breakthroughinitiatives.org

It’s all very well dreaming up technologies that will let us travel quickly to other star systems, but how do we apply the brakes on arrival?

Rene Heller of the Max Planck Institute for Solar System Research and independent space researcher Michael Hippke now have an answer: we can slow down a solar sail-powered craft using the stars themselves.

The nearest star system, Alpha Centauri, is over four light years away. Chemical propulsion technologies are too heavy to be practical – they would take 100,000 years to get there.


However, lasers fired from Earth can accelerate ultra-light solar sails made from graphene to around 20 per cent of the speed of light within a few minutes. That means an interstellar probe could reach the Alpha Centauri system – including the Earth-mass planet orbiting its companion star, Proxima Centauri – just 20 years after launch.

Last year, billionaire Yuri Milner pledged $100 million to do just that: send tiny “wafersats” to visit the system and investigate its denizens.

Unfortunately, at that speed, the craft would whiz through the whole system in just a few hours. “It’s hardly enough time to take in the view, let alone do serious science on the habitability of a planet,” says Heller.

Hit the brakes

One braking option is to carry fuel and a reverse thruster. Laser technology could also throw the light-sail into reverse. But these would weigh the craft down, and could malfunction.

Now, Hippke and Heller show that a combination of the stars’ gravity and radiation pressure from their photons can bring the craft into a stable orbit around one of the stars. Photon pressure can also be harnessed to transfer the probe into orbit around the planet Proxima b, which sits in the habitable zone of Proxima Centauri.

The idea is “scientifically viable and very interesting”, says Avi Loeb at Harvard University. His main reservation is a practical concern. “One should keep in mind that the sail must be accompanied by electronics which will add significantly to its weight,” he says.

In order to keep the weight down, the sail would have to be just a few atoms thick. That means it would be orders of magnitude thinner than the wavelength of light that it aims to reflect, and so its reflectivity would be low. “It does not appear feasible to reduce the weight by so many orders of magnitude and yet maintain the rigidity and reflectivity of the sail material,” Loeb says.

Hippke acknowledges the problem. “The issue of producing an extremely thin material with high surface reflectivity seems to be a very challenging exercise,” he says. However, he can see solutions coming over the horizon. A one-atom thick coating of silicon would boost the reflectivity of the graphene sail enormously, he points out, and silicon-based metamaterial monolayers are now being designed.

Journal reference: Astrophysical Journal Letters, DOI: 10.3847/2041-8213/835/2/L32