I didn't join the project actually until the following summer because I had other things to do in Princeton, but I took a leave of absence from the Institute for a full year, so I joined the project in the summer of '59 and then – I guess that's right – and worked there for a full year until the fall of 1960 – or is that.... is that correct? No, I think it's wrong, I think we're a year wrong. Sputnik went up in '57, I joined the project in summer '58 and went back to Princeton in fall of '59. So I was 15 months working on Orion. And that was also a very exciting time. We did all the basic physics, and it works, I mean there's no doubt technically the thing works. You have a number of very interesting physics problems; essentially it's radiation, hydrodynamics, the same kind of physics that you need for studying stellar atmospheres. A lot of it's just applied astrophysics, which I... I enjoy, so it's using information you get from astrophysics for this practical application. And there are always stability problems in hydrodynamics. The hard part is understanding the various instabilities. You have this oblique flow; the... the bomb is essentially a point source, the debris is flying off in a cone hitting the bottom of the ship. The bottom of the ship has to be very broad to distribute the impulse over a wide area so you don't get too high a pressure, so a lot of the stuff is coming up at an oblique angle, so it's capable of then giving rise to various kinds of instabilities, Taylor instabilities and Helmholtz instabilities. So we worked out the instabilities and tried to convince ourselves that they were tolerable. And then you have... this is combined with neutron physics and radiation flow, so there was a lot of good physics to be done. And in addition, there was a lot of very interesting engineering. I mean, one of the very big problems is shock absorbers. The bottom of the ship is a very heavy flat plate and it's impulsively... pushed by the bomb debris, once every second or so, so you get this bang, bang, bang on the bottom of the plate, and that impulse has to be transmitted to the rest of the ship, and the passengers if you're actually on board, in a more gentle fashion. So you need huge shock absorbers to transfer the momentum from the bottom of the ship to the rest of the structure. So there's a big engineering problem there. So we went through a lot of different designs of shock absorbers. And then there were very interesting problems connected with the command and control of the orientation of the ship, and especially what you do if one of the bombs is a dud, so the pusher plate is going backwards and the bomb doesn't fire, and you have to have some means of rescuing the pusher plate before it disappears! So, anyway, there were lots of problems, but after a year and a half I had to make the choice either to stay with this or go back to my scientific life in Princeton, and it was obvious to me by that time that I'd done what I could for the project and from then on it had to fly by itself. It was already by... by that time pretty clear that this wasn't going to be our road into space. It was a competition between this kind of nuclear propulsion and the big chemical rockets, the von Braun brigades, and it was fairly clear that von Braun was going to win. He had the chemical rockets and he had a huge organisation already in place, and he had a lot of political support, and we weren't going to push him out, and if we couldn't push him out then we didn't have really a great and glorious future. And so it was fairly clear that this thing wasn't going to fly, although it was still alive. The project actually continued for 7 years and it developed considerably after I left, but by the time I left the... sort of the real fire had gone out of it. The dream that we would be the ones actually to fly to Mars I think had more or less disappeared.