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Ion Propulsion for the Nearby Interstellar Medium

When scientists began seriously looking at beaming concepts for interstellar missions, sails were the primary focus. The obvious advantage was that a large sail need carry no propellant. Here I’m thinking about the early work on laser beaming by Robert Forward, and shortly thereafter George Marx. Forward’s first published work on laser sails came during his tenure at Hughes Aircraft Company, having begun as an internal memo within the firm, and later appearing in Missiles and Rockets. Theodore Maiman was working on lasers at Hughes Research Laboratories back then, and the concept of wedding laser beaming with a sail fired Forward’s imagination.

The rest is history, and we’ve looked at many of Forward’s sail concepts over the years on Centauri Dreams. But notice how beaming weaves its way through the scientific literature on interstellar flight, later being applied in situations that wed it with technologies other than sails.

Thus Al Jackson and Daniel Whitmire, who in 1977 considered laser beaming in terms of Robert Bussard’s famous interstellar ramjet concept. A key problem, lighting proton-proton fusion at low speeds during early acceleration, could be solved by beaming energy to the departing craft by laser.

Image: Physicist A. A. Jackson, originator of the laser-powered ramjet concept.

In other words, a laser beam originating in the Solar System powers up reaction mass until the ramjet reaches about 0.14 c. The Bussard craft then switches over to full interstellar mode as it climbs toward relativistic velocities. Jackson and Whitmire would go on in the following year to confront the problem that a ramscoop produced enough drag to nullify the concept. A second design emerged, using a space-based laser to power up a starship that used no ramscoop but carried its own reaction mass onboard.

The beauty of the laser-powered rocket is that it can accelerate into the laser beam as well as away from it, since the beam provides energy but is not used to impart momentum, as in Forward’s thinking about sails. In the paper, huge lasers are involved, up to 10 kilometers in diameter, with a diffraction limited range of 500 AU.

But note this: As far back as 1967, John Bloomer had proposed using an external energy source on a departing spacecraft, but focusing the beam not on a departing fusion rocket but one carrying an electrical propulsion system bound for Alpha Centauri. So we have been considering electric propulsion wed with lasers as far back as the Apollo era.

Now we can swing our focus back around to the paper by Angelo Genovese and Nadim Maraqten that was presented at the recent IAC meeting in Paris. Here we are looking not at full-scale missions to another star, but the necessary precursors that we’ll want to fly in the relatively near-term to explore the interstellar medium just outside the Solar System. The problem is getting there in a reasonable amount of time.

As we saw in the last post, electric propulsion has a rich history, but taking it into deep space involves concepts that are, in comparison with laser sail proposals, largely unexplored. A brief moment of appreciation, though, for the ever prescient Konstantin Tsiolkovsky, who sometimes seems to have pondered almost every notion we discuss here a century ago. Genovese and Maraqten found this quote from 1922:

“We may have a case when, in addition to the energy of ejected material, we also have an influx of energy from the outside. This influx may be supplied from Earth during motion of the craft in the form of radiant energy of some wavelength.”

Tsiolkovsky wouldn’t have known about lasers, of course, but the gist of the case is here. Angelo Genovese took the laser-powered electric propulsion concept to Chattanooga in 2016 when the Interstellar Research Group (then called the Tennessee Valley Interstellar Workshop) met there for a symposium. Out of this talk emerged EPIC, the Electric Propulsion Interstellar Clipper, shown in the image below, which is Figure 9 in the current paper. Here we have a monochromatic PV collector working with incoming laser photons to convert needed electric power for 50,000 s ion thrusters.

Image: An imaginative look at laser electric propulsion for a near-term mission, in this case a journey to the hypothesized Planet 9. Credit: Angelo Genovese/Nembo Buldrini.

Do notice that by ‘interstellar’ we are referring to a mission to the nearby interstellar medium rather than a mission to another star. Stepping stones are important.

Genovese and Maraqten also note John Brophy’s work at NASA’s Innovative Advanced Concepts office that delves into what Brophy considers “A Breakthrough Propulsion Architecture for Interstellar Precursor Missions.” Here Brophy works with a 2-kilometer diameter laser array beaming power across the Solar System to a 110 meter diameter photovoltaic array to feed an ion propulsion system with an ISP of 40,000 seconds. That gets a payload to 550 AU in a scant 13 years, an interesting distance as this is where gravitational lensing gets exploitable. Can we go faster and farther?

Image: John Brophy’s work at NIAC examines laser electric propulsion as a means of moving far beyond the heliosphere, all the way out to where the Sun’s gravitational lens begins to produce useful scientific results. Credit: John Brophy.

An advanced mission to 1000 AU emerged in a study Genovese performed for the Initiative for Interstellar Studies back in 2014. Here the author had considered nuclear methods for powering the craft, with reactor specific mass of 5 kg/kWe. Genovese’s calculations showed that such a craft could reach this distance in 35 years, moving at 150 km/s. This saddles us, of course, with the nuclear reactor needed for power aboard the spacecraft. In the current paper, he and co-author Maraqten ramp up the concept:

The TAU mission could greatly profit from the LEP concept. Instead of a huge nuclear reactor with a mass of 12.5 tons (1-MWe class with a specific mass of 12.5 kg/kWe), we could have a large monochromatic PV collector with 50% efficiency and a specific mass of just 1 kg/kWe… This allows us to use a more advanced ion propulsion system based on 50,000s ion thrusters. The much higher specific impulse allows a substantial reduction in propellant mass from 40 tons to 10 tons, leading to a TAU initial mass of just 23 tons instead of 62 tons. The final burnout speed is 240 km/s (50 AU/yr), 1000 AU are reached in just 25 years (Genovese, 2016 [26]).

In fact, the authors rank electric propulsion possibilities this way:

  • Present EP performance involves ISP in the range of 7000 s, which can deliver a fairly near-term 200 AU mission with a cruise time in the range of 25 years.
  • Advanced EP concepts with ISP of 28,000 s draw on an onboard nuclear reactor, and produce a mission to 1000 AU with a trip time of 35 years. The authors consider this ‘mid-term development.’
  • In terms of long-term possibilities, very advanced EP concepts with ISP of 40,000 s can be powered by a 400 MW space laser array, giving us a 1000 AU mission with a trip time of 25 years.

So here we have a way to cluster technologies in the service of an interstellar precursor mission that operates well within the lifetime of the scientists and engineers who are working on the project. I mention this latter fact because it always comes up in discussions, although I don’t really see why. Many of the team currently working on Breakthrough Starshot, for example, would not see the launch of the first probes toward a target like Proxima Centauri even if the most optimistic scenarios for the project were realized. We don’t do these things for our ourselves. We do them for the future.

The Maraqten & Genovese paper is “Advanced Electric Propulsion Concepts for Fast Missions to the Outer Solar System and Beyond,” 73rd International Astronautical Congress (IAC), Paris, France, 18-22 September 2022 (available here). The laser rocket paper is Jackson and Whitmire, “Laser Powered Interstellar Rocket,” Journal of the British Interplanetary Society, Vol. 31 (1978), pp.335-337. The Bloomer paper is “The Alpha Centauri Probe,” in Proceedings of the 17th International Astronautical Congress (Propulsion and Re-entry), Gordon and Breach. Philadelphia (1967), pp. 225-232.

{ 24 comments… add one }
  • James Davis Nicoll October 18, 2022, 9:57

    Supplying power to such vehicles was the inspiration for the Nicoll Dyson Laser. Being able to evaporate planets at a million light years was not the purpose and came as a surprise.


    • ljk October 18, 2022, 16:32

      If ETI are anything like humans in their evolution, they may find more reason to build a Dyson Shell first as a megaweapon before funding any other projects such as pushing starship sails, Matrioshka Brains, or habitats.

      Just look at what humans used rockets for first before replacing warheads atop them with scientific instruments and astronauts. Or I should say in many but not all cases.

      These YouTube videos go into wonderful detail on the concepts, with in-depth scientific analysis…

      Nicoll-Dyson Beams:


      Matrioshka Brains:


      • DCM October 20, 2022, 12:55

        Conflict powers technology and understanding. We wouldn’t be in space if it weren’t for the US and USSR competing for the high ground.
        If some aliens are peaceful and understanding they’re probably on the cultural level of orangs.

  • Benjamin R Stockton October 18, 2022, 10:27

    I totally enjoy your series – stimulates a lot of discussion with my sons. But I worry that all the designs for interstellar and interstellar precursors proceed without consideration of keeping people alive, shielding and life support. Of course these make no sense for a trip to e.g. Alpha Centauri. IMHO humans are never going to another star anyway, so I understand the trade off over load and speed. But shouldn’t we look at sending humans anyway, somewhere? I like the Plotonic Ionic Mission; send a ship with people to flyby Pluto, Solar circumnavigation, drop off payloads on the way by various places, return to Earth/Moon. To me this emulates parts of a star mission (dropping off payloads on the way by thus avoiding de-celleration) and builds human experience in long duration trips (35 years!). Anyway, fun stuff.

    • ljk October 18, 2022, 16:52

      What advantages do human crewed space missions have over robotic ones if the goal is scientific exploration? AI and robotic technology is only getting better and even in the earliest days of the Space Age we saw benefits of sending machines into the Final Frontier over humans: Much cheaper, less complicated, last longer, enter more dangerous environments, and so on.

      Obviously humans will keep going into space regardless of the various costs and the extra resources and time required just to keep them alive in a place that would otherwise kill them in seconds, forget about exploring and scientific returns. The main reason for doing this as time progresses is for settlement purposes to preserve the species.

      Of course it won’t be long before even that changes as bioengineering and general technology will mold humanity into new forms in our attempts to improve the species. This will include better adapting our descendants for space.

      I still think Artilects will dominate space exploration in the coming generations as well as be the type of beings we will encounter from other star systems. However, there are those species which may prefer to manipulate the organic into becoming fully adapted to raw space and alien worlds that no current human could last upon, This includes our own species.

      If nothing else, it will solve the major problems of contemporary humans trying to adapt to confining ships and settlements for very long periods of time among a host of other potential issues:



  • Alex Tolley October 18, 2022, 14:05

    Small niggle with the paper. Figure 8 is a laser thermal propulsion concept, not a laser electric propulsion.
    The relevant URL on the projectrho site is Engine List 1/Beamed Power/Laser Thermal

  • Alex Tolley October 18, 2022, 14:55

    The LEP concept relies on Brophy’s assumption[?] of a 0.2 Kg/KW PV system. Directed-Energy Propulsion Architecture for Deep-Space Missions with Characteristic Velocities of Order 100 km/s This is about 25x the mass/power output of the best solar PV in space at 1 AU and is primarily expected to be due to new ultra thing perovskite PV arrays. Areal density is 100 g/m^2 . The proposed PV array generates about 0.33 KW/m^2 (140 m dia array, 10 MW laser, 50% conversion efficiency), about 2x that of a ground-based solar PV array assuming all the 10MW laser power is captured. As the beam divergence eventually increases beyond the array area, the electric power output by the array declines.

    If that performance can be realized, it looks very attractive. Such lightweight laser PV arrays would have more applications than deep space missions, and could conceivably make in-system probes faster and more flexible than currently. It really will depend on having those MW lasers available to power the craft. If refueling from depots is possible, this would make a shipping line possible to deliver cargo throughout the system.

    I would like to see any work looking into these possibilities for developing the solar system exploration and economy using beamed power coupled with these lightweight laser PV arrays and electric engines.

    • Randy Chung October 19, 2022, 13:25

      Here’s the abstract I have submitted to the Interstellar Research Group. If it’s accepted, I hope to present the paper in Montreal in July.
      Title: Ion Beam Propulsion Using A DC Particle Accelerator
      A lightweight high performance engine can be built using basic physics principles and in-space assembly. The initial stage of ion generation would occur at relatively low voltages, allowing the use of a low power electric or magnetic nozzle before the ions are accelerated. The engine’s ion accelerator is gridless, so it does not suffer from grid erosion as the acceleration voltage is scaled up. The ion beam is periodically re-focused using electrostatic or magnetic lenses. The acceleration voltages are directly generated from thin film photonic panels by a tapped serpentine array of series connected photoelectric or rectenna cells. The accelerator can be built using metal foils and thin plastic sheets and tubes, and can be powered from solar photons, remotely beamed photons (including microwave frequency photons), and locally beamed photons. Our calculations show that an ion engine for low Earth orbit applications with specific impulse of 12,000 and 10 kilovolt DC acceleration is achievable, and an engine for interstellar applications with specific impulse of 1.2 million and 100 megavolt DC acceleration is feasible.


      • Alex Tolley October 20, 2022, 13:25

        specific impulse of 12,000 and 10 kilovolt DC acceleration is achievable

        An Isp of 12,000s implies a Ve of 120 km/s. That would be enough for an interstellar flight, although I wonder at the thrust this generates. I would guess very low, although it does seem very efficient.

        Is this based purely on calculations, or is there a demonstration prototype that indicates that these values are real?

        • Randy Chung October 20, 2022, 20:23

          It’s just calculations for now. We do intend to build a solar powered one for LEO and asteroid missions, but it will take time and money. Perhaps we will try for a NIAC grant, which isn’t enough to build one, but would be enough to design one. And we have something else going on that should allow us to build one in maybe five years.

  • Ivan Vuletich October 18, 2022, 21:41

    One thing that strikes me about these studies is how massive improvements in the system being advocated are assumed, while only minor improvements in competing systems are also assumed.

    In this case, why not consider a hybrid scheme? Use the laser for the initial boost and have a small nuclear reactor (<1MW?) take over when beam divergence becomes a problem. The solar arrays and ion engines could then be jettisoned to save mass and increase the efficiency of the nuclear stage. Alternatively, could the PV arrays be made to do double duty as radiators? Once the probe is no longer under laser power and in the cold environment somewhere beyond the asteroid belt, most likely.

    The advantages of this is that you an optimise your systems for what they do best, initial boost for the laser, sustained operation in cold environments for the nuclear reactor and you now have a power source for decent data rates at your destination.

    On a related note, I recently came across a recent electric thruster called an Electrodeless Lorentz Force Thruster.

    If the boosters are to be believed, its inherently light weight, rugged and runs on just about any propellant. It also doesn't need the heavy power processing systems of conventional ion drives. This could be a good topic for a future CD post.

    • Michael October 19, 2022, 7:03

      This mirrors my thoughts, a high thrust booster perhaps to cause a drop into the inner solar system where there is more energy and back out again.

    • Alex Tolley October 20, 2022, 13:19

      The Weber Ph.D. thesis was from 2010. Yet there is very little I have been able to dig up since then. Is there any current development, or has it proved a discontinued or dormant project? There are other electrodeless engines, which may have taken on that mantle.

    • Adam October 22, 2022, 23:46

      Hi Ivan
      As my previous CD post discussed, high-thrust/high-power is a combination that nuclear reactors, sadly, can’t supply for electric propulsion of any kind. Uranium fuel can only get so hot and put out so much power per kilogram. At least, the only kind of reactors we’ve had experience making.

      Plasma Core Fission Reactors, which are essentially controlled nuclear explosions, might have enough power per unit mass, but our experience in making them is very limited. People often drag out Fission Fragment Rockets or Orion as alternatives, but both options are horribly anti-social in low orbits. Any such system would need to fire up past the Moon’s orbit to reduce their nuisance value for orbital assets, and independent explosive fission devices in space still violate the Test Ban Treaty.


  • Robin Datta October 18, 2022, 22:14

    “We don’t do these things for our ourselves. We do them for the future.”

    “We” seem to do them. They get done by processes that seem to be controlled “us”: the “we/us” appears very real, but cannot be pinned down. Like a ball of yarn, when unrolled, there is nothing at the center.

    • henry cordova October 20, 2022, 16:52

      Emergent complexity?

      You have a point. Things happen because people make decisions to do things, but there are also other causes. The pre-conditions have to exist, and attitudes and technology have to be in place. There must be pre-existing demands and capabilities, and lots of just plain random luck. History is not a smoothly flowing stream, it is an avalanche of disparate forces and motives, sometimes reinforcing and other times c9nflicting. And in this chaotic flow there are eddies and turbulence, interference and standing waves

      Why didn’t the Romans have an Industrial Revolution? Why didn’t Chinese civilization develop scientific method the way the Europeans did? Historians are always searching for the reasons and causes why things do or don’t happen, and sometimes they pick on the individual genius who precipitates a landmark event, and sometimes they hand-wave and mutter about how “conditions were ripe”, or the “breakthrough was inevitable”, “it was only a matter of time”..

      Of course, with the perspective of history we often delude ourselves into thinking we have traced out how and why things occur, but have you noticed how the actual participants in these great revolutions never seemed to realize they were unfolding, or that they were themselves responsible for them? When historical events or processes are studied, the actual causes seem to depend as much on the historian who is studying the event as to any actual empirical narrative that might be gleaned from the data. Does this mean the historians are all wrong? Or that only one of them is right? Or perhaps that they are ALL correct, that their separate interpretations are simply different perspectives of some truth too complex for us to properly understand.

      History is a fractal, chaotic process, populated by multiple strange attractors. And it looks very different depending on the time scale you choose, or the resolution of your investigative tools, And there are no simple obvious dualities at work like diffusion vs independent invention, nature vs nurture, free will vs determinism, cause vs effect, or for that matter, particle vs wave. We invent them to help us think, they may have no actual referents in reality. Human behavior, both individual and collective, is a quantum process. It is non-Newtonian, and relativistic. We have lots of experience with it and can navigate it fairly well, but we do so only by instinct and intuition, not by reason.

      History is fuzzy and statistical, and causality doesn’t necessarily apply. And even the individual actors all seem to have an exaggerated opinion of their own importance and relevance: ” I think I think, therefore, I think I am.”

  • Neil Stahl October 20, 2022, 13:18

    I wonder about searching for such activities as part of SETI.
    There’s the narrowcast energy being sent out to spacecraft, likely from somewhat near the star (issues of distinguishing it from stellar radiation and it’s usually not being pointed at us) and the radiation from the exhaust of the spacecraft (often well separated from the star but much fainter; possibly the sum of such activity over the part of the stellar system we can separate from the star would be detectable?).

  • Kevin in WI October 20, 2022, 23:54

    A 10MW laser is a strategic asset with military implications. The dual use issue will have to be addressed before it could be built without controversy/unwanted reactions. Is there an otherwise benign location that doesn’t compromise its usefulness? Something like the far side of the moon comes to mind.

    • Antonio October 21, 2022, 7:25

      Exactly. In the end, it will be more difficult to develop than a fission rocket.

    • Alex Tolley October 21, 2022, 7:59

      A 10 MW space-based laser would be a large beast. While it would clearly have military applications, even a stealthed unit would be vulnerable to countermeasures after its first shot. Unstealthed, it would be an easy target to disable. As for their military use, they would be effective against stationary targets, slow-moving targets like ships or tanks, and targets that move predictably like ballistic missiles. My guess is that as a weapon it would be too expensive and vulnerable to deploy, IOW, a military boondoggle.
      Analysis of a 10 Megawatt Space-Based Solar-Pumped Liquid …

      But I agree with you that there would have to be some safeguards for civilian use, even planetary defense from asteroids. Maybe the simplest solution is to park it in LEO behind a shield to monitor it and prevent it from being pointed at terrestrial and satellite targets.

    • Mike Serfas October 21, 2022, 17:27

      In 1996, I think, Air Force University put out a declassified version of an “AF2020 report”, predicting the use of a space-based laser system. It was later repudiated and (like most disturbing things) seems to have dropped out of sight on the internet in the past few years, but as I recall the plan was rather straightforward: use two or more space mirrors to bounce a ground-based laser to the desired destination. Given subsequent strides in adaptive optics and the quality of mirrors available, the notion wouldn’t seem out of the question, at least when the option of testing the aim with a guide beam before firing laser pulses is taken into account. A very powerful laser might not even be necessary for some applications: wildfires grow more common and destructive every year, undoubtedly due to climate change.

      Though a ground-based laser introduces some technical issues, it would allow such devices to be built out of sight in nuclear-hardened bunkers, perhaps linked to the surface only by narrow boreholes oriented to intersect with satellites passing above.

      • Alex Tolley October 22, 2022, 5:44

        it would allow such devices to be built out of sight in nuclear-hardened bunkers, perhaps linked to the surface only by narrow boreholes oriented to intersect with satellites passing above.

        If the mirror[s] is/are key to directing the beam from anything other than nearly vertical, doesn’t that imply that destroying the mirror above the laser renders the laser useless? I suppose one could build and launch a swarm of these mirrors to create redundancy, but they should be easily trackable and easy targets. It would be rather “James Bond” style to disguise the mirror as a regular comsat which then quickly unfurls a [multi-diamond] mirror just before it is used to reflect the beam.

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