Imagine that we take a journey by a supersonic jet flying at twice the speed of sound, starting at the Sun and fly out across the Solar system towards the planets.
After flying for 5 years
and travelling 58 million km we would arrive at Mercury. It is a smaller planet than the Earth
and much hotter. There is no air there and it is so hot that lead would melt in the mid-day
sun! Space probes show that the rocky surface is covered in craters like our
Moon.
Mercury can sometimes be seen from the Earth as a small star-like object near
the Sun at dusk or dawn.
Nine years and 108 million km
after leaving the Sun we would arrive at Venus. This is a planet about the same size as the
Earth but it is not a pleasant place to be.
Instead of air it is covered with clouds of hot
acid fumes that would kill you if you breathed them, the clouds keep it very hot and also
make it impossible to see any details of the surface from Earth. Space probes show that the
surface is covered with rocks.
Venus is an example of a 'runaway' greenhouse
effect. The radiation from the Sun is trapped beneath the thick atmosphere causing the
planet to heat up.
Venus can be a very bright object when seen from the Earth
because of the light reflected from its dense atmosphere. Like Mercury it would be near the
Sun in the evening or morning. However apart from the Sun and the Moon Venus is the
brightest object in the sky.
Venus has no known
moons.
When our high speed jet had been flying for
about twelve years we would have gone some 150 million km and we would reach the Earth.
Think how lucky we are that the Earth is where it is. The Sun is far enough away so that we
don't fry and yet not so far away that we freeze. There have been some very chilling science
fiction movies about what would happen if the Earth were to move a little bit nearer the
Sun.
The Earth has a thick layer of air around it which we breathe and it also
protects us from harmful radiation from space. It is most important that we look after our
planet because the balance between survival and destruction is actually very
delicate.
Many of you like to get a good suntan but too much sun can be dangerous.
Your skin gets browner because radiation called ultra violet falls on it. This comes from the
Sun. The amount of ultra violet falling on the Earth is kept to a safe level by a gas called
ozone in the upper atmosphere. This absorbs some of the ultra violet and so not too much
reaches the ground.
Recently people have become very worried about some man-made chemicals called CFC s. These can destroy the ozone layer and so let in too much
ultra violet - this would cause skin cancer. CFC s are found in some aerosol sprays and also
in fridges and freezers. It is hoped to ban their use in the sprays and also to find another gas
that can be used in the fridges. If your family gets rid of a fridge or freezer do try and make
sure that they get somebody to take the CFC type gas out of it
safely.
The other major threat to our atmosphere is the GREENHOUSE EFFECT. This is the rise in temperature of the Earth that will be caused by the build up of gases such as carbon dioxide in the atmosphere. The heat from the Sun can get in but can't escape so the Earth will get hotter and hotter just like being in a greenhouse. The polar ice caps will begin to melt and many of the world's major cities would be flooded because of the rise in sea level.
Carbon dioxide is
produced by burning, and it is then taken in by green plants that convert it to oxygen. You
can therefore see why the burning of the Brazilian rain forests will have such a disastrous
effect on the world. The forests are being burnt to provide cattle grazing land to produce
meat for American hamburgers and to build roads and houses The trees are also being cut
down to make paper. It is therefore most important to try and use some recycled paper to
help save the forests.
The Earth has one large Moon orbiting it. A small telescope will show
that it is covered with craters. These carters were almost certainly made by the impact of
meteorites on the lunar surface. It is a very desolate place but it is the only other place in the
Universe when man has been, the first astronauts in Apollo 11 landed on the Moon in July
1969.
The first photograph (both taken with my 300 mm reflector) shows part of the
Mare Imbrium and the huge curved mountain chain known as the Appenines. Towards the
bottom of the picture you can see the crater Clavius and the Alpine valley making a scar
through the mountains.
The second
photograph shows a cratered region near the Moon's southern pole. The giant crater Clavius
with a diameter of 245 km is visible near to the top of the picture in the centre. Its
mountainous walls are dented by smaller craters – formed by meteorite impact after the
main crater was made.
The Apollo astronauts who first landed on the Moon in
July 1969 took a few days to travel there and in our jet it would be a ten day flight from the
Earth.
A little further out from the orbit - of Mars there are a large number of tiny planets called ASTEROIDS. The biggest of these is only a few miles across and most of them are much smaller. You can't see any of them from the Earth without a telescope. It is possible that they are the remains of a large planet that broke up millions of years ago or they could be parts of a planet that never formed.
Fifty-five years after leaving the Sun our pilot
would be getting old but he would then have reached Jupiter, the giant of the solar system,
nearly 800 million km from the Sun. Jupiter is so large that you could fit over 1000
planets the size of the Earth inside it. However the surface of Jupiter is not solid at all, it is
made of very cold, heavy gas.
Astronomers think that it may be solid inside and
some even think that it may produce its own heat like a very small sun! From the Earth
Jupiter looks like a very bright yellow star.
Jupiter has at least thirteen 'large' moons
circling it and some of these are so big that they can be seen with a pair of binoculars. It was
the discovery of these moons by Galileo in about 1610 that convinced him that the Earth
was not the centre of the universe.
Recently a space probe has discovered an active
volcano on one of these moons !
The surface of Jupiter is covered with coloured
bands and there is also a huge red spot. Nobody is still quite sure what this spot is and why
it does not seem to break up over the years. However it is thought to be a huge storm that
has been raging on the planet for hundreds of years. The sheer size of it has made it
relatively stable over that time.
Jupiter is thought to play one really vital part in the
history of the Solar System and therefore of our own planet. If it were not for the huge
gravitational pull of Jupiter which 'gobbles' up comets, some of them would probably have
collided with the Earth and so ended civilization as we know
it!
Our next stop on our journey would be another
giant planet, Saturn. Our journey would have taken over 100 years and we would have
travelled nearly 1500 million km. Saturn is a very cold planet with a surface of heavy gas. It's
so cold there that some of this gas may freeze solid! However Saturn is most famous for its
impressive set of rings. They are made of dust and small rocks and although they are about
250 million km in diameter they are only some 20 km thick.
Saturn has ten 'large' moons, and over 100 smaller ones, one of the larger ones is bigger than Mercury.
As we journey out past Saturn we have to go twice as far from the Sun
before we reach the next planet. This is Uranus and with Neptune and Pluto it is one of the
three most distant planets.
Our journey from the Sun to Uranus would take 200
years, to Neptune over 300 years and to Pluto about 420 years. Uranus is twenty times the distance of the Earth from the Sun and Neptune around thirty times.
Both Uranus and Neptune are giant 'ice giant' planets, both with a diameter about four times that of the Earth but still small when compared with either Jupiter or Saturn.
They are so far away that it is very difficult to see even Uranus without a
telescope. Uranus has five ‘large’ moons and nine smaller ones. Neptune has fourteen moons, the largest, Triton is the only one big enough to be approximately spherical.
Our journey ends for the present at Pluto, a tiny planetoid nearly 6000
million km from the Sun. It is a frozen world covered in ice and from there the sun would look
like a bright star in a black starlit sky. In the last decade it has been realized that in fact Pluto
is a 'double planetoid'. It has a smaller dark companion that has been named
Charon. Pluto is no longer thought of as a 'proper' planet because it is so small.
Although you might think that this journey has been a long one, if our
high speed jet continued at the same speed then the descendents of the original crew would
eventually reach the nearest of the stars - about 2 million years after their flight had
begun!