NASA is pumping funding into the development of a radical approach to solar sail construction – a new way of exploiting the radiation pressure of sunlight to generate spacecraft propulsion.
Most theoretical designs for solar sails have relied on coating microscopically thin textiles – sometimes in arrays measured in square-kilometres – with reflective metallic surfaces. These coatings transfer the momentum of solar photons to the sails themselves, resulting in propulsion.
And while NASA plans to test this technology in its forthcoming Near-Earth Asteroid Scout mission – which will feature CubeSats fitted with 86-square-metre sails – it is also putting weight behind a second, alternative approach.
The organisation has announced a tranche of funding to assist research led by Grover Swartzlander of the Rochester Institute of Technology, in the US, to develop what are known as diffractive solar sails.
Diffractive sails avoid metallic coatings and use newly designed metamaterials to create textiles that alter the angle of incoming photons across a broad range, or grate, of possibilities.{%recommended 1195%}
(This is easily, if crudely, visualised by looking at the rainbow effect generated by shining light on the playing surface of a compact disc. The coating that produces the visible light spectrum is an example of diffraction grate.)
Because diffraction grates are capable of reflecting photons at different angles, they can be used, and reused, for different purposes. Metallic solar sails use photons only once – to propel the sail forward – after which they are either reflected back into space, or absorbed by the material.
Either way, they are lost – and in the second scenario contribute to warming the sail and the craft to which it is attached, potentially raising a slew of downstream problems.
A diffraction grate sail, in contrast, does not warm up. It can use some photons for momentum, and others to generate solar-electric power, for instance. Correctly angled photons could also be recycled, being fed back into the system to generate additional momentum.
“We’re embarking on a new age of space travel that makes use of solar radiation pressure on large, thin sail membranes,” says Swartzlander.
“The conventional idea for the last 100 years has been to use a reflective sail such as a metal coating on a thin polymer and you unfurl that in space, but you can get a force based on the law of diffraction as well. In comparison to a reflective sail, we think a diffractive sail could be more efficient and could withstand the heat of the sun better.
“These sails are transparent so they’re not going to absorb a lot of heat from the sun, and we won’t have the heat management problem as you do with a metallic surface.”
NASA’s interest in Swartzlander’s work was piqued after he delivered the result of a nine-month proof-of-concept trial in 2018.
With the latest range of funding, announced recently, the researcher hopes to develop a demonstration mission of a working satellite powered by a diffraction sail within five years.
His ultimate aim, he says, is to dispatch a fleet of sail-satellites towards the sun, where they would settle into different orbits and provide a 360-degree view.
He estimates travel time from launch to destination would be about five years.
