Science Questions

Will a laser work to propel a spacecraft?

Sun, 15th Aug 2010

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Question

Ben Luckie asked:

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 thereís 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 Ė 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 thatís equivalent to the output of several hundred power stations, all going into one light source.  So, this isnít 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 thereís just recently been an announcement that the solar sail of the Icarus spacecraft is being unfurled.

Dominic -   So this is a serious question because itís 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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Ben Luckie asked the Naked Scientists: Hi Chris, Just wondering whether or not you could propel a craft at light speed in space or close to c with a laser or light beam source from Earth once the craft had been deployed in space? As to do so it 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? And would electro-static propulsion work in space on a craft, like it does for the B2 stealth bomber here on Earth? If so would that also provide greater protection inside the craft for the astronauts from deleterious cosmic radiation etc. Regards Ben Luckie What do you think? Ben Luckie, Fri, 28th May 2010

The problem is that you need 300 MW to produce 1 Newton of thrust the good news is that this thrust can be maintained up to near light speed ( When you would be fried by blue shifted CMBR Photons ).  syhprum, Fri, 28th May 2010

Q. Will a laser work to propel a spacecraft?

A. Yes if it is within the Earth's atmosphere ... http://www.youtube.com/watch?v=5_9ac-w4DW8&feature=related

I don't fancy being in a Myrabo "light craft" though: being spun at 100's of rpm and being booted up the backside every half second, (and if the beam and craft are not aligned it's curtains). RD, Fri, 28th May 2010

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