On the Wings of the Sun

I’ve been reading about solar sails the past couple of weeks. These are spacecraft that get their main “thrust” (if you will) from sunlight. It’s pretty simple in theory — photons of light hit the reflective surface of a solar sail and as they bounce off that “travelling mirror”, their momentum gives a little “push” to the mirror, and the spacecraft attached to it.

Solar sails must capture a lot of sunlight in order to give constant velocity or even acceleration to an instrument-laden spacecraft. This means they need to be big, but lightweight. And, for now, they have to be launched into space from Earth aboard chemically propelled rockets. This is where solar sails get complex.

There are two examples of attempted solar sail experiments that failed due to launch vehicle problems, and one that was successful as a test. The Cosmos 1 solar sail, launched in 2005 (by the Planetary Society and Cosmos Studios) never had a chance to prove itself. The rocket carrying it to space never reached orbit and the sail was lost. Had it reached orbit, it faced a number of challenges, including a first unfurling (which would have allowed designers to measure the “outgassing” a sail would experience as atmospheric gases from Earth rushed out to the vacuum of space.

Even though the experiment was not the success the designers wanted, it did advance the science of sail design, and other groups — including some Japanese teams (who successfully tested a solar sail in 2004) as well as designers at NASA and private concerns in the U.S. — are actively studying sail technology for future solar system missions.

The other failed launch was for NASA’s Nano-Sail D, which was supposed to go up and be tested in the summer of 2008. It ended in failure when the SpaceX launch vehicle failed to achieve Earth orbit. In the case of both Cosmos 1 and NanoSail-D, the solar sails weren’t at fault, but the delivery systems failed them. So, humanity’s attempts to deploy these sails are proceeding in baby steps.

Solar sails have long been a staple of science fiction stories. I recall reading about them as a teenager and marveling at the idea of a diaphanous “fabric” giving a space ship the same ability to “tack” on the wind that a sailboat does here on Earth’s lakes, oceans and rivers. Today, they are a reality just waiting to be deployed. And, as mission planners look to them for a number of uses, it’s interesting to learn more about them.

This is where the book Solar Sails: A Novel Approach to Interplanetary Travel, by physicist Gionvanni Vulpetti, NASA scientist Les Johnson, and physics teacher and writer Gregory L. Matloff, comes in handy. Their book is a very rich overview of solar sails and the science principles behind them. It’s divided roughly into three parts: a pretty approachable overview of all the ways we get into space (covering both chemical and non-chemical propulsion systems); and two sections that take more detailed looks at past efforts to build solar sails and what the major technical challenges have been.  This is not a highly technical book, but it also isn’t a quick read.  However, if you’re interested in learning more about how a solar sail might be used to keep spacecraft sitting above the poles of other planets for long-term studies (or even around our own), or how such technology might help blaze the way across the solar system for eventual human exploration, then this is a book you should read. Solar sails will have their day in space, just as alternatives like ion propulsion systems are getting.