“We know of almost four thousand ‘home-grown’ comets that formed, like the planets, in the leftover cloud of swirling gas and dust that fell in on itself to form our Sun,” explains Marco Micheli, Astronomer in ESA’s Near-Earth Object Coordination Centre.

“118 of these are currently classified as ‘near-Earth comets’, and they’re being monitored closely as their orbit brings them across Earth’s own.”

Made up of dust, rock and ice, these "dirty snowballs" are the most pristine and untouched objects you can find, having changed very little over billions of years and so offering a window back to the forming of the Solar System. It is also thought that at least some of the water on Earth originated in comets that impacted our planet in its early years.

Not only does their chemical composition reveal a great deal, but just 70 years ago the motion of their tails revealed the presence of the solar wind.