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.”
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.