Boaters come in two major camps: sail versus motor, wind versus diesel. Fuel gets you farther faster, and is more reliable, but then runs out. Sail is slower, can be cheaper, and go travel forever as long as the wind blows and the fragile craft survives. Space travel has relied on fuel, but it doesn’t have to. Sails can ride on photons from the Sun. The craft are incredibly light (in terms of weight) and don’t accelerate very quickly, but the Sun is more constant than the wind, and sailing through space doesn’t have to deal with shoals and reefs that run sailboats aground. After it is launched, a solar sail can travel throughout the solar system without fuel. Limitations fall away, as long as the mission allows patience. A test sail is being launched. It will only be a few meters across, but the eventual designs can reach sails measured in square kilometers. The cost of travel, and therefore commercialization and colonization, drops dramatically.