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Nasa is developing a harpoon capable of taking samples from comets.

The space agency has already built a prototype capable of launching test harpoon tips across a distance of a mile (1.6km).

The engineers believe it would be safer torather than trying to land on the celestial bodies.

Nasa said that the samples could reveal the origins of the planets and how life was created on Earth.

Comets are made up of frozen chunks of ice, gas and dust. They orbit the sun and, if they are close enough to the star, project a tail in the opposite direction made up of ionised gases.

Particle samples recovered byin 2002 were found to include an amino acid, glycine, which is used by living organisms to create proteins. The agency said the discovery supported the theory thatand had been delivered to Earth by meteorite and comet impacts.

To gather more material, the agency is developing a sample-collecting space harpoon which could be projected "with surgical precision" from a spacecraft hovering above the target.

Experts said this would avoid the risk of trying to anchor the craft to a comet's rugged surface.

Comets are much smaller than planets and have much lower gravity as a consequence, so a landed spacecraft would have to find some way of attaching itself to the object to avoid floating off.

Engineers at Nasa's Goddard Space Flight Center in Greenbelt, Maryland, have built a trial harpoon that is 6ft (183cm) tall.

The bow is made out of a pair of springs normally used to provide the suspension for trucks. The bow string is made out of steel cable half an inch thick.

It can fire projectiles at speeds of more than 100ft per second. Test projectiles are fired into large drums filled with sand, rock salt, ice or pebbles.

"We had to bolt it to the floor, because the recoil made the whole testbed jump after every shot," said the project's lead engineer, Donald Wegel.

"We're not sure what we'll encounter on the comet - the surface could be soft and fluffy, mostly made up of dust, or it could be ice mixed with pebbles, or even solid rock.

"Most likely, there will be areas with different compositions, so we need to design a harpoon that's capable of penetrating a reasonable range of materials."

Data collected from the experiments will be used to determine which design and explosive powder charge should be used on the mission.

The scientists are also developing a hollow harpoon tip to contain a sample chamber in which the gathered material would be stored.

"It has to remain reliably open as the tip penetrates the comet's surface, but then has to close tightly and detach from the tip so the sample can be pulled back into the spacecraft," said Mr Wegel.

The team added that it expected several harpoons with different powder charges would be put on the spacecraft to ensure materials could be recovered from different parts of the same comet.

The researchers said the work could also help discover the best way to destroy comets.

Science-fiction stories have described using nuclear weapons to change the direction of comets set on collision course with Earth.

However, Nasa warned the idea could backfire if the explosion only shattered the object into smaller, but still deadly, fragments.

Mr Wegel suggested that if scientists could gather material from the comet, they might be able to work out its composition and thus be able to work out how to deflect it.

Nasa is not the only space agency to have the idea of using a space harpoon.

The, launched in 2004, plans to use one. It is set to rendezvous with a comet named Churyumov-Gerasimenko by October 2014.

It will use the harpoon to anchor a lander to the ground. The lander contains equipment to analyse the comet's surface and subsurface.

Mr Wegel described the ESA's harpoon as "ingenious", but noted that it could not collect samples of its own.