Bridging the Gap: Part II

A Discussion with Freeman Dyson

Going to another star is a terribly powerful idea, just as going to the Moon was originally. At some point in human history, there will be a leap across the great void not just to the nearest star but to any star that might be interesting to explore.

Renowned physicist, educator, and author Freeman Dyson joined Planetary Society Chairman of the Board Bruce Murray and Executive Director Louis Friedman at Society headquarters for an informal discussion about interstellar flight.

Their discussion dovetails to a proposal for sailing on solar wind. Nearly 400 years ago astronomer Johannes Kepler observed comet tails blown by a solar breeze and suggested that vessels might likewise navigate through space using appropriately fashioned sails. It is now widely recognized that sunlight does indeed produce a force which moves comet tails and a large, reflective sail could be a practical means of propelling a spacecraft. In fact, one concept explored by NASA centers is to develop an interstellar probe pushed along by sunlight reflected from an ultra-thin sail. Nearly half a kilometer wide, the delicate solar sail would be unfurled in space. Continuous pressure from sunlight would ultimately accelerate the craft to speeds about five times higher than possible with conventional rockets — without requiring any fuel.

In collaboration with the Planetary Society, Cosmos Studios, has funded the first solar sail. which had its initial trial launch from an intercontinental ballistic missile [ICBM] on a Russian submarine in the Barents Sea. The launch unfortunately had a third-stage separation failure, which was a problem of the ICBM rather than the spacecraft. They are launching again. Solar sailing is a kind of technology which enables probes to move through space ten times faster than even the Voyager spacecraft,–38,000 miles an hour. To go ten times faster than that begins to get to a potentially practical rapid transit system for our local neighborhood in space, but also even to go to other stars.

Lou: Do you think we’ll be putting the lasers in space or on the Moon?

Freeman: I would say in space, but I think that’s a matter of convenience. One of the problems with the Moon is day and night 14 days of night is inconvenient if you’re talking about solar energy.

Freeman Dyson, physicist, author, educator
Image Credit: Trustees of Dartmouth College

Lou: A nuclear-powered laser doesn’t make much sense.

Freeman: It makes no sense at all. Nuclear power is fine for getting around the solar system. But it’s no good if you’re talking about really high speeds you’re only using 1 percent of the mass. That’s no good at all.

With the power available from nuclear reactors, whether fission or fusion, you can comfortably reach speeds on the order of 100 kilometers [60 miles] a second or so which allows you to go more or less anywhere you want in the solar system within a couple of years, maybe even quicker.

But if you’re serious, you really want to travel at something like half the speed of light, which is tens of thousands of kilometers per second. So, the amounts of energy you need are enormously larger, and neither fission nor fusion has that much energy. At the very most, if you consume the fuel at 100 percent efficiency and have no other machinery on board, you’re using a little less than 1 percent of the mass.

Lou: Freeman, you said you weren’t intimidated by lasers, and I’m not intimidated by the sail. However, we agree that the biology, the challenge of human space travel long distance, is very hard. One revolution that may make interstellar flight more practical is the information revolution. We might be able to build brains and communication power into the wispy structure of the sail itself. You wouldn’t need to send humans at all.

Freeman: But people would like to go.

Bruce: Still, there may be 200 years’ difference between sending robotic spacecraft and sending humans.

Freeman: Oh, certainly. It’s the same with exploring the solar system. We’ve done that with robots very beautifully, making human pilots, from a scientific point of view, irrelevant. We’ll send humans for the human adventure.

When I said it will take 500 years to go interstellar, I was thinking of humans. When you’re talking about instruments only, then we could probably cut that in half.

Lou: Probably the biggest challenge will be to get a data rate that’s kilobits per seconds; megabits would be better.

Bruce: Mariner 4 was 8 bits per second. So, don’t knock it. We learned a lot. Eight bits per second from Alpha Centauri would be wonderful.

Lou: Only if we can use Bob Forward’s scheme of slowing down. Eight bits per second zooming through the Alpha Centauri system wouldn’t be good. Forward came up with an idea for a detachable sail that, after positioning itself out in front, reflects the light back so it can be used as a brake system.

Freeman: I think you can probably do better using magnetic fields, but that we will learn in due time.

Lou: Do you mean generating a magnetic field?

Freeman: Well, interacting with interstellar plasma. But we don’t really know how to do that yet.

It’s not energy you need then, but mass. Something to drag you to a halt. If you want to decelerate, you don’t need energy, you’ve got more energy than you want. You’ve got to have some way of dissipating the energy something massive to absorb your momentum. So, if you could couple yourself into the interstellar plasma, which has lots of mass, you could use that to brake.

The question is, can you couple yourself magnetically to the plasma in such a way that you use it as a cushion to bring yourself to a halt? In principle, it looks as if it would work, but whether it really does, we don’t know yet.

Lou: Two things have happened since the interstellar flight conference more than 20 years ago: the advancement of information processing and the microminiaturization of spacecraft. With that in mind, Freeman, would you describe yourself as more optimistic or about the same in terms of robotic interstellar flight in this century?

The ion-propelled Deep Impact 1 spacecraft.
Credit: NASA

Freeman: Well, I would say about the same. Judging from what we know now, I would say we’re not going to make it this century, but that could easily be wrong.

Bruce: I have a question for Freeman that I want to be sure to get in. Carl Sagan wrote about wormholes or worm tubes as a way to travel the solar system and beyond. The physics seems to be relatively stable. What do you think?

Freeman: That certainly could change things totally. But, in fact, I don’t think our understanding of wormholes has improved at all in the last 30 years. As far as we know, there’s absolutely no way they could actually function. All the models with imaginary wormholes don’t allow you to travel through them. There are all sorts of impossibilities you have to deal with in order to get from one end to another.

I would say that one of the best features of the universe, as far as I’m concerned, is the speed limit. It’s a guarantee of privacy you just get far enough away and you’re out of sight. I find that very consoling.

Lou: But it’s philosophically not very acceptable. It’s a limit.

Freeman: I find it very acceptable.

Bruce: Let me ask you another off-the-wall question. By your reasoning, it will take our civilization maybe 250 years from now to send a payload, and another 250 years to send a human, to another star. If that’s the case, there are presumably other planets out there with civilizations. They must have had the same opportunity. Where is everyone?

Freeman: It is a paradox. I tend to believe that life is much more difficult to get started than people seem to imagine. Of course, we know nothing about the origin of life, it is still a total mystery.

The simple explanation is that life is very rare, and that to me would be quite plausible as this planet does seem to be very suited to life or life is very suited to this planet.

It’s not a big surprise that we don’t see anybody out there. To me, it makes absolutely no sense to calculate probabilities. The exciting thing is to look whether or not we find anything.

Our solar system is so big and there’s so much variety out there, I think we’ll be less excited about interstellar travel because there will be exciting things we’ll discover along the way. It won’t be such a big jump once we’re traveling farther and farther from Earth and finding unexpected surprises. There may be many things going on out there of which we have no conception at the moment.

Bruce: I think that is very true, and this kind of exploration will happen because it’s doable, and the technology, both the miniaturization and the computing electronics, is making it easier and easier to do.

An artist’s rendition of Mars after terraforming the Red Planet.
Credit: Thinkquest

Freeman: And I would say that biology is going to be even more important. We’ll find ways of growing crops on Mars and growing potatoes on Europa and so on. As soon as we have a little better control of biology, most of these worlds will be habitable but in very different ways. We’ll have totally different ecologies in each place.

Lou: We’ll learn a lot about the search for life and the habitability on other planets from our experience here in the solar system especially on Mars and Europa and we’ll make some conclusions about the rest of the universe based on that.

Freeman: The fact that we’re getting stuff from Beta Pictoris also changes one’s view of panspermia the idea of life moving from place to place in space. If there are creatures living around Beta Pictoris, then they’re probably already here. If an organism is already adapted to living in a vacuum, interstellar travel is not all that big a problem.

Lou: I’m glad to hear you say that, Freeman. You’re well-known for being provocative and creative, with a lot of ideas that are intellectually stimulating, and yet in most of this conversation, you’ve been the conservative, pessimistic one.

Freeman: I only think you’re asking the wrong questions. My point is that sending humans on an interstellar trip is not really the interesting thing. There are so many more interesting things you can do in the time you have available.

Bruce: Like getting material from Beta Pictoris in the lab, where we could really look at it carefully– that’s exciting.

Related Web Pages

Freeman Dyson: Biography
Carl Sagan
Cosmo Solar Sail Project
Mars terraformation
Planetary Society