A ‘Sundiver’ mission may offer the best acceleration we can muster given the current state of our technology. New Horizons is currently moving toward Pluto/Charon at roughly 19 kilometers per second, but back of the envelope calculations can pull out 500 kilometers per second for a solar sail that makes the optimum close approach to our star and then unfurls to full diameter, riding the photon storm outward to the edge of the Solar System and beyond in record time.
But Sundivers are tricky missions even on paper (we have yet to attempt one). Gregory Benford (UC-Irvine), who coined the ‘Sundiver’ term, and brother James (Microwave Sciences) have studied the matter in depth, and bring a unique perspective. They’ve not only theorized about sails and acceleration, but have actually tested the concept in the laboratory. Specifically, they’ve used an intense beam of microwaves to lift a carbon sail vertically in a vacuum chamber, and have studied how to spin and control it.
A Sail Takes Flight
This work took place in 1999 and 2000, and while I wrote the experiments up in my Centauri Dreams book some years back, I hadn’t seen the final report on it (“Wireless Power Transmission for Science Applications” NAS8-99135) until a conversation with James Benford last fall resulted in his passing along a copy. It’s absorbing reading because it’s the kind of essential laboratory work that leads to new thinking in propulsion on a practical level. It’s startling that these significant experiments have received as comparatively little attention in the space community as they have to this point.
The sail material chosen was a carbon fiber ten times thinner than a human hair. A carbon sail — the Benfords used small sails just inches across — has ‘memory,’ able to regain its shape after being rolled or folded. That could make deployment easier in space. And carbon fiber has other benefits that make it stand out as opposed to aluminized mylar sails. A micro-fiber mat like this can handle high temperatures even though it’s lighter than tissue paper. Putting this material close to the Sun poses no risk to the sail’s survivability.
Image: Carbon disk sail lifting off of truncated rectangular waveguide under 10 kW microwave power (four frames, 30 ms interval, first at top). Credit: James and Gregory Benford.
But laboratory work on sail materials is tricky indeed. Trying to get a sail to lift off against the force of gravity poses serious temperature problems, for the beam intensity (the Benfords used a 10 kW, 7 GHz microwave beam in a vacuum chamber) would melt conventional materials. But the Benford’s carbon fiber microtruss reached temperatures above 2000 kelvin from microwave absorption without melting. The concept of microwave beaming to push a spacecraft has been initially validated, and the requisite material tested to ensure it could handle the temperatures involved in a close solar pass.
A Sundiver Concept Emerges
The general shape of the Sundiver mission begins to take on substance. Surely we could use microwave beam technologies to launch a sail into a trajectory that, over time and multiple orbits, would reach the vicinity of 0.1 AU, at which point the sail receives the mighty wallop of solar photons after rotating to face the Sun. It’s an idea with a pedigree in propulsion studies, but one to which the Benford’s laboratory work has added a significant new dimension.
For there seems to be a way to kick in a new form of propulsive ‘burn’ at perihelion to maximize the resultant acceleration. Remarkably, in their experiments both at the Jet Propulsion Laboratory and UC-Irvine, the Benfords found that when they turned their microwave beam on the test sail, it experienced accelerations well beyond what photon pressure alone could account for. Tomorrow I want to look at how this effect could be modified and enhanced, capable of being used both at the initiation of a Sundiver mission and at the critical moment of closest Solar approach.