Beaming ‘Wafer’ Probes to the Stars

by Paul Gilster on June 25, 2015

The last interstellar concept I can recall with a 20-year timeline to reach Alpha Centauri was Robert Forward’s ‘Starwisp,’ an elegant though ultimately flawed idea. Proposed in 1985, Starwisp would take advantage of a high-power microwave beam that would push its 1000-meter fine carbon mesh to high velocities. As evanescent as a spider web, the craft would use wires spaced the same distance apart as the wavelength of the microwaves that drove it, which is how it could be so lightweight and yet maintain rigidity under the microwave beam.

Throw in sensors and circuitry built-into the sail itself and you had no need for a separate probe payload — Starwisp was its own payload. This was conceived as a flyby mission, in which the microwaves would again bathe the craft as it neared its target, providing just enough energy to drive its communications and sensor array to return data to Earth. What a mission: Starwisp would accelerate at 115 g’s, its beam pushing it up to 20 percent of lightspeed within days.

Addendum: I had two typos above when this ran, both now corrected thanks to a note from James Jason Wentworth who caught my error re Starwisp’s acceleration (115 g’s, not 11) and diameter (1000 meters, not 100). I need to put the coffee on earlier in the morning…

Significant cost advantages emerge in comparison to beamed laser ideas. Microwaves, Jim Benford once reminded me, are typically two orders of magnitude cheaper than lasers in terms of the optics used to broadcast and the power efficiency of the laser. But Geoffrey Landis’ subsequent work on Forward’s original concept revealed that the craft’s wires would absorb rather than reflecting the microwaves, destroying the craft in a fraction of a second. The beaming system in Forward’s paper, incidentally, called for a lens 50,000 kilometers in diameter, another strike against the notion, but the idea of lightweight probes remains alive.

A Laser Beaming Infrastructure

In fact, beamed propulsion continues to have many advocates at both laser and microwave wavelengths. Philip Lubin (UC-Santa Barbara) has been awarded one of fifteen proof-of-concept grants from the NASA Innovative Advanced Concepts office. Lubin’s project is called Directed Energy Propulsion for Interstellar Exploration (DEEP-IN), and like Forward he sees it as a way to achieve, through laser beaming, a very fast mission to the nearest stars.

“One of humanity’s grand challenges is to explore other solar systems by sending probes — and eventually life,” says Lubin. “We propose a system that will allow us to take the first step toward interstellar exploration using directed energy propulsion combined with miniature probes. Along with recent work on wafer-scale photonics, we can now envision combining these technologies to enable a realistic approach to sending probes far outside our solar system.”

Laser Sail Adrian Mann

Image: Beamed propulsion leaves propellant behind, a key advantage. Coupled with very small probes, it could provide a path for flyby missions to the nearest stars. Credit: Adrian Mann.

A major talking point for beamed strategies is that they allow designers to keep the propellant back here in the Solar System. Moreover, build Lubin’s photon driver and you can go to work on many problems, not all of them interstellar. The beaming technology he advocates is called DE-STAR, for Directed Energy Solar Targeting of Asteroids and Exploration. Any laser powerful enough to drive even a highly miniaturized craft to 20 percent of lightspeed is also powerful enough to serve as a planetary defense against asteroids on wayward trajectories, an idea Lubin’s group studied last spring at the Planetary Defense Conference in Italy.

Imagine an orbital planetary defense system in the form of a modular phased array of kilowatt class lasers, the modular design promoting incremental development and upward scaling of the system over time. The idea is to use laser energies to raise spot temperatures on a dangerous asteroid to roughly 3000 K, which would create enough ejection of evaporated material to alter the asteroid’s orbit. A basic DE-STAR 1 could handle space debris, but the system, in Lubin’s view, scales up in useful ways. A DE-STAR 2 could divert 100-meter asteroids at distances up to 0.5AU, and the phased array configuration could create multiple beams, so a single DE-STAR could engage several threats simultaneously. Lubin says that a DE-STAR 4 would produce energies sufficient to completely vaporize a 500-meter asteroid over a year of beaming.

The NIAC grant will presumably help Lubin flesh out the details. Given this kind of power, an interstellar application seems worth investigating. In a talk Lubin gave at 2013’s Starship Congress in Dallas, he explained that a system as powerful as his DE-STAR 4 would be able to propel a 102 kg spacecraft up to 2 percent of the speed of light, shutting down after 30 AU, after which the spacecraft would coast. In the abstract for this talk, Lubin said that a far more powerful DE-STAR 6 system could push a 104 kg probe “to near the speed of light allowing for interstellar probes.” If you’d like to view the talk, it’s available here.

But the NIAC work operates at the other end of the size spectrum. The description on the NIAC site says this: “We propose a system that will allow us to take a significant step towards interstellar exploration using directed energy propulsion combined with wafer scale spacecraft.”

Couple nanotechnology with highly reflective, extremely thin sail films and an Alpha Centauri flyby with flight time of 20 years becomes feasible. How long will it be until we have the needed sail materials and laser capabilities? Lubin will speak on this in August at the SPIE conference on Nanophotonics and Macrophotonics for Space Environments IX in San Diego. The session is called “Directed energy propulsion of wafer scale spacecraft for interstellar missions.”

What NIAC funding gives Lubin’s group at UCSB is the ability to create a detailed roadmap for such a craft, one with fully contained communications, power systems and controllable photon thrusters. Both the spacecraft and the photon driver that would propel it will be studied in terms of necessary technology development. It will be interesting to see how the Lubin team will handle the huge problems of communications at this scale, and what sort of time frame they propose for deployment of a prototype DE-STAR system.

Centauri Dreams‘ take: Miniaturized probes that could accomplish interstellar flybys would not be one-off events. The potential for ‘swarm’ technologies that would take advantage of such a laser infrastructure is immense. Here we can imagine interstellar probes one day being sent by the thousands, an interactive mesh that could give us our first close-up images of extrasolar planets.

The original Starwisp paper is Forward, “Starwisp: An Ultra-Light Interstellar Probe,” Journal of Spacecraft 22 (1985b), pp. 345–50. And see Geoffrey Landis’ significant follow up, “Advanced Solar- and Laser-Pushed Lightsail Concepts,” Final Report for NASA Institute for Advanced Concepts, May 31, 1999 (downloadable from the older NIAC site). For more on DE-STAR and propulsion, see Bible et al., “Relativistic Propulsion Using Directed Energy,” in Taylor and Cardimona, ed, Nanophotonics and Macrophotonics for Space Environments VII, Proc. of SPIE Vol. 8876, 887605 (full text). A UCSB news release on Lubin’s grant from NIAC is also available.

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Andrew Palfreyman June 25, 2015 at 18:22

Very inspirational. It seems as though Lubin thinks that direct solar-powered lasers are not as efficient as PV->exciter->laser using ytterbium lasers. I’d like to see the numbers.

J. Jason Wentworth June 25, 2015 at 20:31

These interstellar probe concepts–particularly beam-propelled ones–attract my interest not only because those who launched the probes would likely live to see their results, but also because we can “work up” to them while gaining new scientific data in the process. For example:

There are no funded missions to Uranus and Neptune (or to Pluto, after New Horizons), two interesting planet/satellite/ring systems that we’ve only briefly visited. Individual and “swarm” wafer probes could be sent to them as lower-powered “exercises” for interstellar precursor and interstellar missions. Such missions would also provide opportunities to work out the production engineering, instrument integration, launching, communication, remote beamed power, “swarm operations,” and other requirements of the probes. Also:

Possible mass-production of wafer probes (whose “at-home engines” would be reusable and upgrade-able, of course) could make possible inexpensive, regular survey missions to worlds throughout our solar system. While flyby probes aren’t as efficient as orbiters regarding data gathering, if the cost of flyby probes (the wafer probes) came down dramatically due to automated quantity production, affordable frequent flyby flights would largely compensate for this. Centauri Dreams’ beam propulsion “footer logo” may yet be replaced with an actual photograph in the lifetimes of most readers…

John Freeman June 26, 2015 at 5:23

Propulsion is one problem, communications another – could some portion of the propelling beam be used to power a return signal across interstellar distances?

Angelo June 26, 2015 at 8:04

Andrew, I am very interested in the numbers of direct solar-powered lasers, do you have any recent reference/link?

Paul D. June 26, 2015 at 15:32

Powerful space-based lasers would also be applicable to power beaming down to the Earth’s surface. There are some markets where the light need not be converted back to electricity but could be used directly, which could reduce the complexity/cost of the system.

J. Jason Wentworth June 26, 2015 at 18:50

Paul D., are you speaking of “piped light” (via lucite tubes and optical fibers) to illuminate buildings, or “solar” cells whose peak sensitivity is to the laser light’s wavelength, or both (or perhaps something else)? Being able to use laser light beamed from space to the Earth in its ‘as-sent’ form would avoid conversion losses. Having interstellar probe “engines” (space-based laser and/or maser arrays) that could earn their own keep by doing other things between probe launches and encounters would be very advantageous.

Michael June 26, 2015 at 20:09

In keeping the huge fennel lens ridged just spinning and applying a charging device (like charges repel) could be enough, but keeping it aligned over a long time would be a huge hurdle. As for the carbon burn-up issue when a carbon wafer is exposed to microwaves that issue could be resolved by a vapour deposition of silver or high temperature microwave reflective material in a grid like configuration onto the carbon back bone thereby protecting the carbon from absorbing to much microwaves.

Paul D. June 27, 2015 at 15:07

JJW: the markets I am thinking of are:

(1) Photochemical processes that use stoichiometric quantities of photons. The largest of these is the Toray PNC process for the production of caprolactam, although as currently designed it uses near UV light that would be unacceptable for beaming to Earth due to scattering losses. The photochemical step (dissociation of NOCl) still proceeds with longer wavelength light though.

(2) Very high temperature heating, as what is now done in arc furnaces. This is potentially a very large market.

(3) Lighting, as you suggest, in particular enclosed greenhouses. We are already seeing high efficiency LEDs being used to grow high value crops near cities in some places. Beamed light delivered through a receiver + optical fibers could be used instead.

Alex Tolley June 27, 2015 at 15:48

IIRC, the development of high power lasers was supported by other uses, e.g. asteroid defense, which would attract DoD funding.

I also recall that the mirrors for propulsion needed to be almost 100% perfect reflectors given the power of the lasers. He was a bit hand wavy about that, but such mirrors may be available.

Eric Tolle June 27, 2015 at 16:57

Technical considerations aside, the fundamental problem with a fast flyby mission is the question of whether it would actually get any data we couldn’t acquire from Earth using advanced telescopes. After all, the principles behind the huge fresnel lens for the laser could also be used to make telescopes hundreds of miles in diameter.

J. Jason Wentworth June 28, 2015 at 17:39

Paul D., I’m totally lost regarding your first example (it sounds like a pharmaceutical compound of some kind–but it sounds like visible laser light could still be effective in its production, just more slowly than with UV light), but I definitely grok the other two. I imagine all kinds of other uses for satellite-delivered laser and/or microwave beamed power will be developed after it/they become available. Also:

Eric Tolle, you raise a very interesting point! Why go to distant star systems when the launching apparatus (the enormous fresnel lens) could make it possible to look in closely upon them and take spectrograms of them (and speaking of spectroscopy, imagine being able to resolve spectroscopic binary stars as discrete components)? But:

Even with such a prodigious telescopic viewing/spectroscopic capability, sending beam-pushed wafer probes would still be worthwhile, as they could directly sample the stellar winds and magnetic fields of their destination stars, and just possibly–if one or more probes in a “swarm” was aimed correctly for this, perhaps by chance–directly sample the outermost atmosphere of one or more of the exoplanets in a star system (provided that it/they didn’t pass *too* close to the planet or planets). Also, a “stream” or “line” of probes could observe different sectors of each star system, to obtain complete coverage. Being able to observe the systems at high resolution from here would further enhance the probes’ utility, by making it possible to time their launches and aim them to pass close to specific planets in each star system.

ole burde June 29, 2015 at 11:53

The isue of the powersuply is not a minor one in this relation . So far the biggest existing powerplant in space ( 100KW range) , i the ISS solar panels , ant THAT was not cheap …
It may be cheaper to beam up the elektrical powersuply from earth by microwave . For a big existing powerstation this would be a minor event , and the microwave antenna in space could be much smaller and more lightweight than the alternative solar panels . The reason for this difference is , that the microwave beam (when directed upwards ONLY to LEO !) could have a MUCH higher energy density than natural sunshine . If necesary , the microwave antenna in LEO could re-transmit the energy as laser power to GEO .

Paul D. June 30, 2015 at 11:49

JJW: caprolactam is the monomer that is used to make nylon 6.

https://en.wikipedia.org/wiki/Nylon_6

It’s a much higher volume (in mass) market than any pharmaceutical (4 million tons in 2010 according to that link.)

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