INNER SOLAR SYSTEM
Close to the Sun are four hard, rocky planets: Mercury, Venus, Earth and Mars. Earth has one Moon, Mars two moons, while Mercury and Venus have no moons. Together these four planets are called terrestrial planets and form the inner solar system.
The size of each inner planet and moon compared to Earth and Australia.
| Diameter |
4,878 km |
| Average distance from Sun |
58 million km |
| Orbital period |
88 days |
| Rotation period |
59 days |
| Number of known satellites |
0 |
Mercury is the closest planet to the Sun and the second smallest planet in the solar system. It has been visited by only one spacecraft, Mariner 10, revealing a planet covered in craters like the Moon. Since there is no atmosphere the temperature can vary from 420 C in day light to -180 C at night. Its distance from the Sun varies from a close 46 million km to a distant 70 million km. A mercurian 'day' is 59 days long, making a mercurian 'year' equal to one and half mercurian 'days'. Mercury has no moons. On 3 August 2004 a space probe called Messenger was launched into space on a mission to study Mercury. Messenger will fly past Earth once, Venus twice and Mercury three times before easing into orbit around Mercury in March 2011.
| Diameter |
12,104 km |
| Average distance from Sun |
108 million km |
| Orbital period |
225 days |
| Rotation period |
243 days |
| Number of known satellites |
0 |
Venus is named after the Roman goddess of love and beauty; however, it is a hostile rocky planet, with thick layers of cloud which trap the heat from the sun. Venus is the hottest planet with a surface temperature of 482 degrees Celsius and an atmospheric pressure 90 times higher than Earth's. Venus has no moons. On 15 December 1970 the Russian Venera 7 became the first spacecraft to land on a planet (Venus) and send data back to Earth.
| Diameter |
12,756 km |
| Average distance from Sun |
150 million km |
| Orbital period |
365.256 days (1 year) |
| Rotation period |
24 hrs (1 day) |
| Number of known Satellites |
1 |
Every living thing on Earth is a space traveller. Our spaceship Earth travels about 108,000 km every hour or 30 km every second, yet still takes 365.256 days to travel around the Sun. The atmosphere is made from 78 percent nitrogen, 21 percent oxygen and 1 percent other gases. The atmosphere, along with a magnetic field generated by the Earth’s rapid spin and its molten nickel-iron core, shields life on Earth from most of the harmful radiation coming from the Sun. Earth’s own moon is about the same size as Australia.
| Diameter |
6,787 km |
| Average distance from Sun |
228 million km |
| Orbital period |
687 days |
| Rotation period |
24 hrs 37 min |
| Number of known satellites |
2 |
Mars, named after the Roman god of war, has a reddish appearance in the night sky due to its iron-rich surface and gigantic seasonal dust storms. It is about half the width of Earth and has two moons thought to be captured asteroids. Space probes sent to Mars discovered it to be a cold desert planet, with a thin atmosphere. River-like channels on the surface show water once flowed on the surface, suggesting Mars was wet in the distant past. NASA's Odyssey spacecraft has discovered large quantities of water ice under the planet's surface, which leads to the question: is there or has there ever been life on Mars? Two robots named Spirit and Opportunity landed on the surface of Mars in January 2004, and are exploring the surface looking for evidence of ancient lakes and oceans.
