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How to Sail on Sunlight

For thousands of years,humans have used the power of wind to sail across our planet and discover new lands thatwere seemingly out of reach. What if I told you thatthere's a 400 year old idea that applies the concept of sails to space that could make the edge ofour solar system accessible in under 10 years of travel time? In the hands of the Sith Lord Count Dooku, a version of this concept was used when he escaped fromGeonosis without a trace.

How could we use sunlight to assail our travel woes in real life? (upbeat electronic music) The sun is a source ofplentiful free energy, and ever since 1608,harnessing it was an idea that BFF's Johannes Keplerand Galileo Galilei shared, and it's since inspired tabletopRPG's like Star Traders, short stories, TV shows, movies, and actual solar sail missions. One example of solar sails in movies is Count Dooku's Punworcca116, or the Solar Sailer. It's somewhat of a misnomersince it's not being pushed by solar pressure, but absorbs energy, similar to a solar sail toprovide energy to the ship. For someone on the DarkSide, he sure knew how to leverage clean energy. Maybe he wasn't so dark after all. (lightning zaps)Okay, maybe he was! As space Biggie once said,"Mo-photons, mo-mentum." Solar radiation allowsus to harness the sun like sailboats use wind. Each photon that hits yoursail is transferring momentum onto your sail, whichaccelerates your ship.

Based on the way the sail is angled, you can change directions, decrease, or increase a spacecraft's velocity. In the vacuum of space, youdon't have the pesky air that we have on Earthadding to atmospheric drag and slowing us down. There are several designs that solar sails use in pop culture,like Count Dooku's ship, and other concepts designed byspace agencies here on Earth. Each one providing an advantage depending on the maneuvers you need to do and how you plan on deploying the sail. The principle of sunlightpressure pushing things around in space has been harnessed and applied already inseveral satellite missions.

For example, Mariner 10 ran out of gas in their attitude control system, but flight controllers used this principle to allow the sun to spinstabilize the spacecraft by orienting the reflective solar panels to generate the spin. Unlike traditionalrockets or ion propulsion, solar sailing is a veryunique form of propulsion that was once thought to be impossible and still proves to be difficult even though it has beendemonstrated in the past. Scientists at NASA'sMarshall Space Flight Center are researching ultralightweight materials to use as sails, which canadvance the use of this method in the future. There are many factors that contribute to the forces that canpropel a solar sail. The largest factor'selectromagnetic radiation emitted by the sun. Other factors that generate a force are solar wind, which is100 times less effective as electromagnetic radiation, albedo, which is the reflection and scattering of solar radiation off the Earth, and Earth's infrared emissions. Calculating the amount offlux, or density of pressure that the electromagneticradiation from the sun contributes per square meter at ourdistance from the sun, the linear momentum thatthis flux generates is, this is a million timesless than the pressure exerted from a dollarbill sitting on the table.

This flux is so small that it falls below the threshold pressurerequired for us to hear, and so, just like that bugthat's crawling behind you, it's too small to detect its presence with our fancy ear holes. Well, if our ears were out there in space listening for pressure exertedby electromagnetic waves, not hearing the sun would bethe least of your concerns. It's not just about the pressure, but the total force exertedover the full area of the sail. The larger the sail, thelarger the total force. The acceleration thatyou get from a solar sail can be calculated using this equation where nu is the sail efficiency considering that the opticalproperties of the sail is not perfect. Just like a fun house mirrorversus a regular mirror, you want it to be as closeto perfect as possible to get the most efficient use out of it. Based on this equation, we can make all kinds of fun calculations. Let's say you wanna knowhow big of a solar sail you would need totransport humans to space.

A solar sail two microns thick has an approximate acceleration of around one millimeterper second squared. This is what you get whenyou solve the equation for the area. Assuming the mass of thespacecraft is divided equally by the payload structure and sail, the area needed is 387meters squared per kilogram of payload mass. That means if your payloadis a tiny five kilogram or 11 pound payload, you'dneed a solar sail the size of 1,935 meters squared,or a sail the size of 4.5 basketball courts combined. If you wanted to transportthe Orion capsule, which can hold two to six humans and weighs about 10,000 kilograms, you'll need a solar sail aboutfour million square meters, which is the size of 1.2 Central Parks. Well if you were one of the crew members accelerating at onemillimeter per second squared, how long would it take foryou to get to the moon? Assuming at constant acceleration,the distance traveled is calculated by this formula.

 To see how much time it takes to travel a 384.4 millionmeters at that rate, it will take approximately 10 days. At the 10 day mark, thevehicle will be traveling over 877 meters per second, oraround 2,000 miles per hour, which is ridiculously fast. Likely if this was a realexample of sending humans to the moon on a solarsail, they would have to start their deceleration upon approach, and so, it will likely take a bit longer. Traditional rocketscan get you to the moon in just under three days versusthe estimated 10 or so days using the solar sail. However, if you wantto travel even farther in our solar system or beyond,the perks of solar sails really begin to shine. As payloads continue tobe pushed by the sun, it can reach speeds that are far beyondtraditional rocket speeds. Similar to the age oldstory about the tortoise and the hare, eventually thesolar sail tortoise zips by.

This can be a huge savingsin both cost and mass. No matter what method you use, it still costs somewherebetween $9,000 and $50,000 per pound of payload thatyou're launching into space. If you don't have to bringall of that rocket fuel, you're already saving atremendous amount of money. It's been calculatedthat using solar sails to make routine deliveries to Mars can save 10's of billionsof dollars in mission costs. That's a lot of cash. The solar radiation pressure on the sail also decreases as you travelfarther away from the sun with this pressuredecreasing by about half as you get to Mars's distances and 10 times less as you pass Jupiter. Even with that decay, the free rays still make a huge impact. Voyager 1 has been trekkingalong for 44 years, and it's currently 18.8billion miles away from Earth. Scientists calculate thatwe could catch up to Voyager in less than a decade usingthe solar sail technology. Cool! Scientists are also looking at Sirius A as the best target forsolar sail traveling outside of our solar system. Since the large star can provide a way to start putting on thebrakes for the approach. Using solar sails, youcan get to Sirius A, 8.6 light years away,in just under 70 years. You can imagine addinglasers pointed at the sail, which provides a niceboost from us Earthlings. (bright music) Is someone shining a light on me (scoffs)?

Space is dark and full of terrors. The only factor limiting how fast you cruise the galactic seas is how well you're material can last against the elements of space,the mass of your vehicle, and how far away you are from the sun or the star that you're using. LightSail 2 solar sailis made out of Mylar, which is a polyester film used often in space-fairing missions for reasons, such as its dimensionalstability and reflectivity. This Mylar sail has ripstop sown in, just like a parachute, sothat if a micrometeoroid by chance punches a hole in the sail, you can still continue to function while preventing the holefrom growing in size. This Mylar sail is 4.5 microns thick, or one-five thousandth of an inch. It's also pretty light.

The entire spacecraft, including the sail, comes in at 11 pounds. So, so tiny!(bright music) With LightSail 2's launch onthe SpaceX Falcon Heavy rocket currently set for late June,we'll be one step closer to realizing the dream ofaffordable long-distance travel by harnessing the free andever-present rays from the sun. It's been a long time coming, and this tech is ready to take us places because space!(upbeat electronic music) Thanks for watching thisepisode of Because Space. Remember to give us a thumbs up and if you want to learnmore about the LightSail and The Planetary Society,you can join the mission over at planetary.org.(bright music) 

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