If you haven’t read George Dyson’s fascinating history of Project Orion, let me recommend it to you highly. Project Orion: The True Story of the Atomic Spaceship (Henry Holt, 2002) fires the imagination with the audacity of the project, a nuclear pulse rocket that would have exploded atomic bombs behind the vehicle, using the world’s ultimate shock absorbers to ride the wave to the outer planets. There was talk of going to Saturn (to Enceladus, no less) in the late 1960’s, but those dreams were quickly quashed by treaties forbidding nuclear testing.

The Problem with Orion

Kelvin Long, who heads up the ambitious Project Icarus attempt to revisit and extend Project Daedalus, notes in a recent post on the Icarus blog that Freeman Dyson (George’s father) ultimately gave up on Orion (a fact that surprised me when I did a telephone interview with him on the prospects for interstellar propulsion back in 2003). Here’s what Dyson says about the subject in his book Disturbing the Universe (1979):

Sometimes I am asked by friends who shared the joys and sorrows of Orion whether I would revise the project if by some miracle the necessary funds were suddenly to become available. My answer is an emphatic no…. By its very nature, the Orion ship is a filthy creature and leaves its radioactive mess behind it wherever it goes…. Many things that were acceptable in 1958 are no longer acceptable today. My own standards have changed too. History has passed Orion by. There will be no going back.

People still advocate concepts like Orion, but the Icarus team draws on the extensive work that has been done on pulsed fusion using a high intensity laser beam or an electron driver to produce detonation of fusion fuel pellets. This was the Daedalus approach in the 1970s, capitalizing not only on the research performed for Orion but also the intense scrutiny on pulsed micro-explosions that began in the early 1970s. The crucial fact is that this body of knowledge can be tapped to produce realistic performance assessments.

Image: Daedalus arrives at Barnard’s Star. Credit: Adrian Mann.

Fusion Alternatives and Their Uses

Long goes through various space propulsion systems in his post, noting two obvious categories. The first involves electric propulsion, which is power-limited; i.e., these methods can produce high specific impulse and high exhaust velocity, but at very low levels of thrust. The second covers both chemical and nuclear methods, which are energy-limited and can produce high thrust with high exhaust velocity, although at the cost of a short specific impulse. If we can develop fusion technologies sufficiently, we should be able to combine the advantages of both these approaches, getting more out of our fuel and increasing efficiency.

The fuel itself? Long talks about the same Deuterium/Helium-3 reaction that interested the Daedalus designers, although he notes the potential for Deuterium/Tritium. The latter involves the production of large amounts of neutrons and could therefore be problematic. On the other hand, we’re talking about huge demands on the civilization that develops D/He3 methods, too, as he goes on to note when discussing recent advances in fusion research:

The physics of fusion research has moved forward dramatically in recent years with the US National Ignition Facility now operational and others such as Laser MegaJoule in France under construction. Fast ignition proposals such as HiPER are also under consideration. The chances for scientists finally solving the ‘fusion problem’ are greatly increased. With this in mind, thinking about the implications to deep space missions is timely. It is quite possible that the demands of a fusion based drive will necessitate a sophisticated space based infrastructure for resource acquisition, processing, manufacture and construction. Especially if He3 mining of the gas giant Jupiter or even the Moon is considered. However, as a theoretical exercise in the application of science and engineering Project Icarus has a large amount of intrinsic worth.

Icarus and Competing Studies

We’ll see what the Project Icarus team comes up with (disclaimer: I am a consultant on the project, and obviously an advocate of Icarus). Given that Daedalus was a design study that changed the game, producing the first detailed investigation of what it would take to build an interstellar probe, it makes sense to build upon that work to re-evaluate its key assumptions and improve the design. It would be excellent to see such efforts paralleled by a study of an interstellar probe based on solar sail technology coupled with laser or microwave beam propulsion. Icarus is one attempt to expand our knowledge of interstellar engineering. It is not meant to rule out studies of competing concepts by others.

Ultimately [writes Long] the aim would be to improve the Technological Readiness Level for this sort of engine design type. If other teams used the same approach, and say built upon historical projects like Vista, Longshot, TAU or Starwisp it is a personal belief that the credibility of engineering designs for interstellar missions would be vastly improved. The historical link with both Orion and Daedalus also captured the hearts of the Icarus team and made for a strong support base upon which to galvanize both academic and public interest; a necessary condition to inspire people that this design study is worth doing. Although it is also true that after having questioned the original assumptions of Daedalus, the final Icarus design may look very different with technology not envisioned in the 1970s.

Indeed. Are hybrid designs possible? Long mentions antimatter-catalyzed fusion techniques (Penn State did early work on these possibilities), and that notion resonates given the relatively small amounts of antimatter needed and the potential for mining naturally occurring antimatter in the Solar System (James Bickford has studied the matter extensively — see this earlier Centauri Dreams story for more on his work). Whatever the case, just as the solar sail concept was refined through key papers in the 1980s, leading to our current readiness to deploy small sails for testing in space, fusion methods continue to be investigated in hopes of pushing our research into designs that will eventually be practical.