Laser Propulsion Could Satisfy Our Spacecraft’s Need For Speed

There are many wonderful places we’d like to visit in the universe, and probably untold numbers more that we haven’t even seen or heard of yet. Unfortunately…they’re all so darn far away. A best-case-scenario trip to Mars takes around six months with present technology, meanwhile, if you want to visit Alpha Centauri it’s a whole four lightyears away!

When it comes to crossing these great distances, conventional chemical rocket technology simply doesn’t cut the mustard. As it turns out though, lasers could hold the key to cutting down travel times in space!

45 Days To Mars on the Laser

Laser thermal propulsion is a relatively simple concept, and could get our spacecraft travelling our celestial neighborhood quicker than ever. A powerful laser beam fired from Earth targets a large heat exchanger aboard the craft, through which is pumped a propellant. As the heated propellant expands, it’s ultimately exhausted out of a nozzle in much the same way as in a traditional rocket. It’s also similar to the concept of nuclear thermal propulsion, but instead of using heat from a nuclear reaction, it relies on externally-supplied laser energy.

A recent paper suggests such a propulsion system could run at a specific impulse of around 3000 seconds. This is essentially a measure of how much thrust a engine develops per mass of fuel. At 3000 seconds, a laser thermal propulsion system could be said to be at least 12 times as fuel efficient in thrust terms as the solid rocket boosters (SRBs) on the Space Shuttle.

A rendering of a laser thermal propulsion spacecraft. Note the large reflector and rocket nozzle at the rear. Credit: Duplay, 2022, CC BY 4.0

This allows a laser thermal propulsion to achieve far greater changes in velocity with less fuel, which gives a space mission the ability to send payloads farther and faster. Calculations show that with an idealized mission plan, a payload of around 1000 kg could be sent to Mars in just 45 days, far quicker than the usual 6-7 months possible in typical chemical-fueled missions.

A conceptual laser thermal propulsion space mission to Mars. Credit: Duplay, 2022, CC BY 4.0

The technology involved is complex, as you’d expect. A large laser array with power on the order of 100 MW would be required for the mission. The spacecraft itself would be launched out of the atmosphere on a conventional chemical rocket, whereupon it would separate and reveal a large inflatable parabolic reflector. The ground-based laser would then fire for up to an hour, using adaptive optics to counter the effect of the Earth’s atmosphere on the beam. The parabolic reflector on the spacecraft would then focus the energy on to a chamber to heat hydrogen propellant that would be expelled out of a nozzle at great velocity, providing thrust.

If so desired, the spacecraft could be designed to release the payload capsule on its path towards Mars, with the laser thermal propulsion unit separating off and returning to a stable Earth orbit for refueling. This has the benefit that the propulsion system itself could be used multiple times in quick succession to loft payloads far beyond Earth.

Such a system has one major flaw that stands out. While a laser on Earth is used to accelerate the spacecraft to great velocity, there is no corresponding laser array on Mars that can decelerate the craft on arrival. Nor is using chemical propulsion a practical way to slow down, as this would take up far too much of the craft’s useful payload. Researchers instead have determined that a very careful aero-braking maneuver in the Martian atmosphere could be used to slow an arriving craft. However, it’s a delicate operation that must be executed flawlessly to ensure success.

Overall, such a system could be readily developed in the near term. While nobody has a 100 MW laser array just lying around, modern fiber optic laser technologies mean that such a power figure is not outside the realms of possibility. Similarly, much work would be required to create a reliable laser thermal spacecraft and ground system capable of sending payloads in useful directions in space not solely limited by the relative positions of spacecraft and ground laser.

Ride The Laser To The Stars

If you want to go as far as our nearest star, Alpha Centauri, you’ll need to travel even faster. Even going at the speed of light, it would take four years to get there. Thus, a probe intending to travel that far would want to be going as close to that speed as is possible to make it there in a reasonable period of time.

Laser sails may just hold the answer to this problem. They rely on the concept of photon radiation pressure, where light hitting a surface actually creates pressure and pushes it along. They’re referred to as sails because the concept is exactly the same as that of a sailing ship of centuries past. Instead of cloth and wind, though, a laser sail substitutes in advanced nano-materials and powerful laser light.

A artist’s conception of the laser sail idea. Such a craft could potentially carry a gram-scale payload at relativistic speeds, up to around 0.2c. Credit: Breakthrough Initiatives

Recent research suggests a laser sail on the scale of a few meters could propel a gram-weight craft at velocities up to 0.2 times the speed of light. This would enable reaching Alpha Centauri in around 20 years, rather than the tens of thousands of years it would take with conventional rocketry.

The concept would require the use of a sail made of exceedingly thin sheets of materials like aluminium oxide, silicon nitride, and molybdenum disulfide. Measuring thousands of times thinner than a sheet of paper, the sail would have to be strong enough not to tear, and also be capable of dissipating heat so as not to melt from the power of the laser propelling it along.

Advanced nano-patterning of the sail would be key to achieving this goal. The idea is to produce a sail with high reflectivity to maximize acceleration due to photonic pressure, while also maintaining high thermal emissivity to keep the sail cool enough not to melt. With a 100 GW laser array firing at the sail, that’s no mean feat. Much like a conventional sail on a sailing ship, the material would be allowed to billow out under the pressure of the incoming light. This reduces the chance of tears significantly.

At best, the sail would only be able to carry a tiny payload weighing a few grams. It’s hoped that advanced fabrication methods could create a microprocessor, cameras, and communication hardware for the probe that would be able to communicate over the vast distances between Alpha Centauri and Earth.

It’s a bold plan, and one that could enable space research to tackle subjects farther afield than ever before. However, the challenges involved are great. The requirement for hugely powerful laser arrays is beyond our current capabilities, and the issue of materials still needs to be solved. Furthermore, any message sent from a probe at Alpha Centauri would take four years to arrive back on Earth, so communications issues present themselves as well.

Regardless, the research run thus far by Breakthrough Initiatives shows that laser sail concepts aren’t necessarily just a matter for science fiction. With the right investment and development, they could prove to be a useful propulsion method for research craft one day in the future.


Unlike other seemingly sci-f tech, like ion thrusters, these laser propulsion methods are still quite a ways off being fielded in real space missions. There are huge challenges to overcome, and it also bears sparing a thought for any birds or other unlucky wildlife that finds itself in the beam of a megawatt- or gigawatt-class laser.

However, if we are to open up the heavens, it’s going to require more than our existing technology can achieve. Thus, these projects, or perhaps other fancy new ideas, could one day take us far beyond our own solar system.

51 thoughts on “Laser Propulsion Could Satisfy Our Spacecraft’s Need For Speed

  1. Well the 4×10^26 Elephants in the room, is that big flaming ball in the middle of the solar system room… no lasers required.

    When we go interstellar, maybe, but for schlepping around between the planets, it’s all we need.

      1. No extra bird deaths required, you just put solar concentrators on the craft, no ground installation (beyond normal traking and control)

        Need more area than concentrating the laser, but drops the exotic heatproof requirement of the reflector down by a lot. Probably it has a PR problem in that “absolutely erroneous gut feeling physics” department, that thinks a vacuum craft needs to have a low cD.

      2. If you put the laser array on the Moon, flock all birds would be incinerated.

        Maybe a soup dragon or two.

        Also it would skirt around the adaptive optics problem.

        But – why are we using lasers at all? Can’t lower-frequency EM do the same job, and also be easier to generate at high power? E.g. microwaves.

        1. Do the arithmetic: How big do you need to make your transmitter array and receiver array (or mirror) if you switch from a laser wavelength of 1-10 microns to a microwave wavelength of 1-10 cm?

    1. Without focusing and directing its rays somehow (might be an idea to do that from earth orbit) how many watts per square meter do you get out of the sun? You can beat that with a laser.
      If you put big, wide collectors for concentration on the craft itself, you incur way too much weight penalty for efficient interplanetary travel. Space is merciless towards unnecessary mass. Plus the heat and thus ISP you’d get would probably still be lower than a system optimized from stem to stern

  2. A 100 MW laser that can precisely target high-speed objects in space would be very handy for de-orbiting debris in low earth orbit, simply by ablating the leading face of it, imparting enough momentum to de-orbit it.

    And it will be even handier to de-orbit or disable at will other orbiting assets the laser operator might consider “debris”.

    1. Or replace that “debris” with a mirror and “disable” things that are not in orbit. Old idea, new disguise to get public funding for another military weapon.

        1. This answer is for Robert. We need lasers to attain very high speeds because missiles depend on chemical reactions for propulsion and chemical reactions are much too slow to propel a spacecraft anywhere near the speed of light. Missile speeds are adequate for travel to nearby planets, but Interstellar distances are so vast that we need to approach the speed of light to get to the nearest star in a reasonable time. So we need to use light itself to accelerate a material object close to the speed of light and a laser beam is a highly concentrated beam of light. It all comes down to Newtons third law of motion. In interstellar space, gravity is weak, so it’s not how much mass you throw out the back that determines your top speed, it’s how fast you can eject the reaction mass that determines how fast you can go. Only photons of light can do the job. Electrically propelled ions are an alternative, but then you have to carry the ejection mass with you and laser propulsion solves that problem.

          1. Ah, the argument circles back around. The above comment was in response to the idea of using lasers to shoot satellites out of the sky. The real problem with missiles in that context is that they tend to make more debris instead of less, so lasers are better for de-orbiting debris, but anyone with a missile can take out a satellite.

            You aren’t talking about missiles, you’re talking about rockets. Nobody is going to make it to a significant percentage of the speed of light any time soon. It’s just not practical. With the energy required to get a small craft to .1C, you can research and outfit an entire generation ship. The inherent problem with this thought isn’t that we can’t do it, it’s that anybody with this kind of funding will immediately ask “what problem are we solving?” If we aren’t solving a problem with the attempt, it’s no better than Elon Musk shooting a Tesla into outer space — it’s not star-faring so much as ego-faring.

            Second point: At no point does the initial concept use lasers for propulsion. It’s just an energy delivery system; a replacement for a power source. It still accelerates hydrogen out the back as a reaction mass. You probably knew this.

            Third point: The current proposals for this (e.g. specify megastructure lenses, minute payload size, unreachable cryogenic presumptions, and zero-gravity construction techniques.

            Building a laser into a spacecraft and pushing yourself around with a laser beam out the back would require a fusion reactor to generate the mass-to-velocity conversion rates you’re discussing. If you look at the cost and size of the current fusion reactor projects, you’ll note that this isn’t going to happen any time soon.

            So, overall, if we “need” this for anything, we’re going to have to live without it for the foreseeable future.

  3. SciFi…
    And even more SciFi: Humans taking care of the planet they already have!

    Do we really want to see this immature species in space?
    Come on!
    This planet *and* the universe deserve better!

    1. Outer space is a harsh environment. Just being there would teach its inhabitants important things like “don’t shit where you eat.” The real problem will come in with the wealthy back on earth insisting that they can send miners to Mars, but can’t afford to pay for proper toilets.

        1. I think he (or she) means that we don’t take care of Mother Earth, so why should we go about fucking up other parts of the solar system (and beyond).

  4. I would like some pork in the restaurant at the end of the universe.

    Some of the intro’s here on hackaday have become so far off-topic that I’m not even sure what the rest of the article is about.

  5. So I am sure this question is born out of my ignorance of physics. But wouldn’t a large nuclear power plant on the ship and its own local laser array hitting a sail allow the vessel to set its own course both here in our solar system as well as beyond? Do we need 100 MW if the lasers are located at the rear of the vessel and the sail at the front? Could lesser strength lasers at such a short distance make that a more obtainable goal?

        1. Nope. The recoil of the departing photons on the light source *is* the thrust. Bouncing the photons off a mirror might change the direction of the net force but no way, no how, is it going to “improve” it. TANSTAAFL.

      1. That’s definitely cool, but I think the projects there were cancelled for other reasons too. I imagine you’d need a fairly closed loop with the nuclear reactor with maybe exterior cooling for the water used to keep the reactor in shape. But jettisoning the liquid used to cool the reactor would mean needing to find replacements to keep going. Something like an array of nuclear powered ION reactors or a nuclear powered laser for the sail would seem to have greater longevity, but, again, I profess my ignorance here on the details.

        1. Yeah, scrapped nonetheless but I don’t think he discriminates according to scrapped for what reason. Budgetary or just plain don’t work. As long as it wasn’t just some crank stuff. He does NOT like the emdrive for instance—and for good reason :)

        2. The nuclear thermal rocket was tested on the ground, in the 60s I think. It does run out of coolant (the coolant being considered an expendable, like fuel but it’s not the actual nuclear fuel so it was called “propellant”). Open-cycle cooling can cool a lot better in the short run because you just jettison all the heat completely, along with the mass that contains it, so you don’t have to cool back down the liquid and then radiate all the heat it had from the craft’s own hull. You just leave it in space.
          The US tests were named NERVA and KIWI. The KIWI test did eventually work, but nuclear space treaties (and JFK being utterly horrified by project Orion, thank God) put it down.
          The nice thing about open-cycle cooling is that you can just toss all the heat into space and leave it behind, instead of having to re-cool the coolant—transferring all the heat back into the spacecraft—before re-using it. Then you have to radiate all of it from the hull somehow. Of course it does run out eventually, but it was considered a consumable like any other rocket fuel or propellant.

  6. For deceleration why not build the craft so the mirror stays in place but the propulsive end can rotate 180 degrees and begin to decelerate using the same laser energy?

    then build a laser on Mars to accelerate loads away from there toward Earth.

    Once both destinations have lasers, they can decelerate the craft with relative ease (not that ANYTHING about this is EASY.)

    1. The inherent problem here isn’t that the laser is coming from the wrong direction. There are a few laser-boosting technologies where it would (e.g., laser-pushed solar sails or rockets that use the laser energy to vaporize inert material to produce thrust), but this isn’t one of them. The problem would be that the beam would be too attenuated by the time it reached Mars for the receiving end to use it. We could put a second laser in Mars orbit, and that would work, but then you’d be using half of your reaction mass for deceleration and wouldn’t get the same top speed.

      1. I’ve seen some interesting potential solutions for this using drastically different laser wavelengths and diffraction-based lenses out in space (think a diffraction grid perhaps a kilometer across sitting out at a Lagrange point) but it’s over my head as to whether it would be feasible or not

  7. It should be pointed out that 100 MW * 1-3 hrs of time you might have to push on your target is ~ 10^12 J, about the amount of electrical energy produced in a fission reactor from burning about a teaspoon (40g) of uranium or plutonium. Sure, the mass of the engine is significant, but you can make it *much* smaller (1 MW) and push a *lot* longer than the every short time window you have for a ground-based laser.

  8. A ground based laser would be silly for something like this. Atmospheric losses and the fact that the planet keeps spinning throwing off your aim… By the time we can get the power capacity built up for a 100MW laser, we can put something smaller into orbit to push around the craft.

    1. and if you don’t want to energize your laser by burning coal, you get the atmospheric loss twice. Once to capture sunlight to generate energy, and again to push the laser light back out. Better to put solar panels and the laser in a Lagrange point.

    1. *60s
      And yes, it was. Problem is you have to get a working nuke reactor into orbit, any screwups along the way result in said reactor uncontrollably falling back to Earth, which is bad. Very bad.

      1. But we already send quite a few satellites out there using radioactive material as fuel (basically most ion drives). Couldn’t we safely send up the parts on more than one flight and Lego the reactor together in space?

    2. Solid-core nuclear-thermal tops out at the 600s-700s-ish ISP level due to the temperature limit of the core materials. Other NTR designs (namely the ‘nuclear ‘lightbulb’ gas core) get into the thousands of seconds range, but rely on materials not yet invented to actually work.

      The tested NERVA and ROVER NTRs also require HEU (High Enriched Uranium), which has proliferation concerns around its use (and transportation, and handling, and export, etc) but was a viable option at the hight of the cold war when those engines were being developed. Those designs would need to be converted to LEU (Low Enriched Uranium) or the new intermediary category of HALEU (High Assay Low Enriched Uranium) to really be viable today.

  9. Left out of the discussion is the need for some form of shielding. Interstellar space maybe be “empty” but it’s not *empty*. Hitting a stray atom or two at even low relativistic speeds will vaporize your gram-weight spacecraft. IIRC the need for shielding puts a speed limit near 0.2c, barring some Star Trek tech deflector. That shielding will increase the gram weight to ???

  10. “Such a system has one major flaw that stands out.”

    That’s not the flaw. The big issue to solve is coupling the input laser light into the Hydrogen propellant. You want to make the Hydrogen as optically opaque as possible, because every watt that is not absorbed is not only wasted but is attempting to melt the chamber the propellant is being heated within. And as ISP is proportional to propellant exit velocity, and propellant exit velocity is proportional to propellant heat, and laser heating is proportional to beam intensity, to achieve the desired ISP means a very intense incident beam. Hot enough to melt any solid chamber if the propellant does not capture the vast majority incident energy. The paper proposed doping the Hydrogen flow with carbon particles to increase opacity, but that’s a technique that would first need to be demonstrated to actually work at the target power levels.

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