A concentrated beam of particles and photons could push us toward Proxima Centauri

Getting to Proxima Centauri b will require a lot of new technologies, but there are increasingly exciting reasons to do so. Both public and private efforts have been seriously looking for ways to make it happen, but so far there’s been one major obstacle in the way: propulsion.

To solve that problem, Christopher Limbach, now a professor at the University of Michigan, is working on a new type of beam propulsion that uses both a particle beam and a laser, which aims to overcome the technology’s greatest weakness.

First, let’s look at why conventional propulsion systems wouldn’t work to get a spacecraft to Proxima b. Conventional rockets are out of the question because their fuel is too heavy and burns up too quickly to get a probe anywhere near the speed it would need to reach Proxima b. Conventional solar sails also fail because once they get far enough away from the sun, only minimal push is applied to them.

Other non-conventional solutions could work, such as nuclear propulsion or ion drives. However, they fall victim to the tyranny of the rocket equation: since they have to carry their fuel, they have to carry more mass to go faster, losing much of that advantage.

That leaves beamed propulsion, which essentially creates a giant beam into space that keeps pushing a spacecraft with a collector on it, which can keep pushing the entire time the spacecraft is en route to its destination. Typically, there are two types of beams used in these systems, particle beams and light beams. However, each has a weakness: diffraction.

Both light and particle beams tend to scatter over large distances, making them much less effective at focusing on a single small object that may be light years away. Even lasers, if allowed to point far away, eventually scatter into unusable light. There is a way around this, however.

Recently, optical research has developed a way to combine particle and laser beams that virtually eliminates diffraction and beam spreading when both are used simultaneously. This would allow a bundled propulsion system to keep its beam focused on exactly the right spot without slowly losing its thrust as the probe gets farther away.

Dr. Limbach used this underlying technology to develop what he calls PROCSIMA, a novel propulsion method that uses a coherent combined particle and laser beam propulsion system.

Calculations by Dr. Limbach and his collaborator, Dr. Ken Hara, now a professor at Stanford, show that it is possible to make a coherent beam that can persist effectively out to Proxima b, but diffracts only out to about 10 m, at least in theory.

According to their calculations, a 5G probe, such as the one the Breakthrough Initiatives project is working on, could reach 10% of the speed of light, which would allow it to reach Proxima b in 43 years.

Alternatively, they calculated that a much larger probe, weighing about 1 kg, could reach the system in about 57 years. That would allow for a much more exciting payload, even if the probe were to zoom through the Proxima Centauri system at a significant fraction of the speed of light.

There is still some work to be done, such as developing cold atomic particle sources and improving the functionality of the beam systems.

So far, however, the project has not received any other funding, although Dr. Limbach’s lab at UM is still working on similar ideas, such as a nanoNewton propulsion system. Development of a starshot method to eventually get a probe to another star continues, and it seems that, for better or worse, beamed propulsion is the way to get us there.