New NASA spacecraft runs on light

In 2018, a small space probe will unfurl a sail and begin a journey to a distant asteroid. It’s the first NASA spacecraft that will venture beyond Earth’s orbit propelled entirely by sunlight. This technology could enable inexpensive exploration of the solar system and, eventually, interstellar space.

The $16 million probe, called the Near-Earth Asteroid Scout, is one of the 13 science payloads that NASA announced Tuesday. They will hitch a ride on the inaugural flight of the Space Launch System—the megarocket designed to replace the space shuttle and, one day, send the Orion spacecraft to Mars.

It will take 2.5 years for the NEA Scout to reach its destination, a smallish asteroid named 1991 VG. But it won’t be a leisurely cruise. The continuous thrust provided by sunlight hitting the solar sail will accelerate the probe to an impressive 63,975 mph (28.6 km/s) relative to the sun.

Given enough time, a spacecraft equipped with a solar sail can eventually accelerate to higher speeds than a similarly sized spacecraft propelled by a conventional chemical rocket.

Solar sails are made of ultrathin, highly reflective material. When a photon from the sun hits the mirror-like surface, it bounces off the sail and transfers its momentum to the spacecraft—the same way that a cue ball transfers its momentum when it smacks into another ball in a game of pool.

The solar sail concept has been around since 1924, when Soviet rocket pioneers Konstantin Tsiolkovsky and Friedrick Tsander speculated about spacecraft "using tremendous mirrors of very thin sheets" and harnessing “the pressure of sunlight to attain cosmic velocities.”

NASA began investing in solar sail technology in the late 1990s. In 2010, it successfully launched a small, sail-propelled satellite into Earth’s orbit, where it remained for 240 days before reentering the atmosphere.

That same year, the Japanese space agency demonstrated the feasibility of solar sails for interplanetary travel. A test craft hitched a ride aboard the Venus probe Akatsuki. The solar sail, dubbed the Interplanetary Kite-craft Accelerated by Radiation Of the Sun (IKAROS), was released into space by the probe when it was 4.3 million miles away from Earth. Six months later, IKAROS made history when it successfully flew by Venus.

Solar sails have become feasible thanks to the revolution in electronics. NASA sees such reconnaissance as an essential first step for future crewed missions to asteroids.