Sailing with LighT
the science behind solar sails
There have been a myriad of myths and stories where people have been able to walk or glide on light, such as the Norse myth of Bifrost, the legendary rainbow bridge that connects Asgard, the realm of the Aesir, with Midgard, our world. Of course, this is all fantasy. However, while we may not be able to walk on light, we are able to sail with it.
About Solar Pressure
Solar or radiation pressure is the force exerted on a surface due to the exchange in momentum between the object (which is usually reflective) and an electromagnetic field, as Wikipedia states. Basically, this means that light can exert forces on things. Usually, these forces are too small to be considered, but in certain circumstances, like in outer space, they play a huge factor.
Now solar pressure seems impossible; photons (packets of light) have no mass, so how could they exert force? Here’s a little math: using de Broglie’s equation, λ = h/mv (λ being wavelength, h being Planck’s constant, and mv being mass x velocity, or momentum), we can rearrange it to become mv, or momentum = Planck’s constant / wavelength of the photon. So although photos don’t have mass, they do have momentum.
Even so, the momentum of a single photon is tiny. But when the vacuum of space and the sheer number of light particles are taken into account, the principle of conservation of momentum means that there will be a rather large force on the affected object. When launching probes and satellites into space, solar pressure is significant enough to be taken into account.
Uses of Solar Sailing
Here is where the concept of solar sailing takes centre stage. We take what seems to be a nuisance or a setback, and we turn it into an advantage. Using large reflective, retractable metal sails, spacecrafts can use solar pressure to (for example) correct their orientation without expending too much fuel, increase their orbital height, or even propel themselves further into space. The Ikaros spacecraft was the first to actually use solar sailing technology, using the generated force to visit Venus and return back after three years. LightSail2, a small satellite funded by the Planetary Society, used solar sailing to increase its orbital distance from Earth by rotating its sails to maximise the force resulting from the sunlight. (Refer to the Real Engineering video for a full explanation)
Major Problems and Setbacks
For starters, the spacecraft has to be incredibly light for solar sailing to be effective, all the while holding the material for the sails. The sails themselves have to have huge surface areas to maximize the number of photons colliding with the structure; they also have to be super reflective to minimize the amount of momentum that converts into heat instead of kinetic energy.
On top of that, the solar sails are incredibly difficult to manoeuver; the constant adjustments needed to maximize the force from sunlight as the satellite spins in orbit has to be automated, which in of itself is a feat to be feared.
The force from the photons also decreases exponentially the farther away the light source is, due to the law of inverse square (intensity ∝ 1/distance^2). The farther the satellite drifts from the Sun, the less force the Sun can provide. Moreover, the angle of the spacecraft matters: the farther it tilts from the sun, the less force from the photons.
Overall, the science behind solar sailing, albeit very interesting, is quite complicated. I just thought it was a cool concept, and I wanted to write about it. Hope you enjoyed the article!
Written by Sonam Okuda, Grad 21 student from Bangkok Patana School
Real Engineering (2019) How to Sail on Starlight [accessed September 2019, online]
Available at: https://www.youtube.com/watch?v=j8umNI8gtlA&t=225s
Wikipedia (2019) Radiation pressure [accessed September 2019, online]
Available at: https://en.wikipedia.org/wiki/Radiation_pressure
ThoughtCo. (2018) de Broglie Equation Definition [accessed September 2019, online]
Elizabeth Howell (2014) Ikaros: First Successful Solar Sail [accessed September 2019, online]
Available at: https://www.space.com/25800-ikaros-solar-sail.html
Emily Henderson (2019) There's Now A Satellite Sailing In Earth's Orbit On The Rays Of The Sun [accessed September 2019, online]