Drag sails take out the trash in space
What happens when two spacecraft collide? At orbital velocities of more than 17,000 miles per hour, collisions of space vehicles in low-Earth orbit are high-energy events that create thousands of pieces of shrapnel, similar to the breakup of a racecar when it hits a track wall. The shrapnel then spreads out, creating a hazardous cloud of debris that becomes an orbital keep-out zone. A 4-inch piece of debris packs the explosive punch of three pounds of TNT.
Low-Earth orbit (an altitude of 2,000 km — about one-third of Earth’s radius — or less) is getting crowded. Satellite constellations to provide global internet service are driving dramatic growth in low-Earth orbit applications. The SpaceX Starlink constellation will consist of more than 30,000 satellites; competitors OneWeb, as well as Kuiper Systems (owned by Amazon), plan to launch thousands more.
The stakes of orbital collisions are high. A 2007 Chinese anti-satellite test resulted in more than 2,000 new pieces of orbital debris that are still being tracked. An accidental collision in 2009 between two communications satellites, one active and one defunct, also produced thousands of debris fragments.
To maintain the sustainable use of low-Earth orbits, spacecraft and rockets need to deorbit once their mission is complete. The Federal Communications Commission (FCC) ensures that each U.S. space vehicle has an approach to deorbiting within 25 years of end-of-mission. For orbital altitudes below 500 km, atmospheric drag is generally sufficient to meet the 25-year guideline. Vehicles deorbiting from higher orbits often use propulsion to lower the altitude to the point where atmospheric drag can take over.
However, some spacecraft don’t have propulsion systems. Using propellant to deorbit depletes a precious consumable, limiting the lifespan of the craft. And propulsion systems don’t work if the spacecraft has failed; a significant fraction of satellites die unexpectedly shortly after being deployed into orbit or during their primary missions.
Drag sails represent a passive, failsafe approach to meeting deorbit guidelines. The concept is simple: increase the surface area of the vehicle so interactions with atmospheric molecules cause more atmospheric drag and speed up the deorbit timeline.
The sweet spot in low-Earth orbit for drag sails is the 600-to-800-km altitude range, where unaided spacecraft will take decades to deorbit, but enough atmosphere exists to accelerate deorbit with a drag sail. Atmospheric drag provides a constant force that causes the vehicle’s orbit to slowly decay, spiraling down until it reaches atmospheric densities that cause the vehicle to break apart and burn up, typically at an altitude of around 80 km.
The drag sail itself is a thin-film membrane, held in shape by a set of lightweight deployable booms. Packaged onboard the spacecraft into a small volume, with the sail material folded and stored in compartments, the booms when activated are extended, pulling the sail into shape. The drag sail can be deployed on command or via a backup timer, providing reliable deorbit capability even if the host vehicle is inoperative.
My startup, Vestigo Aerospace, is developing a commercial product line of drag sails, sized to deorbit host vehicles ranging from toaster-sized CubeSats to launch-vehicle upper stages on the scale of a Mini Cooper. We’re developing two new prototype drag sails through a NASA Small Business Innovative Research contract; commercial marketing is scheduled to begin later this year. Vestigo licenses the drag sail technology from the Purdue Research Foundation.
Deployable tethers are another approach to deorbiting satellites. Electrodynamic tethers, deployed to tens of meters of length, generate an electric charge that interacts with the Earth’s magnetic field, resulting in a drag force. Both drag sails and tethers are considered passive deorbit systems because once they are deployed, they do their work without any required interaction with the host vehicle or ground controllers.
The Defense Advanced Research Projects Agency (DARPA) and industry partners are pursuing active debris removal, using dedicated spacecraft to rendezvous with large pieces of space junk, harness it, and then tug it down to deorbit altitudes. This active method of debris removal is much more costly that the preventative approach of launching with a built-in deorbit system. Active debris removal spacecraft will cost in the hundreds of millions of dollars, while the price point for passive, “bolt-on” systems generally is tens of thousands of dollars.
There is a need for both types of systems. Studies have shown that we have reached a tipping point, where the amount of space debris in orbit can lead to a cascade effect of unintended collisions, each creating a debris cloud resulting in more collisions. Termed the “Kessler Syndrome” — after NASA researcher Donald J. Kessler, who first published simulation results showing the impending cascade effect while working at the Johnson Space Center — this risk to the orbital environment is a top priority of the U.S. Space Force.
After more than 60 years of space flight, the accumulation of spent rocket engines and defunct satellites in low-Earth orbit has the potential to hamper continued economic development of space. Active removal of large pieces of space junk will go a long way toward reducing that risk.
Launching new satellites with reliable deorbit systems like drag sails is the most sensible “ounce of prevention” approach to this challenge. Global reliance on communication, navigation, and imaging services from low-Earth orbit is increasing rapidly. The benefit of maintaining the sustainable use of the orbital environment is worth the investment.
David Spencer
Adjunct Associate Professor
School of Aeronautics and Astronautics
College of Engineering, Purdue University
Mission System Manager, Mars Sample Return Campaign
NASA Jet Propulsion Laboratory
