Solar Sails and Space Pirates

Cruising through the stars on a sailboat... just watch out for Matt Damon.

Jan 07, 2021

There are a few ways to get to space, but none of them are very efficient compared to propulsion methods we have here on Earth. Engines we use for planes burn a combination of fuel and air, and they need oxygen for the fuel to burn. This becomes an issue when we get to space, because there is no atmosphere, and thus no oxygen. As such, rocket engines need to bring both fuel and oxygen (called oxidizer in rocket propulsion) with them on their journey. Space is unfortunately BYOO. Because of this, rockets need to carry much more fuel relative to their cargo compared to planes. Making their job even harder, rockets rely solely on their engine’s propulsion for upwards acceleration—readers of last week’s newsletter remember that planes have wings to generate lift. This in part is what makes getting to space so hard. There’s a lot of gravity to overcome. All of this makes rocket engines the least efficient of their counterparts. We use them, however, because of the amount of thrust force they are able to generate. They have much higher pure thrust capabilities than jet engines, and that means they can get things big things off the ground with relative ease. Google “rocket launch” and you’ll see what I mean.

Once we get into space though, we have more freedom. When already in orbit, satellites and spaceships don’t have to deal with air resistance, since there’s no air in space. Think of a soccer ball on the ground. If you kick it lightly, it will speed up, roll for a bit, then slow down and stop. That’s because there is friction. Without friction, if you kick it lightly, it will speed up, roll for a bit, roll some more, and … just keep going. (Technically without friction things wouldn’t roll, they would just kind of slide, but let’s ignore that for now) In space, without air friction, little “kicks”, when repeated, can result in a lot of speed gained over time. Therefore, satellites can get away with using engines that produce very little thrust but are much more efficient. One extremely popular form of propulsion for deep space probes and satellites is ion propulsion. This is when a radioactive isotope decays, and the particles that are released are accelerated and shot out to produce thrust. Thrust is based on of conservation of momentum where:

Where M1 and V1 are the mass and velocity of the exhaust that is ejected, respectively. M2 and V2 are the mass and velocity of the satellite, respectively. Even though the particles can be ejected at a very high speed, they don’t weigh very much, so the satellite, which weighs a lot more, only gains a small amount of speed. This method produces a small amount of thrust but over time this adds up, and space probes can gain a significant amount of velocity from this system. The huge benefit of ion propulsion is efficiency: because these elements decay very slowly, a small amount of fuel can last for years. But what if we had a propulsion method that requires zero fuel?

In comes the solar sail. This technology is still in development, and has not been employed on an operational mission, but has the potential to accelerate satellites to a significant fraction of the speed of light. Solar sails work similarly to how you would think a sailboat works – wind hits the sail and propels the boat forward. (This isn’t quite how sails work so @ me for another newsletter) Sunlight, which obviously travels at the speed of light, technically has no mass but does have some energy and momentum. So, using the same equation introduced earlier, sunlight can impart momentum to a satellite. Because individual photons do not have a lot of momentum, we want to expand the area for sunlight to hit, hence the sail. An artist’s rendering of what they may look like can be seen below. [1]

The larger the sail, the more sunlight momentum we can capture, but increasing size also increases mass. So, in order to maximize our propulsive force, we want to make these sails as thin as possible. This is doable because we don’t have to worry about air resistance messing up the sail. This technology, all things considered, is projected to be able to accelerate mini satellites to up to 20% of the speed of light. That is insanely fast. To put that into perspective, the fastest object that humans have sent to space is the Parker Solar Probe which will top out at about 430,000 mph. [2] That’s about 120 miles every second. On our scale that is practically inconceivable, but on the universe’s scale, that’s only 0.064% the speed of light. 20% the speed of light is 37,000 miles per SECOND. The implications of this technology are immense. We could get probes to Saturn in 7 and a half hours, as opposed to the Voyager Probe which took 3 years. The nearest star to us, Alpha Centauri, is 4 light years away. With the fastest thing we have so far it would take over 6,200 years to get there. Solar sails have the ability to get us there in 20. This turns science fiction into reality. That being said, don’t expect to sail to Venus anytime soon—this is just to get probes there. Solar sails would need to be inconceivably huge to get a space craft with humans on it going that fast. Regardless, sensor technology has improved greatly, and we can get a surprising number of instruments on a pretty small satellite. The limit here is how much light energy can we capture. As the probes get farther from the sun, the sunlight is not as strong. This means that at a certain distance away from the sun we are no longer gaining speed. This is the maximum speed we will be able to achieve.

Okay great, this is theoretically possible. But have we tried it yet? Yes. There have been a few missions that have deployed solar sails like LightSail 2, which successfully orbited the Earth with solar sails. Breakthrough Starshot is another mission that hopes to use a fleet of small satellites powered by solar sails to explore the outer reaches of space. They propose a unique idea of not only using sunlight, but lasers. We can impart further momentum to the space craft by shining extremely powerful lasers at it from Earth. This can get the satellites moving faster, in a shorter period of time, reducing the time they need to speed up.

All in all, solar sails may be our best bet at getting probes to points of interest beyond our solar system. While they are extremely expensive and difficult to build, they require next to no fuel, and can theoretically get space probes to speeds we haven’t been close to achieving thus far. Will the next interstellar spaceships be sailboats? Will we see a significant uptick in space piracy? Is it scary to go that fast? How much damage would we cause if one of these probes accidentally hit a planet? Do aliens exist? Unsure. But we can figure out the answers to maybe one or two of these questions in a future newsletter.

Okay, no, let’s quickly get into the space piracy thing. While funny, legally, it’s very possible. One of my favorite parts of the movie The Martian is Matt Damon calling himself the first space pirate. Damon’s logic is as follows: There is a treaty that has been signed by several nations stating that no country can lay claim to any celestial body (any piece of rock that’s not Earth), and because any territory not claimed by a country follows maritime law, being on the surface of Mars is technically international waters. Resultantly, because he cannot directly communicate with NASA at this point in the movie, and he is taking a vehicle that is NASA’s property without explicit permission, that means he is technically committing piracy. Space Pirate. Pretty cool. Now there are a few holes in this story, as pointed out by Ben Adams from Overthinking It such as: piracy requires a threat or force, and because there is no one else there, he can take the vehicle without it. Additionally, he is technically a US employee taking a US vehicle so realistically if they tried him it would be for theft and/or robbery. So, if there were multiple countries on Mars, and you stole a ship by force, that would do it. That will probably be the most realistic scenario because docking to another orbiting spaceship without that spaceship’s permission is incredibly hard, not to mention they just wouldn’t open the door once you dock. So, if you want to be a space pirate, your best bet is to get on out to Mars. Now that that tangent is out of the way... Thank you for reading!

It's Not Rocket Science

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