NASA’s Pioneering Solar Sail Hits a Snag While Tumbling in Space

A microwave-sized spacecraft with large silver wings is tumbling in orbit as engineers work to fix a glitch that’s causing it to lose orientation.

After deploying four composite booms on board the experimental solar sail mission, NASA engineers observed a slight bend in one of the poles that extends along the spacecraft’s wings, the space agency revealed in a recent update. The team behind the mission is hoping that the bent boom does not hinder the remainder of the mission.

The Advanced Composite Solar Sail System (ACS3) launched on April 23 on board Rocket Lab’s Electron rocket from New Zealand. The solar sail was delivered to a Sun-synchronous orbit around 600 miles (966 kilometers) above Earth’s surface (more than twice the altitude of the International Space Station) to test a novel form of spaceflight that uses sunlight for propulsive power.

Four cameras aboard the Advanced Composite Solar Sail System spacecraft show the four reflective sail quadrants supported by composite booms. The in the boom pictured in the left corner of the bottom left image has a slight bend. Credit: NASA

NASA’s ACS3 is designed to test new materials and deployable structures for solar sail propulsion systems, including new composite booms that were used to unfurl the solar sail in orbit. They work like a sailboat’s boom, but are designed to hold the sails, which catch the propulsive power of sunlight rather than wind. The composite booms are made from a polymer material; they’re lightweight while still being stiff and resistant to bending and warping when exposed to different temperatures.

The spacecraft has fully unfurled its sail to its square shape, stretching across 30 feet (9 meters) per side, or roughly half the size of a tennis court. However, the mission team noticed what appears to be a slight bend in one of the four booms, which span the diagonals of the square to reach 23 feet (7 meters) in length.

NASA engineers are assessing the bend, which may have occurred when the booms and sail were pulled taut against the spacecraft during deployment, according to the space agency. “The primary objective of the Advanced Composite Solar Sail System demonstration is to test deployment of the booms in space to inform future applications of the composite boom technology for large-scale solar sails and other structures,” NASA wrote in its update. “Data collected from this flight test has already proven highly valuable, and the demonstration will continue producing critical information to enable future solar sail missions.”

Solar sails run on photons from the Sun, causing small bursts of momentum that propel the spacecraft farther away from the star. If a spacecraft is able to surpass the drag from Earth’s atmosphere, it could potentially reach very high altitudes. The composite boom technology used for this demo mission could be used to deploy solar sails up to 500 square meters (5,400 square feet), or about the size of a basketball court, according to NASA.

Since its launch, however, things have not been smooth sailing for the experimental mission. A few months after its launch, the mission’s sail became stuck when an onboard power monitor detected higher-than-expected motor currents, pausing the unfurling process. In late August, the mission team was successful in its second attempt to fully deploy the spacecraft’s sail and booms.

Before rolling out the booms, however, the team had to deactivate the spacecraft’s attitude control system in order to accommodate its changing dynamics as the sail unfurled. The attitude control system helps the spacecraft maintain a particular orientation relative to another location in space, essentially keeping it oriented. Solar sail’s attitude control has not yet been reengaged, and as a result, the spacecraft is currently tumbling through space.

“Now, mission operators are working to reposition the spacecraft, keeping the Advanced Composite Solar Sail System in low power mode until its solar panels are more favorably oriented toward direct sunlight,” NASA wrote. “The team is conserving the spacecraft’s energy for priority operations – such as two-way communications with mission control – until its attitude control system is reactivated.”

Once its attitude control system is reengaged, the better oriented spacecraft will be able to point its radio antenna toward mission control on Earth and communicate with the NASA engineers on the ground. “At this stage, the team will be able to gather even more data, calibrate the precise shape of the sail, and prepare to begin its sailing maneuvers,” according to NASA.

NASA engineers want to test the maneuvering capabilities of the spacecraft, raising and lowering its orbit using only the pressure of sunlight acting on the sail. The mission’s initial flight phase is designed to last for two months. Throughout its time sailing in space, NASA is hoping to collect as much data as possible from its ACS3 mission to help inform future spacecraft design.

Other solar sails have come before it, namely the Planetary Society’s LightSail 2, which launched in June 2019 and gained 2 miles (3.2 kilometers) of altitude just two weeks after unfurling its 344-square-foot (32-square-meter) solar sail. In November 2022, the mission came to a fiery end after it began losing altitude and reentered through Earth’s atmosphere. The legacy of LightSail 2 inspired several other missions in its wake, including ACS3, as well as NASA’s NEA Scout mission to a near-Earth asteroid and NASA’s Solar Cruiser (scheduled for a 2025 launch).

The technology is still in its infancy, and it’s clearly not without its challenges, but the potential to reach farther destinations deep in the cosmos could be powered by our host star.