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

Question

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?

Answer 1

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.