The Sun and the planets

1. At the centre of our solar system is the Sun, a main sequence star that contains more than $99\%$ of all the mass in the solar system.
2. The Sun's gravity holds everything in the solar system in orbit around it.
3. Eight planets orbit the Sun, in order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
4. The planets' orbits are ellipses that are very close to circles, and they all lie in nearly the same flat plane.
5. All eight planets orbit in the same direction, which is a clue that they formed from the same spinning disc of dust and gas.
6. Planets do not give out their own visible light, so we see them only because they reflect sunlight.
7. The further a planet is from the Sun, the longer it takes to complete one orbit, so a year on Mercury lasts 88 Earth days while a year on Neptune lasts about 165 Earth years.

Inner and outer planets

1. The four inner planets, Mercury, Venus, Earth and Mars, are small rocky planets with solid surfaces made of rock and metal.
2. The inner planets are close to the Sun, so they are much warmer than the outer planets.
3. The four outer planets, Jupiter, Saturn, Uranus and Neptune, are much larger and are known as the giant planets.
4. Jupiter and Saturn are gas giants, made mainly of hydrogen and helium, with no solid surface to land on.
5. Uranus and Neptune are ice giants, containing large amounts of water, ammonia and methane ices beneath their atmospheres.
6. All four giant planets have ring systems, although Saturn's rings are by far the brightest.
7. Jupiter is the largest planet, with a diameter about eleven times that of the Earth.
8. The planets therefore share some features, such as orbiting the Sun and being nearly spherical, but they differ in size, composition, temperature and number of moons.

Minor planets and comets

1. The solar system also contains many smaller bodies that orbit the Sun, called minor planets.
2. Dwarf planets are big enough for their own gravity to pull them into a nearly round shape, but they have not cleared other objects from around their orbits.
3. Pluto was called the ninth planet until 2006, when it was reclassified as a dwarf planet because it shares its region of space with many similar icy bodies.
4. Ceres, the largest object in the asteroid belt between Mars and Jupiter, is also a dwarf planet.
5. Asteroids are smaller, irregularly shaped lumps of rock and metal, and most of them orbit the Sun in the asteroid belt.
6. Comets are lumps of ice, dust and rock that follow very stretched elliptical orbits, spending most of their time far beyond Neptune.
7. When a comet passes close to the Sun, some of its ice turns to gas and streams away with dust to form a glowing tail that points away from the Sun.

Moons

1. A satellite is any object that orbits a larger body.
2. Moons are natural satellites, and most of them orbit planets.
3. Mercury and Venus have no moons, the Earth has one, and Mars has two small moons called Phobos and Deimos.
4. Each of the giant planets has dozens of moons, and new small moons are still being discovered, so the totals keep changing.
5. Some moons are larger than the planet Mercury, such as Ganymede, which orbits Jupiter.
6. Like planets, moons do not give out their own light, so we see them by the sunlight they reflect.
7. Our Moon orbits the Earth about once every 27 days and always keeps the same face turned towards us.

Artificial satellites

1. Artificial satellites are human-made objects carried into orbit by rockets.
2. Thousands of artificial satellites now orbit the Earth.
3. They are used for communications, such as satellite television and phone links, and for navigation, such as the GPS signals used by phones and car sat-navs.
4. They are also used for weather forecasting, for monitoring the Earth's surface by tracking floods, crops and ice sheets, and for military surveillance.
5. Some carry telescopes, such as the Hubble Space Telescope, which observes space from above the blurring effect of the Earth's atmosphere.
6. The International Space Station is a large artificial satellite where astronauts live and carry out experiments.
7. The orbit chosen for each satellite depends on the job it has to do.

Geostationary and polar orbits

1. A satellite in a geostationary orbit circles the Earth directly above the equator.
2. It takes one day to complete each orbit and moves in the same direction as the Earth spins, so it stays above the same point on the Earth's surface.
3. Geostationary satellites orbit at a height of about $36000km$, which is a very high orbit.
4. Because they never move across the sky, a fixed dish aerial on a house can stay pointed at one, which makes them ideal for communications and satellite television.
5. Weather satellites in geostationary orbit can also watch the same large area of the Earth continuously.
6. A satellite in a polar orbit passes over, or close to, both the North Pole and the South Pole on each orbit.
7. Polar orbits are low, typically between a few hundred and about a thousand kilometres above the surface, and one orbit takes roughly 100 minutes.
8. The Earth spins beneath the satellite as it travels, so on each orbit the satellite passes over a different strip of the surface.
9. Within about a day the satellite can scan the whole surface of the Earth, which makes polar orbits ideal for mapping, weather monitoring and surveillance.
10. Being close to the Earth also lets a polar satellite take much more detailed images than a geostationary one.

Natural and artificial satellites compared

1. Moons and artificial satellites are both satellites, so both orbit a larger body.
2. Both are kept in orbit by the gravitational attraction of the body they orbit.
3. Both reflect sunlight, which is why the International Space Station can be seen from the ground as a bright point of light moving across the night sky.
4. A moon is natural, having formed alongside its planet or been captured by it, while an artificial satellite is human-made and placed in orbit deliberately.
5. Most moons are far larger than any artificial satellite, and a few are larger than the smallest planet.
6. Artificial satellites are placed in orbits chosen for a purpose, such as geostationary or polar orbits, while moons stay in the orbits they formed in or were captured into.
7. Moons orbit planets and dwarf planets across the solar system, while almost all artificial satellites orbit the Earth.
8. Planets orbit the Sun directly, while moons orbit planets, so our Moon is a satellite of the Earth rather than a planet in its own right.