A solar sail is a revolutionary type of spacecraft propulsion, like a sail on a boat uses wind to propel the boat a solar sail uses light to propel an object in space. This technology has many exciting possible applications maybe one day...
Believe it or not light can be used to push objects. Light is essentially made up of tiny packets of energy that are called photons. Photons have no mass but they do have momentum. This does go against the formula for calculating momentum which is p=mv (p-momentum, m-mass, v-velocity) however when objects travel near the speed of light the formula becomes E²=(pc)² + (mc²)² this allows photons to have momentum and therefore can push objects such as a solar sail.
Still finding it hard to believe? Below is a video of light propulsion in action:
A solar sail is a vast reflective surface, they are extremely thin to minimise mass. Solar sails are attached to spacecraft and allows them to move through space without using any fuel. To help visualise how vast a solar sail is compared to its payload look at the specifications of a solar sail spacecraft is The Lightsail Spacecraft. It is about the size of a loaf of bread, the solar sail attached to this is the size of a boxing ring, but has a thickness of only 4.5 microns which is 1/5000 of an inch! Solar sails are only really a viable propulsion option in space because there is no friction which is caused by our atmosphere here on Earth.
So as we have already established photons carry momentum, solar sails being reflective capture the momentum of photons as they reflect off of the sail. This momentum transferring onto the sail pushes the sail away from the light source, this acceleration is minute but constant. In theory with a big enough sail and a light enough payload a spacecraft can accelerate up to near the speed of light. Realistically though it is accepted that we can create a solar sail spacecraft that can be accelerated up to a fifth of the speed of light, this means that using solar sail technology we could reach our neighbouring solar system (Proxima Centauri) in 20 years. In fact there are probes in development at the moment which promise to do just that using solar sails (Breakthrough Starshot). Lasers can be used to create a focused stream of photons to give an additional boost to accelerating solar sail spacecraft rather than just relying on photons from the Sun, this concept is called beam sailing.
The first ever interplanetary solar sail spacecraft was launched in May 2010 by the Japan Aerospace Exploration Agency (JAXA), it was the Interplanetary Kite-craft Accelerated by Radiation of the Sun or IKAROS for short. It was the first ever spacecraft truly propelled by the power of the Sun, before this no solar sails had ever been used successfully in space to propel spacecraft. It has a 200m² solar sail that consists of a diagonal spinning 7.5 micrometer sheet of polymide. Embedded within the sail are eighty liquid crystal panels that can change how reflective they are to control the spacecraft. IKAROS is still in space at the moment, it successfully completed all intended experiments by the end of 2010 then was tasked with a flyby of Venus, which took six months, before flying to the other side of the Sun, which took three years. There has been numerous issues with loss of communications with the spacecraft and it has frequently been dipping in and out of hibernation mode due to power shortages. From communications that have been received the solar sail has been providing the spacecraft with continuous acceleration. Communications with the solar sail spacecraft were lost for a fifth time in May 2015, 110 million kilometres away from Earth. JAXA expected to regain communications towards the end of 2016 as it heads back towards Earth, I cannot find any update on IKAROS's position in space so I believe there has still been no communications with the solar sail spacecraft, but if I find something I'll be sure to edit this article.
Solar Sail spacecraft can be extremely lost cost when compared with other methods of spacecraft propulsion. This is because they require no propellant and have a small number of moving parts. This combined with a long lifetime and the ability to reuse spacecraft for numerous space operations provides a inexpensive method of space exploration.
Solar Sail spacecraft might sluggishly accelerate but the acceleration is constant. This allows them to achieve higher speeds than other forms of spacecraft propulsion. Using the Lightsail spacecraft as an example after a month of constant sunlight it would achieve a speed of 549kmph (about the cruising speed of a jet airliner) after 16 months this would increase to 8556kmph. Conventional rockets travel at speeds in excess of 28440kmph but acceleration is not constant a solar sail spacecraft will continue to accelerate throughout its entire voyage. The Lightsail spacecraft relies on solely the sun as a source of light if lasers were to be fired at a solar sail spacecraft then it would accelerate much faster and therefore be able to surpass the speed of conventional rockets in a shorter space of time.
The force of solar pressure (force created by sunlight's photons) is equal to drag pressure at an altitude of around 800km, so for solar pressure to be greater than drag pressure and therefore move away from Earth the spacecraft must be higher than 800km. How high is 800km? Well it is higher than the Troposphere, higher than the Stratosphere, the Mesosphere, Thermosphere, it is actually 100km into the Exosphere. The Exosphere mainly consisting of low densities of hydrogen and helium.
A solar sail can only be used within certain temperature limits due to the limitations of the materials it is made of. Sail operating temperatures are influenced by distance from the sun, the angle of the sail and how reflective the sail is. This being said a solar sail spacecraft can operate up to around 0.25 AU away from the Sun. To put this into perspective, Mercury is 0.39 AU away from the Sun and Earth is 1 AU from the Sun.
So what can solar sail spacecraft actually be used for? Solar sail spacecraft can applied to a wide variety of space operations, be it operations involving our neighbouring planets or those towards the edges of the solar system and ambitiously even places beyond our solar system.
Solar sail spacecraft being reusable and requiring no propellant means that they can be an effective and economic way of traversed to and from Mars. They can be applied to transport both humans and cargo it is thought using solar sail spacecraft opposed to traditional chemical rockets it is estimated over $10 billion could be saved in mission costs.
At the end of 2013 The Gossamer Deordit Sail was proposed by the European Space Agency. This is a solar sail intended to help recover satellites from low orbits. The Gossamer Deorbit Sail only weighs 2kg and takes up 15x15x25cm of space when the solar sail is activated it expands to 5x5m. Once activated the solar sail captures light to propel the satellite it is attached to closer to Earth this will help free up room in Earth's orbit for new satellites.
The Breakthrough Starshot project involves sending 1000 miniature cameras with solar sails attatched. When the probes are launched lasers will be fired at them from Earth to increase acceleration and to reach Proxima Alpha Centauri within 20 years. The nano spacecraft will travel at speeds up to one fifth the speed of light. The Breakthrough Starshot project was announced in April 2016.
The Near-Earth Asteroid Scout for NEA Scout for short is a joint project been NASA and the Jet Propulsion Laboratory. The project involves attaching a solar sail to a CubeSat. A CubSat is a minute cube satellite only 10x10x10cm in size they have a mass of just over 1kg, they are satellites specifically for space research. The solar sail CubSat is intended to scout the near Earth object 1991 VG. This object was first observed in 1991 and again in 2017, in the past there was speculation that this object could have been a spent rocket part floating around in space after the moon landings but after further analysis this notion has been discarded. At the moment it is thought to be a naturally occurring asteroid. The NEA scout will take high resolution photographs of 1991 VG and other near Earth objects to give us a better understanding of the sorts of objects that pass near Earth's orbit and potential asteroids we could utilise in the future. Maybe even mine some near Earth asteroids that contain valuable materials.The Near-Earth Asteroid Scout is scheduled to launch in 2019.
So to wrap things up, solar sails are continuously proving to be a viable form of spacecraft propulsion. They are a cheap and extremely effective way to propel objects through space and they are most certainly going to appear more in our future space operations, at the moment using a solar sail is the only viable option for sending spacecraft outside of our solar system and might even one day send humans to Proxima Centauri B.