Using Light to Propel Spacecraft

A surprising thing to note is that this is not a recent idea. In fact, the idea was put forth during the discussions of  General Relativity. But news of recent development has surfaced, and it’s certainly a subject sparking the general interest of the public; never has such a vehicle been launched without requiring the burning of fuel.

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With this method of propulsion a spacecraft could attain speeds up to 10% the speed of light (30 million meters /second) and deliver a robotic payload to mars in as little as three days. A human crew on the other hand would take about a month due to the additional mass. The main source of energy for takeoff and the acceleration throughout the duration of the trip (the lasers) would be stationed on Earth, and the spacecraft would not need to carry the fuel required to get into orbit, meaning that a spacecraft would weigh significantly less and could possibly designed to be smaller. In contrast to current rocket systems that utilize a three stage takeoff, a photonic propulsion system wouldn’t require the spacecraft to carry fuel.  the launch sequence would be reduced into a single stage, minimizing the amount of waste produces by the vehicle.

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The amount of energy to do this, however, would be nearly equivalent to the amount of energy it takes currently to launch spacecraft using rocket fuel. 50 to 100 GW (Giga-Watts) to be precise . But how can something with virtually no mass be used for propulsion? Well we know that photons carry momentum and energy, so all the spacecraft has to do is steal any incoming photons coming from the directed laser source from earth. This is a process called direct momentum transfer.

 

Acceleration to 10% the speed of light would be a gradual process, where speed would be procured over a long distance. So what enables us to achieve this speed with the proposed light station on earth is the long distance between Earth and Mars, or really any other planet in our solar system. Lasers have a very high magnitude of specific impulse : 3.06 x 10^7 seconds. The highest specific impulse of any fuel:  thrust to power ratio. At relativistic speeds (speeds near 3 x 10^8 meters/second), particles used for propulsion achieve nearly the same amount of efficiency. But it would be an inherent problem for particles that are used in current widespread propulsion systems.

This method of propulsion does sound very promising, and it’s the kind of idea that leads others to imagine. But the reality is that laser propulsion would be very inefficient. It would require speeds above 0.5c (half the speed of light) for photons to redshift (which would indicate that most of their energy has been transferred to the spacecraft). Some solutions to this would involve the use of photon recycling, where a spacecraft would use an active resonant optical cavity that would allow the a recently used photon to bounce back to the spacecraft and propel it a little more. But more testing will have to be done.

 

References:

 

http://ykbcorp.com/downloads/Bae_photon_propulsion_STAIF2_Paper_Circulation.pdf

http://www.sciencealert.com/nasa-scientists-are-investigating-a-propulsion-system-that-could-reach-mars-in-3-days

http://www.astrobio.net/news-exclusive/beaming-rockets-into-space/

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http://www.centauri-dreams.org/wp-content/uploads/2015/06/Laser-Sail-Adrian-Mann.jpg

https://vertassets.blob.core.windows.net/image/1fae04f1/1fae04f1-fa4e-48a0-b623-5721026358c8/laser_rocket.jpg