|
|
| Fig. 1: This is an image of a solar sail that was being used for tests at the NASA Glenn Research Center's Plum Brook facility in Sandusky, Ohio. (Source: Wikimedia Commons, courtesy of NASA) |
There are many types of propulsion systems that allow spacecraft to travel through space. These include (but are not limited to) using solar power, nuclear power, fuel cells and batteries. [1] One of the more novel methods of spacecraft propulsion that had only been experimental until recently is whats known as solar sails or photon sails. This type of space propulsion works by applying radiation pressure from sunlight to giant mirrors on the spacecraft, pushing it along through space much like how winds are able to propel boats along water. [2] Radiation pressure as applied by an electromagnetic wave is given by
where I is intensity of light (measured in W/m2) and c is the speed of light. The force F on a sail of area A is then
Using Newton's law F = ma, where m is the spacecraft's mass and a is it's acceleration, we find that the acceleration is
| a | = | 2IA mc |
Using the approximate area of the solar sail shown in Fig. 1 of 405 m2, a mass of 300 kg, and asuming that the craft is 1 AU from the sun (the distance of earth's orbit), the acceleration is
| a | = | 2 × 1388 W m-2
× 405 m2 3.00 × 108 m s-1 × 300 kg |
= | 1.25 × 10-5 m s-2 |
This is an incredibly small acceleration, and if the solar sail craft accelerated at this rate over a year, by the end it would have sped up by 391 m/s. Until 2010, the viability of launching a vehicle powered only by photon propulsion was only known in theory, but that changed completely with the historic launch of IKAROS (Interplanetary Kite-craft Accelerated by Radiation Of the Sun) by the Japan Aerospace Exploration Agency (JAXA). [3]
Compared to conventional rockets, solar sails have a few advantages. Firstly, rockets require a fuel source (nuclear, chemical, or other) in order to create propellant that will be discharged from the rocket. Due to conservation of momentum, that discharged fuel creates thrust, propelling the rocket forward. [4] Eventually the fuel source will be exhausted, which is why solar sails have an advantage. Photons are moving everywhere and at all times through space, meaning they are able to provide thrust for a spacecraft using sails indefinitely or at least until the destination is reached. [5] A second advantage is that sails can be reused, just like sails for vehicles on Earth. This is very important for the future of space travel as being able to reuse parts of spacecraft has proven to be an important cost saving measure. A third advantage of using solar sails for spacecraft is that they can create a small but continuous amount of thrust that makes them more efficient over long distances. Rockets accelerate to their final velocity very quickly and then glide towards their target for the length of their journey, whereas solar sailed powered vehicles being more slowly but accelerate constantly over time. The one caveat of this advantage is that solar sails cannot generate enough thrust to escape the Earths surface, meaning they would have to be deployed once in space. [5]
As stated before, the Japanese launched IKAROS (Interplanetary Kite-craft Accelerated by Radiation Of the Sun) set sail in 2010. It was released into space as part of the payload of Japan Aerospace Exploration Agencys (JAXA) H-IIA No. 17 launcher. With the successful deployment of IKAROS, it marked the first time a solar sail craft was used in practice. [5] With JAXA proposing to use the new method of propulsion for deep space exploration in the future, the IKAROS voyage showed that the sail could be deployed, that the vehicle was actually accelerating due to light induced radiation pressure, and that it could be guided and navigated. [3]
© Andrew Chun. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] B. Johnson, "Power Sources for Space Exploration," Physics 240, Stanford University, Fall 2012.
[2] B. Fu, E. Sperber and F. Eke, "Solar Sail Technology - A State of the Art Review," Prog. Aerosp. Sci. 86, 1 (2016).
[3] Y. Tsuda et al., "Flight Status of IKAROS Deep Space Solar Sail Demonstrator," Acta. Astronaut. 69, 9 (2011).
[4] N. Nguyen, "Space Propulsion Technology and Energy Expenditures," Physics 240, Stanford University, Fall 2011.
[5] G. Vulpetti, L. Johnson and G. L. Matloff, Solar Sails: A Novel Approach to Interplanetary Travel (Springer, 2015).
