LightSail: Using sunlight to propel a spacecraft.

Using Sunlight to Propel Spacecraft: LightSail Launches Wednesday!

Using Sunlight to Propel Spacecraft: LightSail Launches Wednesday!

Bad Astronomy
The entire universe in blog form
May 19 2015 7:00 AM

Tripping the Light: Fantastic!

light sail
When fully deployed, the light sail will use sunlight to move among the planets. Note the cubesat for scale in the middle.

Drawing by the Planetary Society

Update, May 20, 2015 at 15:15 UTC: The Atlas V rocket launch went well, taking to the skies just after 15:00 UTC. 

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

N.B. Scroll to the bottom for launch info.


On Wednesday, if all goes well, the Planetary Society will test a new technology that could open up a new way to explore the solar system. On board a United Launch Alliance Atlas V is a tiny cubesat, a parallelepiped just 10 x 10 x 30 centimeters in size—roughly the same size as a loaf of sandwich bread.

Packed into that tiny enclosure is a prototype called LightSail, a spacecraft that carries no fuel. Instead, it will use sunlight to propel it.

Light of any kind can exert pressure, called radiation pressure, when it interacts with matter. The amount is incredibly small, but it’s there. And if you’re out in space, where sunlight is eternal and there’s no atmosphere to counteract it, that pressure exerts a teeny force that can be used to accelerate a probe.

To be useful, you need a probe that has very little mass (to reduce overcoming its own inertia) but a lot of surface area, to catch as much sunlight as possible. This naturally leads to the idea of a light sail, a huge but very thin sail made of Mylar (4.5 microns thick; a human hair is about 20 times thicker). Attach this to a small satellite and you have yourself a space probe.

light sail
A full-size model of LightSail held by Planetary Society CEO Bill Nye.

Photo by the Planetary Society


To be sure, the acceleration is small, and it takes a while to build up speed, but that acceleration can be applied over long periods of time. Months. Interplanetary speeds are achievable this way!

This test is sponsored by the Planetary Society, whose purpose for existence is to “Empower the world's citizens to advance space science and exploration.” They do a lot of great work educating the public and advocating for space exploration, including funding actual projects.

LightSail is one of them. The launch Wednesday is to test a prototype of this technology and show that it can be deployed—it won’t be up high enough in orbit to overcome the very tiny but persistent drag caused by the very thin atmosphere. But if this goes as planned, the next step will be to build the full-fledged LightSail demonstration craft itself in 2016.

Here, let Bill Nye, CEO of the Planetary Society, explain:

This is amazing stuff, right out of science fiction … but it’s fact. By all accounts, this should work. The prototype should go a long way toward proving that.

As for the next step, the Planetary Society needs your help. It’s raised most of the funds needed to build the full-up LightSail for next year, but they’re looking for donations from the public to complete it. They’ve set up a Kickstarter for it that blew through the $200,000 initial goal pretty quickly, but the more the merrier. If you’ve got a few extra bucks lying around, you could do worse than help a group of really good folks try to create a new way to explore the solar system.

ULA will webcast the launch of the LightSail prototype live. The exact time of the launch window has not yet been announced, but the launch is scheduled between 14:45 to 18:45 UTC (10:45–2:45 p.m. Eastern time). You can keep up with the latest news and articles about the LightSail project at the Planetary Society, too.