If you have questions about beamed energy concepts, James Benford is your man. A plasma physicist who is CEO of Microwave Sciences, Benford has designed high-power microwave systems for the likes of NASA, JPL and Lockheed. Now Chairman of the Sail Subcommittee for Breakthrough Starshot, he is deep into the investigation of sail materials and design, as he explains below. After reading Greg Matloff’s Near-Term Interstellar Probes: Some Gentle Suggestions, Jim passed along his comments, which highlight the need for a dedicated laboratory facility to explore the Starshot possibilities. He offers as well his thoughts on where sails stand in the overall propulsion landscape, a position of growing significance.

By James Benford


My colleague and old friend Greg Matloff has given us a well-informed broad survey of propulsion options for interstellar flight. I’m going to contribute a few comments.

Even a century-long flight to Alpha Centauri requires a velocity of ~10,000 km/sec, which is about 500 times the fastest velocity that any human object has reached by rocket propulsion — Voyager 1 is at about 17 km/sec. On the way to getting to a velocity like 10,000 km/sec, we’re going to enable fast missions for exploration and development of the solar system and the near-interstellar region around us.

Beamed Energy Propulsion

The cubesat experiments described by Greg would probably have to be fairly massive in order to produce any substantial velocity. It’s far more expeditious and far less expensive to do such experiments in evacuated chambers in the laboratory. After all, the only flight experiments that been conducted to date, which I participated in 16 years ago, were conducted in just such apparatus. That’s the most straightforward way to demonstrate stability and high acceleration. No such laboratory exists at present.

I’m hoping that Breakthrough StarShot will soon develop such a Beam-Driven Sail Test Facility, because the pressing matter of stability and high acceleration will take some time to be researched. Of course such a laboratory has to be designed and built and that would take a year at least. The most suitable beam source will be either microwave or millimeter-wave, because they can be obtained off-the-shelf at high power and have quite reasonable costs of a few dollars per watt. (Lasers are now in the hundreds of dollars per watt range.)

Such a Beam-Driven Sail Test Facility would conduct experiments on the key Starshot issues, in order of priority:

    1) Beam-riding stability of various sail shapes.

    2) Sail material and its properties: to maintain a high reflectance with very little beam absorption or transmission.

    3) Ability to sustain high acceleration, which is necessary in order to achieve high velocities.

The Cosmos-1 experiments we (the Planetary Society, Microwave Sciences and JPL) had planned to do, and which unfortunately didn’t happen because of the launcher failure, should certainly be looked at again. We were ready to carry out an experiment to irradiate the sail with the Deep Space Network beam from Goldstone. This could have demonstrated beamed propulsion of a sail in space. The 450 kW microwave beam from the large 70-m dish can show direct microwave beam acceleration of the 30-m sail by photon pressure, and we can measure that acceleration by on-board accelerometer telemetry.

The key thing to do is to measure the acceleration on the spacecraft with an on-board accelerometer. The alternative, deducing acceleration from orbit changes, would depend upon multiple transits of the sail through the beam. It would be hard to winkle out of orbital data, especially if the acceleration is small. [see “MAX-Microwave Acceleration eXperiment with Cosmos-1,” James Benford, Gregory Benford and Tom Kuiper, Proc. Fourth IAA Symposium on Realistic Near-Term Advanced Scientific Space Missions, also JBIS 59, pg. 68 (2006).]

Greg Matloff is certainly correct to point out that the sail material for Starshot must be able to reconfigure its shape. It must have very precise and accurate pointing and tracking, at both Earth and the star that it is approaching. The sail must be a very smart material with embedded and distributed artificial intelligence. How to provide the energy for reconfiguring the shape of the sail is one of the many challenges of Starshot.

Thermonuclear Fusion Rockets

Greg gives a good list of the many technical problems that face fusion rockets. Of course the greatest problem is to produce a fusion reaction at all! For interstellar flight they will be very large, very inefficient and very costly.

For comparison, here are some key parameters of a US supercarrier, two Icarus design concepts and the Starshot sailship:

  • Aircraft Carrier 0.3 km, 105,000 tons, 0.01 T$
  • Firefly 0.75 km, 23,550 tons, mass fraction 0.0064, rocket cost 40 T$, 4.7% c
  • Ghost 1.2 km, 154,800 tons, mass fraction 0.0008, cost 0.02-34 T$, 6% c
  • Large Sailship 10 km, 10 tons, mass fraction ~0.1, Beamer cost 40 T$, sailship capital cost ~ 1B$, 10% c [operating cost, electricity to drive the Beamer at today’s rate (0.1 $/kW-hr) is 0.5 T$.]
  • Starshot ~3 m, ~1 gram, capital cost ~10B$ [operating cost ~6 M$.]

And for a size comparison, see Figure 1. The ships that explored the oceans, such as the Santa Maria (19 m) and Kon-Tiki scale Polynesian rafts (45 m), as well as the Breakthrough Starshot sail (~3 m) could not be visible on this scale.

With the fusion rocket approach, the infrastructure necessary to build such huge vessels and supply nuclear fuel is a fixed cost. It is not easy to estimate, but will be quite huge.


Figure 1. Scales of Starships compared with largest Earth vessels. From top, largest Earth ocean ships, Firefly, Ghost, 10 km diameter Sailship, all to scale. The ~3m Breakthrough Starshot sail is actually not visible on this scale. (Figure from Michael Lamontagne).

The concept that Freeman Dyson originally proposed, using nuclear materials out of the nuclear arsenals to make explosives to drive starships, is a bit out of date now. As you can see in figure 2, the Cold War adversaries have been radically reducing the warheads they have, the rapid progress beginning during the Reagan-Gorbachev era. The Nunn-Lugar Cooperative Threat Reduction program deactivated more than 7,600 nuclear warheads. Highly enriched uranium contained in them was made into commercial reactor fuel which was purchased by the U.S. Few Americans realize that during the last several decades a fair amount of the electrical power the United States was generated by burning up Russian nuclear materials in fission power reactors!

There are now a few thousand nuclear explosives compared to the hundred thousand at the peak of the Cold War.


Figure 2: History of Nuclear Stockpiles.


Beam-driven propulsion is more firmly grounded and credible than nuclear fusion propulsion, as in Project Icarus. Fusion rockets remain far-term, distant on a timescale of decades, if not centuries. In today’s funding environment, that’s not likely to change: Due to Starshot, sails are becoming near-term.