The comet cloud

Beyond the planets, in the darkness far from the Sun, orbits a swarm of blocks of ice and dust – inactive comets. No one has actually seen this swarm, for at that distance the blocks are too faint to detect with any telescope. Rather, the swarm’s existence has been deduced from the fact that virgin comets, making their first approach to the Sun, arrive on highly elongated orbits that must originate partway to the nearest star.

According to the most widely accepted theory, comets are debris remaining from the formation of the Solar System 4600 million years ago. The comets tag along unseen in cold storage at the perimeter of the Solar System until diverted onto new routes by the gravity of passing stars. Some comets are elbowed out of the Solar System entirely, but others head sunwards.

Astronomers estimate that the cometary cold store, known as the Oort Cloud after the Dutch astronomer Jan Oort who proposed its existence in 1950, lies about 50,000 times farther from the Sun than does the Earth, a distance that light takes nearly a year to cross. Even that far away, the Oort Cloud is still held by the Sun’s gravity, albeit loosely. In recent years, astronomers have come to recognize that there is an inner extension of the Oort Cloud, called the Kuiper Belt, which is concentrated in the plane of the Solar System. Pluto, formerly classified as the ninth planet until demoted in 2006, is simply a large and close-in member of that belt, orbiting where the outer planets meet the realm of the comets.

Every few hundred thousand years a star passes close enough to disturb the comets of the Oort Cloud onto new orbits. To account for the regular supply of comets heading inwards to the Sun, there must be around a million million of them in the cloud. Yet the mass of all these comets is, in total, only a few per cent of the mass of the Earth. Although comets advertise themselves impressively, in truth they are disappointingly insubstantial.

No one knows when the comet that was to bear the name Halley made its first approach to the Sun, although one estimate suggests it was over 100,000 years ago. Left to its own devices, the comet would have swung around the Sun and headed back out into the Oort Cloud, not to be seen again for millennia. But, during the comet’s passage through the inner Solar System, its path was tweaked by the gravitational pull of the giant planet Jupiter which reined it into a tighter loop that has brought it back to the Sun regularly ever since. Halley’s Comet is the only really bright example among the comets that return regularly to the Sun.

When far from the Sun, a comet resembles a dirty snowball a few kilometres wide. Only when it reaches somewhere around the orbit of Mars do the Sun’s light and heat awaken the snowball from its hibernation so that it displays itself for the admiration of astronomers.

A comet’s regalia has four components: a nucleus; a coma, which together with the nucleus makes up the head; a tail (although not in all cases); and, around the whole comet, a bubble of hydrogen, millions of kilometres across, invisible to the eye but detectable by instruments in space.

The fount of all cometary activity is the nucleus, the dirty snowball, composed mostly of frozen water caked with dark dust. Under the warming influence of the Sun, fountains of evaporating ice burst through the crust to form an enveloping halo of gas and dust, the coma. An ordinary comet’s coma is about 10,000 kilometres wide, big enough to engulf the Earth, but in the greatest comets, Halley’s among them, the coma spans more than 100,000 kilometres, half the distance from the Earth to the Moon. For all its size, the coma is still transparent – stars can be seen shining through it, as they can through the tail. The gases of a comet are far thinner than your breath.