I’ve learned that you can’t assume anything when giving a public talk about the challenge of interstellar flight. For a lot of people, the kind of distances we’re talking about are unknown. I always start with the kind of distances we’ve reached with spacecraft thus far, which is measured in the hundreds of AUs. With Voyager 1 now almost 156 AU out, I can get a rise out of the audience by showing a slide of the Earth at 1 AU, and I can mention a speed: 17.1 kilometers per second. We can then come around to Proxima Centauri at 260,000 AU. A sense of scale begins to emerge.
But what about propulsion? I’ve been thinking about this in relation to a fundamental gap in our aspirations, moving from today’s rocketry to what may become tomorrow’s relativistic technologies. One thing to get across to an audience is just how little certain things have changed. It was exhilarating, for example, to watch the Arianne booster carry the James Webb Space Telescope aloft, but we’re still using chemical (and solid state) engines that carry steep limitations. Rockets using fission and fusion engines could ramp up performance, with fusion in particular being attractive if we can master it. But finding ways to leave the fuel behind may be the most attractive option of all.
I was corresponding with Philip Lubin (UC-Santa Barbara) about this in relation to a new paper we’ll be looking at over the next few days. Dr. Lubin makes a strong point on where rocketry has taken us. Let me quote him from a recent email:
…when you look at space propulsion over the past 80 years, we are still using the same rocket design as the V2 only larger But NOT faster. Hence in 80 years we have made incredible strides in exploring our solar system and the universe but our propulsion system is like that of internal combustion engine cars. No real change. Just bigger cars. So for space exploration to date – “just bigger rockets” but “not faster rockets”. [SpaceX’s] Starship is incredible and I love what it will do for humanity but it is fundamentally a large V2 using LOX and CH4 instead of LOX and Alcohol.
The point is that we have to do a lot better if we’re going to talk about practical missions to the stars. Interstellar flight is feasible today if we accept mission durations measured in thousands of years (well over 70,000 years at Voyager 1 speeds to travel the distance to Proxima Centauri). But taking instrumented probes, much less ships with human crews, to the nearest star demands a completely different approach, one that Lubin and team have been exploring at UC-SB. Beamed or ‘directed energy’ systems may do the trick one day if we can master both the technology and the economics.
Let’s ponder what we’re trying to do. Lubin likes to show the diagram below, which brings out some fundamental issues about how we bring things up to speed. On the one hand we have chemical propulsion, which as the figure hardly needs to note, is not remotely relativistic. At the high end, we have the aspirational goal of highly relativistic acceleration enabled by directed energy – a powerful beam pushing a sail.
Image: This is Figure 1 from “The Economics of Interstellar Flight,” by Philip Lubin and colleague Alexander Cohen (citation below). Caption: Speed and fractional speed of light achieved by human accelerated objects vs. mass of object from sub-atomic to large macroscopic objects. Right side y-axis shows γ − 1 where γ is the relativistic “gamma factor.” γ − 1 times the rest mass energy is the kinetic energy of the object.
Thinking again of how I might get this across to an audience, I fall back on the energies involved, for as Lubin and Cohen’s paper explains, the energy available in chemical bonds is simply not sufficient for our purposes. It is mind-boggling to follow this through, as the authors do. Take the entire mass of the universe and turn it into chemical propellant. Your goal is to accelerate a single proton with this unimaginable rocket. The final speed you achieve is in the range of 300 to 600 kilometers per second.
That’s fast by Voyager standards, of course, but it’s also just a fraction of light speed (0.1 percent), and the payload is no more than a single proton! We need energy levels a billion times that of chemical reactions. We do know how to accelerate elementary particles to relativistic velocities, but as the universe-sized ‘rocket’ analogy makes clear, we can’t dream of doing this through chemical energy. Particle accelerators reach these velocities with electromagnetic means, but we can’t yet do it beyond the particle level.
Directed energy offers us a way forward but only if we can master the trends in photonics and electronics that can empower this new kind of propulsion in realistic missions. In their new paper, to be published in a special issue of Acta Astronautica, Lubin and Cohen are exploring how we might leverage the power of growing economies and potentially exponential growth in enough key areas to make directed energy work as an economically viable, incrementally growing capability.
Beaming energy to sails should be familiar territory for Centauri Dreams readers. For the past eighteen years, we’ve been looking at solar sails and sails pushed by microwave or laser, concepts that take us back to the mid-20th Century. The contribution of Robert Forward to the idea of sail propulsion was enormous, particularly in spreading the notion within the space community, but sails have been championed by numerous scientists and science fiction authors for decades. Jim Benford, who along with brother Greg performed the first laboratory work on beamed sails, offers a helpful Photon Beam Propulsion Timeline, available in these pages.
In the Lubin and Cohen paper, the authors make the case that two fundamental types of mission spaces exist for beamed energy. What they call Direct Drive Mode (DDM) uses a highly reflective sail that receives energy via momentum transfer. This is the fundamental mechanism for achieving relativistic flight. Some of Bob Forward’s mission concepts could make an interstellar crossing within the lifetime of human crews. In fact, he even developed braking methods using segmented sails that could decelerate at destination for exploration at the target star and eventual return.
Lubin and Cohen also see an Indirect Drive Mode (IDM), which relies on beamed energy to power up an onboard ion engine that then provides the thrust. My friend Al Jackson, working with Daniel Whitmire, did an early analysis of such a system (see Rocketry on a Beam of Light), The difference is sharp: A system like this carries fuel onboard, unlike its Direct Drive Mode cousin, and thus has limits that make it best suited to work within the Solar System. While ruling out high mass missions to the stars, this mode offers huge advantages for reaching deep into the system, carrying high mass payloads to the outer planets and beyond. From the paper:
…for the same mission thrust desired, an IDM approach uses much lower power BUT achieves much lower final speed. For solar system missions with high mass, the final speeds are typically of order 100 km/s and hence an IDM approach is generally economically preferred. Another way to think of this is that a system designed for a low mass relativistic mission can also be used in an IDM approach for a high mass, low speed mission.
We shouldn’t play down IDM because it isn’t suited for interstellar missions. Fast missions to Mars are a powerful early incentive, while projecting power to spacecraft and eventual human outposts deeper in the Solar System is a major step forward. Beamed propulsion is not a case of a specific technology for a single deep space mission, but rather a series of developing systems that advance our reach. The fact that such systems can play a role in planetary defense is a not inconsiderable benefit.
Image: Beamed propulsion leaves propellant behind, a key advantage. It could provide a path for missions to the nearest stars. Credit: Adrian Mann.
If we’re going to analyze how we go from here, where we’re at the level of lab experiments, to there, with functioning directed energy missions, we have to examine these trends in terms of their likely staging points. What I mean is that we’re looking not at a single breakthrough that we immediately turn into a mission, but a series of incremental steps that ride the economic wave that can drive down costs. Each incremental step offers scientific payoff as our technological prowess develops.
Getting to interstellar flight demands patience. In economic terms, we’re dealing with moving targets, making the assessment at each stage complicated. Think of photovoltaic arrays of the kind we use to feed power to our spacecraft. As Lubin and Cohen point out, until recently the cost of solar panels was the dominant economic fact about implementing this technology. Today, this is no longer true. Now it’s background factors – installation, wiring, etc. – that dominate the cost. We’ll get into this more in the next post, but the point is that when looking at a long-term outcome, we have a number of changing factors that must be considered.
Some parts of a directed energy system show exponential growth, such as photonics and electronics. And some do not. The cost of metals, concrete and glass move at anything but exponential rates. What “The Economics of Interstellar Flight” considers is developing a cost model that minimizes the cost for a specific outcome.
To do this, the authors have to consider the system parameters, such things as the power array that will feed the spacecraft, its diameter, the wavelength in use. And you can see the complication: When some key technologies are growing at exponential rates, time becomes a major issue. A longer wait means lower costs, while the cost of labor, land and launch may well increase with time. We can also see a ‘knowledge cost’: Wait time delays knowledge acquisition. As the authors note in relation to lasers:
The other complication is that many system parameters are interconnected and there is the severe issue that we do not currently have the capacity to produce the required laser power levels we will need and hence industrial capacity will have to catch up, but we do not want to be the sole customer. Hence, finding technologies that are driven by other sectors or adopting technologies produced in mass quantity for other sectors may be required to get to the desired economic price point.
System costs, in other words, are dynamic, given that some technologies are seeing exponential growth and others are not, making a calculation of what the authors call ‘time of entry’ for any given space milestone a challenging goal. I want to carry this discussion of how the burgeoning electronics and photonics industries – driven by power trends in consumer spending – factor into our space ambitions into the next post. We’ll look at how dreams of Centauri may eventually be achieved through a series of steps that demand a long-term, deliberate approach relying on economic growth.
The paper is Lubin & Cohen, “The Economics of Interstellar Flight,” to be published in Acta Astronautica (preprint).
