Somehow I usually expect famous people to be big-sized ('larger than life'!), but Dyson is a fairly small person, maybe 5 ft 5', and he looks very sprightly for his 80+ years. Despite spending over half a century in this country, he has still managed to keep his English accent. We all introduced ourselves, and when I told him that I was working on primordial black holes in the radiation era, he said, 'Wow, that's very speculative, best of luck on that...'
Introducing himself, he described himself as a mathematician with an interest in solving problems in physics. Being in the midst of a bunch of astrophysics grad students, he started off telling us that his closest brush with astronomy was when he helped to developed the technique of adaptive optics* some 25 years ago. He was working with a group of scientists linked to the military called JASON (apparently the name didn't stand for anything, and was randomly chosen), and the USAF wanted a system that would allow ground-based telescopes to observe hostile satellites.
*Adaptive optics is the technique of using a bright natural star or a laser beam as a reference for atmospheric turbulence, and adjusting the telescope surface in real-time to correct for the turbulence.
He wrote one of the first paper outlining the feasibility of adaptive optics systems, and then "...the idiots in the Air Force went and classified it". However, since all the Air Force required was to observe satellites which were pretty bright, they never pushed the limits of the technology. Meanwhile, astronomers were either denied access to the technology, or didn't bother trying to develop it in the first place since they assumed that the Air Force would do it better (when actually the Air Force technology stayed stagnant). It wasn't until about a decade ago that adaptive optics technology was declassified, and since then it has caused a renaissance in ground-based optical astronomy.
I then asked him about Project Orion, which was the scheme to propel interplanetary spacecraft by dropping nuclear bombs behind it to provide impulse. He worked on the project in the late '50s, and the idea at the time was to have a 2000 ton spacecraft with a 1000 ton payload, with about 1000 ton's worth of nuclear bombs as propellant. The propulsion would involve dropping about 2 bombs per second, each with a yield of a couple of kilotons. The spacecraft would basically be hemispherical, with a buffer on the flat bottom to absorb the impulse from the blast, and provide shock absorption and some radiation protection. They were also planning to launch the thing directly from the ground!
I asked him about the safety of the astronauts, and he said 'Well, we were all hoping to get on it ourselves, so we did think about keeping the astronauts alive!'. The Orion spacecraft would have been a bumpy ride, but the maximum acceleration experienced by the astronauts would have been only about 3-4 G's, since the spacecraft could be accelerated relatively gradually on average compared to chemical rockets. During the propulsion phase, the astronauts could take shelter in a hardened compartment in the middle of the spacecraft to get additional protection from the radiation.
The Orion spacecraft was intended to be launched from the surface, and the idea at the time was to launch it from a barge in the middle of the ocean to avoid too much contamination, with the spacecraft propped up on legs prior to launch to avoid being vapourised by the first blast! However, by that time Von Braun's rocketeers were already in full swing, and the Orion group never got funding to do a test where the test vehicle would be propelled to the upper atmosphere, and then a single bomb would be detonate to test the concept ("At the time the United States was testing 100 megatons a year, and our test only involved a couple of kilotons!", he complained). The project dragged on until the mid-60s, when it finally died from being overtaken by the new political environment and the success of chemical rockets.
*To be continued*
