When I told my older brother, who is considered the real science and science fiction expert in the family, that I was planning a novel about solar sailing, he scoffed at me. “It doesn’t produce enough force to actually move a whole ship,” he said.

As the younger child, many of the cool things I’ve done — learning Latin, getting ahead in math, learning to ride a bike — were because my brother said I couldn’t, so of course that didn’t stop me at all. At first I just said the book would be soft sci-fi. I’d mention sails but not really worry about whether or not the whole concept was plausible. I mean, I’ve written about humanoid aliens with faster-than-light engines and grav plating. Not everything in SF has to actually work.

But, the more I wrote and researched, the more plausible I saw it was. It’s true, the force put out by the sun is tiny — 9µN, or nine millionths of a newton, per square meter of sail.¹ One newton can accelerate one kilogram one meter per second. However, solar sailing has been done!² The trick is either making the sail massively big or the ship incredibly light.

How big? How light? Well, I decided my ship would be about 90 feet by 40 feet. A 90-ft yacht weighs about 70,000 kg, but my ship would be wider and have a thicker hull. So let’s say it weighs 140,000 kg. Add in all the stuff they have to bring (did I do the math for that? of course I did) and make it an even 200,000 kg.

And how big are the sails? I decided they’re two kilometers across, and rounded, so the area would be 13,600,000 square meters. (I’m going to go ahead and assume the materials the sails and masts are made out of are both strong and light–I mean, it is the future after all.)

That gives us 122 newtons. How fast will 122 newtons accelerate 200,000 kg? I’m getting .00061 meters per second. That’s not very fast at all.

However, there’s one more big number we get to multiply in: the number of seconds the ship spends accelerating. The ships NASA uses today burn their engines for a very short time and drift the rest of the way. This ship could accelerate, cost-free, for the entire journey when traveling away from the sun.³ Say the journey takes six months. That’s 15,780,000 seconds. Every one of those seconds, the ship is speeding up just the tiniest bit. By the end of the journey, they’d have sped up 9625 m/s from their initial speed!

Initial speed, of course, is important. No object in the solar system stays still. A ship traveling from, say, Venus to Mars, is starting off at the blistering pace of 126,077 km/h. All it needs is to change its velocity a little bit, both in speed and direction. I had a heck of a time finding out how much change is required on any given trip, but 3,000 m/s seems about average for an interplanetary trip, if you leave at the right time.⁴ That’s a lot–almost 7000 mph–but it’s less than we got from our solar sails.

So, it can be done. It requires keeping weight to an absolute minimum, having sails that stretch incredibly far, and calculating orbits to require the least acceleration possible, but you absolutely could fly between the inner planets with a solar sailing ship.

Take that, big brother. Wait till you hear my opinions about terraforming Mars!

1. That’s at the distance the Earth is from the Sun, the only number I could easily find. Of course it would be greater at Venus, and less when you were near Mars.
2. https://www.planetary.org/explore/projects/lightsail-solar-sailing/what-is-solar-sailing.html
3. I only addressed going away from the sun in the math, but you can also go closer in to the sun by setting the sails at an angle that degrades your orbit. Basically, you’re aiming to slow yourself down; as your speed decreases, gravity will pull you closer toward the sun.
4. https://en.wikipedia.org/wiki/Hohmann_transfer_orbit