Finally, we have arrived at a technology which unequivocally exists right now. See that picture up there? That is a depiction of the IKAROS solar sail probe. The IKAROS is real! It is out there, in space, at this very moment, over 100,000,000 miles from Earth.
IKAROS is a solar sail and currently it can only transport small probes. But someday, conceivably, you could ride a solar sail through the solar system.
In order to understand solar sails, you first need to understand what propels a solar sail. In this first part, we're going to discuss solar sail basics and learn about what fuels a solar sail journey.
Space is a vacuum. Not a perfect vacuum, hence the dispersed hydrogen the Bussard Ramjet relies on. But, relative to Earth's atmosphere there isn't as much stuff in outer space. So if outer space is mostly empty, what the hell kind of "wind" is a solar sail harnessing?
There are only two contenders, so let's take a look at both.
1. The Solar Wind
In the 1950s Ludwig Biermann made a peculiar observation: while looking at various comets he noticed that no matter which direction a comet was headed, the comet's tail always pointed away from the sun. In doing so, Biermann joined a small chorus of scientists who, for over a century, hypothesized that the sun was expelling material into space all the time.
Since the 1960s this phenomenon has been confirmed beyond any doubt. The sun is constantly expelling large amounts of high energy charged particles released by the nuclear fusion which fuels the star. This constant barrage of high energy particles is called "Solar Wind".
The solar wind is mostly protons and electrons which shoot out from the sun across the entire solar system, moving at relativistic speeds, over hundreds of millions of miles. These particles do exert a force on objects in the solar system. For example, when we send a probe to another planet, the solar wind has to be taken into account during the planning stages, otherwise the probe will be off course.
The Earth itself has to deal with the solar wind, as our planet is constantly being bombarded by it. Luckily Earth has a magnetosphere which diverts or filters most of the solar wind and prevents it from stripping away our atmosphere and leaving us barren.
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Mars, on the other hand, does not have a strong magnetosphere, and the result is that the solar wind has stripped away most of the planet's atmosphere, leaving the atmosphere only 1/100th the density of the earth. You can see the meager remnants of Mars's atmosphere in the photo to the right. Plus, the strength of the solar wind increases the closer you get to the Sun. This helps explains why poor little Mercury, constantly bombarded by the full brunt of solar wind and radiation, has almost no atmosphere at all. It has had almost all of its atmosphere blown away by the solar wind over countless millions of years.
There is a lot more detail to dive into regarding the solar wind - how it's created, its various speeds, densities and energy levels - but we won't be getting into the weeds on any of it because solar wind is only a minor propellant of a solar sail.
2. Solar Radiation
In 1690 Johannes Kepler made a similar observation about the tails of comets, but Kepler attributed the behavior to a different phenomenon, "Radiation Pressure."
In addition to the solar wind, the Sun is constantly emitting tons of radiation, at almost all wavelengths and strengths along the entire electromagnetic ("EM") spectrum. A big part of that radiation energy falls into the visible range of the EM spectrum - i.e. sunlight. The remainder of the solar radiation falls within non-visible ranges on the EM spectrum. Some is lower energy, like radio waves, and some is much higher energy, like destructive gamma radiation.
What is radiation?
Radiation is photons imbued with varying amounts of energy. A photon is one of the elementary particles that defines how our universe works. For more info on elementary particles, check out @lemouth's excellent post on the topic recently.
This is a diagram of the Electromagnetic ("EM") Spectrum. All radiation falls within this spectrum. The further to the left on the spectrum, the lower the energy of the radiation - e.g. low energy radio waves. The further to the right on the spectrum, the higher the energy of the radiation - e.g. high energy gamma rays. But no matter where the radiation falls on the EM spectrum, it is still just photons at varying levels of excitement.
When photons interact with a physical object, either:
- The photons pass through the object
- The photons are absorbed by the object
- The photons are reflected by the object
- Or some combination of all three things happens
For instance, the reason you can see anything at all is because your eyes are built to see visible light. Photons with an energy level on the EM spectrum equal to visible light radiate out of the sun, or a light bulb, and those photons hit things. Some of these photons are absorbed by the things they hit, but a lot of them are reflected. Everything you see is your eyes catching the reflection of photons coming off everything around you.
Alternatively, consider Ultraviolet radiation - when UV rays hit your skin, some UV rays are reflected by your skin and some are absorbed. The absorbed UV damages your skin cells while the reflected UV bounces off you and makes you shine nice and bright for a butterfly, which can see UV radiation.
How does all this radiation make a solar sail fly through space?!
A photon has no mass. But photons are almost always moving, and so they have momentum.
As a result of this momentum, when photons absorb into or reflect off of things, the photons impart a physical pressure - albeit a very small amount: many, many orders of magnitude less than, say, a child's push.
This is called Radiation Pressure and this stuff is the gasoline which propels a solar sail spaceship through space!
Here on Earth you would never be able to feel solar radiation pressure. But out in space, where there is no atmosphere and we are dealing with travel over huge distances, radiation pressure becomes more noticeable. Over many millions of miles, even the small amount of pressure from solar radiation can make a big difference in the trajectory - or speed - of a moving object. The combination of radiation pressure and solar winds shape the various solar systems in our universe, constantly shifting and crafting the shape of things, including nebulae like "The Pillars Of Creation."
Reflection = Maximum Propulsion
What a solar sail does, using super shiny, super thin materials stretched over lots of space, is interact with and reflect as much solar radiation as possible. A solar sail attempts to reflect the solar radiation because the reflection of a photon results in the highest amount of radiation pressure. There are a number of formulas used to prove this and calculate pressure amounts, which you can take a look at if you don't believe me or want to understand in even more detail.
By reflecting as much solar radiation as possible, solar sails can theoretically accellerate to really fast speeds over enough time. In fact, this is more than theoretical, as the IKAROS solar sail has already proven radiation pressure propulsion to be physically possible. But we'll discuss IKAROS more next time
For now, we know the solar sail runs mostly on radiation coming off of the sun, along with a small percentage of propulsion from the solar wind. The question next time is how far can we go on just solar radiation alone? And, if limitations exist, how can we push past them?
Next time - Solar Sails Part II - IKAROS, Super lasers, and Super Masers.
Information Sources
https://en.wikipedia.org/wiki/Solar_sail
https://en.wikipedia.org/wiki/Photon
https://en.wikipedia.org/wiki/Radiation_pressure
https://en.wikipedia.org/wiki/Solar_irradiance
https://en.wikipedia.org/wiki/Solar_wind
Photo Sources
[1]By Andrzej Mirecki Own work CC BY-SA 3.0
[2]By NASA via Wikimedia Commons
[3]By NASA via Wikimedia Commons
[4]By Inductiveload, NASA GFDL or CC-BY-SA-3.0], via Wikimedia Commons)
[5]By NASA, ESA, and the Hubble Heritage Team STScI/AURA Public domain, via Wikimedia Commons
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