Why are comets interesting?

Comets are ancient cosmic icebergs. They are roughly 4.6 billion years old and formed at the same time as the Sun, Earth and the other planets.

Gravitational interactions fling them towards the inner planets from out beyond the orbit of Neptune, making them some of the most chaotic and dynamically interesting objects in the Solar System.

A comet’s nucleus is typically between 1 and 50 km across with the structure of a dirty snowball. They are made of dust and ice, which partly goes from solid to gas when the comet is warmed by the Sun. Cometary and asteroid impacts may be responsible for a fraction of the water present on Earth and for the icy reservoirs that are thought to exist at the bottom of permanently shadowed craters on the Moon.

The defining characteristic of a comet is its tails. Tails come in two main types, a dust tail and an ion (or plasma) tail. When sunlight warms the ice on a comet, gas and dust are released into space.

The dust falls into orbit around the comet and is gently pushed into fan-shaped tail by incoming sunlight.

The gas is ionised by ultraviolet sunlight and the particles of the solar wind, and these charged particles (ions) form a narrow ion tail that is pushed directly away from the Sun by the solar wind.

Comet tails are the reason for Earth’s annual meteor showers, during which Earth passes through clouds of dust left behind by comets.

Comet 12P/Pons-Brooks is a ‘near-Earth comet’, which means it orbits the Sun in less than 200 years and comes close enough to the Sun that it could cross Earth’s orbit. ESA’s Planetary Defence Office keeps a close eye on near-Earth comets to monitor any risk of collision.

Near-Earth comets are much rarer than near-Earth asteroids – their less icy cousins. We know of only 122 near-Earth comets, but of almost 35 000 near-Earth asteroids.