Everyone knows that solar energy is free and almost limitless here on Earth. The same is true for spacecraft operating in the inner Solar System. But in space, the Sun can do more than provide electrical energy; it also emits an unending stream of solar wind.
Solar sails can harness that wind and provide propulsion for spacecraft. NASA is about to test a new solar sail design that can make solar sails even more effective.
Solar pressure pervades the entire Solar System. It weakens with distance, but it’s present. It affects all spacecraft, including satellites. It affects longer-duration spaceflights dramatically. A spacecraft on a mission to Mars can be forced off course by thousands of kilometres during its voyage by solar pressure. The pressure also affects a spacecraft’s orientation, and they’re designed to deal with it.
Though it’s a hindrance, solar pressure can be used to our advantage.
A few solar sail spacecraft have been launched and tested, beginning with Japan’s Ikaros spacecraft in 2010. Ikaros proved that radiation pressure from the Sun in the form of photons can be used to control a spacecraft. The most recent solar sail spacecraft is the Planetary Society’s LightSail 2, launched in 2019. LightSail 2 was a successful mission that lasted over three years.
The Red Sea and the Nile River, from the LightSail 2 spacecraft. LightSail 2 was a successful demonstration mission that lasted more than two years. Image Credit: The Planetary Society.
Solar sail spacecraft have some advantages over other spacecraft. Their propulsion systems are extremely lightweight and never run out of fuel. Solar sail spacecraft can perform missions more cheaply than other spacecraft and can last longer, though they have limitations.
The solar sail concept is now proven to work, but the technology needs to advance for it to be truly effective. A critical part of a solar sail spacecraft is its booms. Booms support the sail material; the lighter and stronger they are, the more effective the spacecraft will be. Though solar sails are much lighter than other spacecraft, the weight of the booms is still a hindrance.
“Booms have tended to be either heavy and metallic or made of lightweight composite with a bulky design – neither of which work well for today’s small spacecraft.”
NASA is about to launch a new solar sail design with a better support structure. Called the Advanced Composite Solar Sail System (ACS3), it’s stiffer and lighter than previous boom designs. It’s made of carbon fibre and flexible polymers.
Though solar sails have many advantages, they also have a critical drawback. They’re launched as small packages that must be unfurled before they start working. This operation can be fraught with difficulties and induces stress in the poor ground crew, who have to wait and watch to see if it’s successful.
This image shows the ACS3 being unfurled at NASA’s Langley Research Center. The solar wind is reliable but not very powerful. It requires a large sail area to power a spacecraft effectively. The ACS2 is about 9 meters (30 ft) per side, requiring a strong, lightweight boom system. Image Credit: NASA
ACS3 will launch with a twelve-unit (12U) CubeSat built by NanoAvionics. The primary goal is to demonstrate boom deployment, but the ACS3 team also hopes the mission will prove that their solar sail spacecraft works.
To change direction, the spacecraft angles its sails. If boom deployment is successful, the ACS3 team hopes to perform some maneuvers with the spacecraft, angling the sails and changing the spacecraft’s orbit. The goal is to build larger sails that can generate more thrust.
“The hope is that the new technologies verified on this spacecraft will inspire others to use them in ways we haven’t even considered.”
The ACS3 boom design is made to overcome a problem with booms: they’re either heavy and slim or light and bulky.
“Booms have tended to be either heavy and metallic or made of lightweight composite with a bulky design – neither of which work well for today’s small spacecraft,” said NASA’s Keats Wilkie. Wilke is the ACS3 principal investigator at Langley Research Center. “Solar sails need very large, stable, and lightweight booms that can fold down compactly. This sail’s booms are tube-shaped and can be squashed flat and rolled like a tape measure into a small package while offering all the advantages of composite materials, like less bending and flexing during temperature changes.”
ACS3 will launch from Rocket Lab’s launch complex 1 on New Zealand’s north island. Image Credit: Rocket Lab
ACS3 will be launched on an Electron rocket from Rocket Lab’s launch complex in New Zealand. It’ll head for a Sun-synchronous orbit 1,000 km (600 miles) above Earth. Once it arrives, the spacecraft will unroll its booms and deploy its sail. It’ll take about 25 minutes to deploy the sail, with a photon-gathering area of 80 square meters, or about 860 square feet. That’s much larger than Light Sail 2, which had a sail area of 32 square meters or about 340 square feet.
As it deploys itself, cameras on the spacecraft will watch and monitor the shape and symmetry. The data from the maneuvers will feed into future sail designs.
“Seven meters of the deployable booms can roll up into a shape that fits in your hand,” said Alan Rhodes, the mission’s lead systems engineer at NASA’s Ames Research Center. “The hope is that the new technologies verified on this spacecraft will inspire others to use them in ways we haven’t even considered.”
ACS3 is part of NASA’s Small Spacecraft Technology program. The program aims to deploy small missions that demonstrate unique capabilities rapidly. With unique composite and carbon fibre booms, the ACS3 system has the potential to support sails as large as 2,000 square meters, or about 21,500 square feet. That’s about half the area of a soccer field. (Or, as our UK friends mistakenly call it, a football field.)
With large sails, the types of missions they can power change. While solar sails have been small demonstration models so far, the system can potentially power some serious scientific missions.
“The Sun will continue burning for billions of years, so we have a limitless source of propulsion. Instead of launching massive fuel tanks for future missions, we can launch larger sails that use “fuel” already available,” said Rhodes. “We will demonstrate a system that uses this abundant resource to take those next giant steps in exploration and science.”
A solar flare as it appears in extreme ultraviolet light. The Sun is a free source of energy that’s not going away anytime soon, yet it’s also hazardous. Credit: NASA/SFC/SDO
Solar sail spacecraft don’t have the instantaneous thrust that chemical or electrical propulsion systems do. But the thrust is constant and never really wavers. They can do things other spacecraft struggle to do, such as taking up unique positions that allow them to study the Sun. They can serve as early warning systems for coronal mass ejections and solar storms, which pose hazards.
The new composite booms also have other applications. Since they’re so lightweight, strong, and compact, they could serve as the structural framework for lunar and Mars habitats. They could also be used to support other structures, like communication systems. If the system works, who knows what other applications it may serve?
“This technology sparks the imagination, reimagining the whole idea of sailing and applying it to space travel,” said Rudy Aquilina, project manager of the solar sail mission at NASA Ames. “Demonstrating the abilities of solar sails and lightweight, composite booms is the next step in using this technology to inspire future missions.”