I do mean propulsion (not gravity assist or the like). All the "rocket" engines I've seen (chemical, nuclear, ion, etc) seem to eject material one way to get the craft pushed the other way. Other than the Alcubierre drive (warp drive) which is purely theoretical/fictional at the moment, is there any other propulsion system that does not involve ejecting material?

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    $\begingroup$ Lightsails might count. $\endgroup$ Commented Jun 29, 2021 at 17:16
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    $\begingroup$ You didn't ask for a good alternative :) $\endgroup$ Commented Jun 29, 2021 at 18:07
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    $\begingroup$ A lunar railgun pushes the moon away to accelerate the projectile. $\endgroup$ Commented Jun 29, 2021 at 19:12
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    $\begingroup$ @DavidHammen Bussard ramjets (theoretically) accelerate matter and eject it. Gravity assists were ground ruled out in the question. $\endgroup$ Commented Jun 29, 2021 at 19:26
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    $\begingroup$ @Rodo Do Bussard ramjets count? While they do work by ejecting matter, the matter they eject is captured from the interstellar medium and then accelerated in the spacecraft. Bussard ramjets escape the tyranny of the rocket equation by not carrying the propellant they eject with them. In theory, at least. (I wrote "in theory" because nobody has built a Bussard ramjet, or has even proposed how to build one beyond the science fiction stage of building.) $\endgroup$ Commented Jun 29, 2021 at 19:38

3 Answers 3


[This is an expansion of @sno's answer about the dipole drive]

Robert Zubrin's dipole drive is a refinement of the idea of the electric sail, taking advantage of the ubiquity of plasma in space and the difference in mass between the plasma's positively and negatively charged components. As described in the abstract of the paper that describes it:

The dipole drive is a new propulsion system which uses ambient space plasma as propellant, thereby avoiding the need to carry any of its own. The dipole drive remedies two shortcomings of the classic electric sail in that it can generate thrust within planetary magnetospheres and it can generate thrust in any direction in interplanetary space. In contrast to the single positively charged screen employed by the electric sail, the dipole drive is constructed from two parallel screens, one charged positive, the other negative, creating an electric field between them with no significant field outside. Ambient solar wind protons entering the dipole drive field from the negative screen side are reflected out, with the angle of incidence equaling the angle of reflection, thereby providing lift if the screen is placed at an angle to the plasma wind. If the screen is perpendicular to the solar wind, only drag is generated but the amount is double that of an electric sail of the same area. To accelerate within a magnetosphere, the positive screen is positioned forward in the direction of orbital motion. Ions entering are then propelled from the positive to the negative screen and then out beyond, while electrons are reflected. There are thus two exhausts, but because the protons are much more massive than the electrons, the thrust of the ion current is more than 42 times greater than the opposing electron thrust, providing net thrust. To deorbit, the negative screen is positioned forward, turning the screen into an ion reflector. The dipole drive can achieve more than 6 mN/kWe in interplanetary space and better than 20 mN/kWe in Earth, Venus, Mars, or Jupiter orbit. In contrast to the electric sail, the ultimate velocity of the dipole drive is not limited by the speed of the solar wind. It therefore offers potential as a means of achieving ultra-high velocities necessary for interstellar flight.

  • $\begingroup$ Wow, I just wrote this comment and refreshed the page, and voila! Journal of the British Interplanetary Society :-) $\endgroup$
    – uhoh
    Commented Jun 30, 2021 at 14:15
  • $\begingroup$ This is still pushing on matter to produce thrust, yes? It's just that it uses the matter found in the ambient environment. $\endgroup$
    – Harabeck
    Commented Jul 1, 2021 at 19:46

Of all the stuff that makes up the universe, we generally don't consider light as matter. Light has no rest mass at least.

Intercept incoming photons; keep and/or redirect their momentum

Light sails that intercept the momentum of photons don't eject matter. They use the momentum being thrown at them by the Sun or by absurdly powerful lasers. You can decide if you want to absorb the light gaining momentum in the same direction that the light was traveling, or reflect it which allows you to double the force if you reflect it directly backwards, or change the direction of the force by reflecting it somewhat sideways. But remember if you absorb the photons in a black "sail" it will get hot and re-radiate the same amount of power. If they're insulated on the back you'll still get a 50% boost from hemispherical radiation, but if they radiate from both front and back it will cancel.

Make your own photons; direct their momentum any way you like

You can radiate your own home-made photons in any direction you like! You could get power from that space laser that's hitting you, from solar power (which is pushing your solar-sails nee solar panels already) or from on-board generation (RTG, nuclear reactor, chemical batteries, etc.) It's profoundly inefficient in terms of thrust per megawatt compared to carefully using a little bit of reaction mass and electrostaticaly (or otherwise) accelerating it, but if you've got power but no mass to spare, this will do.

More about that heat:

Pioneer Anomaly screenshot

Fig. 1: Illustrative representation of the thermal model of the Pioneer 10 spacecraft evaluated at 40 AU. Top left: spacecraft body interior (temperature range: blue -16° C, red +10° C); Bottom left: spacecraft exterior (blue -155° C, red -108° C); Right: entire spacecraft (blue -213° C, red + 136° C). Un-modeled struts that connect the RTGs to the spacecraft body are indicated with yellow-black dashed lines.

Source (click for larger)

What can I do with a million watts of light?

If it's coming at you and you can reflect it straight back, you can make 6 milli-newtons, which is about the weight of a mosquito on Earth.

If you have a parallel beam of photons and you know the power (wavelength doesn't matter) the momentum per unit time (units of force but be careful it's not force per se) they carry is

$$\frac{d\mathbf{p}}{dt} = \frac{\mathbf{P}}{c}A = \frac{I}{c}A \mathbf{\hat{n}} = \frac{P}{c} \mathbf{\hat{n}}$$

where the vector quantities $\mathbf{p}$, $\mathbf{P}$ and $\mathbf{\hat{n}}$ are momentum, the Poynting vector and the normal vector in the direction of the beam of light and it's Poynting vector respectively, and the scalar quantities $P$, $I$, $A$ and $c$ are power (e.g. watts), intensity (e.g. watts/m^2), area and the speed of light.

Or just plain $F \approx dp/dt \approx P/c$. Amazing right!?

As discussed above the actual resulting force vector can have a different direction and vary by factor depending on albedo, emissivity and some angles and shape factors.

Also note: that if your velocity is appreciably relativistic in the frame in which you are doing your calculation, use the appropriate math and do not assume that F = ma = dp/dt with hard equal signs, otherwise you will accidentally propose a perpetual motion machine. For more on that see this answer to Could the helical engine work?

Popular Mechanics perpetual motion machine Source (click for full size)

Perpetual motion has long been a holy grail of tinkerers and inventors, but it violates the laws of physics, including Newton's 3rd law and the laws of thermodynamics. Burns' new 'helical engine' is just the latest example of a self-deception making its way into mainstream science discussions. NORMAN ROCKWELL / POPULAR SCIENCE

Can I "push against" a magnetic field?

That would probably be cheating as it really uses the matter that's generated the magnetic field to push against, and would be absurdly weak because you can't push against a uniform dipole field, you need a field gradient before you can have a potential energy gradient and therefore a net force. Put a dipole magnet in a uniform magnetic field and it will spin wildly and eventually damp to an equilibrium attitude if it has any loss mechanisms available to it, but it will hover in space as the video below (from Why doesn't the ISS orient itself to the Earth's magnetic field like a compass needle?) nicely demonstrates with a bar magnet inside the ISS over Australia.

Screen shot from "Magnets in Space" https://youtu.be/G_uKt2i2jvc

Screen shot from Magnets in Space


There is Zubrin's dipole drive


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