Forget the Starship Enterprise, the Battlestar Galactica, the Millennium Falcon and the Jupiter 2. In all likelihood, the first spacecraft to make the voyage from our solar system to neighboring stars won’t be big gleaming vessels under the command of heroic astronauts. This old-school space-opera scenario has inspired some great science fiction, but it ignores the fundamental challenge of interstellar travel: Even the stars closest to our sun—the three stars in the Alpha Centauri system—are more than 25 trillion miles away. Without warp drive or wormholes to shorten the journey, it’s going to take a long, long time to get there.
On Tuesday, Russian billionaire and science-minded philanthropist Yuri Milner held a press conference in New York City to propose a more realistic strategy for interstellar travel, one that takes advantage of Isaac Newton’s second law of motion. You might remember this formula from high-school physics: force equals mass times acceleration. A spacecraft would need to accelerate to about twenty percent of the speed of light to travel to Alpha Centauri in twenty years. Propelling a typical NASA probe to such high speed is beyond the capabilities of any foreseeable technology. The spacecraft are simply too massive. But the task would be a lot easier if the craft could fit in the palm of your hand and was as light as a penny.
The concept of a lightweight interstellar probe isn’t new; in fact, I edited a story about the idea for Scientific American in 1999 and more recently I wrote a science-fiction novel about it. But over the past 15 years, rapid advances in microelectronics have made it possible to pack all the essential components of a spacecraft—radio, camera, power supply, and so on—into a chip that weighs less than a gram. (Milner calls it the StarChip.) Best of all, this tiny craft wouldn’t need to carry a bulky propulsion system or a heavy load of fuel; instead, it could be attached to an ultrathin lightsail and propelled by a powerful laser beam.
Under Milner’s plan, thousands of lasers arrayed at a site on the Earth’s surface would fire in precise synchrony so that their outputs can merge into a single beam shooting upward through the atmosphere with a power of 100 billion watts. The beam would focus on the lightsail of a StarChip previously launched into orbit. If the sail reflects the laser light instead of absorbing it, then the force delivered by all the colliding photons could swiftly accelerate the lightsail to twenty percent of light speed, propelling the spacecraft hundreds of thousands of miles in a mere two minutes. After that point, the craft would be so distant from the Earth that the laser beam could no longer push the craft, but the StarChip would already be cruising at 134 million miles per hour toward whatever star system the laser had targeted. And if the lightsails and StarChips can be manufactured cheaply enough, they could be launched into orbit by the hundreds, and the laser array could accelerate a new one toward the stars every day.
Milner announced at his conference that he would devote $100 million of his Internet fortune to preliminary research for this plan, which he calls Breakthrough Starshot. (Last summer, Milner committed an equally enormous sum to Breakthrough Listen, a ten-year effort to hunt for signals from intelligent extraterrestrials.) He gathered an all-star cast of scientists and space enthusiasts to express their support for Starshot, including famed physicists Stephen Hawking (who has warned about the dangers of contacting extraterrestrials) and Freeman Dyson (who has studied proposals for interstellar travel since the 1950s). Starshot’s director is Pete Worden, who formerly headed NASA’s Ames Research Center and will be in charge of awarding Milner’s funds to scientists involved in the project.
The technical challenges are enormous, of course. Researchers would have to learn how to keep the laser beam tightly focused as it punches through the atmosphere. The lightsail and StarChip would have to be sturdy enough to withstand the sudden acceleration, which would be equivalent to 60,000 g-forces. But the biggest problem might be ensuring that these tiny spacecraft can do some useful exploration once they finally reach their destinations. There’s no way to decelerate the StarChip, so the spacecraft will zoom past its targeted star system in a matter of hours. The craft might be able to slightly adjust its trajectory, but it won’t have much time to photograph the planets, asteroids and comets orbiting the star. And then there’s the challenge of transmitting the data back to Earth.
The ultimate cost of Starshot would surely run into the billions, more than Milner can pay for, which means the project would eventually need other sources of funding. But once researchers begin to design the lightsails and StarChips and laser arrays, NASA and other space agencies might find the technology appealing because they could use it for interplanetary missions as well as interstellar voyages. A laser-propelled StarChip could reach Mars in hours and Pluto in days. And swarms of the tiny spacecraft could work in concert to explore our solar system.
But interstellar travel will require more than just cutting-edge research and ample funding. To reach the stars, we’ll need to rethink the very nature of space exploration. For example: when an interstellar probe approaches a distant star system, it won’t be able to communicate with the mission controllers on Earth in a timely fashion, because it would take years for a signal to travel back and forth. The craft would have to work autonomously as it observes the planets and asteroids and comets in the star system. It would have to make its own decisions about the best targets to explore during its brief flyby, and therefore it would require some kind of artificial-intelligence system.
So here’s my take on Breakthrough Starshot: We need to adapt our exploration strategies to the vast distances of interstellar space. For the foreseeable future, Earth’s emissaries to the stars won’t be human. Our galactic explorers will be our machines and AIs.