Will a laser work to propel a spacecraft?

15 August 2010

Question

Hi Chris,

Just wondering whether or not you could propel a craft at light speed in space, with a Lazer or light beam source from Earth, once the craft had been deployed in space.

To do so would obviously reduce a lot of issues with respect to on board fuel storage, and propulsion system requirements etc and be a lot faster then conventional propulsion. Is it theoretically feasible? Regards

Ben Luckie

Answer

We put this question to Dr Dominic Ford:

Dominic - Well yes, in theory, you could. The way that any rocket works is that it ejects material backwards and there's a principle of physics - the conservation of momentum - which says that if the rocket exerts a backward force on that material to accelerate it backwards, there must be an equal and opposite push, pushing your rocket forwards, accelerating it to move faster. And what matters is how much momentum the ejected material has, and that affects how great the push is forwards on the rocket.

So, could you substitute the exhaust gases of a conventional rocket with a laser or light beam?

Well, yes you could, because light is made up of photons which - although they have no mass, do carry a very small amount of momentum. So for example, if you put your hand underneath a light, then the light is actually exerting a very tiny downward force on your hand. You don't notice it because the force is so small, but it is there.

So likewise, a light on the back of a rocket would push it forwards ever so slightly, but the problem is, this force is so incredibly small. I did a quick calculation this morning of what power of light source you would need to replace the thrusters on the Cassini Spacecraft in orbit around Saturn, which can produce a force of 440 Newtons, and the answer is you would need 130-Gigawatt light source. So that's equivalent to the output of several hundred power stations, all going into one light source. So, this isn't a terribly practical way of propelling the rocket.

Andrew Pontzen - But of course, there are actually ways you can use the pressure of light. For instance, in solar sail technology, and I think there's just recently been an announcement that the solar sail of the Icarus spacecraft is being unfurled.

Dominic - So this is a serious question because it's very inefficient to carry large volumes of rocket fuel around the solar system. So people will continually looking for new ways of controlling spacecraft and solar sails are one promising idea. How they work is you have a reflector which the sun exerts an outward force on, due to radiation pressure. Also, the solar wind exerts an outward force. What the Japanese Space Agency are doing at the moment, is trialing this experimental solar sail Icarus, which is 20 metres across. And they're going to try and to glide it down in the solar system towards Venus, using the solar radiation pressure to control their direction as they glide through the solar system. It will be fascinating to see how it goes.

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