Chemical rockets are loud, noisy and can only get us so far. If we want to reach another star system, we’ll need something better—either super energy-dense fuel to improve the efficiency of chemical rockets or a way to push spacecraft using beams of energy, like a photonic lightsail. A new paper looks at the pros and cons of lightsails, figures out the best setup to carry a small payload to another star while humans are still alive to see it, and checks out what materials might actually work for this kind of mission.

Interstellar travel, or journeying between stars, represents one of our most ambitious challenges. While current technology limits us to exploring the solar system, the dream of reaching distant star systems drives scientific innovation and imagination. Such journeys would require advanced propulsion systems, like nuclear fusion engines, solar sails, or theoretical concepts such as warp drives and wormholes (must resist any reference to Star Trek.) The immense distances between the stars present enormous challenges in terms of time, energy, and resource management. Shielding from radiation, life support and the psychological effects of isolation are among the challenges yet still, the pursuit of interstellar travel continues to inspire.

A new paper authored by a team led by Jadon Y. Lin from the University of Sydney explores one possible technology that may get, if not us then our technology, to the stars. They explore the principles of lightsail technology and how the application of photons of light could drive spacecraft the immense distances. Starting with the desired outcome, the team use a computational method which starts with a desired outcome and work backwards to get the best solution to achieve it. 

Just what is the problem. Travelling even relatively short distances among the stars, such as to Proxima Centauri ‘just’ 4.2 light years away, a spacecraft would need to travel at over 10% the speed of light to get there in a human lifetime! That’s approximately 30,000 km per second when our fastest probe has only achieved 194 kilometres per second! We need to go faster! According to the Tsiolkovsky rocket equation, chemical propulsion to accelerate a single proton to that speed would require more fuel than the entire observable universe! That means any spacecraft aiming for such enormous speeds needs an external source of momentum and energy. Enter light sail technology which could, according to recent research propel a probe to Proxima Centauri in just 21 years!

Fundamental to the success of a functional lightsail for interstellar travel hinges on finding the right materials and fabrication methods for the sail itself. There are some promising options available such as silica, silicon nitride and molybdenum disulfide although their full properties in ultra-thin membranes have still to be tested. The team conclude that molybdenum disulfide is currently the best contender but further testing is needed. Shifting the focus to design, the traditional sail shapes show potential but the paper concludes that they are outperformed by nano-structured designs like diffraction gratings, which optimise propulsion, thermal control, and stability. 

Sadly interstellar lightsails might yet take decades to become a reality. The technology isn’t quite there yet, not just in material science but progress is needed in areas like metalenses and high-powered lasers too. We have already seen light sails used successfully in space but, as interest develops and technology advances, slowly, interstellar spacecraft designs may at least one day becoming a reality. 

Source : Photonic Lightsails: Fast and Stable Propulsion for Interstellar Travel