A note from James Benford, soon to be followed by e-mail from other interstellar advocates, tells me of the death of Robert Bussard. The creator of the Bussard ramjet concept, Bussard (1928-2007) died of cancer in Santa Fe just a few days ago. Benford, who knew Bussard for forty years, recalls his open attitudes and deep technical insight, adding “He was still sharp as a pin into old age.” We should all be so lucky.

Recently we’ve seen a lot of discussion about Bussard’s fusion ideas, but it’s the ramjet that I return to as I think about him. If you collect classic papers, as I do, here’s one for you: Bussard’s “Galactic Matter and Interstellar Spaceflight” in Acta Astronautica 6 (1960), pp. 179–94. Imagine a scoop created by a magnetic field that sucks in interstellar hydrogen ionized by a forward-firing laser. The result is fed into a fusion reactor. Get the vehicle up to about six percent of light speed and you could light that engine, with presumably amazing results.

Bussard ramscoop

At least, that was the idea. Assume one hydrogen atom per cubic centimeter as the density of the interstellar medium and a one-ton probe would demand a scoop 6,000 kilometers in size, but to be realistic about interstellar densities, you’d probably need one as large as 60,000 kilometers to account for hydrogen density variations. The concept was indeed breathtaking even if later work made its success dubious (drag is a major problem, leading to the idea of using magnetic sails for deceleration into a target system).

But imagine going at one g continuously. When Carl Sagan went to work on that one, he got to Alpha Centauri in three years (ship time, obviously), but as the vessel picked up speed, things got more and more interesting. The Andromeda galaxy could theoretically be reached in about 25 years, assuming you’re willing to live with the two million years that passed in the interim back on Earth. Poul Anderson’s starship Leonora Christine in the novel Tau Zero grew directly out of Bussard’s idea.

Image: A Bussard ramjet could theoretically get a crew to Andromeda. Credit: ESA/ITSF/Mancu.

Anderson’s description of the Bussard ramjet at work is superb:

The ship was not small. Yet she was the barest glint of metal in that vast web of forces which surrounded her. She herself no longer generated them. She had initiated the process when she attained minimum ramjet speed; but it became too huge, too swift, until it could only be created and sustained by itself … Starlike burned the hydrogen fusion, aft of the Bussard module that focused the electromagnetism which contained it. A titanic gas-laser effect aimed photons themselves in a beam whose reaction pushed the ship forward—and which would have vaporized any solid body it struck. The process was not 100 per cent efficient. But most of the stray energy went to ionize the hydrogen which escaped nuclear combustion. These protons and electrons, together with the fusion products, were also hurled backward by the force fields, a gale of plasma adding its own increment of momentum . . . The process was not steady. Rather, it shared the instability of living metabolism and danced always on the same edge of disaster…

Suffice it to say that if you haven’t read Tau Zero (and yes, we did name the Tau Zero Foundation after the book, and Poul’s wife Karen is on our board of directors), then you should make haste to your nearest bookseller or library. I think Bussard must have taken great pleasure from reading Tau Zero, as well as knowing that his stunning starship concept would galvanize theoretical research into interstellar propulsion for decades to come. He’s been part of my thinking on interstellar matters for the last forty years, a loss everyone involved in our deep space future will take to heart. Our thoughts and condolences go out to his family.