Asteroids


In this diagram you can see the orbits of Mars (closer to the Sun) and Jupiter. They are shown so that you can locate the biggest band of rocks in the Solar System.
Between the orbit of Mars and the orbit of Jupiter you can see a broad ring made by rocks. The rocks are called asteroids and the ring is known as the asteroid belt.


What is the largest asteroid?
It is called Ceres. It is 940 kilometres across and was discovered in 1801.

Can any asteroids be seen with the naked eye?
One, called Vesta. Its surface reflects a large amount of sunlight. It is also the second largest asteroid.

Do asteroids have moons?
Some asteroids may have moons. One asteroid, called Ida, definitely has one. Ida is block shaped and 56 kilometres long. Its moon is called Dactyl and is almost cube shaped, with two sides about 1.5 kilometres long. Dactyl is 90 kilometres from Ida.

Are all asteroids made of the same material?
No. Some asteroids, like Ceres, are made of a dark stoney material and reflect little light. Other asteroids may be rocky with some metal in them or may be made almost entirely of metal. Metallic asteroids are made of a mixture of iron and tin. In the future, people may be sent to the asteroid belt in spaceships to mine the metals there and take them to Earth or to colonies in space to use as raw materials.

Do asteroids spin like planets?
Yes. They may spin completely around in as quickly as two hours sixteen minutes or as long as sixty-two and a half days.

If all the asteroids were put together how big a planet would they make?
It would be about a thirty-third of the size of our Moon.


Comets



What are comets made from?
They are made from lumps of rock and particles of grit that are held together by ice and frozen carbon dioxide.

Does a comet always have a tail?
No. It grows a tail as it comes nearer the Sun. The heat from the Sun melts the ice and the solid gases. These are blown backwards by the Solar Wind which comes from the Sun. The particles of grit are also blown into a tail so that most comets have two tails.

Which was the largest comet?
This appeared in 1811. The head of the comet, called the coma, was two million kilometres across. This distance is larger than the diameter of the Sun. The comet had a tail that was one hundred and sixty million kilometres long.

Which is the most famous comet?
Halley's comet, which returns to a position where it can be seen from the Earth every 76 years. It is named after a famous British scientist, Sir Edmund Halley.


What is the greatest length of a comet tail?
This is three hundred and thirty million kilometres long. When the head of the comet approached the Sun, the end of its tail was beyond the orbit of Mars. The comet was known as the Great Comet of 1843.

Do comets crash?
Some comets crash into the Sun. In 1994 a comet called Shoemaker-Levy 9 crashed into Jupiter. The planet's strong gravity tore the comet into pieces before it hit the atmosphere.

How many trips does a comet make?
A comet keeps orbiting round the Sun like the planets and asteroids. It may take thousands of years to travel all the way round its orbit.

What happens when a comet runs out of ice?
It is thought that it just becomes a rocky lump in space like an asteroid.

Which way does a comet's tail point?
A comet always keeps its tail pointing away from the Sun – even when the comet is going away from the Sun. The reason for this is the Solar Wind, which blows particles from the Sun onto the materials in the tail.