Starshot: Concept and Execution

Because I get irritable when I don’t get my walking in every day, I made sure when I arrived in Palo Alto to cram as much as I could into the day before Breakthrough Discuss began. That meant heading out from the hotel just after noon and putting in about five miles. Palo Alto is a very walkable place and I found myself ambling up and down shady streets past gardens bright with spring flowers. We had superb weather for the entire conference, but naturally when things got going, both days were crammed with talks and long walks were out of the question.

But the day before, as I walked, I pondered the schedule of the conference, wondering how a mission to Alpha Centauri fit into the overall plan. In addition to the $100 million going toward Breakthrough Starshot, Breakthrough Initiatives has also put up $100 million for its SETI project, which has already begun operations at Green Bank (West Virginia) and is slated to operate at the Parkes dish in Australia as well, giving SETI the services of two of the largest steerable dishes in the world. But as we’ve seen, SETI fit into the Starshot discussion because beamed laser technologies are an observable, and one that can be seen from a long way off.

The focus on Alpha Centauri also turned out to be a natural for Starshot. If you’re hoping to build a mission, one of your first priorities is to understand the target. Here we’re very close to learning what kind of planets are available around the three stars in the Alpha Centauri system, and what other challenges by way of dust and debris we may encounter with a fast probe through the plane of any planets found there. And of course a technology that can get us to Alpha Centauri is an enabler for a gravitational lens mission, which I’ll discuss more on Monday.

Miracles Required?

In more than one overheard conversation a sentence more or less like this one occurred: “Just how many miracles are required to get this plan to work?” The theory being that one miracle is OK, because we can’t discount how fast technology is developing, and two miracles might also work, because we can’t know in which direction we’re going to be surprised. But as miracles pile up, the odds become more daunting. That’s why Breakthrough Starshot is spending $100 million. That’s far less than the actual mission would require, but it’s a more than workable amount to create the kind of concept study which is what is in Starshot’s immediate future.

As Pete Worden, former director of NASA Ames and now executive director of Breakthrough Initiatives made clear, there is a process here through multiple grants, research and experiments that will attack the problem of sail beaming at every level, addressing along the way the issues of data return, dust mitigation during the flight, beamer construction, sail materials technology and all the other matters that bedevil so futuristic a concept. Breakthrough Starshot will hope to complete the study with a scaled-down prototype of the mission hardware.

Kepler’s famous letter to Galileo came up not just in Worden’s keynote on day two but in several other talks, it being the first time using sails in space in any realistic way was introduced. Here’s the relevant quote:

There will certainly be no lack of human pioneers when we have mastered the art of flight… Let us create vessels and sails adjusted to the heavenly ether, and there will be plenty of people unafraid of the empty wastes. In the meantime, we shall prepare for the brave sky-travelers maps of the celestial bodies. I shall do it for the moon, you Galileo, for Jupiter.

That was written in 1610, and it reminds us that despite their misconceptions about what they would find in space, the early pioneers of astronomy could draw sound conclusions from their observations. Kepler had studied comets and viewed the fact that comet tails always pointed away from the Sun as evidence for a ‘solar wind’ that could be harnessed by space travelers. We might also remember that Kepler wrote an early science fiction tale called the Somnium, a journey to the Moon accomplished through supernatural means, but with an attempt to be as scientifically accurate as possible given knowledge of the day.

Image: In a new and much smaller room, day two of Breakthrough Discuss began with Pete Worden’s keynote.

Today’s beamed sail draws on trends that were not obvious even decades ago when science fiction writers and physicists alike described the sail missions we might one day fly. Today nanotechnology is helping us build materials one molecule at a time, Worden said, while we have a continuing decrease in size and increase in complexity in electronics, along with advances in lasers and fiber optics that point toward creating phased laser arrays. A key question, of course, is this: Does something like Moore’s Law apply to laser technologies? And how far can we extrapolate it taking us?

The Starchip we would like to see deployed on the Starshot is about the size of a postage stamp, for as I’ve said before, this is a spacecraft more like a smartphone than a macro-scaled capsule with beams and struts. In Worden’s words:

“The Starchip. enabled by phone technologies, is under 1 gram in mass. That makes it 1 million times lighter than an ordinary spacecraft, and on this chip we can carry cameras, thrusters, navigation and communications equipment at the cost of an iPhone. NASA doesn’t like to build anything small, but I’m now seeing JPL doing all sorts of things with smaller satellites. As to the sail, nanotechnology is key to building a sail 300 atoms thick that has to hold together and not absorb much energy, but transmit and reflect energy. Experiments will start soon.”

These are huge requirements, for we need a sail as light as smoke that can nonetheless maintain its structural integrity when hit by the beam from a phased laser array that will accelerate it at 60,000 g’s to twenty percent of the speed of light. The slightest absorption of heat will fry the sail, and it had better be engineered to the tolerances needed to stay on the beam, which is going to involve a great deal of investigation. The only laboratory work on microwave sail beaming I’m aware of, conducted by the Benford brothers, implies the need for a cone-shaped sail of a specific topology that will also spin, riding the beam rather than being deflected from it. What we need now is lab work involving laser-beaming to clarify these matters.

Can the sail and chip withstand the beating they’ll take from the laser array? 60,000 g’s seems astonishing, but Worden noted that the chip has to survive only a few minutes of this, and the technology to protect it is developing rapidly, there being artillery rounds that do guidance and propulsion and withstand 100,000 g’s even now. The starships would be deployed by a ‘mothership’ in a highly elliptical orbit, after which the mothership would depart the area to allow the beam to work its magic on the sails. Two minutes to 20 percent of the speed of light.

A Full Array of Challenges

A four-meter sail produced in great numbers and capable of riding a powerful laser beam would be the kind of enabler Robert Forward used to think about, though in the days when he took an interstellar program to the US Congress (see his “A National Space Program for Interstellar Exploration,” as discussed in Roadmap to the Stars, a 2013 Centauri Dreams entry), he was talking about much larger ships. In his document, which was published by the House Committee on Science and Technology, Forward talked about sending automated probes to nearby star systems by the turn of the 21st Century, beginning with a 15-year period of mission definition studies and work on key technology areas. This was in 1975, and I can only imagine what Forward would have thought of having $100 million to pursue the idea.

Now we think small on the one end — the sail, the payload — and large on another — the beamer. Phase locking combines the output of an array of lasers into a single focused beam, with Worden describing “network effects that amplify the beam into a laser wind of 50 gigawatts.” If we can build something like this, perhaps in Chile’s Atacama desert, perhaps in Antarctica, we could theoretically create a modular and scalable system that could make Forward’s dream a reality, sending fast flybys to a host of nearby stars at tens of thousands of kilometers per second.

It was great to talk to Mason Peck at the Breakthrough Discuss meetings, especially since his work at Cornell on the tiny spacecraft he calls ‘Sprites’ has led directly to ideas like Starchip. Both Peck and Zac Manchester, also at the conference, have worked in the Cornell lab on the Sprite technology, each of these 32 x 32 x 4mm in size and weighing less than 7.5 grams. You can see a rundown of these chip-sized satellites in Sprites: A Chip-Sized Spacecraft Solution, which I wrote in 2014. At lunch on the second day of the conference, Peck showed several of us a Sprite, whose implications in swarm technology missions continue to fascinate me.

But back to the beamer. Worden said that the Atacama desert might be the best location of those considered — the Kalahari, the Australian outback, Antarctica — because it has more infrastructure, and power might be bought from utilities or produced through systems built by the project itself, perhaps solar, perhaps nuclear. Space hasn’t entirely been ruled out, but it’s hard to see us ready to build a huge, privately funded laser installation in space even within several decades. For that matter, you can imagine the political issues involved in placing a laser that could be weaponized anywhere near Earth orbit.

The Starshot sail would fly edge-on to minimize the cross-section exposed to matter in the interstellar medium. Here we’re dealing with a lot of unknowns because we’ve only gotten one mission out beyond the heliopause, and it — Voyager — wasn’t designed to do the kind of measurements we’d like to have about the Local Interstellar Medium (LISM). But based on what we do know about local ‘bubbles’ in the medium and our Sun’s position in them, a fast mission to Alpha Centauri seems survivable at least by some of the craft thrown at it. Redundancy thus becomes crucial, which is why the plan is to send a large number of sails.

And here we arrive at yet another challenge, or ‘miracle’ if you will. We’ll look at getting a signal back to Earth on Monday, but the plan is to use the sail itself as an optical element, turning it into a phased receiver as well as a transmitter. The tolerances needed in doing this, and the technologies required to shape the sail at its destination, remain unexplored territory. We have to ensure that this element is not the showstopper. As you might expect, data reception back on Earth is to be handled through the enormous laser array that sent the craft.

That array also serves as a kilometer-class telescope, meaning it would have a useful future of continuing astronomical observation. And as a beamer, says Worden, the laser array is multi-purpose. A successful beamer could make possible any number of missions within the Solar System and beyond, including the gravitational lens FOCAL mission. We have to remember we’re not just targeting Alpha Centauri. “We’re convinced we can contemplate in this century, and perhaps in a single generation, expanding the human reach to the stars.” Note the plural.

I’ve only mentioned some of the larger challenges Breakthrough Starshot will face. At the Yuri’s Night celebration, Worden showed a slide listing nineteen, as seen in the image above. These suggest and certainly do not exhaust the list of issues that Starshot raises. In my last Breakthrough Discuss report on Monday, I’ll look at how the FOCAL mission to the Sun’s gravitational lens fits into the mission, with thoughts on where we go from here.