All About Space

Interstell­ar ramjet

Travelling between stars takes a lot of energy, but we might be able to pick some up on our way through space


The trouble with rockets is that you have to pack everything before you go. While earthly modes of transport all work within their environmen­t, shooting around air or water or rolling along a surface, rockets must carry whatever they are going to shoot out the back – reactive mass – with them. This effect is exacerbate­d because the reactive mass you’ll be using at the end of the flight is just dead weight at the start. So you need even more mass and energy at the start just to lift the mass and energy you will need later on. This is why it is so challengin­g and expensive to get into space. Rockets are so big compared to the payloads they launch because they need to be more than 90 per cent full of propellant­s at take off.

Once you’re in space you have more options: craft don’t need to be aerodynami­c and the engines don’t need to support a craft against gravity, so a small thrust over a long time is equivalent to a large one over a short time. However, if we want the space travel of fiction, to voyage across the stars to find other planets and life, the challenge gets even greater.

Proxima Centauri, the nearest star beyond the Sun, is 4.2 light years away, so traditiona­l rockets would take thousands of years to get there.

Worse still, if we are to approach light speed to minimise the travel time we have to drag even

more propellant up to super-high speeds. As a result the Daedalus fusion-powered interstell­ar probe concept would stand nearly as tall as the Empire State Building and weigh 54,000 tonnes – 50,000 tonnes being propellant. But space isn’t as empty as it appears, so what if we could make use of the resources already out there?

In 1960 nuclear physicist Robert Bussard proposed such a system: the Bussard ramjet. A ramjet is an engine that uses its forward speed to ram air into the engine, instead of the fan blades seen on jet engines. With the Bussard ramjet a craft would be initially set moving by a fusionpowe­red rocket using internal fuel, and then it would generate a huge funnel-shaped magnetic field. This could collect the free hydrogen molecules that float around in interstell­ar space and duct them back to the spaceship. Once collected the hydrogen molecules could be used as fuel for a fusion-powered rocket.

Nuclear fusion is the most powerful reaction we have available from ordinary matter. Current nuclear power stations use nuclear fission where energy is released by splitting heavy atoms, but in nuclear fusion energy is released by combining light atoms, which is cleaner and more powerful. The hydrogen molecules would be fused together, producing a hot jet of helium gas to push the spacecraft along while it collects more fuel.

There are challenges of course: the scoop would have to gather one trillion cubic kilometres (240 billion cubic miles) of space to pick up one kilogram (2.2 pounds) of hydrogen. Scooping up the hydrogen may create more drag than the engine can overcome, and hydrogen itself is not easy to fuse. Some studies have suggested it might be better to just use the interstell­ar hydrogen as a reactive mass, heated up by a separately fuelled fusion reactor.

Though the Bussard ramjet is still a theoretica­l concept, it has already had a cultural impact as the ‘Bussard Ramscoop’ on the front of the Starship Enterprise in Star Trek.

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