Voyager's Edward Stone remembered

First this week, tributes have been paid to Edward Stone, the man who oversaw the remarkable Voyager missions. He’s died. He was 88. Stone - who was originally from Knoxville in Iowa - attended the California Institute of Technology before joining the NASA Jet Propulsion Laboratory. It was there that he became project scientist for the Voyager space missions - which would define his career. To find out about Edward Stone and his remarkable achievements, I went to meet someone who knew him, and knows a lot about the cosmos, our Astronomer Royal, Martin Rees…

Martin - Yes, I'm privileged to know him because I went to Caltech for academic reasons quite often. And he was an important presence around there. And of course I was aware of his great achievements, the Voyager Project in particular. But the value of his leadership was very important because he was very genial and engaging, a very good team leader. And of course, it's remarkable that he was in charge of these Voyager projects for 50 years, more or less.

Chris - It was NASA's kind of golden decade, wasn't it? The seventies. But why did they do Voyager when they did in 1977?

Martin - Well, as Ed in particular realised, and others did, there was at that time an interest in a possible grand tour. This is the idea that the alignment of the outer planets was such that by bouncing off them, by going close to them, it might be possible to give an extra push to a spacecraft headed to the edge of the solar system and get to the edge rather quicker. And so this alignment, which happened every 170 years or thereabouts, was a good opportunity and it would be sad if there weren't efforts made to send some probes to the outer planets at that time. And Ed was one of the people who actually implemented this.

Chris - Why two Voyager probes, though?

Martin - They went on slightly different orbits and one of them overtook the other. And I think one was concentrating more on Saturn and Uranus and one hoped to get further.

Chris - What did they teach us?

Martin - Well, of course we got some pictures of those outer planets. And of course there have been some other probes that have given us later pictures, the outer planets and Pluto. But of course, what's been really special about Voyager is that it is still going after nearly 50 years. It's way beyond Pluto. And it's sending back data. And the particular thing which is important for a very remote probe is that it gets out of the inference of the solar system. The solar system, obviously we regard as the planets and the asteroids and Kuiper belt and all that. But there is a region where the solar wind pushes out the interstellar medium. And so we're not really in interstellar space until we get to the edge of this region. And it was generally thought that you had to get further away than Pluto before you would encounter the transition between seeing the wind of the sun still going out and genuine, pristine interstellar matter. And it's clear that the voyagers did get across this barrier and found changes in the density, the temperature, and the magnetic field at that time. So that was the distinctive achievement to actually be the first human based objects that got outside the influence of the solar system.

Chris - I had a look at NASA's website this morning because you can chart the progress of the Voyagers and it says they're 24 billion kilometres away, so that's roughly four times the Earth to Pluto distance at closest. It's a long way <laugh>.

Martin - Yeah, that's right. And it takes more than a day to get a signal to them. And what's been remarkable, and that's been in the news in the last few months, is that the signal from Voyager went dead. Something went wrong. But amazingly they seem to have managed to revive it. And if you just think of what's involved sending a signal to something that far away from a radio transmitter, some dish here, and getting back a signal from that enormous distance from something where the battery gives just a few watts of power. It is amazing really. And of course, the other inference I draw is if we think of the worthwhileness of what can be done even with the technology of the 1970s, think how much better we could do if we were able to launch a whole flotilla of probes with the kind of material that we have in our mobile phones. Much more sophisticated, much more compact.

Chris - I'm in awe just of the engineering. 50 years later, well 46 years to be precise, in the worst imaginable environment really in terms of radiation temperature and so on. This stuff is still working. Mostly they've had to shut down some of the systems. There's even a tape recorder on Voyager, one magnetic tape that records things and then sends it back at the princely rate of two kilobits, I think <laugh>. And it sends it back. But that's amazing. It still works.

Martin - The constraint is the weight limit, which means you can't be as robust as you'd like to be with these instruments because of the weight limit. And incidentally, that's going to be a problem that's going to be much eased when we have the new SpaceX rocket, which can launch up to a hundred tonnes into space. So at the moment, we are looking forward to much bigger projects where we can launch heavier things, but this was a very small object, which had to be simple and use technology that's 50 years out of date.

Chris - What I found amazing about Ed Stone was that he was still going through a time where many scientists have hung their test tube rack up. He was still going, still leading a mission at the age of 88. Or thereabouts. That's pretty good.

Martin - Well, that's right. And of course, very few of us have the privilege of having work. We started when we were young, still going and still being fruitful and still being deemed interesting when we were as old as he was. But he was a great guy. He continued in fairly good health.And he was a very good leader. And over his career, he was the director of the big lab at Pasadena, and he was involved in more than a dozen different projects over his time. So he made a great contribution to space exploration in general.