LightSail, a project by The Planetary Society, demonstrates and explores the capabilities of solar sailing by putting it in low Earth orbit by using a miniature satellite called CubeSat.

What is Solar Sailing?

Although something straight out of Treasure Planet might pop-up in your mind when you hear the words, Solar Sailing, with Jim Hawkins sailing around the cosmos on this rocket-propelled sail boat, it is quite the opposite of that. It omits the use rocket propulsion and harnesses the power of Sun to transit through space. The spacecraft essentially functions like a sail ship which captures tailwind to move forward while the mirrored sails of the spacecraft capture photons. Photons are packets of energy in light that do not possess mass but have momentum. The reflective sails made from Mylar capture the photons, which transfer all their momentum and get reflected to push the spacecraft forward.

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Can Humans transit through the cosmos by Solar Sailing?

It is theoretically possible for humans to use solar sail spacecrafts for interstellar travel. To put it in perspective, the key to the weight of payload such a spacecraft can carry is the area to mass ratio (A/m), that is the forward momentum required to move the spacecraft depends on the relation between the area of the sails and mass of the vehicle. So higher A/m ratio would mean better functionality of the spacecraft. With human travel, the weight of the spacecraft will be exponentially higher than the 5 kilograms than the LightSail 2 weighs, which would mean that the area of Mylar sheets required will also be exponentially higher.

Japan has launched its own Solar Sail called IKAROS in 2010 with a A/m ratio of 1.3 while the LightSail 2 boasts a A/m ratio of 7 because of the much smaller payload of CubeSat and the 32-square-meter sails.

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How fast can these Solar Sail move through space?

The Planetary Society says that, on assumption of perfect reflectivity by the Mylar sails the Sun exerts a force of 2.91x10⁻⁴ N/m² on the 32-square-meter sails of the LightSail 2 which results in an acceleration of 0.058mm/s2 which seems like it is not that big, but in a month, LightSail would be moving at 549 kmph and 8,556 kmph in a period of 16 months. As it moves away from the sun, the acceleration will decrease because the number of photons incident on the sail will decrease. But the speed gained by the sails will be retained as there is no resistance in space. The acceleration depends on the incident photons, thus is directly related to amount of energy transferred or simply energy flux. The mean energy flux near Mars is 43% of the mean energy flux near Earth while it drops to just 4% near Jupiter.

In short, the LightSail will be ineffective as we move away from the Sun, thus for interstellar travel methods like building up speed in the Solar System or using support lasers to keep up the acceleration can be used.

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What is LightSail project?

The project has already launched two spacecrafts in space namely LightSail 1 and LightSail 2. The LightSail 1 was launched on 20th May 2015 and the mission lasted for 25 days after which it re-entered the atmosphere. The mission’s goal was to check the deployment of solar sails in space, it did not perform solar sailing. The LightSail 2 that was deployed on 25th June 2019 and will remain in a low Earth Orbit for a year. This spacecraft is fully capable of solar sailing and is expected to demonstrate the capabilities of this relatively new method to travel in space. The LightSail project is also completely crowd funded, i.e. the monetary funds required for the project were raised by the public. The project was possible because of funds from about 50,000 people, which is an amazing accomplishment.

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At the moment, we might be unable to travel through the cosmos at hyperspace speeds of Millennium Falcon. But soon, we might be able to send satellites or observers to far reaches of the galaxy without the limitations of rocket fuel propulsion.