Nasa goes all Ahab and plans to harpoon a comet

Man has always hunted. Even before prehistory ditched the ‘pre’ part of its name and became just history, man has used harpoons to make the hunt easier – especially when there was water involved.

Before history even considered dropping its prefix, hunters used long sharp pointy things to spear fish. But sometimes the fish slipped off the end. Then some bright spark had the idea of putting a barbed end on the sharp pointy thing and the harpoon was born.

For centuries, the harpoon was the weapon of choice for hunting at sea but, lately, it has fallen out of vogue.

Nasa are planning to rehabilitate the harpoon but, instead of hunting whales at sea, they will be hunting comets in space.

Astronomers are fascinated by comets. These frozen chunks of dust and ice were formed when the solar system was still a baby (that’s well before history, prehistory or any other sort of history) and they have remained unchanged ever since. As such, they are like frozen time capsules, crammed full of information about the origin of the solar system.

Astronomers would love to get their hands on a sample of comet and unlock its secrets.

To make their wish a reality, Nasa will be equipping a comet-hunting spacecraft called OSIRIS-REx with a harpoon and, to complete the historical synergy, they will fire it from a crossbow.

A comet can move through space quite quickly (about 240,000kph) so landing a craft on its surface is a bit tricky. The craft, which is planned to launch in 2016, will use a two-metre crossbow to fire a high-speed harpoon with a special hollowed-out tip into the comet’s surface. The harpoon will grab a sample from inside the comet and then the sample will be winched back to OSIRIS-REx and returned to Earth.

But comet hunting isn’t as straightforward as you’d think – you can’t just grab a harpoon and go at it.

So, ‘Call me Ishmael’ and check out our comet-hunting guide.

The hunter’s guide to comets: Expert tips to help you bag your prize

So you think you know a comet when you see one?

Well, think again. They only adopt their full comety plumage when they pass close to the Sun. Without the Sun’s warmth, the water and gases that form the comet’s tell-tale tail stay frozen solid – locked away in the comet’s nucleus (during long hunts, entertain yourself by repeating “tell-tale tail” as fast as you can).

Because comets have such huge orbits, they spend most of their time a long way from the Sun’s warmth. This means that comets can spend 99 per cent of their lives looking an awful lot like asteroids, so if you’re not careful a comet could pass right by you and you’d never know.

When you do get your quarry in your sights, don’t be fooled by its size –it’s not as big as it looks. Most of what you can see is just a cloud of gas.

The comet’s nucleus is just a tiny speck somewhere in the middle. The cloud is created by gas that vents out of the comet when it is warmed by the Sun. So, although your target might appear to be hundreds of thousands of kilometres across, if you don’t aim for the comet’s tiny heart, your harpoon will just sail harmlessly through.

If you think you can wait for a comet to pass by before you shoot it, think again. Comets can come from deep space, which gives them plenty of time to pick up speed. Some could be moving as fast as 70km/sec (relative to Earth), whereas the fastest bullet can only reach about 1km/sec. You will either need to get in front of the comet to shoot it as it streaks towards you (not recommended), or you’ll need to match the comet’s speed yourself.

Intuition tells us that the tail of gas and dust trailing from a comet should be an indication of the comet’s direction of travel. After all, if you throw a ball with a streamer attached to it, the streamer will drag behind the ball. But if you try to anticipate a comet’s movements by looking at its tail, your ambush could be doomed to fail.

A comet’s tail is actually being blown away from the comet by the solar wind. So, all the tail can tell you for certain is where the Sun is (and if you can’t see the Sun already, you shouldn’t be hunting). To make matters worse, comet orbits are hard to predict. All that gas venting from a comet’s surface can act like the manoeuvring thrusters on a space craft – suddenly pushing the comet into a new course.

Comets can have multiple tails. Usually, they have a blue tail, which is made up of ionised gases (their atoms have been stripped of their electrons by the solar wind). These atoms get all excited by the Sun’s radiation and emit blue light. This tail always points away from the Sun.

Another tail is made of dust and gas that, because it contains more mass than the ion tail, can be dragged behind the comet and curve slightly.

A successful hunter is a patient hunter, but don’t be too patient or your prize will vanish before your eyes. Because comets are essentially giant lumps of ice, every time they pass the Sun, they melt a little bit. All the gas that makes a comet so spectacular is actually its life blood venting away into space. Some comets can lose hundreds of tonnes of material a second so, eventually, all the ice and gas that holds them together will be gone and your comet will disintegrate.

Happy hunting!