Breakthrough Starshot: A Sail to the Stars
Water, water everywhere, nor any drop to drink…
When Samuel Taylor Coleridge wrote these words in the poem The Rime of the Ancient Mariner in the late 1700s, he probably wasn’t thinking about the depths of our cosmos. Rather, the idiom refers to the irony of being surrounded by something one needs, but somehow cannot make use of, like sailors being desperately thirsty while finding themselves in a sea of saltwater.
Space, in a way, is an infinite ocean. Looking up at the sky on a clear night – if you’re lucky enough to avoid light pollution – you are greeted with an overflowing sea of stars, twinkling enticingly with their siren song of mysteries. Stars and their planetary systems are like islands; as the oases of the universe, they harbor energy, warmth, and perhaps even life. Reaching them is vitally important; it could not only answer our most fundamental of questions – such as whether we are alone in the universe – but also ensure humanity’s very existence in the long term.
And yet, despite this heavenly kaleidoscope seeming near enough to grab, reaching even our sun’s closest neighbor – Proxima Centauri – is beyond our current technological capability. Stars, it seems, are everywhere, but not one is within our reach.
That might change soon. A project known as Breakthrough Starshot, announced in 2016, aims to ‘demonstrate proof of concept for ultra-fast light-driven nanocrafts’ that would travel at dizzying speeds to traverse the gap of four light years between the sun and Proxima Centauri (part of the Alpha Centauri star system); it could even conduct a flyby of Proxima b, a somewhat Earth-like exoplanet. If all goes to plan, the spacecraft would travel at speeds of up to 20% the speed of light, reaching Alpha Centauri in twenty years, with another four years for the information – which may even include pictures of the faraway world – to be beamed back to Earth.
The theory behind the project relies on a bafflingly simple method of propulsion: light. It uses something known as a light sail, which is not unlike the cloth ones found on ships. But instead of wind, photons – massless particles of light – hit these sails, which, ideally, would be over 90% reflective, if not more. As the photons bounce back off the sail, they pass their momentum onto the spacecraft, propelling it forwards. Though all photons travel at the speed of light, their momentum also depends on the level of energy, essentially meaning that a less energetic photon, such as an infrared, would result in less propulsion than, say, an ultraviolet one. This method produces very small amounts of thrust; as a result, unthinkable amounts of energetic light would be needed to accelerate a large spacecraft.
This idea of using light for propulsion is far from new; Johannes Kepler mused over the possibility in 1610, followed by many others, including Konstantin Tsiolkovsky, father of the rocket equation, as well as Carl Sagan. As such, light sails are not completely untested; as explained here, concepts such as the microwave-powered Starwisp spacecraft – the brainchild of science fiction writer Robert L. Forward – and the solar-powered Cosmos 1 employed the concept, but neither came to fruition (though the latter did launch, it never reached orbit). In 2010, the Japan Aerospace Exploration Agency (JAXA) succeeded in proving the concept’s viability with its IKAROS mission, which used a quadratic solar sail 20 meters diagonally across, but only 0.0075mm thick (about one-sixth of a sheet of paper).
But while the concept has been proven to work, the technology required for Starshot is in a league of its own. For one, as explained here, to reach desired speeds, it needs to save on mass; the reason for this is as simple as Newton's second law of motion, which states that an object’s acceleration depends both on the force exerted onto the object and on its mass. Therefore, the smaller an object is, the better it can accelerate; one spacecraft used for Starshot would, including its sail, only weigh two grams. This is made even trickier by the fact that Starshot will be powered not by the sun, but lasers; 100 gigawatts’ worth of lasers, to be exact. That’s about the equivalent of powering one billion 100 watt bulbs, and could take its toll on the delicate light sails. To minimize absorption of these high-energy photons, the sail’s reflectivity may have to be higher than 99.999%, something that has not yet been achieved. And though the light beam – consisting of an array of powerful lasers – will only need to hit the light sails for a few minutes to accelerate it to desired speeds, the spacecraft will likely encounter radiation and dust on its twenty year journey, which need to be accounted for in its design.
On the plus side, the brains of the spacecraft – the so-called StarChip – look a lot more promising due to extensive improvements in technology, allowing for much smaller and lighter computers. As stated on the project’s website, Moore's law – which predicts rapid technological developments in that doubling of transistors on microchips occurs roughly every two years – ‘creates the possibility of a gram-scale wafer, carrying cameras, photon thrusters, power supply, navigation and communication equipment… constituting a fully functional space probe’. Though these ambitions seem lofty, they are far from impossible to achieve; one-gram computer chips containing many of these features already exist (such as in an iPhone). In terms of space, precursors to the StarChip – known as Sprites – were launched into Low Earth Orbit in 2017, hitching a ride on some satellites and performing as expected. This feat was followed by a larger deployment in 2019, and while the satellites (intentionally) burned up in the atmosphere several days after their release, they proved that the concept works.
Clearly, while the initial design is there, interstellar travel is no walk in the park, and several significant obstacles are yet to be overcome; the project lists these challenges on its website, encouraging anyone to pitch in with ideas. Ideas, however, do not seem to be in low supply, given that the project was founded by Facebook’s Mark Zuckerberg, Russian-Israeli billionaire Yuri Milner, and Stephen Hawking; other leaders include executive director Pete Worden (former Director of NASA Ames Research Center), chairman Avi Loeb (chair of the Harvard Astronomy department) and Nobel Prize winner Steven Chu, among many others. Milner initially invested $100 million into the project – which, as of now, is still a proof-of-concept mission – predicting that it could cost up to $10 billion, which might come from outside investors such as NASA or ESA.
According to Professor Loeb, most of that money would go towards the ‘kilometer-scale’ array of lasers that would send the spacecraft on their journeys (as well as the solar cells that would ideally power it). Apart from that, he estimates the actual spacecraft would only cost around $100 each, remarking that once the laser infrastructure is in place, it would make sense to send some probes out every day; not only would this ensure greater chances of success, but it could also allow for more missions within our own solar system, made easier by the system’s speed (it would, for example, only take around five days to reach Pluto). As stated on the project’s site, thousands of the craft would be launched on a ‘mothership’ and deployed from there; due to the rise in private space companies, which result in an ever-increasing frequency of launches, this is certainly possible.
Whether or not the rest of the many hurdles will be overcome by the 2036 launch date predicted by Milner during the project’s announcement remains to be seen. Given the sheer audacity of the idea and the unprecedented challenges it poses (it’s already a struggle to send things around our own solar system), the fact that it is being considered at all is already quite the achievement, owed to its star-studded society of supporters; if it weren’t for them, the mere possibility of interstellar flight might have remained confined to the pages of science fiction. Why, then, do the greatest minds of astrophysics throw their support behind something so (literally) outlandish?
Well, we have to start somewhere. When it comes to the search for extraterrestrial life, there is only so much we can do from here: look for signals, keep an eye out for UFOs, or use telescopes such as James Webb to analyze the atmospheres of exoplanets. But actually traveling to the stars is a different story.
It’s far from a guarantee that the planet Proxima b – whose parent star, Proxima Centauri, has just 12% of the mass of our sun – hosts any life, or is even theoretically able to sustain it. From what we know so far, it orbits within the habitable zone of its star, but is tidally locked, meaning one side is constantly facing its sun – which, as it happens, might have a habit of pelting the planet with radiation from solar flares, tearing apart its atmosphere.
Finding intelligent life would be groundbreaking (when asked what that would look like in 2016, Hawking theorized that ‘judging by the election campaign, definitely not like us’). But even if Proxima b is less than idyllic for life, its proximity to Earth counts for something. As explained here, we simply do not know which percentage of star systems harbor intelligent life – we haven’t even found one form of it outside of Earth – so we cannot assign distinct probabilities to individual systems. By that logic, Proxima has as good a chance of supporting life as any star we can see; again, we have to start somewhere.
Going there increases our realm of awareness from one star system to two; who knows if, on the off chance that someone out there wanted to reach us, an alien civilization sent a signal to Proxima instead? It is simply the most reasonable place to start looking, habitable or not, and when we go, we’ll know a little more about whether we are indeed alone. Whether or not that is the case is something we may not know for a while yet, but either scenario will forever change our perception of ourselves, the universe, and life itself, going far beyond the nerdy niche of astronomy.
But fundamental questions aside: it is a fact that one day, the sun will die. And even before it does, it will increase in brightness by about 10% in the next billion years, shifting the habitable zone from Earth to Mars. Hawking, one of the founding members of Starshot, believed that avoidable, human-caused problems such as climate change or nuclear warfare (as well as an asteroid strike, which humans can also theoretically prevent), could all wipe out human civilization on Earth within the next thousand years. Staying on Earth and keeping all of one’s eggs in one basket means that when that happens, all life (that we know of) could be permanently extinguished. ‘Spreading out,’ Hawking said, ‘may be the only thing that saves us from ourselves’.