If we want to travel to the stars anytime soon, the simple solar sail is probably our best bet. These sails harvest momentum from light as photons bounce off the sail’s surface. Last summer, the Japanese space agency, JAXA, launched a solar sail into space. Named Ikaros, the sail is constructed from thin sheets of aluminum-coated plastic, and it will help JAXA engineers determine how fast the sails can accelerate and how much energy they can collect.

There are several designs for space-based sails, and not all use solar radiation. In 1984, American physicist and science-fiction author Robert Forward proposed a deep-space probe he called Starwisp. Its sail, originally conceived as a mesh of fine metal wire, would be more than half a mile wide but weigh just a few hundredths of a pound. A space-based, 10-million-gigawatt microwave laser would bombard the sail with photons, eventually accelerating Starwisp to a fifth the speed of light. Forward continued fine-tuning the design of the sail until his death in 2002.

The proposed Starwisp sail was more than a half mile wide.

Solar sails are already in space, but the sail proposed for the hypothetical deep-space probe Starwisp would harness the photons from a huge laser to accelerate to the stars.

HOW A PHOTON SAIL WOULD WORK

1. Solar panels provide power for a 10-million-gigawatt microwave laser, which is housed on a satellite.

2. A 600-mile-diameter lens focuses the microwaves onto the photon sail.

3. The force of the microwave photons reflecting off Starwisp’s sail causes the craft to accelerate to about one fifth the speed of light over a two-week period.