Although we frequently talk about beamed sails for interstellar missions, the fact is that spacecraft on the scale Robert Forward used to talk about that could take us to Alpha Centauri in 40 years won’t come out of nowhere. The evolution of the solar sail into the beamed sail will involve all kinds of experimentation and a variety of mission concepts developed for use right here in the Solar System. Consider just one, a microwave-driven sail that could reach Mars in one month, and Pluto in five years. I wrote about this one in A Microwave-Beamed Sail for Deep Space.

The idea comes from Jim and Greg Benford, who discussed it in a 2006 issue of the Journal of the British Interplanetary Society. The scenario involved a phenomenon the duo had discovered in their laboratory work on microwave beaming. Experimenting with a 7.5 g/m2 carbon sail, they had uncovered the fact that molecules evaporating from the sail created accelerations beyond what would have been expected from photons alone. Painting various compounds on the sail would allow it to take advantage of this ‘desorption’ to produce an extra kick.

A mission like this would be a mixture of the familiar and the unknown. The sail would be deployed by conventional rocket into low Earth orbit, then pushed by a microwave beam from Earth to cancel its solar orbital velocity and create a close flyby of the Sun for a gravitational slingshot boost augmented by deploying the sail at perihelion. The Benfords work has shown that no sail damage results from desorption of the materials painted onto the sail. Having expended its desorbed materials, the sail would then operate as a conventional solar sail for the rest of its interplanetary journey, still using a solar push as far out as the orbit of Jupiter.

Near-Term Uses of Beamed Sails

Fast missions to Mars are desirable, but let’s consider may be the first practical assignment for microwave beaming: Shortening the amount of time a solar sail needs to escape Earth orbit. Let me quote from Jim Benford’s new paper on microwave beaming:

Computations show that a ground-based or orbiting transmitter can impart energy to a sail if they have resonant paths — that is, the beamer and sail come near each other (either with the sail overhead an Earth-based transmitter or the sail nearby orbits in space) after a certain number of orbital periods. For resonance to occur relatively quickly, specific energies must be given to the sail at each boost. If the sail is coated with a substance that sublimes under irradiation, much higher momentum transfers are possible, leading to further reductions in sail escape time.

Benford believes that resonance methods can reduce escape times from Earth orbit by over two order of magnitude compared to a solar sail using only the pressure of solar photons. And while microwave transmitters require much larger apertures for the same focusing distance than lasers, they make up for this by producing higher acceleration. This is why the carbon microtruss material that the Benfords used in their laboratory work is so significant. It can be heated to high temperatures without damage, allowing a stronger beam and higher acceleration. That means more velocity in less distance, which in turn allows the aperture size to be reduced.

A number of missions for microwave beaming are in the literature that develop the idea for use here in the Solar System, with 175 kilometer per second speeds produced during a relatively short period of acceleration. Missions like these could be critical supply channels for getting small payloads to human crews on Mars or the asteroid belt, with deceleration by aerocapture in the case of Mars, or perhaps through a decelerating microwave beam. Here we’re talking about potential travel time to Mars as low as 10 days, though not for human crews. Scientists have explored missions to the outer planets as well, including Jordin Kare’s work on a beamed energy mission to Jupiter, and Benford envisions 250 km/sec for interstellar precursor work.

Assessing an Interstellar Precursor

But Jim Benford’s studies of cost-optimization offer a number of examples, beginning with a slower precursor mission moving at 63 km/sec. The 1-kilometer sail is driven through a ground-based array supplied by power from the Earth grid from a site chosen to optimize the effect of the 100 GHz waves — this implies a high altitude location with low humidity. A capital cost of $144 billion can be reduced by high-volume manufacturing and economies of scale to $21.6 billion, in a range comparable to Flagship missions like Gaileo and Cassini. Benford works through the hardware and plots the ‘learning curve’ — the decrease in unit cost of hardware with increasing production — that accounts for increasing economies in the various technologies the mission will require. The cost savings ratio (CSR) is what brings the mission within reach.

Moreover, this kind of precursor mission creates a lasting infrastructure:

The operating cost, i.e., the electrical cost to launch out of the Solar System is 17 M$. This is surely far less than the capital cost of building the sail itself, so once built, the beaming facility can send many probes into the interstellar medium. With a launch cost less than the cost of the 1 km sail, the strategy will be to use the system to launch sequences of sails in many directions to sample the Interstellar Medium and flyby Kuiper Belt and Oort Cloud objects, such as Sedna. As the facility grows, the sails will be driven faster and can carry larger payloads.

Several other mission concepts are examined in this paper, and I’ll deal with them briefly tomorrow, along with some of Jim Benford’s observations on cost-optimized scaling. Ultimately, we’re interested in a development path that will support what he calls ‘directed energy propulsion,’ and his paper is a clarification of what a roadmap for sail technologies leading to beamed energy sail missions must include. Further work on cost-optimization should help us examine other interstellar concepts from Robert Forward, Greg Matloff and others to find the optimal development path.

The paper is Benford, “Starship Sails Propelled by Cost-Optimized Directed Energy,” soon to be made available on arXiv. Also relevant is G. Benford and P. Nissenson, “Reducing solar sail escape times from Earth orbit using beamed energy,” Acta Astronautica Vol. 58, Issue 4 (February 2006), pp. 175-184. The Benford brothers’ paper cited at the top of the article above is “Power-Beaming Concepts for Future Deep Space Exploration,” in the Journal of the British Interplanetary Society Vol. 59 No. 3/4 (March/April 2006), pp. 104-107.