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Counterintuitively, solar sails may be best suited for carrying spacecraft toward the Sun.
Sailing across the known universe on beams of light is about as romantic as the space industry gets.
But while the whimsical dream of harnessing the Sun’s photons with solar sails has existed for more than 100 years, the concept has been limited mostly to science fiction by the level of engineers’ ability to pack metallic booms into spacecraft that could unfurl and control a sailing canvas.
- NOAA is interested in a “space weather buoy”
- The propulsion type offers acceleration that is continuous but initially slow
To overcome that obstacle, NASA has developed a novel expanding tubular boom system made of flexible polymer and carbon-fiber materials that can be rolled up inside a cubesat for launch and then unrolled when deployed. NASA’s Advanced Composite Solar Sail System (ACS3) was launched in April and fully deployed in August. The agency has started testing it as part of a 6-9-month demonstration.
The ACS3 solar-sail-equipped 12U cubesat was made by NanoAvionics. Its fully deployed sails—there are four arrayed around the spacecraft—cover 860 ft.2 (80 m2), about half of a tennis court.
“The composite boom architecture that we’re demonstrating can support up to a 2,000-m2 sail. It has a lot of range,” Justin Treptow-Miller, deputy program executive of small spacecraft technology and flight opportunities programs at NASA Ames Research Center in Mountain View, California, tells Aviation Week. “It makes the overall structure very stiff. You both have a lot of flexion stability, and you have a lot of torsional stability utilizing this composite structure.”
The ACS3’s solar sails are made of reflective polymer sheets and harness the energy of photons emitted from the Sun to propel the spacecraft in the opposite direction. When light particles hit the sails, they impart momentum; the photons redshift as they reflect and lose some of their energy. The spacecraft maneuvers by angling the solar sail, using reaction wheels, for example.
Solar sails could be particularly useful for non-Keplerian orbits that would otherwise require large, expensive and impractical propulsion systems. NASA sees the type as useful for long-duration, deep space travel. Although solar sails are an initially slow form of propulsion, the systems promise to accumulate momentum over time.
“You don’t have to burn propellant, you don’t have to have a high-voltage power system, and you don’t run out of propellant,” Treptow-Miller says. “It enables a very high-reliability, continuous propulsion system.”
Counterintuitively, solar sails may be best suited for carrying spacecraft toward the Sun rather than away from it. Solar sails can be used as a sort of drag chute for spacecraft by angling the system so that photons hit the sail and decelerate the vehicle’s orbital velocity, allowing the Sun’s gravity to pull the spacecraft toward it.
Spacecraft pulled along by solar sails could journey on heliophysics missions for NASA. The National Oceanic and Atmospheric Administration has pondered using a solar sail to maintain a “space weather buoy” between the Earth and the Sun for space weather missions of 3-5 years, Treptow-Miller says.
“Think of it as a propulsion system to pace along with the Earth,” he explains. “The sensor could be between the Sun and the Earth and provide that early warning system for coronal mass ejections that would be coming at the Earth.”
The ends of a solar sail’s composite booms would also offer a place, far away from the electromagnetic interference of a spacecraft’s electronics, for a highly sensitive magnetometer to detect incoming geomagnetic storms—a phenomenon that can disrupt Earth-based communications systems and power grids as well as orbiting satellites.
Although solar sails have been theorized by physicists and imagined by science fiction authors for decades, the first solar sail system, the Ikaros (Interplanetary Kite-craft Accelerated by Radiation of the Sun), was launched by the Japan Aerospace Exploration Agency in 2010. In the years since, only a handful of solar sail spacecraft have been launched.
Treptow-Miller says advances in carbon-fiber manufacturing may make the type more practical. A NASA Ames Research Center team is currently observing how the ACS3’s carbon-fiber booms and reflective polymer solar sails behave in space, and the team plans to attempt orbit-raising and -lowering maneuvers before year-end.
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