But in the near-vacuum of space, even a minute pressure can have a major effect because it’s pushing all the time, hour after hour, day after day. And unlike rocket fuel, there’s a free and unlimited supply of sunlight. If we can harness this power, we can use it; all we need are sails. Like seafarers of old, future spacecraft (and space travelers) could propel themselves into the vast cosmic expanse with the help of the sun—or at least that’s how Sagan envisioned it.