Comets, like the planets and moons, were born from the cloud of dust and gas before the Sun flared into existence. Often described as “dirty snowballs”, they consist of a solid nucleus up to ten kilometres across, with a vaporous tail that can be millions of kilometres in length. The dramatic tail is actually composed of two, often overlapping parts: a curved dust tail trailing behind, and a gas tail pushed by the solar wind pointing directly away from the Sun.
The observed comets are classified as either short or long period. The short period comets orbit in regions of space between 35 and 100 astronomical units (AU) from the Sun; each AU is the distance from Earth to the Sun, or about 150 million kilometres. Just beyond the orbit of Neptune, these regions known as the Kuiper Belt and the Scattered Disk are filled with objects similar to the asteroids that exist between Mars and Jupiter. These comets are flung onto their dramatic orbits by the gravity of Neptune, and every so often one is deflected towards the Earth and the Sun.
Even more exotic are the long period comets from the Oort Cloud. This distant region of space between 3,000 and 50,000 AU – almost a quarter of the way to the closest other star – is hypothesised from the orbits of these far-travelling comets with orbital periods of millions of years. In it, trillion of comets are travelling on their distant ellipses, still gravitationally tied to the Sun. In some instances, such as Halley’s Comet, they are captured by the gravity of the outer planets and bound to much shorter orbits.
When a comet comes within the orbit of Mars, the energy of the Sun begins to heat up its atmosphere to form the dust and gas tails. When a comet comes close enough to Earth, we can train our telescopes on them. If we plan and cooperate, we can send robots after them to take a more careful look.