NASA’s Solar Sails

Sailing into the sunset gets a whole new meaning.
CubeSats and solar sailing have been announced by NASA as the new and exciting way to travel in space. CubeSats are basically tiny spacecrafts built in units of 10cm cubed. They are efficient, low cost and have great potential for space exploration. Once coupled to a substantial solar sail they become the perfect little explorers. The solar sail harnesses the Sun’s energy to power its propulsion in space. Place the CubeSat on a rocket and voilà, the hitchhiker in space will unfold its wings and start its journey.
Until now, space travel has been a very lengthy process; NASA’s Voyager I probe took 35 years to reach the edge of our Solar System. But the new CubeSat with its space sail could make the same journey in only 20 years. NASA announced that the CubeSat NEA Scout will be launched in 2018 using the scheduled Space Launch System rocket, and once it leaves the Earth’s orbit NEA Scout will detach and go off on its mission. It will unfold its solar sail and through it will be propelled through space to its destination asteroid 1991 VG.
The solar sail basically runs on light: “Instead of a traditional, chemical propellant, it’s powered by photons – or light particles – from the Sun pelting its ultrathin sail, which is about the size of a school bus. The sail is made of plastic with aluminium coating, and is as thin as a single strand of human hair.” (Adam Epstein for qz.com) When the photons bounce off of the aluminium coating, they transfer a small amount of momentum, as governed by Maxwell’s theory of electromagnetism. This idea has also been touched on in Jules Verne’s novel “From the Earth to the Moon” in 1865. (Kenneth Chang for nytimes.com)
National Geographic suggests such a sail can travel up to 28.6 kilometers per hour. As the sun never stops shining in space, the fuel for the sail never runs out. The only drawback is that the load the sail is carrying has to remain light, as acceleration must come from the constant light source, not a from any sudden increase of fuel boosters.
NASA are not the only ones sailing into space. The non-profit organisation Planetary Society, led by Bill Nye “The Science Guy”, have launched their own “LightSail” mission in 2015. In 2016 their LightSail will be carried into space by Falcon Heavy rocket, which is built by private spaceflight company SpaceX.
Solar sailing doesn’t stop there. NASA is already developing the concept further to create an E-sail, or electric sail, which was invented by Dr. Pekka Janhunen who teamed up with NASA to develop it. The aim is to test whether a scientific spacecraft can be propelled to the edge of our Solar System in less than 10 years.
The full and proper name for the E-sail is “Heliopause Electrostatic Rapid Transit System” (HERTS). Its technology is based on using the solar wind. As explained in NASA’s video, “the Sun is continually releasing protons and electrons into the solar wind”. Once the spacecraft is deployed it “grows” twenty spinning spider-like arms and legs which are positively charged. Each of these arms has a length of 20 km.
As the arms spin in space they are bombarded with the positively charged protons flying past in the solar wind. The propulsion comes from the momentum exchange of the zooming protons with the positively charged arms, which are electrostatically repelled by the arms. This gives rise to a thrust. The electrons in the solar wind are collected and discharged via an electron gun to stop the arms from neutralising (electrons are negatively charged particles).
A brief comparison of the E-sail to the solar sail shows that the E-sail surpasses its mechanical predecessor. The solar sail is 200m by 200m in size, while the E-sail’s effective area (in essence the sail equivalent) is 600 square kilometres at 1 AU (Astronomical Unit, or distance of the Earth from the Sun), and increases to 1200 square kilometers at approximately 5 AU. The solar sail stops accelerating at 5 AU whilst the E-sail continues its acceleration up to 30 AU.
NASA say they still require more modelling, simulation and ground testing to be done before a “Mission of Opportunity” test flight will be tried in cislunar space.
Are we making a giant step towards space travel as a reality?