Extension: Moons, Asteroids and Comets

This is an extension lesson. It goes beyond the Stage 4 syllabus dot points for interest and enrichment. You will not be tested on it in the core unit, but it is a brilliant way to explore the smaller members of our Solar System and the rocks that travel between them.

A moon is a natural satellite, which means a natural object that orbits a planet rather than the Sun directly. Earth has just one moon, which we simply call the Moon. It is the brightest object in our night sky and the only other world humans have ever walked on.

Some planets have many moons. The gas giants in the outer Solar System are circled by whole families of them.

• Jupiter has dozens of moons. The four largest, Io, Europa, Ganymede and Callisto, are called the Galilean moons because the scientist Galileo first spotted them through a telescope in 1610. Ganymede is so big it is larger than the planet Mercury.
• Saturn also has many moons. Its largest, Titan, is bigger than our own Moon and is the only moon known to have a thick atmosphere and lakes of liquid (made of cold methane, not water).
• Mars has two tiny, lumpy moons called Phobos and Deimos, which look more like potatoes than spheres.

Moons come in many sizes and surfaces. Some are rocky and cratered, some are icy, and a few may even hide oceans of liquid water beneath a frozen crust. Mercury and Venus are the only planets with no moons at all.

When the Solar System formed about 4.6 billion years ago, most of the dust and rock clumped together to make the Sun and the planets. Asteroids are the rocky leftovers from that building process, lumps of rock and metal that never joined a planet. They range from tiny pebbles to giant boulders hundreds of kilometres across.

Most asteroids are found in a wide ring called the asteroid belt, which sits between the orbits of Mars and Jupiter. Even though there are millions of them, the belt is mostly empty space, so the asteroids are usually very far apart. Space probes have flown straight through it without trouble.

Asteroids matter because they are like a time capsule. Since they have barely changed since the Solar System formed, studying them tells scientists what the early Solar System was made of. Australia plays a part in this work: a tiny sample of the asteroid Itokawa, brought back by Japan's Hayabusa spacecraft, parachuted to the ground at Woomera in South Australia in 2010.

A comet is sometimes called a "dirty snowball". It is a small body made of ice, frozen gases, dust and rock, left over from the cold outer edges of the Solar System. For most of its long, stretched orbit a comet is just a dark, frozen lump too faint to see.

Everything changes when a comet swings close to the Sun. The Sun's heat warms the ice and turns it straight into gas. This gas and dust stream off the comet and form a glowing tail that can stretch for millions of kilometres. A bright comet with a long tail can be one of the most beautiful sights in the night sky.

Here is the surprising part. A comet's tail does not trail behind it like smoke behind a car. Instead, the tail is pushed by sunlight and the stream of particles flowing out from the Sun, so the tail always points away from the Sun. That means when a comet is heading back out into space, its tail actually points ahead of it. The most famous comet, Halley's Comet, sweeps past Earth only about once every 76 years, and many Australians remember seeing it in 1986.

Space is full of tiny pieces of rock and dust. When one of these pieces hits Earth's atmosphere at very high speed, it heats up and burns, making a bright streak of light across the sky. That streak is called a meteor, and most people know it as a "shooting star". A meteor is just the flash of light, not the rock itself.

Usually the piece burns up completely before it reaches the ground. But if a lump is big enough to survive the fiery fall and actually land on the surface, that landed rock is called a meteorite. So the simple rule is: a meteor is the streak in the sky, and a meteorite is the piece that lands on the ground.

When a large meteorite slams into the ground, it can blast out a bowl-shaped hollow called a crater. One famous example is right here in Australia. Wolfe Creek Crater, known to the Djaru People as Kandimalal, lies in the Tanami Desert of Western Australia. It is one of the best-preserved meteorite craters on Earth, about 880 metres across, formed when a large iron meteorite struck the ground long ago. The Djaru People of this Country hold their own knowledge and stories about the crater. Many other meteorites have been found across the dry plains of Australia, where the pale space rocks are easy to spot against the red soil.

The Solar System is far more than just a Sun and eight planets. It is a busy place filled with smaller bodies, each with its own story.

• Moons orbit the planets. Our Earth has one, while gas giants like Jupiter and Saturn have whole families of moons.
• Asteroids are rocky leftovers from the Solar System's birth, gathered mostly in the belt between Mars and Jupiter.
• Comets are icy travellers from the cold edges that grow glowing tails, always pointing away from the Sun, when they visit the inner Solar System.
• Meteors and meteorites remind us that space rock reaches Earth, sometimes leaving craters such as Wolfe Creek Crater (Kandimalal).

Together, these smaller bodies help scientists understand how the whole Solar System formed and how it still changes today. The next time you spot a "shooting star", you will know exactly what you are really seeing.