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Blog-a-Day #5 -- Sailing Away


OK. We got ourselves off the ground into low earth orbit (LEO) using a Screamer. We’re 90% of the way to anywhere. Yay! But, where do we go from here? With a bit of extra fuel, we can push off and coast for a few days to get to the Moon. That might be a relaxing vacation for some, but would we want to take more than a year to go to Mars and back? Space is really empty. Distances are vast, and traveling from one planet to another is a long haul.

How can we get there quicker? Acceleration. It’s acceleration that speeds us up and slows us down. And, thanks to Sir Isaac Newton, we know that acceleration is directly proportional to force applied and inversely proportional to the mass being pushed, as in the famous equation F=ma. So, to get there sooner, we need to go faster, and to go faster we need to accelerate, and…you get it. It’s all about force. But, where will the force come from?

Well, we could haul extra fuel into orbit, but that darn Rocket Equation is always present. Is there some way to generate force, and hence acceleration, without burning fuel? Of course there is, or this would be a very short piece and series. What follows is a description of a real technology that has been successfully tested in orbit—the light sail.

More specifically, the “LightSail 2”, which is a crowd-funded experiment launched by the Planetary Society in 2019. The light sail concept has been around for decades. The idea is a giant version of a Nichols radiometer. Don’t confuse a Nichols radiometer with the Crookes radiometer, which is a lightbulb shaped device with vanes mounted on a spindle and pained black on one side and white on the other. The vanes rotate when you shine a light on them because air heated on the black side of the vanes slides past them and as we saw in the Screamer article, when you move air, you get a push in the opposite direction.

It was once thought that light pressure—energy imparted by the light photons—was turning the vanes, but subsequent experiments proved that wasn’t the case, but those experiments did demonstrate that light pressure does, indeed generate a force, just not enough to spin the vanes. But, as they said in “Alien”, in space no one can hear you scream, because there is no air and hence no air resistance. In the vacuum of space, any force, no matter how small, will produce an acceleration. And, in space, sunlight is free.

The name LightSail explains exactly what it is, a big piece of very thin, shiny Mylar plastic, like a Happy Birthday balloon, which reflects sunlight and getting a push from it. It’s a tiny push, but it’s also a constant push, and even a tiny push that goes on and on over days, weeks, and months makes a huge difference in speed.

Remember Newton’s F=ma formula? Flipping it around to solve for acceleration, ‘a’, give us F/m=a. The force, slight as it is, is counterbalanced by the mass of the spacecraft. Since the force is proportional to the size of the light sail, to get appreciable acceleration, you need either a small mass or a very big sail, or both. A light sail big enough to significantly shorten the flight of a crewed spacecraft would need to be miles across.

Today, a sail that large isn’t practical. But, these ideas are meant to stimulate speculative fiction ideas, so let’s assume it will be practical someday. What type of stories could light sails enable? Stories are built around tension and conflict. A small crew in confined quarters on a long spaceflight driven by a fragile piece of plastic with no backup? What tension or conflict could possibly arise from that situation?

Let’s get started.

Rob Johnson

May 20, 2020

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