A solar sail or interstellar ramjet is one of the great unrealized ideas in interstellar travel. The idea is this: catch the hydrogen of intersteller space in an enormous sail and use it for propulsion. Also using a big sail is a lightsail, which is propelled by an onboard laser beam. These ideas are more than 60 years old at thsi point — where are we now with ramjets? Here’s more from Centauri Dreams:
The idea of beaming a propulsive force to a sail in space is now sixty years old, if we take Robert Forward’s first publications on it into account. The gigantic mass ratios necessary to build a rocket that could reach interstellar distances were the driver of Forward’s imagination, as he realized in 1962 that the only way to make an interstellar spacecraft was to separate the energy source and the reaction mass from the vehicle.
Robert Bussard knew that as well, which is why in more or less the same timeframe we got his paper on the interstellar ramjet. It would scoop up hydrogen between the stars and subject it to fusion. But the Bussard ramjet had to light fusion onboard, whereas a sail propelled by a laser beam – a lightsail – operated without a heavy engine. The idea worked on paper but demanded a laser of sufficient size (Forward calculated over 10 kilometers) to make it a concept for the far future. His solution demanded very large lasers in close solar orbits, and thus an existing system-wide space infrastructure.