Centauri Dreams first ran across Jordin Kare’s remarkable SailBeam concept in a report called “High-Acceleration Micro-Scale Laser Sails for Interstellar Propulsion” that the astrophysicist prepared for NASA’s Institute for Advanced Concepts (NIAC). The idea seemed outrageously simple: if you accelerate vast numbers of tiny sails rather than one enormous one, you can bring the same amount of mass to high speeds with a less complex optical system. Using dielectric rather than metal sails, you can accelerate the sails much closer to their power source. The stream of microsails then becomes a source of propulsion as it is vaporized into plasma behind a departing starship.

Dana Andrews, who has worked with Kare on magsail concepts, notes that a SailBeam Boosted Magsail (SBBM) solves a key problem of particle beam propulsion. A neutral particle beam will disperse as it travels, but a stream of low-mass microsails is not limited by such diffraction. Andrews’ MagOrion concept explored some of the same territory, driving a magsail starship by the plasma pulses of small nuclear explosions. But ionizing a stream of incoming microsails should work as well, provided you find an efficient way to do it.

We looked at Andrews’ recent paper “Interstellar Propulsion Opportunities Using Near-Term Technologies” yesterday. In its final sections, Andrews analyzes SailBeam concepts and discusses three ways to ionize the incoming sails. Lasers could be mounted on the starship that would vaporize the sails as they approached. Particle beams could perform the same function, a method Andrews sees as more efficient than lasers. Finally, the microsails could simply impact against a small mass of solid, gas or plasma placed directly behind the starship. This is the least effective method. As Andrews writes:

This approach has the advantage of requiring little complex vehicle hardware, but the disadvantage that the vehicle must supply mass to intercept each sail, at least some of which is lost. The effective specific impulse of the vehicle propulsion is thus no longer infinite, and unless the impact mass lost is comparable to or less than the sail mass, the maximum vehicle velocity will be a fraction of the sail velocity.

All told, SailBeam concepts are cost effective because of their lower average power requirement and the simplicity of the SailBeam accelerator, which Andrews believes could be built with near-term optics. For manned missions, SailBeam would not require — as a Particle Beam Boosted Magsail would — a string of particle accelerators along an ‘interstellar runway’ to achieve adequate acceleration at a slow enough pace to avoid killing the crew.

Andrews advocates a Life Cycle Cost analysis for each kind of mission he discusses to determine which offers the lowest cost per spacecraft. And consider this exotic possibility: an interstellar vehicle could be launched as a number of small payloads at high acceleration. These could then be linked up during the first year of coast to form a larger vehicle that would use magnetic sail braking upon arrival at its destination star system. Andrews calls this the ‘wagon train’ approach to interstellar transportation. It seems a promising concept for interstellar robotic probes.

In summary, the laser-propelled lightsail or the slightly more efficient SBBM alternate, while not as efficient as the PBBM [Particle Beam Boosted Magsail], could well be the system of choice if the price of solar panels continues to drop over the next 30 years. Either way, we can watch the cost of space-based energy fall and predict the time when interstellar exploration becomes affordable.

The bottom line is that interstellar travel looks expensive, but possible. Therefore, the Fermi paradox is still a paradox. It is possible that we are alone in the galaxy, or that no civilization has acquired the necessary technologies, but the former is more likely than the latter.

The Andrews paper is “Interstellar Propulsion Opportunities Using Near-Term Technologies,” in Acta Astronautica Vol. 55 (2004), pp. 443-451. Jordin Kare’s report on SailBeam concepts is “High-Acceleration Micro-Scale Laser Sails for Interstellar Propulsion,” Final Report, NIAC Research Grant #07600-070, revised February 15, 2002. And be aware of the classic paper by C.E. Singer, “Interstellar Propulsion Using a Pellet Stream for Momentum Transfer,” Journal of the British Interplanetary Society 33 (March 1980): 107-115.

Gerald Nordley has done fascinating work on the concept of self-steering pellet propulsion. I’ll discuss his ideas in an upcoming entry.