Taking a drag
Of the many competing concepts for mitigating space junk, drag sails are one of the most intuitive. They are synthetic sails deployed from a spacecraft to create additional drag when its mission ends, slowing and deorbiting the craft sooner than would otherwise be possible.
“Drag sails work by using the slightest, thinnest layers of atmosphere, existing at the higher levels, as a mechanism that will hit the sail and slow down the spacecraft,” says Rohan Sood, director of UA’s Astrodynamics and Space Research Laboratory. “We are deploying a sail that is perpendicular to the direction of the spacecraft’s motion, which slows down and deorbits the craft.”
One of the first craft to demonstrate the concept was NASA’s Nanosail-D2, a water bottle-sized nanosatellite that deployed a 108-square-foot (10 square meters) sail in 2011. Instead of taking an estimated 50 years to fall to Earth from its 400-mile-high (650 kilometers) orbit, the satellite reentered the atmosphere and burned up in less than one year.
Because drag sails work passively, using nothing but atmospheric resistance, drag sails present a unique means of preventing future space junk. Competing deorbiting technologies like electric propulsion — used by SpaceX in their Starlink constellation — require a power source onboard the spacecraft, which could be vulnerable to complications.
“If the spacecraft works well all the way through the mission, that’s fine, but if that host spacecraft dies or loses functionality, that electric propulsion system isn’t going to be effective in deorbiting,” says David Spencer, a Mission Systems Manager with NASA’s Jet Propulsion Laboratory and director of Purdue’s Space Flight Projects Laboratory. “That’s where drag sails provide a more fail-safe approach, in our opinion.” (For its part, SpaceX says it doesn’t fly Starlink satellites any higher than 370 miles [600 km], which reduces the time that inoperable craft deorbit to five or six years.)
The concept is intuitive enough in theory, but the key challenge is designing the deployment system, Spencer says. “We’ve got these long booms that are stowed in a very tight area, we’ve got large sail materials that have to be crammed in these small storage compartments for years, and when that deployment command comes, it all has to work properly.”
Even successfully deployed drag sails may encounter problems as orbits decay and spacecraft pass into lower, denser regions of the atmosphere. When this happens, drag sails must remain perpendicular to the motion of the craft to sustain effect. “If the sail starts to tumble and is no longer perpendicular, the drag effect decreases, and if it is parallel, can be zero,” Sood says. “So, the challenge is also controlling the attitude of the sail.”
Spacecraft typically use active methods of attitude control, which require a source of power — like thrusters or reaction wheels. But a drag sail for a defunct satellite wouldn’t be able to count on having power.
Spencer’s team turned to badminton for inspiration. Their Spinnaker-1 sail model takes the shape of a pyramid once it’s deployed. “That pyramid acts like a shuttlecock would,” says Spencer, which automatically turns the vehicle to the orientation that provides maximum drag.