# Could protons in the Sun's solar wind be used to create a photonic laser thruster for a spacecraft?

I have a conceptual idea for a photonic laser thruster for a spacecraft and I am wondering if this particular idea has already been thought of and whether or not it is capable of generating enough thrust to propel a spacecraft.

Please reference the drawing below. This photonic laser thruster would be attached to the stern of the spacecraft, although it could also be attached to the side of the spacecraft.

In a nutshell, protons (or other positively charged particles) flowing in the Sun's solar wind would enter into a round opening in the thruster's hull, these protons would then be decelerated by an electric field within the thruster resulting in an lower energy level, which means they will lose kinetic energy and start emitting photons, thus satisfying the law of conservation of energy. These photons will then hit a mirror and be reflected back and out of the opening in the thruster's hull. In theory, these photons will transfer their momentum to the thruster/spacecraft by reflection and propel the spacecraft forward.

Also, theoretically the protons could be brought to a complete stop by this electric field and then their direction will be reversed by the same electric field and be accelerated to a high velocity until they exit the opening in the thruster's hull.

Could protons in the Sun's solar wind be used to create a photonic laser thruster for a spacecraft?

• I think you just won the competition for a concept of engine with worst thrust ;-) Seems like it would work but with scarce piconewtons of thrust. – SF. Jan 16 at 8:23
• Isn't this akin to shining a laser out of the back of a spacecraft-- or a flashlight, or am I over-simplifying? – Magic Octopus Urn Jan 16 at 14:30
• @Magic Octopus Urn, I don't think it is exactly the same thing, even though laser light would be coming out of the thruster. From what I've studied about photonic laser thrusters, when the photons are reflected, some of their momentum/kinetic energy will be transferred to the mirror and thus to the spacecraft. As far as I know, there is no momentum/kinetic energy transfer to a flashlight when the light it creates exits from it. – user28781 Jan 16 at 16:34
• @HRIATEXP Indeed there is momentum exchange from a flashlight's photon emission. The photon's momentum is given by p = h/lambda, where p is momentum, h is Planck's constant, and lambda is the photon's wavelength. The atom emitting the photon rebounds with the same magnitude of momentum but in the opposite direction, and that momentum is eventually transferred to the flashlight as a whole. Most flashlights have collimating mirrors that exchange momentum when the photons reflect from it. The net reaction momentum has the same magnitude as the emergent beam's. – Tom Spilker Jan 16 at 19:31
• @MagicOctopusUrn See my response to HRIATEXP above. – Tom Spilker Jan 16 at 19:32

Indeed such a thruster is possible, but it is wildly impractical.

The problem is in the energy density of the solar wind as compared to the energy density in the sun's emitted photons. The laser thruster is using the energy contained in the oncoming solar wind particles, converting that kinetic energy into electromagnetic energy (photons) and then using the momentum of the photons.

At 1 AU from the sun the intensity of sunlight is ~1360 $$\frac{W}{m^2}$$ (but you have to scroll down quite a ways in that reference to find that figure). The Wikipedia article on the solar wind serves as a good general reference for the following analysis. At 1 AU the density of the solar wind, which is dominated by protons, is ~$$10^7$$ protons per cubic meter and moves radially from the sun at an average of ~400 $$\frac{km}{s}$$. $$10^7$$ protons per cubic meter is a mass density of ~$$1.67 \times 10^{-20} \frac{kg}{m^3}$$. That density at that speed yields an energy intensity of ~$$5.4 \times 10^{-4} \frac{W}{m^2}$$; that of sunlight is two and a half million times larger!

If you're going to use the energy radiated from the sun, using the energy of the photons, as solar electric propulsion does, is far better than trying to use the energy of the solar wind.

Looking at momentum flux instead of energy flux (which would require a different kind of propulsion system than the laser thruster), the pressure of sunlight at 1 AU is ~$$4.5 \times 10^{-6} \frac{N}{m^2}$$ while that due to the solar wind is $${1–6} \times 10^{-9} \frac{N}{m^2}$$. That ratio is ~1000-5000 times better than the energy flux ratio, but sunlight is still about a thousand times higher.

I've talked with a group in Finland (with some interest from a couple of my ESA friends) that is trying to design a system that would use the momentum of the solar wind directly. The idea is to deploy a huge net (hundreds of square km) of exquisitely thin, electrically conducting wires that are charged to a high potential. In theory the electric field established by the net reflects the oncoming ions (again, mostly protons) back at the sun, yielding the momentum exchange to produce a radially outward force. Unfortunately my friends who are magnetospheric physicists told me, "Those guys don't understand the behavior of magnetoplasmas," and they claim the net wouldn't work as proposed.

• I would upvote your answer but I don't have a high enough reputation score yet. I agree with you that using the energy radiated from the Sun would be better than using the energy of the solar wind. Yet, one advantage that this type of photonic laser thruster would have is if/when the spacecraft would travel out into interstellar space, at which point it could use the charged particles flying around in interstellar space to continue to accelerate the spacecraft. Also, I think a proton gun could add to the thrust by shooting protons directly into the thruster. – user28781 Jan 16 at 23:02
• You might not be able to upvote an answer, but as the author of the question you can accept an answer as answering the question to your satisfaction. – Tom Spilker Jan 16 at 23:07
• You mentioned that the speed of solar wind is 400 m/s. Isn’t that 400 km/s by any chance? If so, are the other calculations stil correct? – grizzly Jan 17 at 0:31
• @grizzly As they say in the medical profession: OOPS!! Yes indeed, that should be km/s—thanks for catching that! I'll edit in the correct units. And yes, the other numbers are all correct. That was just a typo in the units. – Tom Spilker Jan 17 at 4:07