This past Thursday marked the one-year anniversary of LightSail 2’s launch into orbit. LightSail 2 is a crowdfunded solar sail project managed by the Planetary Society and it’s the first craft propelled entirely by sunlight. The Planetary Society hosted a webinar to celebrate the event. Planetary Society CEO Bill Nye, Chief Operating Officer Jennifer Vaughn, Program Manager Dave Spencer, and Project Manager Bruce Betts all spoke. The presentation was moderated by Planetary Radio host Mat Kaplan. I am pleased to have been one of the mission funders and I was delighted to have had an opportunity to attend the webinar live. I gather the presentation will be viewable soon at https://www.planetary.org/multimedia/planetary-tv/. I found the presentation fascinating and I took three pages of notes. I’ll touch on a few highlights below and in Tuesday’s blog post, but if you’re interested in this project, be sure to check out the full video at the link above. You can learn more about the Planetary Society by visiting http://www.planetary.org.
The most exciting news from the webinar is that LightSail 2 is still flying one year after launch. It was placed into an orbit about 720 kilometers above the Earth’s surface, which is still low enough to have a very small amount of atmospheric drag. Despite that, the spacecraft has only lost about 10 km of altitude over the course of a year. It’s not certain how long it will be before it de-orbits, but current estimates say LightSail 2 could continue its mission for another year.
Bill Nye opened the presentation by noting it was 17th century astronomer Johannes Kepler who first speculated on the possibility of solar sails after observing the comet that would ultimately be known as Comet Halley. He reasoned that whatever force from the sun could produce the comet’s tail could propel a sailing vessel to other planets. The specific particle that can be used to propel a solar sail is the humble photon. Light has momentum and that momentum can be transferred to the sail for motion.
LightSail 2 has been able to perform so well because it can be turned like a sailboat’s sail. In this case, the craft is turned by momentum wheels aboard the ship. Momentum wheels are just gyroscopes, but tradition suggests that the word “gyroscope” is used principally when applied to navigation. When LightSail 2 is in a position to get a boost from sunlight, it turns so the sail catches all the light. When the light pressure would work against the sail, the craft turns 90 degrees so it presents the sun with the least amount of surface to push against.
Many great photos were shared during the webinar. My favorite was this one showing LightSail 2 over the Nile and the Red Sea. As you’ll notice, there’s a thin blue line at the Earth’s left edge. That’s our atmosphere, which looks very thin and fragile. I also imagine measuring the spectrum of that thin film on a planet in some distant stellar system from a telescope in our solar system. It’s a real technical challenge, but it looks like we may be getting close to a point where we could do that. This is something we’ll need to do in order to determine whether or not an exoplanet is potentially habitable.
One of the things that makes all of this personally exciting is that I first joined the Planetary Society when I was in high school as a result of a letter sent to Star Trek fan clubs by Gene Roddenberry. I first learned of the Society’s interest to make solar sails a reality in the society’s newsletter, The Planetary Report. The idea caught my imagination and in high school, I started to write a novel called The Solar Sea. I didn’t complete it then, but the idea stayed with me and I made several attempts until I wrote a version that pleased me. That version was published in 2008 and you can learn more about it at: http://davidleesummers.com/solar_sea.html