I don’t want to move past Gregory and James Benford’s interesting sail ideas without pausing to examine another paper that ran in the preceding issue of JBIS. It’s a look at what we might do in the near-term with solar sails, written by Gregory Matloff (CUNY), Travis Taylor (BAE Systems) and collaborators. And it focuses on what inspired Centauri Dreams (the book) in the first place, the question of where we stand right now in terms of deep space propulsion.
In other words, never mind the politics or the economics. If we had to launch a mission soon (obviously with a robotic rather than a human payload), how far could we go and how fast? Matloff and Taylor lay out the near-term possibilities for reaching the heliopause (roughly 200 AU) and the Sun’s inner gravity focus (550 AU) using both sails and other propulsion options. The reference sail used here is a 100 meter disc massing some 157 kilograms, with structure and payload adding up to 100 kg for a total mass of 257 kg.
Let me just quote the paper’s conclusion:
A number of options exist for near-term interstellar exploration using robotic solar-photon sailcraft including sail unfurlment at the perihelion of a parabolic solar orbit, sail unfurlment at the perihelion of an elliptical solar orbit, and Jupiter-gravity-assist after sail unfurlment from an elliptical solar orbit. Although all of these techniques can propel a 257 kg sailcraft with an areal mass thickness of 0.0082 kg/m2 to the heliopause (200 AU) within a human working lifetime, [and] to the Sun’s inner gravitational focus at 550 AU from the Sun in a human lifetime, they are all slower than equivalent missions launched using higher technology sailcraft.
Note what Matloff and Taylor are saying about mission times — this defines the human reach at present using a mix of solar sail and solar-electric technologies. Reaching 200 AU takes about a human working lifetime (i.e., some 40 years), while the gravity focus takes a full lifespan, averaged here at 80 years. A critical factor is the areal mass thickness of the sail. The authors draw on Italian theorist Giovanni Vulpetti, who developed calculations based on areal mass thicknesses that were considerably thinner (0.001-0.002 kg/m2). As you might expect, such vehicles are capable of higher cruise velocities.
The paper is Matloff et al., “Near-Term Interstellar Sailing,” Journal of the British Interplanetary Society Vol. 59 No. 2 (February 2006), pp. 59-62.