Slow boats require little assistance from their home world after launch. But what if the home population of a starfaring society allocates a high priority to interstellar exploration. Extensions of light-sail technologies that may become feasible in the future might decrease interstellar transit times to one or two centuries. But a massive in-space infrastructure would be required—one that would operate seamlessly for decades or centuries.
A fast ship would resemble a slow boat, with one very significant difference. Instead of being propelled by the radiation pressure of solar photons impacting against the sail, the motive force for a fast ship would be a solar-powered energy beam. Conceptually at least, the fast ship could overcome the limitations imposed by the inverse square law that governs solar radiant flux and therefore reach a higher interstellar cruise velocity than a slow boat.
Although a lot of progress has been made lately in demonstrating that the link between energy beam and sail can be maintained over short distances, maintaining this link over trillion-kilometer interstellar distances is far from easy. It will be necessary to either light-levitate the beam-power station in a stationary position between the Sun and the starship for decades, or have the power station follow the starship out of the solar system on a slower trajectory.
In either case, the beam must be maintained on the sail for the decades-long acceleration run. The slightest deviation and the mission will be lost.
Assuming that the engineering challenges can be overcome, there are three options for the power beam. We might consider the most efficient approach (from a momentum-transfer point of view) and project a neutral charged-particle beam against a ship-mounted magsail. Unfortunately, since high-energy particle beams are a component of missile defense systems, it may be difficult to engineer them and convince the world that their use would be for peaceful purposes only.
As an alternative, we might consider a high-power, collimated microwave beam, or maser. Although masers do not suffer from military secrecy requirements, and microwave technology is comparatively inexpensive, maser (and laser) beams are much less efficient than particle beams in transferring momentum to the starship.
A more significant issue affecting microwave beams is beam spread. Unless a corrective optical system is placed between the beam and the starship, the beam will spread too rapidly to be ofuse for accelerating even very large sails. And the application of such an optical system would only complicate the problem of maintaining the beam on the sail.
The third and currently favored alternative is to utilize a huge solar-pumped space laser operating in the ultraviolet, visual, or near-infrared spectral regions. Beam spread would be less of a problem here, but laser-technology is orders of magnitude more expensive than microwaves. And lasers do not solve the problem of maintaining the beam on the distant sail for decades.
There is no fundamental reason why clever engineers cannot ultimately solve these problems. But interstellar beamed-energy sailing cannot be said to currently have a high Technological Readiness Level.
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