# Is it possible to use a laser to propel a spaceship? [closed]

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

• Then practicalities of laser propulsion are discussed on 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) Jan 10 at 10:11
• I’m voting to close this question because it belongs either on Space Exploration, Physics, or Worldbuilding SE. Jan 10 at 15:41

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".

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.

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. 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 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, 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).

• Not unimaginable at all. breakthroughinitiatives.org/news/4 Jan 10 at 17:53
• @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. Jan 10 at 22:43
• @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. Jan 10 at 22:46
• 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. Jan 10 at 23:52
• @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). Jan 11 at 0:01