Asteroids and Comets

Asteroids and comets are the leftover building blocks of the solar system. They formed at the same time as the planets, around 4.5 billion years ago, but never grew big enough to become planets themselves. Asteroids are mostly rocky or metallic and live in the warm inner solar system. Comets are mostly ice and dust and come from the cold outer edges. When a comet swings close to the Sun, the ice starts to boil off and forms the famous glowing tail.

• Known asteroidsapprox. 1.3 millionAnd we find more every year
• Largest asteroidCeres940 km across (it is also a dwarf planet)
• Asteroid Belt329 to 478 million kmBetween Mars and Jupiter
• Kuiper Belt4.5 to 7.5 billion kmOut beyond Neptune, home of comets
• Oort CloudUp to 15 trillion kmThe very edge of the Sun's reach
• Halley's CometEvery 76 yearsNext due back in 2061

What is the difference between an asteroid and a comet?

An asteroid is mostly rock and metal. It sits in the inner solar system (mainly between Mars and Jupiter) and stays roughly the same all the time. A comet is mostly water ice mixed with dust, sometimes called a "dirty snowball". Comets come from the cold outer solar system. When one of them is pulled in towards the Sun, the heat starts to vaporise the ice, and the comet grows a glowing coma (a fuzzy halo) and a long tail that can stretch for millions of kilometres.

Where do they live?

Most asteroids live in the Asteroid Belt, a ring of millions of rocky bodies between the orbits of Mars and Jupiter. Some asteroids stray closer to the Sun and become Near-Earth Asteroids, which is why space agencies keep careful watch for any that come too close.

Comets come from two cold zones far out beyond Neptune. The Kuiper Belt is a flat disc of icy bodies just beyond Neptune (Pluto lives there). The Oort Cloud is a vast spherical shell of frozen objects almost a light year from the Sun, the very edge of our solar system.

Can we stop an asteroid hitting Earth?

Scientists keep careful watch of the sky for any asteroids that might come close to Earth. Most of the really big ones (the kind that could cause a planet-wide disaster) have already been found and tracked, and none of them are due to hit us any time soon. Smaller asteroids hit the Earth's upper atmosphere every day, but almost all of them are tiny grains of dust that burn up as harmless shooting stars.

For larger asteroids, space agencies are working on ways to push them off course. In September 2022, NASA's DART spacecraft deliberately crashed into a small harmless asteroid called Dimorphos to test the idea. It changed Dimorphos's orbit by 32 minutes, proving that a small nudge years in advance could move a dangerous asteroid out of Earth's way.

Fact About 66 million years ago, an asteroid roughly 10 km across crashed into what is now Mexico. The impact threw so much dust into the air that the world stayed dark and cold for years, and an estimated 75% of all species on Earth, including the non-bird dinosaurs, went extinct. The crater is buried beneath the seabed and is called Chicxulub.

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