It was back in 1950 that Arthur C. Clarke looked at electromagnetic methods for getting a payload into space. The concept wasn’t new but Clarke’s paper in JBIS set out to examine what he saw as a practical use of it, an electromagnetic catapult on the lunar surface that could accelerate payloads back to Earth. The system was built around a three-kilometer long electromagnetic launcher that could accelerate payloads at 100 g’s to 2.3 kilometers per second (lunar escape velocity) in a matter of seconds. Gerard O’Neill thought such methods could deliver lunar raw materials to low Earth orbit for delivery to a space manufacturing site.
Clarke’s ideas played naturally into O’Neill’s, for building large space habitats requires vast amounts of raw materials that we’d just as soon not have to lift out of Earth’s gravity well. But Clarke’s thinking wasn’t restricted to near-Earth uses of the technology. He saw no necessary limit to the lengths and accelerations that could be used. Provide enough power to a space-based electromagnetic launcher and it could be configured to send robotic payloads to nearby stars. As with solar sails, we’re accelerating no propellant — only the payload — and that’s magic to any rocket scientist.
By the time E. H. Lemke looked at the idea in 1982, the size of such launchers had become astronomical. Lemke’s accelerator reached 108 kilometers in length, and used a solar collecting array hundreds of kilometers to the side to store the energy needed to boost payloads up to a third of lightspeed. Probes would be flung at 5000 g’s to nearby stars. It’s hard to see how a civilization capable of building on a scale like this (Lemke’s accelerator would be ⅔ of an astronomical unit long!) wouldn’t also have developed smaller and more efficient ways of sending probes on the same journey, but the same could be said for many giant sail concepts.
Clifford Singer’s work at Princeton on ‘pellet stream propulsion’ makes use of both Clarke and Lemke to arrive at a somewhat more adaptable idea. Writing in 1980, Singer proposed using a stream of electromagnetically charged pellets to transfer momentum to a departing interstellar craft. You can think in terms of a lightsail being pushed by a laser or microwave beam, but in this case the beam is composed of macroscopic pellets, each with a mass of several grams, being dispatched at 0.2 c to the starship, where they would impact and impart momentum.
At 105 kilometers in length, Singer’s electromagnetic launcher would be huge, though not on the scale of Lemke’s. In the original work, Singer emphasized that a system like this solved the collimation problem — the spread of the beam over distance — that would be faced by laser-beamed missions. His idea was to use several dozen stations, perhaps deployed by the starship itself, to measure pellet positions and send the needed commands to adjust their course at the launcher. He examined the question of dispersion due to the impact of interstellar dust grains along the route and concluded that pellets of 1 gram and up could be delivered to the starship with accuracy.
Later, Gerald Nordley would take the logical leap of ‘smart’ pellets, with enough technology aboard to adjust their course as required. Nordley’s ‘snowflake’ pellets would be built around nanotechnology and could find their own way to a starship that could itself make small changes in course if required to stay within the stream. And Jordin Kare subsequently produced ‘SailBeam,’ in which Singer’s pellets give way to tiny ‘micro-sails’ that can be accelerated to the departing starship and vaporized upon approach, becoming a plasma that drives the spacecraft.
Image: Jordin Kare’s ‘SailBeam’ concept. Credit: Jordin Kare/Dana G. Andrews.
SailBeam, which was a hybrid system crossing pellet propulsion with lightsails, also offered a major advantage. Kare’s studies demonstrated that you could accelerate a large number of small sails with a far less demanding optical system than required by a huge lightsail. The sails could also be accelerated much closer to their power source, which simplifies the problems of maintenance and sail deployment faced by a large sail under the beam for lengthy periods. Made of diamond film, Kare’s sails would accelerate to close to lightspeed within seconds.
So many concepts, so little time! Digging into interstellar exotica is a refreshing exercise that reminds me how many concepts have been kicked around in the literature. Most are grossly impractical given our level of technology, but the refining of some of them seen in Nordley and Kare’s work shows that we can at least nudge some of these notions in a more practical direction. Practical, that is, for a future with greater energy resources and wider options within the Solar System than we enjoy.
The original Arthur C. Clarke paper is “Electromagnetic Launching as a Major Contribution to Space-Flight,” JBIS 9 (6) (1950), pp. 261-267. It’s reprinted in Clarke’s Ascent to Orbit: A Scientific Autobiography (New York: John Wiley & Sons, 1984). The Lemke paper is “Magnetic Acceleration of Interstellar Probes,” JBIS 35 (1982), pp. 498-503. Cliff Singer’s first pellet paper is “Interstellar Propulsion Using a Pellet Stream for Momentum Transfer,” JBIS 33 (1980), pp. 107–115. And you can find 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) on the NIAC site.