Visiting Nearby Stars Using Tiny Sails

What do Stephen Hawking, Mark Zuckerberg and entrepreneur Yuri Milner have in common – aside from being very rich? They want to build a bunch of tiny spaceships and sail them to nearby stars. They call their project Breakthrough Starshot.

Depiction of a “heliogyro” solar sail made up of multiple blades kept taught by rotating them

What? How do You Sail in Space?

The concept of a solar sail has been around since the earliest notions that space even existed. Johannes Kepler saw that the tails of comets always pointed away from the Sun in the early 1600s. He surmised that the Sun was exerting some sort of “heavenly breeze” and that ships and sails could be adapted to travel using it. By the late 1800s, physicists knew that light exerted pressure – you’ve probably seen it in action in one of those cool, light bulb looking gizmos sitting on someone’s desk at work that has a sort of weather vane with one side of each vane white and the other dark. They spin from the pressure of the light reflecting more off the white sides than the dark.

IKAROS Spacecraft

By the early 20th century the concept of using that pressure to propel a spacecraft to other planets was introduced. It wasn’t until 2010, however, that a spacecraft using a solar sail as propulsion was put into operation. The Interplanetary Kite-craft Accelerated by Radiation of the Sun, or IKAROS, was launched by the Japanese space agency JAXA. It “sailed” past Venus then put itself into an orbit around the Sun.

Can a Solar Sail Really Travel to Another Star?

Solar sails are under constant acceleration – very low acceleration – from the Sun’s rays hitting their reflective sail panels. This allows them to potentially get up to high speeds, albeit very slowly. The problem is that the farther you get from the Sun, the less light there is, and thus the lower pressure there is. Once a solar sail gets a certain distance away, it essentially stops going any faster. This is where the tiny solar sails and small, light weight, little ships come into play. The idea is that instead of using the light of the sun to accelerate, they’d use lasers aimed directly at them. The concentrated beam would exert a lot more force than the dispersed rays of the Sun and get those little ships going fast.

How Tiny is Tiny and How Fast is Fast?

The little robotic ships that the Breakthrough Starshot team envisions are small – really, really small. They call them nanocraft and believe they can be brought down to “gram-scale.” The sails would be “meter-scale” and made of nano materials. The lasers that would push them would be set up in a multi-laser array and scaled up to the 100 gigawatt range – that’s a lot. This would allow them to accelerate the ultra-light weight ships to their target speed within minutes. Hawking, Zuckerberg and Milner hope to have that speed near 20% of the speed of light, or 100 million miles per hour. They could reach the nearest star, Alpha Centauri, in about 25 years. That is about 1000 times faster than more conventionally powered spacecraft could hope to do the same feat.

What are the Technical Challenges?

On the surface it sounds pretty straight forward: build a tiny ship with a little sail and shoot a laser at it. As Facebook reader Bob Waldron brought up, building a tiny ship that can actually communicate back over trillions of miles is a big hurdle. Additionally, if the intent is to accelerate each ship to 20% of the speed of light in “minutes” then the sail must be immensely strong. Assuming “minutes” means more than one and less than 60, the g-force a ship would experience would be anywhere from about 1250 to 70,000! We’ll assume it will be on the lower side. Lastly, that 100 gigawatt laser array they want to use to accelerate the ships is, well, a really big, powerful laser – and it’s getting shot at a little ship. Even though most solar sails are more than 90% efficient, it still means that 10% is getting absorbed – that’s 10% of 100 gigawatts which is 10 gigawatts. Remember that Doc Brown in Back to the Future used a bolt of lightning to generate the 1.21 gigawatts of energy necessary to power the flux capacitor – just 1.21! As it turns out, a bolt of lightning actually gets into terawatts, but 10 gigawatts is still a lot. Those little spaceships need to be pretty robust.

What do They Hope to Accomplish?

Scientists give a fair chance that there is an exo-planet orbiting Alpha Centauri. To be able to get a probe, albeit a really small one, into that star system so it can take pictures and collect other data and send it back would be one of the greatest scientific feats of all time. The fact that it could take place within our lifetimes is nothing short of amazing.

“Earth is a wonderful place, but it might not last forever. Sooner or later, we must look to the stars. Breakthrough Starshot is a very exciting first step on that journey.”

Stephen Hawking

Image credit: Breakthrough Starshot, JPL and JAXA