How sunlight pushes a spacecraft's solar sail

HostI was sitting out on my porch the other day, just feeling the sun on my face. It was warm and bright, but it felt completely weightless. Yet, people are building these huge, silver sails to catch that same light and fly through the stars. It sounds like something from a storybook — using sunbeams to move a giant piece of metal. How does something with no weight actually shove a spacecraft forward?

GuestIt's one of those things that feels wrong because we can't feel it ourselves. But light is actually made of tiny packets called photons. Even though those packets don't have any weight in the way a rock or a car does, they have something called momentum. That's basically just a fancy word for moving power. When a photon hits a surface, it gives that surface a tiny, tiny kick. It's like a grain of sand hitting a sheet of glass. One grain doesn't do much, but if you have billions and billions of them hitting every single second, that shove starts to add up.

HostWait, I'm stuck on the weightless part. If I throw a ball at a wall, the ball has weight, so it hits the wall and pushes it. If light has no weight, what's actually doing the hitting? It feels like saying a ghost could push a swinging door open.

GuestThat's a fair way to look at it. But in the world of the very small, things work a bit differently. Light has energy, and because it's moving so fast — literally the fastest anything in the universe can move — that energy acts like a physical push. Think of it like a stream of water from a garden hose. If you point that hose at a toy boat in a pool, the water pushes the boat. Photons are like the droplets in that stream. They don't need to be heavy to transfer their movement to the sail. They just need to hit it and transfer that energy.

HostOkay, so they hit the sail. But I have seen pictures of these things, and they're always these incredibly shiny, mirror-like sheets. Why do they need to be so shiny? If the light hits it, shouldn't that be enough to move it?

GuestYou could make a sail out of a black sheet that just soaks up the light, but it wouldn't work nearly as well. Here is the trick. If the light hits the sail and just stops there, you get one push. But if you make the sail a mirror, the light hits it and then bounces back the way it came. Think about a person jumping off a small boat onto a dock. If they just stand there, the boat stays still. But if they push off the boat with a lot of force to jump onto the land, the boat gets kicked back in the other direction. When light hits a mirror and bounces away, it gives the sail two shoves instead of one. One shove when it arrives, and another shove when it leaps back off. That double push makes the ship move twice as fast.

HostBut even with a double shove, we're talking about light. If I hold a mirror up in the sun, it doesn't fly out of my hand. This push has to be incredibly weak. I can't imagine it moving a ship that weighs hundreds of pounds. It seems like a lot of work for a tiny bit of speed.

GuestYou're right, the push is very small. If you held a large solar sail in your hand here on Earth, the push from the sun would feel about as heavy as a single postage stamp resting on your palm. That's it. But the secret is that space is empty. There's no air to slow you down and no ground to create friction. On Earth, a car needs a big engine because the air and the road are always trying to stop it. In space, once you start moving, you stay moving. And because the sun never stops shining, that tiny postage-stamp push is steady. It never quits. You keep getting faster and faster, hour after hour, for years. Over time, you can reach speeds that would take a normal rocket a huge amount of fuel to hit.

HostI guess that makes sense for going away from the sun, but it still feels like you're just a leaf in the wind. If the light is always pushing away from the sun, are you just stuck going in one straight line forever? That doesn't seem very useful if you want to actually go somewhere specific.

GuestWe can actually steer them quite well. It works just like a sailboat on the ocean. You don't always sail in the same direction the wind is blowing. You tilt the sail. By changing the angle of that big silver sheet, you can bounce the light off to the side. That pushes the ship in a different direction. You can even angle it so that you slow down and drop closer to the sun, or tilt it the other way to spiral out toward the edge of the solar system. As long as you have light hitting the sail, you can use that bounce to go almost anywhere you want.

HostIt's wild to think we have found a way to sail on something as thin as a sunbeam.

GuestThe real dream is that one day we won't even wait for the sun, but use giant lasers on Earth to push these sails to other stars at speeds we can barely imagine right now.

HostSunlight might feel like nothing on our skin, but in the quiet of space, that tiny pressure is enough to carry us across the solar system.