Space lasers and light sails: the tech behind Breakthrough Starshot

Last year Russian entrepreneur Yuri Milner invested $100 million in the search for alien life. What’s next? Going there, of course.

Alongside famed cosmologist Stephen Hawking in New York, Milner yesterday unveiled Breakthrough Starshot, a plan to send tiny space probes to our nearest star system and check out its planets. The "Starchips" would be carried on light sails propelled by Earth-based lasers in just 20 years, by travelling at a fifth the speed of light.

“Today, we commit to this next great leap into the cosmos,” Hawking added at the announcement. “Because we are human, and our nature is to fly.”

The most astounding part of this idea is that it’s actually feasible. That’s not to say it’s easy – or even sensible – but we may well see it happen, based on existing or extrapolated technology in the coming decade.

The target star system, Alpha Centauri, is around 4.4 light-years away. This means, if a spacecraft travelled at the speed of light, it would take 4.4 years to arrive.

We're nowhere near accelerating a probe to those speeds. Instead, a fifth the speed of light should get a craft across in 20 years or so. But this calls for a dramatic change in design philosophy for a spacecraft.

To date, they carry fuel or propellant that is expelled from rockets (thanks to Newton’s third law, this gives an opposite and equal force in the forward direction).

To go faster you need more fuel. That fuel, though, adds mass to the spaceship, and so it snowballs: you need to carry more fuel to move both it and the spaceship.

Instead, you can use a light sail to reflect a “wind” of photons (particles of light) that bounce off and transfer momentum.

The idea that light can push a spacecraft, much less to 60 million metres a second, seems fanciful. You need to boost the power of the light source to brighter than the Sun and shrink the mass of the spacecraft.

Sure enough, these are the next steps.

Perhaps the most challenging idea of Starshot is constructing a laser powerful enough to focus enough photons on the light sail, so the spacecraft can be accelerated to 20% the speed of light.

Can we build a laser powerful enough to rapidly accelerate the sail to that speed? Starshot suggests an array of lasers, stretching a kilometre across, of total power 100 gigawatts – akin to a blinding billion 100-watt globes – which will act as a giant light “fan” to the light sail.

 

We already have lasers at the National Ignition Facility in California that can focus 500 terawatts of laser power on a point and initiate nuclear fusion. But the blast lasts for less than a microsecond.

To accelerate a Starchip, lasers will need to fire for 10 minutes, transmitting a trillion joules – akin to the energy of a whole Space Shuttle launch – focused on a four-metre-wide sail.

And the lasers will need to stay focused on the light sail for the entire boosting phase of two million kilometres. Such a target at that final distance would appear just 0.4 milliarcseconds across the sky. This is about the size of a DVD on the Moon as seen from Earth.

How much is this laser going to cost? At the moment, laser power costs $10 per watt, but as current usage is driven by telecommunications industries, this cost is halving every 18 months. In a decade, Starshot’s array may cost $10 billion, around the cost of one US aircraft carrier.

Let's not forget the hardware

A tremendous design challenge is constructing a highly reflective light sail just atoms thick. If even a tiny fraction of the laser energy were absorbed, the sail would be destroyed. (That same destructive potential may make governments nervous about allowing such a facility to be constructed.)

But all of the power delivered to the spacecraft will count for nothing if the probe itself is too massive.

Starshot plans to develop a new style of probe that weighs only a gram. A Starchip will include a camera, electronics to transmit and receive signals and a power supply – perhaps a slowly decaying radioactive source such as americium, which is found in home smoke alarms.

One issue, though, is by travelling at a fifth of the speed of light to reach the star in our (and Milner’s) lifetime, the craft will have only minutes to explore the alien solar system before flying by.

As the New Horizons flyby of Pluto showed us, this sort of mission can still be incredibly valuable, even though it would be wonderful to slow down and explore at leisure.

Will Breakthrough Starshot let humanity prove itself an interstellar civilisation? Only time will tell – although the wait may be shorter than we could have dreamed before yesterday's announcement.

Further reading:
Antimatter to ion drives: NASA's plans for deep space propulsion
Launching a solar sail
NASA begins testing solar wind sail technology