A laser beam-riding sail in the shape of a giant trapped ping pong ball could carry the first manmade object across interstellar space towards the recently-discovered Earth-like exoplanet Proxima b, according to Harvard University researchers.
The main idea behind the Breakthrough Starshot Initiative is to send light-propelled nanocrafts to the Alpha Centauri system within the next generation. Image credit: Breakthrough Initiatives.
A sail that harnesses light to propel a spacecraft was first proposed almost 100 years ago.
Following the invention of the laser in the 1960s, it was suggested these new, high power beams could accelerate a sail-bound craft even faster, up to a substantial fraction of the speed of light, opening up the possibility of interstellar travel.
“However, the literature building on these ideas is sparse,” says Professor Avi Loeb from the Breakthrough Starshot Initiative that put forward a laser-propelled sail to carry a single gram-weight spacecraft towards the Alpha Centauri system — the home of the recently-discovered Proxima b.
One of the problems unaddressed by this early theoretical work was how to ensure a sail would remain centered on the Earth-based laser beam despite disturbances, misalignment, and manufacturing imperfections.
“It doesn’t matter how well you design the sail, there will always be fluctuations which will quickly amplify. Just the quantum nature of light produces these perturbations,” Loeb says.
For Starshot these perturbations need to be overcome without feedback sensors and hardware that would add to the mass of the craft.
To start investigating potential solutions a colleague of Prof. Loeb’s at both Harvard, and on the Starshot project committee, Zachary Manchester, analyzed the only shape proposed in those early papers, a conical sail.
However in a new paper posted on the ArXiv.org site, Manchester and Loeb show this geometry soon ends up tilting and being pushed out of the beam.
“It’s like a parachute propelling something with the friction of the wind,” Loeb says.
“However, with a parachute you have a person hanging below which focuses the center of the mass, keeping it stable. We can’t afford this extra weight.”
In proposing a new sail the researchers asked themselves what shape wouldn’t change if tilted by perturbations, and came up with a sphere.
“If you spin around or tilt a sphere it looks the same. This means it doesn’t amplify any problem,” Loeb says.
Loeb and Manchester constructed simulations that bounced light rays off a spherical sail to calculate the forces exerted and how it would react over time.
Based on these, they put forward their theoretical sail which consists of this spherical shell, like a ping pong ball but several meters across, which is trapped by sitting in a well.
“However, there may still be some challenges in the sail design to overcome if the Starshot project is to achieve the required acceleration,” says Pekka Janhunen, the inventor of the E-sail that harnesses the Sun’s ‘wind’ of charged particles.
“I don’t know how a thin spherical shell could withstand high acceleration without collapsing mechanically,” says Janhunen, a researcher at the Finnish Meteorological Institute.
“Unless you fill it with gas to support it, which would decrease performance.”
“Also the radiation load is very high for a fast-moving probe as all interstellar hydrogen atoms are seen as high-energy particles. At a fifth of the speed of light, such radiation would be intense enough to heat the device.”
Loeb has similar concerns about avoiding heating, however in his case it’s the energy of the laser beam that poses a challenge.
“The sail material needs to be an almost perfect mirror, reflecting 99.99% of the photons that reach it, otherwise it will burn up. I am still optimistic,” he says.
“We already have seen a design from industry that could in theory achieve this.”
The next step for Loeb and Manchester is to test their sail in a laboratory which they hope to do in the next 5-10 years.
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Zachary Manchester & Abraham Loeb. 2016. Stability of a Light Sail Riding on a Laser Beam. arXiv: 1609.09506
