in a doughnut-shaped 'main belt' between the orbits of Mars and Jupiter. On the inner edge of this main belt, asteroids take about three years to orbit the Sun. Those near the outer limit of the main belt take twice as long.
The word 'asteroid' means 'star-like'. They were given this name because they can only be seen as points of light in most telescopes. By the early 20th century, astronomers were calling them the 'vermin of the skies'. They were visible in such large numbers that they were spoiling photographs of distant galaxies and nebulas.
The largest of the asteroids, Ceres, was the first to be discovered. It was found by Giuseppi Piazzi from Palermo, Sicily, on 1 January 180
COMETS
![Picture](/uploads/3/7/8/1/37819847/cometsm_orig.jpg)
Ancient people were afraid of comets – they were said to bring bad luck or foretell disaster. We now know that comets are small ice worlds that have been left over from the formation of the planets. Billions of comets are thought to exist on the edge of the Solar System, beyond the orbit of Pluto.
Most comets are so small and so far away that we cannot see them, even in the biggest telescopes. But we can see them when they head inwards toward the Sun and grow tails of gas and dust. (The word comet means 'hairy star'.)
The heat of the Sun makes the ices turn into gas that streams for millions of kilometres into space. Despite their length, the dust particles they contain are no bigger than grains of sand.
Some comets take only a few years to swing around the Sun. Some – like Halley's comet – take much longer. Others appear once and are never seen again. ESA's SOHO spacecraft has been used to discover thousands of small comets that fly very close to the Sun. Many of them crash into the Sun or are pulled apart by its strong gravity.
Most comets are so small and so far away that we cannot see them, even in the biggest telescopes. But we can see them when they head inwards toward the Sun and grow tails of gas and dust. (The word comet means 'hairy star'.)
The heat of the Sun makes the ices turn into gas that streams for millions of kilometres into space. Despite their length, the dust particles they contain are no bigger than grains of sand.
Some comets take only a few years to swing around the Sun. Some – like Halley's comet – take much longer. Others appear once and are never seen again. ESA's SOHO spacecraft has been used to discover thousands of small comets that fly very close to the Sun. Many of them crash into the Sun or are pulled apart by its strong gravity.
METEORS
![Picture](/uploads/3/7/8/1/37819847/meteor_orig.jpg)
The Solar System is full of material left over from the formation of the planets. Small chunks of rock that travel through space are known as meteoroids. If they enter a planet’s atmosphere and burn up, they are then seen as meteors. Sometimes larger pieces of rock survive long enough to crash into a planet’s surface. Space rocks on the Earth’s surface are known as meteorites.
Meteors (often called “shooting stars”) are very common sights. Anyone who stares at a clear night sky will eventually see these brief trails of light. Meteors are caused by meteorites entering the upper atmosphere at speeds of around 15 km/s! Meteor trails usually last for less than a second. Friction with the thin air at an altitude of about 100 km causes meteorites to glow as they are heated and then destroyed.Meteors can be seen at any time, but the best time to look for them is during periodic showers. Such showers appear around the same dates each year, when Earth passes through a stream of dust left behind by a passing comet.
There are about 20 main showers, although only a few of them produce as many as 50 meteors per minute. Occasionally, the sky is filled with shooting stars. The most famous “meteor storms” are linked with the Leonid shower, which takes place every year between 14 and 20 November. The shower has been so active in the past that it resembled falling snow!
Some meteors are too faint to be seen, even at night, but they can be detected by radar. Other planets have meteor showers too. On Mars, meteorites generally burn up in the thin atmosphere to produce meteors. However, there is no air on the Moon, so meteorites hit the ground and explode without burning up.
Meteors (often called “shooting stars”) are very common sights. Anyone who stares at a clear night sky will eventually see these brief trails of light. Meteors are caused by meteorites entering the upper atmosphere at speeds of around 15 km/s! Meteor trails usually last for less than a second. Friction with the thin air at an altitude of about 100 km causes meteorites to glow as they are heated and then destroyed.Meteors can be seen at any time, but the best time to look for them is during periodic showers. Such showers appear around the same dates each year, when Earth passes through a stream of dust left behind by a passing comet.
There are about 20 main showers, although only a few of them produce as many as 50 meteors per minute. Occasionally, the sky is filled with shooting stars. The most famous “meteor storms” are linked with the Leonid shower, which takes place every year between 14 and 20 November. The shower has been so active in the past that it resembled falling snow!
Some meteors are too faint to be seen, even at night, but they can be detected by radar. Other planets have meteor showers too. On Mars, meteorites generally burn up in the thin atmosphere to produce meteors. However, there is no air on the Moon, so meteorites hit the ground and explode without burning up.