The launch of the Dawn mission to the asteroids makes me think about solar sails. I realize that Dawn uses ion propulsion, about which more in a moment, but watching ion methods as they mature makes an emphatic point: We need to bring solar sail technologies up to the same readiness level that ion propulsion currently enjoys. And we need to be shaking out sail ideas in space. The Russian Znamya attempts at a ‘space mirror’ were attached to a Progress supply ship, and interesting mostly in terms of their deployment problems, leaving the 2004 Japanese test of reflective sails in space as the only free-flying experiments I know about.
Which is not to say I’m a skeptic about ion propulsion. It will be fascinating to follow the performance of Dawn’s engines as the mission progresses. 54 feet of solar array produce the needed power to ionize their onboard xenon gas, which is four times heavier than air. The ions are then electrically acccelerated and emitted as exhaust from the spacecraft. The result is an engine of great utility over time, as JPL’s Keyur Patel notes:
“Each of our three ion engines weighs in at 20 pounds and is about the size of a basketball. From such a little engine you can get this blue beam of rocket exhaust that shoots out at 89,000 miles per hour. The fuel efficiency of an ion engine is an order of a magnitude higher than chemical rockets and can reduce the mass of fuel onboard a spacecraft up to 90 percent. It is a remarkable system.”
Remarkable indeed. Dawn’s engines will accumulate 2,000 days of operation during the course of its eight-year investigations, pushing the vehicle with about the same amount of thrust as the weight of a piece of paper in your hand. Days and months of thrusting add up, giving the vehicle an effective change in speed of about 37,000 kph by the end of its mission.
And ponder this. A chemical rocket’s exhaust speed is limited by thermal issues — the rocket nozzle has to be able to bear up to the temperatures involved. The ion thruster’s top exhaust speed is limited instead by applied voltage. Ion propulsion’s future seems bright — the greater the exhaust speed, the more efficient the engine. Dawn needed a boost from a Delta II Heavy to get off the Earth’s surface, but once in space the long push for the asteroids is remarkably stingy on fuel, delivering ten times as much thrust per kilogram as chemical rockets. No wonder JPL has taken to calling Dawn the Prius of space. Ion engines are ingenious and space-tested devices (Deep Space 1 was an early proof of concept, and the SMART-1 lunar mission again validated the basic ion design).
But if something about an ion engine’s gentle push rings a bell, you’re circling, like me, back to the similar situation with solar sails. The momentum imparted by photons falling on a solar sail delivers a vanishingly small push. A sail one-third of a mile square in Earth orbit would be pushed to no more than ten miles per hour in its first hour of operation. The solar flux at that distance is nine orders of magnitude weaker than the force of the wind on the Earth’s surface, so you can see the reason why.
Like ion engines, the speed of solar sails can increase bit by bit, day by day, and unlike them, sails carry no propellant whatsoever. As long as sunlight is plentiful (out to about 5 AU), sails should make an efficient alternative to ion engines, particularly in explorations of the asteroids. And while the continued development of ion propulsion is heartening, it shouldn’t be forgotten that sail strategies continue to be carefully studied. We need to find budgetary help for the work that will lead to the deployment of demonstrator missions. A sail in space will teach us much more about this potentially paradigm-changing form of propulsion.