# How could it be possible to travel fast in space with solar sails? Or even travel with solar sails at all?

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The best idea right now on how to travel fast in space if you would want to reach very far (like outside of our solar system), seems to be solar sails.

But why would light waves alone from the sun be able to push anything at all on a human scale and even more so at really high speeds, like 10-50 % of the speed of light is numbers I’ve seen, which seems insane. Of course this haven’t been done yet, but the basic idea seems to be based on solid science.

I don’t understand how this could work even in theory tho. Like light waves have energy so I could see how that could apply force to an object, but just a small amount. Space have no resistance so if you push something you can keep it going for pretty much forever.

But how do people think that it could get you speeds that even mattered in space and even more so those mind boggling speeds?

In: Physics

Don’t underestimate the time scale of things here, and how very little things over that kind of scale really di add up.

If you use a more traditional rocket burn to get you up to speed, that burn is measured in seconds, and then you coast for months, if not years, if not decades (e.g. voyager).
A solar sail does not have to add a lot of thrust to be equivalent, because it will be adding energy for months/years/decades.

If a solar sail had just 1/31536000th (about 32 million) the energy output of an engine, it could “freely” achieve in a year what the engine could achieve in a second.

Additionally, traditional propulsion methods require engines, tanks, control mechanisms, … all of which increase mass and therefore decrease the velocity you get from kinetic energy.

You need a very light and large sail. Materials we can manufacture right now wouldn’t really be that doable for human space travel. A very tiny probe is maybe almost doable, tho still not quite.

Theoretically, if you can manufacture arbitraly large and extremely light sail, it adds enough solar pressure over a large enough area.

Right now, for human interstellar space travel, nuclear propulsion would prolly be more realistic, tho even that has some issues and pitfalls.

You do it for a very long time. While travelling around the solar system (not orbiting something) you practically have constant access to the sun. If you’re accelerating at 0.000001m/s^2 for months you go really fast

What specifically are you not understanding? You know that light will push on the sail, speeding it up. You also know that there’s nothing to slow you down in space. So you just keep going faster and faster until you’re going that fast. It’s like you understand that someone can walk around the block, but not hike the Appalachian trail. You start walking, and just keep walking until you go 2 thousand miles.

It’s not a very large force, light pressure at 1 AU is about 9.08 μPa and solar wind pressure is 1-6 nPA with a fast speed of 750km/s. but with a large sail it’s free acceleration for a long distance out into space, up to a very fast speed.

Since space is basically a vacuum, there is nothing to cause friction and slow you down.

A solar sail uses light and solar wind to add very very small amounts of acceleration constantly. Over a long time period, that adds up.

It’s like the question “do you want \$1 million dollars now or start with one penny and double the amount every day”. The \$1 million is a rocket that accelerates quickly but once it’s done, that’s how fast you are going. The penny starts really tiny but over time, always adding and never subtracting, you end up going faster.