There aren’t very many, but there are a few! I’ve poked around and found three major projects which are actively being worked on, though there have been more than that in the past, or which were tested on the ground, and never made it to space. If you’re curious to learn about how solar sails work, exactly, check out this post which covered just that!
The solar sail project that might be most familiar to you is the LightSail, brought to you by the people at the Planetary Society. Because the Planetary Society is not a government agency, they’re free to fund their missions however they please, and the LightSail wound up being significantly funded by a Kickstarter campaign. The LightSail Kickstarter in 2015 coincided with their proof-of-concept solar sail deployment test; this was a quick orbit around the planet, just long enough to deploy it from its launch rocket, check that the sail could unfurl as expected, and grab a quick image of itself. The Kickstarter was a tremendous success, raising a million more than their goal of $200,000, meant to help pay for the last bit of unfunded construction and launch costs. Since the proof-of-concept craft worked almost perfectly (there were a few communication glitches), the Planetary Society is going forward with their first full-scale solar sail mission, which should leave the Earth’s atmosphere and hitch a ride on our Sun’s solar wind.
The LightSail 2, as the next one will be called, is scheduled to launch this year, on a SpaceX Falcon Heavy rocket. The LightSail is still in pretty early days, in terms of what kind of science it’s doing - the next launch is primarily to check that the light sail of its size works the way we think it should, and can successfully accelerate a small craft away from the Earth. As each incarnation of the LightSail succeeds, the scientific scope of the missions will increase- which makes sense, you don’t want to put an expensive scientific instrument on a craft if you’re still worried about the craft being able to spread its wings.
The most successful solar sail to date was launched in 2010, by the Japanese space agency JAXA. IKAROS, which stood for Interplanetary Kite-craft Accelerated by Radiation Of the Sun, was launched along with Akatsuki, their craft currently orbiting Venus. IKAROS successfully opened up, and formally operated as a light sail, gaining speed from the solar wind. IKAROS had a few science instruments aboard - a gamma ray burst detector and a dust particle counter. IKAROS was cleverly also equipped with small solar panels embedded in the sail, which provided power to the satellite. IKAROS surpassed its initial mission timeline, which was 6 months, and continued to communicate with Earth (with exceptions for its hibernation periods when the sunlight was too weak to power its instruments) until 2015.
JAXA is planning to launch a much bigger solar sail around 2020. This craft is scheduled to head off to investigate a set of asteroids which share an orbit with Jupiter, making a return trip with a small chunk of an asteroid sometime in 2050. Where IKAROS was 14 meters to a side (by no means small), this new solar sailboat will be 50 meters per side - more than three times the collecting area. JAXA recently showed off a full-sized model of one of its four wings, which took their volunteers a careful 10 minutes to unfold.
The last major ongoing solar sail project is Breakthrough Starshot, but it’s both the one which is aiming the highest, and the least far along in its progress to launch. They’re hoping to construct a solar sail which could be accelerated not just by the solar wind, but by high powered lasers, with the aim of getting a craft near Alpha Centauri in ~20 years. This is ambitious, to put it kindly, and the folks at Breakthrough Starshot are well aware. They have an entire page dedicated to major challenges that we don’t yet know how to solve.
As the technology develops for solar sails, they will become cheaper and easier to produce. I suspect we will start seeing more people attaching them to nanosats and cubesats which are relatively cheap to create and launch!
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