It seems that one of the main hurdles of creating an interstellar propulsion system is the need to carry reaction mass to accelerate while conserving momentum. Pretty much every "fringe science" propulsion proposal tries eliminate this problem (e.g. EmDrive, MEGA drive, Alcubierre drive, almost as if bypassing conservation of momentum is a primary need).

But we know electromagnetic waves can be used as reaction mass to satisfy conservation of momentum (that is how light sail works), so we don't really need to carry reaction mass.

The questions:

  1. Excluding "passive systems", like a light sail accelerated from an Earth based laser, have photon propulsion ever been seriously considered for starship drives?

  2. Considering the time it takes and the amount of fuel required, is there any way an ordinary nuclear fission reactor placed in the focus of a parabolic dish at the back of a starship power it to the nearest star using heat radiation as propellant?

  3. If not this, is there any workable design?

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    $\begingroup$ I fear you're out of luck here, although I would love to be proven wrong :) Anyway, when you have enough energy but need more mass, why don't your space travellers simply mine ice (H2O or similar) off asteroids or moons, and just top it up whenever they're running low? $\endgroup$ Sep 8, 2020 at 14:32
  • $\begingroup$ (a) You're asking questions as if there should be real, practical, empirical science behind the answers. We can't achieve anything at all like what you're asking. So, are you asking from a hypothetical point of view? As in, "mathematically, is something like this plausible?" (b) What do you mean by "seriously considered?" Even Ion thrusters require something in a tank. The inefficiency of today's solar panels means the increasing mass w/larger panels won't allow bussard ramjets. (c) Heat radiation is not electromagnetic. Something must be heated or there's no where for the heat to go. $\endgroup$ Sep 8, 2020 at 14:43
  • $\begingroup$ @JBH (a) I just want ballpark plausibility, something like "for the smallest reactor we can build and without any payload except fuel to accelerate through all journey, it would take about 10000 years to get to the nearest star". (b) Bussard ramjet is a good example of what was seriously considered before proven impossible. Has a few papers published on it, at least. (c) Black body radiation emitted by any hot body is electromagnetic radiation. That is why hot stuff "glows red". $\endgroup$
    – lvella
    Sep 8, 2020 at 14:54
  • $\begingroup$ The thermal radiation spontaneously emitted by many ordinary objects can be approximated as black-body radiation. It isn't black-body radiation. That's why it' so hard to evacuate heat in space. You could heat something to the point where it glows, in which case you're combusting the heated object - but you're trying to not have reaction mass, aren't you? $\endgroup$ Sep 8, 2020 at 15:00
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    $\begingroup$ I believe I have read about using lasers as propulsion. Generally the lasers are located at a fixed point and targeted at ships. This is so the fuel to power the laser does not have to be carried by the ship. Issac Arthur has had several good episodes detailing interstellar ships using laser propulsion. Smaller ships would be accelerated by the primary ship(s) so that they would arrive in a system with enough time to build up some infrastructure to harvest fuel and build up a laser (to either decelerate the primary ship(s), or to further accelerate it as it passes by the system). $\endgroup$ Sep 8, 2020 at 15:01

6 Answers 6


Excluding "passive systems", like a light sail accelerated from an Earth based laser, have photon propulsion ever been seriously considered for starship drives?

Yes. See the Atomic Rockets page on photon rockets. The classic photon rocket is an antimatter-powered drive propelled by gamma rays. Similar drives show up sporadically in science fiction. E.g., the personal rocket devices used by the aliens in Donald Moffitt's The Jupiter Theft are photon drives powered by total matter-energy conversion.

Considering the time it takes and the amount of fuel required, is there any way an ordinary nuclear fission reactor placed in the focus of a parabolic dish at the back of a starship power it to the nearest star using heat radiation as propellant?

Sure. Over a sufficiently long time, you don't need a whole lot of thrust to get up to very high speeds.

If not this, is there any workable design?

If you are using a fission reactor anyway, you might consider a Fission fragment rocket, which gives you a better power-to-thrust ratio than a pure photon rocket does.


The trouble with all those fringe sciences is that eventually one of them sticks, and becomes real. If we end up building interstellar travel, its a safe bet that 2020 physics says the engine we used was impossible, just as 1850 physics said time dilation of fast moving objects was impossible.

That said, I reckon there is one that's feasible within near future science: A Brussard ramjet.

If you can capture the interstellar medium, and accelerate it away from you using a particle accelerator, that thrust is enough to make a difference. The intersteller medium is 70% hydrogen gas, and about 10^6 molecules per cm^3. That's 10^-21kg per cm^3, or 10^-15 per m^3. A 10sqm sucker mounted on the front of a ship can collect a few micrograms every km of travel.

Combine that infinite source of free particles with tech like this single chip can accelerate a particle to 0.94c, and you've got a few N of free acceleration for every km of forward movement.

Yeah it's no star wars battle with sharp dogfights, but that could get a generation ship to a nearby star system.

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    $\begingroup$ This is the only solution I could think of - it's the bussard ramjet solution (the same basic tech as a rail gun, which I remember to be scifi as a teen, the "Gauss rifle"). If we can't use fuel, then we have solar panels powering massive magnetic fields to scoop and propel interstellar mass - I'm not sure the ratios can work in interstellar space. Ivella? Do you require no on-board fuel source, or only no reaction mass? $\endgroup$ Sep 8, 2020 at 14:57
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    $\begingroup$ @JBH Only no reaction mass. $\endgroup$
    – lvella
    Sep 8, 2020 at 15:14
  • $\begingroup$ This is a great concept, and I can't actually find anything else where this has been proposed before. $\endgroup$
    – cowlinator
    Sep 8, 2020 at 19:28
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    $\begingroup$ This is a variant of the Bussard ramjet. $\endgroup$
    – NomadMaker
    Sep 9, 2020 at 0:27

The Atomic Rockets page on photon drives conveys that at perfect efficiency in converting reactor power to a collated gamma ray beam (not happening; there will be waste heat, dispersion...), it takes three hundred megawatts of power to produce one newton of thrust. Unless you'd like to have your building-sized spacecraft accelerate with less than a millionth of a gee, fission reactors are out the window, as are most modern conceptions of fusion reactors. The only really efficient way to convert mass to energy on the scale needed for this kind of drive is a matter-antimatter reaction; proposed photon drive rockets use big tanks of antimatter as their fuel. Since there is no currently feasible way to produce large amounts of antimatter, photon rockets have not been seriously considered by space agencies.


Yes. Using electrodynamic propulsion.

satellite with electrodynamic propulsion


Electrodynamic propulsion relies on a long charged electrodynamic tether. The tether is charged by the spacecraft and by interacting with magnetic fields in space, it can generate propulsion. It still costs energy, but there is no reaction mass and nothing to be hurled behind the spacecraft - just the interaction of charged fields. There are spacecraft now which operate using this principle.


“In other words, it is the sailing ship of space,” says Enrico Lorenzini, a professor of energy management engineering at the University of Padova in Italy, who is not involved in the TEPCE mission. But instead of wind, the electrodynamic tether technology moves thanks to the physical laws that govern electric and magnetic fields. A tether in Earth’s ionosphere—an upper atmospheric layer filled with charged particles such as free electrons and positive ions—can collect electrons at one end and emit them at the other, generating an electric current through itself. The electrified tether’s interactions with Earth’s magnetic field produce an impetus known as the Lorentz force, which pushes on the tether in a perpendicular direction.

There are plenty of magnetic fields in a solar system and especially in the neighborhood of a dynamo like Earth. But are there magnetic fields in deep space? There are, but their provenance remains somewhat mysterious.


magnetic fields

The ISM contains equal numbers of positively and negatively charged particles, so that large-scale electric currents (that could induce large-scale magnetic fields) cannot be maintained. The most promising mechanism for field amplification is the dynamo that transfers mechanical energy into magnetic energy... With a suitable configuration of the gas flow, a strong magnetic field with a stationary or oscillating configuration can be generated from a weak seed field. Seed fields could have been generated in the early Universe, e.g. at cosmological phase transitions, or in shocks in protogalactic halos (Biermann battery), or through fluctuations in the protogalactic plasma.

To traverse interstellar space using electrodynamic propulsion, one would need to identify favorable force lines and align the ship with their path. This lends itself to a fiction because as professor Lorenzini noted in the above quote, it becomes analogous to a ship in the days of sail. And for pushing off against the energy of weak fields one needs a larger sail, or tether. I envision a huge skein of copper wires billowing around the spacecraft, glowing a slight green with its own charge and faint copper ion plasma. And you will need to be ready to take it down if a storm comes...

  • $\begingroup$ Thank you for validating my assertion that reaction mass is NOT necessary for propulsion, the only requirement is energy input. Usually, people refer to the formula as F=ma, but in reality it is the re-arranged formula a=F/m, that is, applied Force per unit mass you want to move at acceleration a. $\endgroup$ Sep 9, 2020 at 16:10
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    $\begingroup$ Shades of the Hornblower inspired series of 'sailing ships and riggers' in space, methinks? Particularly Honor Harrington? $\endgroup$ Sep 10, 2020 at 15:55

I don't know if photon propulsion has been considered, but it's likely not because of the low energy. Photons officially have no mass. If I can quote wikipedia: The total force exerted on an 800 by 800 meter solar sail, for example, is about 5 newtons (1.1 lbf) at Earth's distance from the Sun.

Now interstellar travel doesn't require a lot of energy. Simply putting in more energy is enough, as there is (practically) nothing to slow the craft down. So some engines have been proposed that accelerate very, very slowly, but they can move for decades, if not possibly centuries on end. But light on Earth distance on an 800 by 800 large sheet isn't a small amount of energy. The reason to use light is that it is already abundant, or can be used in beam sailing as you say. With the propelling laser based outside the craft. Putting it on the craft has several very bad implications. You'll have to push against the reactor as well, and in general firing something pushes you backwards with the same amount of force. I don't know if the same applies to light, but you might just put as much force backwards as forwards if you apply it to a sail. Then you might better just aim it backwards and fire away for acceleration.

An alternative is much more likely. Using the energy of the reactor, fire tiny particles as hard as possible backwards. If you push the particle backwards, you'll push the craft forwards with equal force. So with a particle accelerator inside you might expel the particles with as much force as a nuclear reactor can manage, making as efficiently use of the particles as possible.

There are some further alternatives, like they are trying to use curves of spacetime to their advantage, generating a deeper field in front of the craft than at the back, pulling it forwards.

But if you really want to have an EM drive, check out what NASA is doing at this time with it. Already in 2015 they were doing experiments to check if it could really work, as physics said it couldn't. Still it does work a tiny, tiny amount. Seems negligible, but is according to them pretty impressive. More research is obviously needed to make very, very sure it isn't an error, but it might well be possible to use it later.

  • $\begingroup$ Regarding the EM drive, it will be the epitaph of the 20th century that so much progress was killed off by our absolute narcissistic attitude and belief that the real world was restricted by our human-written 'Laws of Physics'. No, the real world is NOT limited by the artificial and arbitrary Laws that us mere humans, in fact, have writen and created to try to explain, control, and command it. We have spent the last 150 years or so, trumpeting from the highest peaks, 'Universe, you MUST obey the Laws that we Humans have cast down in Stone!!!! OBEY, OBEY, OBEY your Mighty Masters!!!!!' $\endgroup$ Sep 9, 2020 at 14:07
  • $\begingroup$ @JustinThymetheSecond for some yes :). Physicist very much know that they describe the world and that their models are an approximation and inadequate. They continously try to improve and get new theories. $\endgroup$
    – Trioxidane
    Sep 9, 2020 at 15:54

The title of the question looked promising, but the body then went somewhat off on a tangent.

In the elaboration on the question, it is still assuming that 'reaction mass' is the only way to propel something.

Yes, the laws of conservation of momentum require energy to be input into a system to create acceleration, but they do not require that reaction mass is necessary, nor that the energy source be inherent or contained in the object that you require to be moved.

There are hundreds of examples of transportation/propuldion systems on earth that do not use 'reaction mass'. That is, they have exactly the same mass at the end of the journey as they do at the beginning, without refilling. They mostly use electricity and electrical motors (those that do not depend on gravity to pull them down to earth). A trolly car, for instance, can go a great distance without its mass being changed one iota. Okay, so it is constantly supplied with energy, but electricity is hardly a 'reaction mass', the electrons are returned to source. Tesla's hyperlopp system, where the propulsion system consists of electromagnets in the tube wall propelling the cars, is another example. No reaction mass need be carried at all by the train. Maglev systems are currently in operation using induction motors that require no energy be supplied to the trains. We even have tractor beams under develpoment as serious no-reaction-mass-required transportation systems.

There are also many propulsion systems currently used in space travel that do not use reaction mass. They use the gravitational boost of planets, in a sling-shot maneuver, to boost speed.

As a future-feasible practical example, using electromagnets (as the title suggests), think of a regular, commercial flight path between say Earth and Mars. Along the path, place super-huge satelite way stations powered by fusion ractors or such. They exert a powerful electromagnetic field (or other such tractor beam) along the route, such that they alternately attract and then repel the spaceship along the route. Newton, of course, woukd insist that their mass would have to be substabtially greater than the 'cars' they pull/push, and substantial development would be required in being able to focus/direct/concentrate the electromagnetic field, but this is an engineering problem more than it is a physics problem. Like Ash said in his (her?) answer, it is only a matter of time before the physics and enginnering textbook get thick enough to allow this scenario, first on an within-star, then an inerstellar system.

But the bottom line is, the laws of the conservation of momentum do not demand reaction mass be used for propulsion, just some form of supplied energy.

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    $\begingroup$ When a car travels on a road, or a maglev train travels on its track, the "reaction mass" is the entire Earth. As the wheels push the car or the magnets push the train forwards, they push the Earth in the opposite direction. A physical system cannot change its total momentum by internal means only. The laws of conservation of momentum absolutely demand that if something accelerates in one direction something else must accelerate in the opposite direction. $\endgroup$
    – AlexP
    Sep 8, 2020 at 22:23
  • $\begingroup$ @AlexP No, the laws of conservation of momentum do not say that at all. That is the most commonly held misconception, that even tops out 'centrifugal force'. The 'law' in reality states 'every action is applied equally in opposite directions'. When you step off a boat, you're foot pushes you forward AND pushes the boat back, equally. The force from your foot is applied in equal and opposite directions. The tractor beam not only pulls the space ship towards it, it pulls the satelite towards the space ship. Hence the need for a very large satelite base station. $\endgroup$ Sep 9, 2020 at 0:46
  • $\begingroup$ The rocket engine 'force' is not just 'ejecting' the reaction mass out the nozzle, it is equally pushing the rocket forward. Same force applied in two opposite directions equally. The propulsion force has no idea it is supposed to only work on the ejected mass, so it works on the rocket as well, equally. $\endgroup$ Sep 9, 2020 at 0:51

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