We’ve all imagined huge starships jammed with human crews, inspired by many a science fiction novel or movie. But a number of trends point in a different direction. As we look at what it would take to get even a robotic payload to another star, we confront the fact that tens of thousands of tons of spacecraft can deliver only the smallest of payloads. Lowering the mass requirement by miniaturizing and leaving propellant behind looks like a powerful option.

Centauri Dreams regular Alex Tolley pointed to this trend in relation to The Planetary Society’s LightSail-1 project. In a scant ten years, we have gone from the earlier Cosmos 1 sail with an area of 600 square meters to LightSail-1, with 32 square meters, but at no significant cost in scientific return because of continuing miniaturization of sensors and components. We can translate that readily into interstellar terms by thinking about future miniature craft that can be sent out swarm-style to reach their targets. Significant attrition along the way? Sure, but when you’re building tiny, cheap craft, you can lose some and count on the remainder to arrive.

The Emergence of SailBeam

I inevitably think about Jordin Kare’s SailBeam concepts when I hear thinking like this. Kare, a space systems consultant, had been thinking in terms of pellet propulsion of the kind that Clifford Singer and, later, Gerald Nordley have examined. The idea here was to replace a beam of photons from a laser with a stream of pellets fired by an accelerator — the pellets (a few grams in size) would be vaporized into plasma when they reached the spacecraft and directed back as plasma exhaust. Nordley then considered lighter ‘smart’ pellets with onboard course correction.

I’m long overdue for a re-visit to both Singer and Nordley, but this morning I’m thinking about Kare’s idea of substituting tiny sails for the pellets, creating a more efficient optical system because a stream of small sails can be accelerated much faster close to the power source. Think of a solar sail, as Kare did, divided into a million pieces, each made of diamond film and being accelerated along a 30,000 kilometer acceleration path, all of them shot off to drive a larger interstellar probe by being turned into a hot plasma and pushing the probe’s magnetic sail.


Image: Jordin Kare’s ‘SailBeam’ concept. Credit: Jordin Kare/Dana G. Andrews.

Kare, of course, was using his micro-sails for propulsion, but between Nordley and Kare, the elements are all here for tiny smart-probes that can be pushed to a substantial fraction of the speed of light while carrying onboard sensors shrunk through the tools of future nanotechnology. Kare’s sails, in some designs, get up to a high percentage of c within seconds, pushed by a multi-billion watt orbiting laser. Will we reach the point where we can make Kare’s sails and Nordley’s smart pellets not the propulsion method but the probes themselves?

In that case, the idea of a single probe gives way to fleets of tiny, cheap spacecraft sent out at much lower cost. It’s a long way from LightSail-1, of course, but the principle is intact. LightSail-1 is a way of taking off-the-shelf Cubesat technology and giving it a propulsion system. Cubesats are cheap and modular. Equipped with sails, they can become interplanetary exploration tools, sent out in large numbers, communicating among themselves and returning data to Earth. LightSail’s cubesats compel anyone thinking long-term to ask where this trend might lead.

A Gravitational Lensing Swarm

In Existence, which I think is his best novel, David Brin looks at numerous scenarios involving miniaturization. When I wrote about the book in Small Town Among the Stars, I was fascinated with what Brin does with intelligence and nanotechnology, and dwelled upon the creation of a community of beings simulating environments aboard a starship. But Brin also talks about a concept that is much closer to home, the possibility of sending swarms of spacecraft to the Sun’s gravitational focus for observation prior to any star mission.

We normally speak about the distance at which the Sun’s gravity bends light from objects on the other side of it as being roughly 550 AU, but effects begin closer than this if we’re talking about gravitons and neutrinos, and in Brin’s book, early probes go out here, between Uranus and Neptune, to test the concept. But get to 550 AU and beyond and photon lensing effects begin and continue, for the focal line goes to infinity. We have coronal distortion to cope with at 550 AU, but the spacecraft doesn’t stop, and as it continues ever further from the Sun, we can be sampling different wavelengths of light to make observations assisted by this hypothesized lensing.

Before committing resources to any interstellar mission, we want to know what targets are the most likely to reward our efforts. Why not, then, send a swarm of probes. Claudio Maccone, who has studied gravitational lensing more than any other physicist, calls his design the FOCAL probe, but I’m talking about its nanotech counterpart. Imagine millions of these sent out to use the Sun’s natural lens, each with an individual nearby target of interest. Use the tools of future nanotech and couple them with advances in AI and emulation and you open the way for deep study of planets and perhaps civilizations long before you visit them.

The possibilities are fascinating, and one of the energizing things about them is that while they stretch our own technology and engineering well beyond the breaking point, they exceed no physical laws and offer solutions to the vast problems posed by the rocket equation. Perhaps we’ll build probes massing tens of thousands of tons to deliver a 100 kilogram package to Alpha Centauri one day, but a simultaneous track researching what we can do at the level of the very small could pay off as our cheapest, most effective way to reach a neighboring star.

More on this tomorrow, as I take a longer look at Clifford Singer and Gerald Nordley’s ideas on pellet propulsion. I want to use that discussion as a segue into a near term concept, Mason Peck’s ideas on spacecraft the size of computer chips operating in our Solar System.

And today’s references: Cliff Singer’s first pellet paper is “Interstellar Propulsion Using a Pellet Stream for Momentum Transfer,” JBIS 33 (1980), pp. 107-115. Gerald Nordley’s ideas can be found in “Beamriders,” Analog Vol. 119, No. 6 (July/August, 1999). Jordin Kare’s NIAC report “High-Acceleration Micro-Scale Laser Sails for Interstellar Propulsion,” (Final Report, NIAC Research Grant #07600-070, revised February 15, 2002) can be found on the NIAC site.