WHERE DO COMETS COME FROM?
Comets obey the same laws of motion as the planets do, which means they travel around the Sun in elliptical orbits. The difference is that while the ellipses followed by the planets are roughly circular, the orbits of comets are far more elongated. As a consequence, while their closest approach to the Sun – called perihelion – may lie here in the inner Solar System, their most distant extreme, or aphelion, is much further away.
Unsurprisingly, the further out a comet’s aphelion is, the longer it takes to complete a single orbit around the Sun. Among the well-studied comets, the one with the shortest period is Comet 2P/encke, which completes an orbit in just under 3.3 years. It has its perihelion at 0.34 AU – where one AU, or astronomical unit, is the distance between Earth and the Sun – and an aphelion at 4.09 AU, inside the orbit of Jupiter.
A more typical cometary orbit is that of Halley’s Comet. With a period of around 75 years, this has an aphelion of just over 35 AU, which is beyond the orbit of Neptune. This puts it in a region of the Solar System known as the Kuiper Belt, which is filled with small, icy comet-like objects – though only a minority of them have orbits that bring them anywhere near Earth. Those that do – together with closer-in comets like Encke – are known as ‘short-period comets’. While Halley’s 75 years may not sound that short, it’s the blink of an eye compared with a genuine ‘long-period comet’, which may take thousands or even millions of years to complete an orbit. These long-period comets – such as Hale-bopp with an aphelion of 363 AU and period of around 2,450 years – originate not in the Kuiper Belt but in an even more distant region of the Solar System called the Oort Cloud.