<p>Yes, it might be possible to send laser-propelled spaceships to the stars.</p>
<p>There are a whole series of technical challenges in the laser technology and the materials that such a spacecraft might be made of.</p>
<p>The basic idea is to accelerate mini-spaceships with very low mass, but a large, reflective surface area, up to relativistic speeds. The project that I know of is called &quot;<a href="https://breakthroughinitiatives.org/news/4" rel="nofollow noreferrer">Breakthrough Starshot</a>&quot;.</p>
<p>Contrary to what you say in your question, the laser would not be aimed at the target star, it would be aimed at the spaceships! These would have to be set off on a carefully calculated trajectory, because during their decades-long journey, the relative positions of the Sun and target star would change significantly. The impulse phase would take place within the Solar System and the craft would cruise most of the way.</p>
<p>The intensity of the lasers required would be immense compared to the light from a star. The baseline plans involve something like <span class="math-container">$10^9$</span> W/m<span class="math-container">$^2$</span> delivered to the light sail for 10 minutes. This is to be compared with about <span class="math-container">$10^3$</span> W/m<span class="math-container">$^2$</span> from the Sun at the orbit of the Earth.</p>

CC BY-SA 4.0
2021-01-10T18:28:32.863Z
2021-01-10T18:28:32.863Z
ProfRob
<p>No. There are at least two crucial problems:</p>
<ol>
<li><p>Even the best lasers diverge after some thousands km. We could not focus the beam to the ship.</p>
</li>
<li><p>With laser, you can give energy to the ship. Most likely, as heat (<em>maybe</em> as electrical energy). You can not give impulse. The only way to give impulse to the ship by lasers, using the impulse of the light, i.e. by <a href="https://en.wikipedia.org/wiki/Radiation_pressure" rel="nofollow noreferrer">light pressure</a>. This is extremely small and it would require unimaginable power to have any measurable effect.</p>
</li>
</ol>
<p>&quot;Lesser&quot; hard problem is that the atmosphere scatters light a little bit, what could make the aiming of the beam impossible (it would also &quot;help&quot; it to diverge). Making the lasers space based would hugely increase the costs.</p>
<hr />
<p>Extension: a quick calculation. The <a href="https://en.wikipedia.org/wiki/Parker_Solar_Probe" rel="nofollow noreferrer">Parker Solar Probe</a> is 8 solar radii away from the Sun in the nearest point of its orbit. The Earth is 200 solar radii away, and gets 1400 <span class="math-container">$\frac{W}{m^2}$</span> light intensity from it. Thus, the PSP gets <span class="math-container">$1400 \frac{W}{m^2} \cdot \big(\frac{200}{25}\big)^2 \approx 900 \frac{kW}{m^2}$</span> energy.</p>
<p>See the best case, if the receiving side is a perfect mirror, which radiates back the whole beam. Using the relevant formulas <a href="https://en.wikipedia.org/wiki/Radiation_pressure" rel="nofollow noreferrer">here</a>, we get the pressure of <span class="math-container">$\approx 0.003 Pa$</span>. If the &quot;laser sail&quot; surface area is <span class="math-container">$1m^2$</span>, this results 0.003N force, also <span class="math-container">$\approx$</span> the weight of 0.3g on the Earth.</p>
<p>But the heat shield of the PSP is 11cm thick reinforced carbon-carbon composite (this was the heat shield also of the Space Shuttle).</p>

CC BY-SA 4.0
2021-01-10T17:53:39.883Z
Not unimaginable at all. https://breakthroughinitiatives.org/news/4
ProfRob
CC BY-SA 4.0
2021-01-10T22:43:14.077Z
@ProfRob I would like if it would not be, but I think it is. See calculation above. Another problem: laser beam defocuses after some thousands km, maybe it can be made longer by very good lasers, but that is all. Yet another problem: interstellar gas, with the density of about 1 $\rm{\frac{atom}{cm^3}}$, causes $\approx$ 100 $\frac{W}{m^2}$ radiation at 0.1c. It causes 8 times more at 0.2 c. It does not happen as light, it comes as $\approx$ 10MeV proton radiation, and no solid matter can withstand it without erosion. Calculate it, you are the prof.
peterh
CC BY-SA 4.0
2021-01-10T22:46:47.347Z
@ProfRob I very like this idea, I just don't believe that it could work. If they can collect $100million, I can believe that they have some chance to make some new Pluto photos.
peterh
CC BY-SA 4.0
2021-01-10T23:52:22.287Z
These problems are discussed via the link I gave you. One of the problems isn't the lack of power in sunlight, since that isn't the proposal. The proposal is to use GW lasers on almost perfectly reflective sails (which therefore absorb very little heat). I believe the sails are jettisoned after use. The remaining craft would be tiny.
ProfRob
CC BY-SA 4.0
2021-01-11T00:01:10.507Z
@ProfRob I used the PSP as an analogy to show one of the impossibilities of the idea. (Other is that laser beams diverge after some thousands of km).
peterh
CC BY-SA 4.0
2021-01-11T00:25:29.653Z
The local ISM has a density of about 0.3 atoms/cc. In each second the craft encounters a cylinder 0.1c in length and thus a mass of about $1.5\times 10^{-14}$ kg/m$^2$ s$^{-1}$. The kinetic energy flux is about $7$ Wm$^{-2}$. The projectile is about 0.1g and area 0.1 cm$^2$ (the high E protons just go thru the sail). It might sweep up $\sim 10^{-5}$g, on a trip to Proxima Cen so this will not slow it. Radiation damage to the chip is more problematic. All this is discussed via the link I gave you.
ProfRob
CC BY-SA 4.0
2021-01-11T00:30:26.113Z
The focusing problem is also discussed. https://breakthroughinitiatives.org/challenges/3 You can argue it is infeasible if you like, but these are just technology, not physics issues, and their solutions are not unimaginable.
ProfRob
CC BY-SA 4.0
2021-01-11T01:31:36.043Z
@ProfRob Ok, I read it. The site looks a little bit chaotic to me. Can you give me a better directed link to it, or summarize, how do they solve the laser focus problem?
peterh
CC BY-SA 4.0
2021-01-11T19:32:39.553Z
@peterh-ReinstateMonica Very short summary -- you use a very wide coherent laser array.  Exactly the same way you make a narrowly focused beam from a phased array radar, or (backwards) the way a radio telescope array can separate out the signal from a very small target. In this case, I think they need something like 100m wide array, all phase-locked, which is challenging, but not inconceivable.
Steve Linton
CC BY-SA 4.0
2021-01-13T20:48:02.220Z
@SteveLinton And is it really calculated by many experts? If yes, it is... amazing.
peterh
CC BY-SA 4.0
2021-01-10T17:47:58.907Z
2021-01-10T22:38:31.267Z
2021-01-10T22:38:31.267Z
peterh
<p>Reference: <a href="http://adsabs.harvard.edu/full/1968BAN....19..421H" rel="nofollow noreferrer">http://adsabs.harvard.edu/full/1968BAN....19..421H</a></p>
<p>It would be necessary to aim a laser to the target star. How would the density of the laser compare with the interstellar radiation?</p>

CC BY-SA 4.0
2021-01-10T10:11:05.060Z
Then practicalities of laser propulsion are discussed on https://en.wikipedia.org/wiki/Laser_propulsion  The details can be asked on space.stackexchange better than here "Would the laser be brighter than the stars" depends on how bright the laser is, and how far you are from the star, but you wouldn't need constant acceleration for the whole trip (indeed you'd probably want to slow down as you approached your destination)
James K
CC BY-SA 4.0
2021-01-10T15:41:25.727Z
I’m voting to close this question because it belongs either on Space Exploration, Physics, or Worldbuilding SE.
WarpPrime
<interstellar>
Is it possible to use a laser to propel a spaceship?
Bingohank
# Is it possible to use a laser to propel a spaceship?

(Asked by: username_0 on 2021-01-10)

Reference: <http://adsabs.harvard.edu/full/1968BAN....19..421H>

It would be necessary to aim a laser to the target star. How would the density of the laser compare with the interstellar radiation?

**Tags:** interstellar
> **Comments:**
> **username_1**: Then practicalities of laser propulsion are discussed on https://en.wikipedia.org/wiki/Laser_propulsion  The details can be asked on space.stackexchange better than here "Would the laser be brighter than the stars" depends on how bright the laser is, and how far you are from the star, but you wouldn't need constant acceleration for the whole trip (indeed you'd probably want to slow down as you approached your destination)  
> **username_2**: I’m voting to close this question because it belongs either on Space Exploration, Physics, or Worldbuilding SE.  

---

## 2 answers

### Answer by username_3 on 2021-01-10. Score: 4

Yes, it might be possible to send laser-propelled spaceships to the stars.

There are a whole series of technical challenges in the laser technology and the materials that such a spacecraft might be made of.

The basic idea is to accelerate mini-spaceships with very low mass, but a large, reflective surface area, up to relativistic speeds. The project that I know of is called "[Breakthrough Starshot](https://breakthroughinitiatives.org/news/4)".

Contrary to what you say in your question, the laser would not be aimed at the target star, it would be aimed at the spaceships! These would have to be set off on a carefully calculated trajectory, because during their decades-long journey, the relative positions of the Sun and target star would change significantly. The impulse phase would take place within the Solar System and the craft would cruise most of the way.

The intensity of the lasers required would be immense compared to the light from a star. The baseline plans involve something like $10^9$ W/m$^2$ delivered to the light sail for 10 minutes. This is to be compared with about $10^3$ W/m$^2$ from the Sun at the orbit of the Earth.


---

### Answer by username_4 on 2021-01-10. Score: -2

No. There are at least two crucial problems:

1. Even the best lasers diverge after some thousands km. We could not focus the beam to the ship.
2. With laser, you can give energy to the ship. Most likely, as heat (*maybe* as electrical energy). You can not give impulse. The only way to give impulse to the ship by lasers, using the impulse of the light, i.e. by [light pressure](https://en.wikipedia.org/wiki/Radiation_pressure). This is extremely small and it would require unimaginable power to have any measurable effect.

"Lesser" hard problem is that the atmosphere scatters light a little bit, what could make the aiming of the beam impossible (it would also "help" it to diverge). Making the lasers space based would hugely increase the costs.

---

Extension: a quick calculation. The [Parker Solar Probe](https://en.wikipedia.org/wiki/Parker_Solar_Probe) is 8 solar radii away from the Sun in the nearest point of its orbit. The Earth is 200 solar radii away, and gets 1400 $\frac{W}{m^2}$ light intensity from it. Thus, the PSP gets $1400 \frac{W}{m^2} \cdot \big(\frac{200}{25}\big)^2 \approx 900 \frac{kW}{m^2}$ energy.

See the best case, if the receiving side is a perfect mirror, which radiates back the whole beam. Using the relevant formulas [here](https://en.wikipedia.org/wiki/Radiation_pressure), we get the pressure of $\approx 0.003 Pa$. If the "laser sail" surface area is $1m^2$, this results 0.003N force, also $\approx$ the weight of 0.3g on the Earth.

But the heat shield of the PSP is 11cm thick reinforced carbon-carbon composite (this was the heat shield also of the Space Shuttle).

> **Comments:**
> **username_3**: Not unimaginable at all. https://breakthroughinitiatives.org/news/4  
> **username_4**: @ProfRob I would like if it would not be, but I think it is. See calculation above. Another problem: laser beam defocuses after some thousands km, maybe it can be made longer by very good lasers, but that is all. Yet another problem: interstellar gas, with the density of about 1 $\rm{\frac{atom}{cm^3}}$, causes $\approx$ 100 $\frac{W}{m^2}$ radiation at 0.1c. It causes 8 times more at 0.2 c. It does not happen as light, it comes as $\approx$ 10MeV proton radiation, and no solid matter can withstand it without erosion. Calculate it, you are the prof.  
> **username_4**: @ProfRob I very like this idea, I just don't believe that it could work. If they can collect $100million, I can believe that they have some chance to make some new Pluto photos.  
> **username_3**: These problems are discussed via the link I gave you. One of the problems isn't the lack of power in sunlight, since that isn't the proposal. The proposal is to use GW lasers on almost perfectly reflective sails (which therefore absorb very little heat). I believe the sails are jettisoned after use. The remaining craft would be tiny.  
> **username_4**: @ProfRob I used the PSP as an analogy to show one of the impossibilities of the idea. (Other is that laser beams diverge after some thousands of km).  
> **username_3**: The local ISM has a density of about 0.3 atoms/cc. In each second the craft encounters a cylinder 0.1c in length and thus a mass of about $1.5\times 10^{-14}$ kg/m$^2$ s$^{-1}$. The kinetic energy flux is about $7$ Wm$^{-2}$. The projectile is about 0.1g and area 0.1 cm$^2$ (the high E protons just go thru the sail). It might sweep up $\sim 10^{-5}$g, on a trip to Proxima Cen so this will not slow it. Radiation damage to the chip is more problematic. All this is discussed via the link I gave you.  
> **username_3**: The focusing problem is also discussed. https://breakthroughinitiatives.org/challenges/3 You can argue it is infeasible if you like, but these are just technology, not physics issues, and their solutions are not unimaginable.  
> **username_4**: @ProfRob Ok, I read it. The site looks a little bit chaotic to me. Can you give me a better directed link to it, or summarize, how do they solve the laser focus problem?  
> **username_5**: @peterh-ReinstateMonica Very short summary -- you use a very wide coherent laser array.  Exactly the same way you make a narrowly focused beam from a phased array radar, or (backwards) the way a radio telescope array can separate out the signal from a very small target. In this case, I think they need something like 100m wide array, all phase-locked, which is challenging, but not inconceivable.  
> **username_4**: @SteveLinton And is it really calculated by many experts? If yes, it is... amazing.  

---

