The news that The Planetary Society is readying the first of its Lightsail spacecraft for a May launch stirs memories of Cordwainer Smith (Paul Linebarger) and mainframe computers. Smith wrote his haunting science fiction in the days when computers filled entire rooms, and the pilot who flies a solar sail thousands of kilometers wide in “The Lady Who Sailed the Soul” is there because, as a technician tells her, “…a sailor takes a lot less weight than a machine. There is no all-purpose computer built that weighs as little as a hundred and fifty pounds. You do. You go simply because you are expendable.”
Despite the anachronisms, Smith’s short stories (collected in The Rediscovery of Man) are as mesmerizing as ever. As computers were big in those days, so have been our sail designs, from Smith’s behemoth (towing 26,000 adiabatic pods containing frozen human settlers) to Robert Forward’s beamed-laser sails. Given the need for harnessing the momentum of photons, all this makes sense, but we’re learning how many interesting things we can do with much smaller sails, like NASA’s NanoSail-D, an experiment in sail deployment and de-orbiting payloads that was a scant 10-meters square. LightSail, in sail terms, is still quite small, with a combined area of 32 square meters.
Both NanoSail-D and LightSail take advantage of the wild card technology of recent times, the CubeSat, which allows sails to be packed into containers no larger than a loaf of bread. Each of the mylar sails aboard the LightSail mission — there are four of them — is about 4.5 microns thick, deploying from four metallic booms that gradually unwind to unfold the triangular sail panels. The craft will use three electromagnetic torque rods to interact with the Earth’s magnetic field to maintain proper orientation. After sail deployment, ground-based lasers will measure the solar photon effect.
Image: LightSail-1 fully deployed. The mission is a precursor to a later LightSail mission to test true solar sailing in a much higher orbit. Credit: Josh Spradling/The Planetary Society.
The Planetary Society is calling this mission a ‘shakedown cruise,’ one that will allow scientists to test out the basic functions of the mission in preparation for the launch of a second LightSail in 2016 aboard a SpaceX Falcon Heavy. A four-week checkout period will be followed by sail deployment, after which, because of its low orbit, the craft will be pulled within days back into the atmosphere. Even so, we should get interesting views of the deployment through LightSail’s two inward-facing cameras, offering time-lapse imagery of the sail’s brief period of operations.
In Jason Davis’ recent article on LightSail, he notes a fact that many of us vividly recall. It will be ten years this June since the Russian Volna rocket carrying The Planetary Society’s Cosmos 1 failed in its attempt to lift the sail to orbit. That left the Japanese space agency JAXA to win the honor of achieving the world’s first operational solar sail when it launched IKAROS in 2010. But interest in small sail technology remains intense, with NASA planning both NEA Scout and Lunar Flashlight for launch in 2018. Both are CubeSat-based, though with larger sails than LightSail. For more on sail projects now in development, see A Near-Term Sail Niche. Note as well that The Planetary Society has created a new website for the two LightSail missions.
But even 85-square meter sails like NEA Scout and Lunar Flashlight are tiny compared to the 1000-kilometer lightsail Robert Forward envisioned for a manned mission to Epsilon Eridani. Can we really do worthwhile science with sails this small? The answer is a resounding yes. By reducing payload mass and maximizing the power of miniaturization, CubeSats give us options like ‘swarm’ missions to the outer Solar System that could be enabled by sail technologies. This could be a low-cost approach to deepening our knowledge of places we’ve only seen in flybys.
So as we continue work on larger designs, let’s see what we can learn from small sails close to home. When LightSail deploys, I’ll probably go back and re-read “The Lady Who Sailed the Soul,” where Cordwainer Smith describes “…the great sails, tissue-metal wings with which the bodies of people finally fluttered out among the stars.” Our CubeSat sails are early steps along the road to the great ships of Smith, Robert Forward and all the researchers who have seen the promise of sunlight and beamed energy as ways to push our payloads into the cosmos.