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IF YOU left Earth now, travelling at the speed of light, you would get to the moon before reaching the end of this sentence. Getting to the sun itself would take 8 minutes at this speed. The furthest tendrils of human activity, Voyagers 1 and 2, which launched in 1977 and are only now reaching the outer edge of the solar system, would be overtaken by this time tomorrow. But getting to Proxima Centauri, our solar system鈥檚 nearest star, would take four years and three months.

And that is travelling at light speed, a velocity well beyond our reach. The quickest we could currently get to Proxima Centauri, using our fastest rockets, is 80,000 years. Small wonder interstellar travel hasn鈥檛 been much of a priority. But what if we could get to the Proxima system in 20 years?

At a highly publicised press conference in 2016, a team claimed to have assembled the scientific know-how to make a mission to Proxima Centauri not only possible, but doable within our lifetimes. Breakthrough Starshot, backed by a Silicon Valley billionaire and tapping into NASA expertise, provoked mostly cautious enthusiasm. Three years later, with a better sense of the challenges and to support the team鈥檚 optimism, the plans are gathering speed. If they succeed, we could be a decade or two away from embarking on the most ambitious mission of all time, and discovering the truth about a solar system different from our own. So what are the key challenges?

Getting to a distant solar system within a human lifetime means travelling fast. Very fast. And the pool of available technologies capable of propelling a spacecraft at a significant fraction of the speed of light is pretty limited.

For decades, there has been only one serious candidate. 鈥淭here鈥檚 no alternative to using light as a fuel if your mission really is to go interstellar,鈥� says of the California Institute of Technology and a member of Breakthrough鈥檚 advisory committee. 鈥淎 few other options, like fusion drives, have been discussed, but aren鈥檛 on the table.鈥� Just as incoming wind can propel a boat by exerting pressure on a sail, beams of light can also drive motion. Make a spacecraft light enough, strap a sail to it, point a powerful light source at it and out in the vacuum of space it could rack up some serious speed.

Solar sails, designed to harness the light of the sun, have been kicking around scientific doodle pads for centuries. Johannes Kepler is credited with inventing the idea in a letter to Galileo. But powerful as the sun鈥檚 rays may be, they can鈥檛 propel anything to near light speed. For that, you need lasers capable of illuminating a given point with millions of times more energy than sunlight alone.

鈥淲e all have these existential dreams in life. What he said was, 鈥榮how me the way鈥欌��

In 2009, had just started a new project at the University of California, Santa Barbara. His primary goal was to design giant laser arrays that could protect our planet by blasting incoming asteroids into smithereens. But he soon recognised their potential for space travel at 鈥渞elativistic velocities鈥�, those approaching the speed of light. 鈥淟iterally within a week,鈥� says Lubin, 鈥渃ame the idea that one could use them for relativistic propulsion.鈥�

By 2014, Lubin had secured funding from NASA to work on that idea full time. Doing the rounds of conferences, he had a chance to speak to Pete Worden, former head of NASA鈥檚 Ames Research Center in Silicon Valley. Worden was interested in Lubin鈥檚 work, and asked him to send over a paper he had been working on. 鈥淧ete emailed me and asked if he could send it to a friend,鈥� says Lubin. 鈥淚 said sure, send it to whoever you want.鈥�

That friend was , a Russian-born billionaire who had recently appointed Worden as head of a new foundation designed to tackle some of humanity鈥檚 biggest challenges. One of those was to travel to another solar system. The mission would eventually be called Breakthrough Starshot, and with Lubin on board Milner had the missing piece needed to pursue his goal. 鈥淲e all have these existential dreams in life,鈥� says Lubin. 鈥淲hat he said to me was 鈥榮how me the way鈥�.鈥�

Shoot for the stars

In April 2016, the project was officially launched. At the One World Observatory in downtown Manhattan, in the presence of physicists Stephen Hawking and Freeman Dyson, and astronaut Mae Jemison, Milner announced the mission to Proxima Centauri.

Lubin鈥檚 plan was breathtaking in its ambition. Hundreds of tiny spacecraft, each equipped with a light sail and the minimum amount of hardware needed to record and transmit information, would be deployed in orbit. An enormous array of lasers on Earth鈥檚 surface would then accelerate them to about a fifth of the speed of light 鈥� some 60,000 kilometres per second 鈥� a velocity thousands of times more powerful than anything experienced by conventional spacecraft. Twenty years later, whichever of the craft had successfully navigated the debris-strewn obstacle course of outer space and reached Proxima Centauri would beam back images of the star system and any potentially habitable planet as they zipped past.

For Avi Loeb, an astronomer at Harvard University who chairs Starshot鈥檚 advisory committee, there is no reason the project shouldn鈥檛 work. 鈥淣othing contradicts the laws of physics,鈥� he says.

That鈥檚 not to say it will be easy. The initiative鈥檚 researchers have to engineer a fabric that can absorb gigawatts of laser light without spontaneously combusting. They have to build electronics that are light enough to cross interstellar space, but sophisticated enough to beam images back across 4 light years. They need to ensure that the lasers on Earth are accurate enough to hit the small light sails accelerating away from Earth at immense speeds and that the spacecraft arrive on target. And then, of course, they have to actually construct this power source. 鈥淲hat we鈥檙e calling for is a laser system that鈥檚 the largest ever built by humans,鈥� says Atwater. As Loeb points out, governments may not be best pleased if what are effectively superweapons suddenly appear on their doorsteps.

But Loeb considers political considerations a bridge to be crossed sometime in the future. 鈥淲e want to focus first on the challenges in terms of the physics,鈥� he says.

The team鈥檚 first priority is the light sail. Most of the necessary technical specifications are already laid out, and the challenge will be finding or making a material that fits the bill. The current plan calls for a fabric that covers maybe 10 square metres, yet weighs no more than 1 gram. That means a thickness of less than 100 nanometres, tens of times thinner even than spider silk. Producing such gossamer-like membranes is perfectly achievable. Ensuring they survive a concentrated laser blast, though, is something else entirely.

鈥淭he Starshot light sail needs to reflect 99.999 per cent of light hitting it鈥�

The key is for the material to absorb almost none of the incoming light. 鈥淵ou鈥檙e trying to build something that acts like a mirror,鈥� says Atwater. But whereas the shiniest metals reflect around 99 per cent of the light that hits them, the Starshot light sail needs to reflect more than 99.999 per cent. Finding a material with the right combination of low density, high reflectiveness and low absorption isn鈥檛 simple. Two promising candidates appear to be silicon dioxide, also known as silica, and a material called molybdenum disulphide that can be manufactured in sheets only an atom thick. A third candidate is diamond, says Loeb. 鈥淚magine having a big diamond in the sky and pushing it with a laser beam. That would be quite remarkable.鈥�

As Starshot鈥檚 light sail specialist, Atwater is trying to encourage research on these and other materials in the hope of identifying the ideal substance. 鈥淏oth in our lab and others around the world, people are tooling up to build experiments to test these materials,鈥� says Atwater. 鈥淲ithin the next year or so, we鈥檒l have results.鈥�

Progress is also being made on building the laser array. According to Lubin鈥檚 2016 road map, the Starshot mission will need a system capable of generating around 60 gigawatts of power. That鈥檚 equivalent to 20 nuclear power plants. By comparison, a standard laser pointer racks up 5 milliwatts and a laser beam strong enough to dazzle pilots needs only a few watts of power.

The system planned for Starshot would be capable of pushing asteroids off course. Applied to a minuscule spacecraft with a laser sail, it could accelerate it to a fifth of the speed of light within 10 minutes, allowing it to coast past Mars 20 minutes later, pass Pluto within 7 hours and get to Proxima Centauri in two decades.

The remarkable thing about making a laser of this scale is that the technology already exists. 鈥淭he problem is not producing lots of power,鈥� says Lubin, it is the difficulty of achieving what is known as parallelisation. Rather than building one enormous laser, the goal is to create an array of weaker lasers and thread their power together. 鈥淚n the past, that was demonstrated with one pair of beams,鈥� says Loeb. The challenge is to do that with several million.

If this effort starts to come together, the team plans to move on to building the spacecraft. Known as StarChips, these tiny craft will be marvels of engineering. Weighing in at less than a gram, they will need an on-board power source, miniature thrusters for course correction, cameras for documenting their destination and a powerful transmitter to beam those images back to Earth.

Existential significance

Combining all that tech on a tiny spaceship designed to be flung across interstellar space is no mean feat. 鈥淲e have gram-scale spacecraft, but right now they鈥檙e at the Sputnik level of capability,鈥� says , a specialist in nanosatellite design and another member of Starshot鈥檚 advisory committee. At the moment, he sees the long-distance communication back to our planet as the major hurdle, potentially requiring a massive ground station on Earth coupled with a high-powered transmitter on the tiny spacecraft.

When asked how long he thinks it might take to get a fleet of mini satellites ready for launch, he hedges his bets. 鈥淚 think that the spacecraft end of it is a lot easier than the laser end,鈥� he says. 鈥淚 would say a decade, maybe, is the timescale over which you could figure it out.鈥�

If the team succeeds, by 2030 all the various technologies will be sufficiently developed for the feasibility of the project to be experimentally demonstrated. Once that is done, an initial prototype must be built, ahead of the full system. The mooted price tag? $10 billion. The ideal timeline? Twenty years. 鈥淭his is an enormous project on the scale of an Apollo-like programme,鈥� says Lubin.

Those outside the core team are cautious in their criticism. 鈥淚 don鈥檛 see a fundamental limit that would make it impossible,鈥� says , a space technology engineer at the University of Strathclyde, UK, 鈥渂ut the challenges are, well, of note.鈥�

, who works with Macdonald, highlights the extraordinary specifications of the proposed laser system, which will need to generate five times more power than the largest power station on Earth. 鈥淚t鈥檚 something that I personally would love to see,鈥� she says, 鈥淚 guess my question is: is it worth it?鈥� Rather than rushing ahead with such an ambitious mission before we know it can succeed, she suggests waiting until the necessary technology is further along. 鈥淭here鈥檚 certainly an argument to be said that maybe we should be putting money into things that really help people on Earth,鈥� she says.

Loeb has little patience for such arguments. 鈥淎 lot of human activity is based on self-fulfilling prophecies. If you tell yourself that you can鈥檛 do something, then you will never do it,鈥� he says. 鈥淒on鈥檛 put chains on your feet and you will reach very far away.鈥� He regards the project as being of almost existential significance. 鈥淲e cannot stay forever on this planet,鈥� he says. 鈥淭he only question is the timescale.鈥�

鈥淥nce we get into interstellar space we will get a message saying 鈥榳elcome鈥欌��

The 2016 discovery of an exoplanet orbiting Proxima Centauri 鈥� making it the nearest one to Earth 鈥� added to the excitement surrounding the project. Later research indicated that the star is prone to nasty outbursts of X-rays and UV radiation, with its planet, Proxima b, unlikely to be habitable. But even if it does turn out to be unfit for humans, the chance of exploring any neighbours it might have is too good to miss.

And then, of course, there is the possibility of encountering alien life (see 鈥�First shots in an interstellar war?鈥�). Once outside the confines of our solar system, Breakthrough鈥檚 spacecraft will be well placed to spot the signatures of any other spacefaring civilisations. 鈥淢y personal belief is that there鈥檚 a lot of traffic,鈥� says Loeb. 鈥淚 would think that once we get into interstellar space, we will get a message saying 鈥榳elcome鈥�.鈥�

Even if that prediction fails to come about, the mission will be a vital first in the history of space travel. 鈥淧eople are really called by grand challenges,鈥� says Atwater. 鈥淭his is a really exciting grand challenge.鈥�

鈥淚t鈥檚 hard not to get excited about something like this,鈥� says Manchester. Even for those not involved, like Macdonald, the mission could yield astonishing benefits. 鈥淚 see the point of a moonshot as being as much about what it inspires en route as the destination itself,鈥� says Macdonald. 鈥淵ou can never be sure where you will end up and we should embrace the journey.鈥�

First shots in an interstellar war?

On the list of things worth worrying about, you might not think that stumbling into an interstellar war with aliens would be too near the top. But according to many luminaries, from Stephen Hawking to and science-fiction writer Cixin Liu, it represents a real possibility.

The argument centres on the danger that human activity in interstellar space could alert unfriendly aliens to our presence and to Earth鈥檚 location. Some believe that Breakthrough Starshot itself could be regarded as an act of war: a tiny armada crossing the empty space between the stars with evil intent. For , president of San Francisco-based METI International, which seeks to communicate with extraterrestrial intelligence, the notion is 鈥減reposterous鈥�. If there are advanced civilisations where we are headed, he says, they won鈥檛 be able to detect and verify the nature of the tiny craft. He compares it to 鈥極umuamua, a mysterious recent arrival in our solar system that sparked widespread debate about whether it was an asteroid or a solar sail.鈥�

鈥淚f there are any technologically savvy inhabitants of Proxima b, they won鈥檛 make first contact with Earthlings by detecting nanoprobes whizzing past their star system at a fifth the speed of light,鈥� says Vakoch. Indeed, any advanced aliens probably know about us already, from decades of TV broadcasts that have leaked into space. Not only is Vakoch not worried, he is keen to reach out directly. He thinks we should actively signal to potential aliens, using the laser array as a beacon to transmit messages. 鈥淢y dream is that once Starshot鈥檚 lasers are up and running, these Earth-based light beams will propel the light sails,鈥� he says, 鈥渨hile also bearing our encoded messages to any extraterrestrials at the other end.鈥�

We come in peace

thinks humanity isn鈥檛 yet developed enough to take on a transmission project. 鈥淎ny finite transmission will pass over the intended recipient for a finite time and they must be looking at us in just the right way during that finite time, or the message will be missed,鈥� she says. Tarter, who was the inspiration for , says messaging must be a long-term strategy, so that when a civilisation starts to explore their sky, the message will be waiting for them to discover. 鈥淚t鈥檚 going to be a while before we can execute on 10,000 or 100,000-year plans, but when we鈥檝e managed to become an advanced technological civilisation ourselves, then we should transmit.鈥�

Liu, whose Remembrance of Earth鈥檚 Past trilogy deals with the consequences of contact with aliens from Proxima b, sees the value of searching for extraterrestrials, but warns of stumbling into a crisis. 鈥淲e are not an advanced civilisation,鈥� he says. 鈥淲e are like toddlers, and even toddlers can cause big problems.鈥� Rowan Hooper