Here on Centauri Dreams we often discuss interstellar flight in a long-term context. Will humans ever travel to another star? I’ve stated my view that if this happens, it will probably take several hundred years before we develop the necessary energy resources to make such a mission fit within the constraints of the world’s economy. This, of course, assumes the necessary technological development along the way — not only in propulsion but in closed-loop life support — to make such a mission scientifically plausible. I get a lot of pushback on that because nobody wants to wait that long. But overall, I’m an optimist. I think it will happen.

Let’s attack the question from another direction, though, and leave human passengers for a later date, as Yuri Milner’s Breakthrough Initiatives, aided by Stephen Hawking, is doing today in a New York news conference. What if we talk about unmanned missions? What if, in fact, the question is: How soon can we put a scientific payload past another star? Let’s not worry about decelerating — this will be a flyby mission. Let’s build it as soon as possible using every breakthrough technology we have at our disposal. How long would it take for that mission to be developed and flown?


Milner, a philanthropist and investor who was an early backer of Facebook, Twitter, Spotify and numerous Chinese tech companies, tells me his goal is to ‘give back to physics’ in developing just such a mission. Part of that giving back is the $100 million he has already put forward to support SETI, a ten-year project that will produce more telescope time for SETI than any other. Milner is also the founder of the Breakthrough Prize, issuing awards in physics, life sciences and mathematics. But in many respects this third Breakthrough Initiative is the most daring of all.

Time for the Stars

Breakthrough Starshot is an instrumented flyby of Alpha Centauri with an exceedingly short time-frame, assuming research and development proceed apace. Milner is putting $100 million into the mission concept, an amount that dwarfs what any individual, corporation or government has ever put into interstellar research. A discipline that has largely been the domain of specialist conferences — and in the scheme of things, not many of those — now moves into a research enterprise with serious backing.

Could an Alpha Centauri flyby mission be developed and launched within a single generation? I think it’s quite a stretch, but it’s the best-case scenario Milner mentioned in a phone conversation over the weekend. He’s enough of a realist (with a first-rate physics background) to know that the challenges are immense. Even so, he sees no deal-breakers.

Let’s walk through the case and see why he finds reason for optimism. “There are major advances that we can now turn to as we develop this proof of concept,” Milner says. “Twenty years ago, none of these things would have been available to far-thinking scientists like Robert Forward. But now we can put them to use and test their possibilities.”

If you’re thinking of an interstellar mission in the near-term, there is really only one choice of propulsion: The beamed sail. Sails have the advantage of known physics, laboratory experiment and actual deployment in space. We could talk about fusion for some indefinite point in the future, but at present, we don’t know how to do fusion even in massive installations on Earth, much less in the tight confines of a spacecraft engine. Interstellar ramjets are a far-future unknown — they may act more effectively as braking devices than engines, according to recent research. Antimatter is nowhere near readiness for propulsion, either in production methods or storage. Chemical rockets fall victim to the mass/ratio problem and are useless for fast interstellar journeys.

That leaves us with sails carrying very small payloads. To cross the 4.37 light years to the Centauri A and B system, Breakthrough Starshot proposes small spacecraft, taking advantage of advances in nanotechnology to reduce payload size. Think Moore’s Law and the reductions in size and cost that have accompanied the vast increases in micro-chip power. “Moore’s Law,” says Milner, “tells us that now is the time.”

StarChip is the Breakthrough Initiatives’ name for a payload measured not in kilograms but grams, a wafer that carries everything you would expect in a fully functional probe. ‘What was once a 300 gram instrument is is now available at three grams,” Milner continues. “What was 100 grams is now 0.5 grams. This is the trend we are riding.”

The StarChip payload includes cameras, power supply, communications equipment, navigation capabilities and photon thrusters. And it would be thrown across the interstellar gulf at 20 percent of the speed of light by a sail that is itself a miniaturized version of the sails Robert Forward used to discuss. Forget the thousand-kilometer sail (much less the continent-sized sails of the science fiction dreamer Cordwainer Smith). Milner’s team believes we can now talk in terms of a laser-driven lightsail that is no more than 4 meters across. This is actually smaller than the first deployed sail craft, the Japanese IKAROS, which boasts a sail measuring 14 meters to the side.

Advances in metamaterials and additional research should be able to produce, Milner believes, a 4 meter sail whose own weight is tallied in grams, and whose materials allow fabrication at a thickness of a few hundred atoms. A sail that small makes its own statement: Clearly, it’s not going to be under the beam for long, which means we need to focus a great deal of light on it for a very brief time. Lasers are another technology that benefits from rising power and falling cost. The trick here will be to create ‘phased arrays’ of lasers that can scale up to the 100 gigawatt level. A phased array involves not one but a group of emitters whose effective radiation pattern is reinforced in the desired direction by adjusting the phase of the signals feeding the antennae.

This is classic Bob Forward thinking rotated according to the symmetries of our new era. Milner aims for a beamer technology that is modular and scalable. And it fits into a larger infrastructure. Breakthrough Initiatives talks about bringing a ‘Silicon Valley approach’ to the problem of interstellar flight. Build a StarChip that can eventually be mass-produced at no more than the cost of an iPhone. For the Alpha Centauri mission, whenever it flies, is itself a proof of concept that could lead to multiple destinations. And if the cost can be driven as low as Milner believes, then we can think in terms of redundancy, with StarChips sent in large numbers to return a full characterization of any destination system. Assemble the light beamer and, as the technology matures, the cost of each launch falls.

These are ideas that are at once familiar but also exotic, for while Forward talked about enormous power stations in close solar orbit to power up his banks of lasers (and a huge Fresnel lens in the outer system to focus the beam), Milner thinks we can build a ground-based beamer at kilometer scale right here on Earth. I was startled at the idea — surely efficiency favors a space-based installation — but Milner’s point is that he thinks we can begin to launch interstellar craft before we have the technology to build the kind of power station Forward envisioned. If you’re serious about a launch within a few decades (again, it’s a best case scenario, and a dramatic one), then you build an Earth-based beamer and use adaptive optics to cancel out atmospheric effects.


Image: A wide-field view obtained with an Hasselblad 2000 FC camera by Claus Madsen (ESO), of a region around the Southern Cross, seen in the right of the image (Kodak Ektachrome 200, 70 min exposure time). Alpha Centauri is the bright yellowish star seen at the middle left, one of the “Pointers” to the star at the top of the Southern Cross. Although it appears here as a single ‘star,’ it is actually comprised of the G-class Centauri A, K-class Centauri B, and the M-dwarf Proxima Centauri. Credit: ESO/Claus Madsen. Original here.

All this will be subject to tightly focused research, which is what the $100 million is for, but what Milner hopes to see are nano craft delivered to orbit and then boosted on their way with a 30 minute laser ‘burn’ that, reaching 60,000 g’s, drives the sail to 20 percent of the speed of light. That makes for roughly a twenty year crossing to Alpha Centauri. With a craft this small, data return is highly problematic, and in fact I think it’s one of the biggest unanswered questions Breakthrough Starshot will have to face (well, this and the challenge of interstellar dust, and key questions related to sail design and the sail’s ability to stay on thee beam during acceleration). The sail is itself the antenna on a craft of this design, and Jim Benford told me in conversation that it will have to be shaped to one-micron precision. Even so, powering up the system to send imagery and data to Earth is going to be tricky. It will be fascinating to see what kind of solutions emerge as this research gets underway, and what alternative methods may be suggested.

Even so, and granting the cost reductions digital technology makes possible, Breakthrough Starshot embarks upon a multi-year research and engineering phase that will focus on building a mission infrastructure. Creating the actual mission will demand a budget comparable to the largest scientific experiments of our time. These are no small aspirations, but what drives them is something that interstellar studies have never had at their disposal: A dedicated, enthusiastic, well-funded effort with the participation of major scientists.

“We have an advisory board of twenty, including Freeman Dyson and other top scientists,” Milner added. “$100 million will be spent in coming years as we look toward concept verification. Multiple grants should flow from this, research and experiments. We need to complete the initial study and see if building a prototype, perhaps at a scale of 1/100, is then the next step.”

At the very least, we can expect the research behind this project to spin off numerous useful technologies, all of which should be applicable not only to star missions but to in-system exploration, along with, potentially, a kilometer-scale beamer that can double as a large telescope for astronomical observations. And while I doubt we can look at interstellar missions within the next few decades (I am open to being convinced otherwise), I believe that the timing for a fast flyby of Alpha Centauri will be considerably advanced by this work.

There is much to be said about all aspects of the Breakthrough Starshot concept, and as you would imagine, I’ll be covering this closely, beginning with a trip later this week to the Breakthrough Initiatives meeting in California. That meeting will have a large SETI component growing out of Milner’s prior commitment of another $100 million, which is already being translated into active observations at the Green Bank observatory in West Virginia. But as you can imagine, the Alpha Centauri mission will be under discussion as well as the research effort begins to be assembled. What spins out of this will keep us talking for a long time to come.