Per nuclear ad astra…

A few years ago I presented a course somewhat ambitiously titled A History of the Universe to a number of WEA groups. One of the most interesting sections (to me) was a discussion of the enormous number of stars and galaxies (and their ‘astronomical’ distances), and the fairly recent discovery that many—probably most—stars have a retinue of planets, including rocky Earth-like planets. Could we even find extra-terrestrial life out there, I wondered? 
 
In one of the course modules, I considered the difficulties of travel to the stars. The great physicist Enrico Fermi suggested that life on Earth was unique. If it wasn’t, and considering the vast age and size of the universe, many other civilizations must have arisen millions of years before humanity, developed space travel, and have come to visit us. Since they have not done so, we can safely assume that they do not exist. A most interesting idea (I said), but it has one fatal flaw: the vast distances between the stars mean that manned (or creatured) spacecraft based on any known scientific principles would take many thousands of years even to travel from here to the nearest star.
 
And then, via good old Wikipedia, I came across a declassified Los Alamos paper from 1955 authored by C J Everett and S M Ulam. Stanislaw Ulam, along with Edward Teller, developed the thermo-nuclear ‘hydrogen’ bomb, and suggested the idea of Nuclear Pulse Propulsion. The paper with Everett developed the idea which was, quite seriously, to propel missiles to very high speeds by exploding a series of H bombs behind them…
 
Following the reference trail, I found an article in Physics Today from October 1968. The author was Freeman J Dyson, a renowned (originally English) mathematician from Princeton, who had worked on the defunct Orion project. Project Orion, funded in the US by DARPA, had examined the possibility of nuclear pulse propulsion as applied to a spaceship for travel to the stars. There is more detail on the Wikipedia page, but the basics can be condensed from Dyson’s paper.
 
Dyson proposed a starship propelled by sequentially exploding one megaton H bombs behind a ‘pusher’ plate connected to a shock-absorber. The bombs would detonate every three seconds over a period of ten days or so, requiring around 300,000 bombs. This would produce an average acceleration of one g, with a final velocity of 10,000 km per second—around one thirtieth of the speed of light. The cost of such a ship at 1968 prices would be 60,000 million dollars, around 0.1 US GNP. The shock absorber would ensure that the crew were not squashed by the force of the explosions, and suitable shielding would protect them from radiation from the bombs.
 
Dyson suggested that it would have been much cheaper to send a spacecraft to Mars powered by nuclear bombs, than it was costing to send the Apollo astronauts to the moon. For travel to Mars, only a few bombs would have been needed, so that’s a relief.