Let’s start the week by talking about gravitational assists, where a spacecraft uses a massive body to gain velocity. Voyager at Jupiter is the classic example, because it so richly illustrates the ability to alter course and accelerate without propellant. Michael Minovitch was working on this kind of maneuver at UCLA as far back as the early 1960s, but it was considered even before this, as in a 1925 paper from Friedrich Zander. It took Voyager to put gravity assists into the public consciousness because the idea enabled the exploration of the outer planets.

Can we use this kind of maneuver to help us gain the velocity we need to make an interstellar crossing? Let’s consider how it works: We’re borrowing energy from a massive object when we do a gravity assist. From the perspective of the Voyager team, their spacecraft got something for ‘free’ at Jupiter, in the sense that no additional propellant was needed. What’s really happening is that the spacecraft gained energy at the expense of the planet. Jupiter being what it is, the change in its own status was invisible, but it lent enough energy to Voyager to prove enabling.

According to David Kipping (Columbia University), the maximum speed increase equals twice the velocity of the planet we’re using for the maneuver, and when you look at Jupiter’s orbital speed around the Sun (around 13.1 kilometers per second), you can see that we’re only talking about a fraction of what it would take to get us to interstellar speeds. But the principle is enticing, because traveling with little or no propellant is a longstanding goal, one that drives research into solar sails and their fast cousins, beamed lightsails. And it has been much on Kipping’s mind.

For gravitational assists from planets are only one aspect of the question, there being other kinds of astrophysical objects that can help us out. Depending on their orbital configuration, some of these are moving fast indeed. In the early 1960s, Freeman Dyson went to work on the physics of gravitational assists around binary white dwarf stars — he would ultimately go on to consider the case of neutron star binaries (back when neutron stars were still purely theoretical). Such concepts obviously imply an interstellar civilization capable of reaching the objects in the first place. But once there, the energies to be exploited would be spectacular.

While I want to begin with Dyson’s ideas, I’ll move tomorrow to Kipping’s latest paper, which addresses the question in a novel way. Kipping, well known for his work in the Hunt for Exomoons with Kepler project, has been pondering Dyson’s notions but also applying them to what would seem, on the surface of things, to be an entirely different proposition: Beamed propulsion. How he combines the two may surprise you as much as it did me, as we’ll see in coming days.

Image: An artist’s conception of two orbiting white dwarf stars. Credit: Tod Strohmayer (GSFC), CXC, NASA, Illustration: Dana Berry (CXC).

Nature of the Question

If we talk about manipulating astrophysical objects, a natural objection arises: Why should we study things that are impossible for our species today? After all, we can get to Jupiter, but getting to the nearest white dwarf, much less a white dwarf binary, is beyond us.

But big ideas can be productive. Consider Daedalus, conceived in the 1970s as the first serious design for a starship. The idea was to demonstrate that a spacecraft could be designed using known physics that could make a journey to another star. The massive two-stage Daedalus (54,000 tonnes) seems impossible today and doubtless will never be built. Was it worth studying?

The answer is yes, because once you’ve established that something is not impossible, you can go to work on ways to engineer a result that may differ hugely from the original. Breakthrough Starshot is built around the idea of using lasers to propel a different kind of spacecraft, not of 54,000 tonnes but of 1 gram, carried by a small lightsail, and designed to be sent not as a one-off mission but as a series of probes driven by the same laser installation.

Once again we’re stretching our thinking, but here the technologies to do such a thing may (or may not, depending on what Breakthrough Starshot’s analyses come up with) be no more than a few decades away. The current Breakthrough effort is all about finding out what is feasible.

Again we’re designing something before we’re sure we can do it. The challenges are obviously immense. Consider: To go interstellar with cruise times of several decades, we need to ramp up velocity, and that takes enormous amounts of energy. Kipping calculates that 2 trillion joules — the output of a nuclear power plant running continuously for 20 days — would be needed to send the Breakthrough Starshot ‘chip’ payload to Proxima Centauri. And that’s just for one ‘shot’, not for the multiple chips envisioned in what might be considered a ‘swarm’ of probes.

Working with Massive Objects

Are there other ways to generate such energies? Freeman Dyson’s extraordinary white dwarf binary gravitational assist appears in “Gravitational Machines,” a short paper that ran in a book A.G.W. Cameron edited called Interstellar Communication (New York, 1963). Conventional gravity assists aren’t sufficient because to be effective, a gravitational ‘machine’ would have to be built on an astronomical scale. Fortunately, the universe has done that for us. So we should be thinking about the principles involved, and what they imply:

…if our species continues to expand its population and its technology at an exponential rate, there may come a time in the remote future when engineering on an astronomical scale will be both feasible and necessary. Second, if we are searching for signs of technologically advanced life already existing elsewhere in the universe, it is useful to consider what kinds of observable phenomena a really advanced technology might be capable of producing.

Dyson’s considers the question in terms of binary stars, specifically white dwarfs, but goes on to address even denser concentrations of matter in neutron stars. Now we’re talking about a kind of gravitational assist that has serious interstellar potential. A spacecraft could be sent into a neutron star binary system for a close pass around one of the stars, to be ejected from the system at high velocity. If 3,000 kilometers per second appears possible with a white dwarf binary, fully 81,000 kilometers per second could occur — 0.27 c — with a neutron star binary.

Hence the ‘Dyson slingshot.’ (As an aside, I’ve always wondered what it must be like to have a name so famous in your field that everything from ‘Dyson spheres’ to ‘Dyson dots’ are named after you. The range of Dyson’s thinking on these matters certainly justifies the practice!).

The slingshot isn’t particularly effective with stars of solar class, where what you gain from a gravitational assist is still outweighed by the possibility of using stellar photons for propulsion. But as Dyson shows, once you get into white dwarf range and then extend the idea down to neutron stars, you’re ramping up the gravitational energy available to the spacecraft while at the same time reducing stellar luminosity. An advanced civilization, in ways Dyson explores, might tighten the orbital distance until the binary’s orbital period reached a scant 100 seconds.

Now a gravity assist has serious punch. In other words, there is the potential here for a civilization to manipulate astrophysical objects to achieve a kind of galactic network, where binary neutron stars offer transportation hubs for propelling spacecraft to relativistic speeds. As you would imagine, this plays to Dyson’s longstanding interest in searching for technological artifacts, and we’ll be talking about that possibility as we get into David Kipping’s new paper.

For Kipping will take Dyson several steps further, by looking not at neutron stars but black hole binaries and coming up with an entirely novel way of exploiting their energies, one in which a beam of light, rather than the spacecraft itself, gets the gravitational assist and passes those energies back to the vehicle. Kipping calls his idea the ‘Halo Drive,’ and we’ll begin our discussion of it, and a novel insight that inspired it, tomorrow.

The Dyson paper is “Gravitational Machines,” in A.G.W. Cameron, ed., Interstellar Communication, New York: Benjamin Press, 1963, Chapter 12. The Kipping paper is “The Halo Drive: Fuel-free Relativistic Propulsion of Large Masses via Recycled Boomerang Photons,” accepted at the Journal of the British Interplanetary Society (preprint). For those who want to get a head start, Dr. Kipping has also prepared a video on the Halo Drive that is available here.