Solar sails are ideal for long missions within the Solar System, but their manifest advantages (no fuel onboard!) are not unalloyed. A major issue is the time needed to escape Earth orbit. Working the numbers on this, Gregory Benford noted that if a sail used the momentum from solar photons alone, unassisted by any other propulsive force, it would require time scales on the order of years to escape from Earth’s gravity. And that’s with sail deployment from an altitude of 800 kilometers, beyond the reach of decelerating air drag.
What we can do to get that sail on its way faster is the subject of Benford’s new paper in Acta Astronautica, written in collaboration with Paul Nissenson. One possibility is to coat the sail with a material that sublimes; when heated, the material vaporizes and is ejected, adding to the momentum transfer of photons (Benford and his brother James at Microwave Sciences have done groundbreaking work on the nature of such sublimation, also called desorption).
But the paper goes further; its authors examine the idea of decreasing sail escape time by using a high-power ground or space-based photon generator — a beamer — that would increase the photon density well beyond solar power alone. Their work on the orbital dynamics of such a beamer/sail combination assumes that an orbiting beamer (ORB) would be deployed behind the sail in the same initial circular orbit. When the beamer illuminates the sail, a resonance between the two is established, with sail and beamer returning to their original positions after a certain number of orbits, where the sail is boosted once again.
The sail trajectories thus created are shown in the image below. From the paper:
“Once the beamer illuminates the sail, the two bodies are no longer in synchronous orbits. After a certain number of orbits, the beamer and solar sail ‘resonate:’ the two bodies meet nearby in space, with the beamer trailing the sail allowing the photon beam to focus well on the sail. A specific energy is given to the sail so that resonance occurs in a relatively small number of orbital periods.”
More economical, perhaps, and easier to maintain would be a ground-based beamer, and the paper offers equivalent calculations for this scenario. In both cases, the effects are startling: using the orbiting beamer method, the sail’s escape time is reduced by over two orders of magnitude if deployed at 800 kilometers. Ground-based beamers also reduce escape time over sunlight alone, with a coating of subliming material capable of producing even higher momentum transfers (the relationship of beamer power and sail temperature gets a close look here). “Engineering optimal desorbed or sublimed fuels in sails made of carbon fiber,” write the authors, “is a promising goal.”
The beauty of beamer technology is that such an installation could drive numerous sails, lowering the cost for each mission. “In this way, it somewhat resembles the railroad, which gives no benefit until the last length of track is laid. Still, a small low power beamer could assist the launch of sails from orbit as a first demonstration of the principles we have examined here, eventually building a utility in orbit, leading to fast deep space missions.”
The paper is “Reducing solar sail escape times from Earth orbit using beamed energy,” in Acta Astronautica Vol. 58, Issue 4 (February 2006), pp. 175-184.
Centauri Dreams‘ take: Solar sails won’t take us to the stars, but sails pushed by lasers — lightsails — just might, as Robert Forward showed convincingly in a series of papers in the 1980s. What this early work on laser-assisted solar sails is doing is building a groundwork for a space-based infrastructure that one day may see large laser installations capable of propelling sails to speeds that could reach the nearest stars in a human lifetime. Forward’s concepts were mind-boggling in their engineering, but I can imagine his pleasure as the far more practical details of interplanetary mission planning are worked out. Beaming technologies have a major role to play as we push outward to the edges of the Solar System and beyond.