# What is a reasonable energy solution for a proposed solar system wide Laser Propulsion array?

In my fiction I'm setting up a civilization which uses Laser Propulsion for moving things between bodies within a solar system (assume Earth/sun analogy). Laser Units have been set up anchored to asteroids throughout the system, such that if you want to get from point A to point B, a range of satellites are always available which can get you going in the proper direction and then slow you back down once you need it.

So if I want to get from Earth Orbit to Mars Orbit in 3 days in a 500,000 kg vessel, I contract with the array operators to boost me and then to stop me in Mars orbit. They'd assign available satellites for the job, accept my bid for the time and energy and away I go. (with much dramatic pew-pewing)

Supposing energy generation at roughly today's power, are we talking football field sized solar collection (at Earth Orbit)? kilometers of solar collectors? More?

Im going to believable science in general. I've been studying Philip Lubin's videos and papers where he talks about setting up something similar. So far the propulsion element sound reasonable I'm just fuzzy on the energy part of things. https://www.nasa.gov/sites/default/files/atoms/files/roadmap_to_interstellar_flight_tagged.pdf

• You'd need to be a Kardashev scale type 2 civilization. – Richard Oct 9 '18 at 0:14
• Is it really Pew-Pew when you're using them as a propulsion system? – Matt Hollands Oct 9 '18 at 7:54
• They definitely go 'pew' when you turn them on. If not, why even do it? – Chris W Oct 9 '18 at 14:36
• Wow. This would be an incredibly complex system to manage. You are effectively pushing off the asteroids so their position and velocity would have to be recalculated every time they are used which would effect the calculations for the next traveler. – Skek Tek Oct 10 '18 at 12:13

# You are going to need a whole lot of power!

1 Gigawatt of laser energy can produce about 7 newtons of force in the way you have described. The largest nuclear power plant on earth produces about 8 gigawatts. A typical solar array can produce 4 gigawatts of electricity using a square mile of panels (though you may be able to do better than this with better panels).

To accelerate your craft at 1g, you would need approximately 5,000,000 gigawatts (5 petawatts) of electricity, this is equivalent to 1.25 million square miles of solar panels, which is about the area of India.

It would depend on when you wanted to go, but I suspect that in most cases you'd need much more than 1g to get from earth to mars in 3 days, but doing so would make it very uncomfortable for the occupants of the ship.

• The longest distance you might travel from Earth to Mars is about half the distance of Mars' orbit around the sun (the farthest point from the Earth). That is equal to $\pi$ times the radius, which is about 227.9 billion meters. The acceleration period is half that (the other half is deceleration). Accelerating and decelerating at 1g would get someone there in less than a day (55,000 seconds or so is less than 86,400). The speed would top out around 275 km/s, less than .1% of light speed. Three days would be around half a g because $d=\frac{1}{2}at^2$. – Brythan Oct 9 '18 at 1:08
• Additionally you have to worry about cooling all of this stuff. You would need an even larger surface area of cooling radiators just to keep the thing from melting itself when activated. Ideally you could distribute the lasers into a network of smaller firing arrays. I honestly can't think of a material you could even build a focusing array out of that could handle that kind of heat. Distributing the laser across a network of arrays would fix this, but you would still need millions of kilometers of heat radiators. – TCAT117 Oct 9 '18 at 1:11
• This is from the paper I linked, It seems to suggest a much better energy to thrust ratio. Here's a quote from the summary (not the proof) As an example, on the eventual upper end, a full scale DE-STAR 4 (50-70 GW) will propel ... a 100 kg payload to about 1% c and a 10,000 kg payload to more than 1,000 km/s My question is asking for more power than that, certainly, but these two assumptions seem to disagree by ... lots. Note this is without assuming photon capture, bouncing photons back and forth between emitter and mirror, which would be a force multiplier. – Chris W Oct 9 '18 at 3:37
• The summary of what you are referencing doesn't indicate anything about the acceleration, only the mass of the payload and the final speed. There is certainly a lot more going on in their estimates than just this. My estimate could be improved by the photon capture method, however there is a limit to the gain in efficiency that this would give. Probably not more than two times (requiring half the energy), but it's hard to say without more research being done. – Mathaddict Oct 9 '18 at 23:00
• Do your calculations utilize what the paper calls "photon recycling" (a "bounce" between the laser driver and the reflector). Each bounce cycle would effectively double the force (barring scattering and alignment issues, which should be negligible for short range travel within the solar system). – Skek Tek Oct 11 '18 at 13:57

I don't have any numbers for you, but I want to address the siting the lasers on asteroids issue. Asteroids move about a lot and they're also pretty erratic. You'd need a large number of individual installations to guarantee coverage. I think a better solution would be fewer lasers at more stable locations. If you put your lasers on the moons of Earth, Mars etc then they could be powered by large fusion reactors built alongside on the surface. It would also make sense to put the Earth laser on the dark side of the moon so that it wouldn't be firing "past" the Earth (and also couldn't be realigned to fire AT the Earth).

Having said all that I went online to find quotes from Larry Niven books which I remember using laser propulsion in some of his stories, and the quote I found talks about mounting the lasers on asteroids. I still think it makes more sense for craft travelling from Earth to be powered from the moon though.

Some info on Larry Niven's approach: http://www.technovelgy.com/ct/content.asp?Bnum=446

• I envisioned an infrastructure project that goes in stages out from Earth. Putting the first ones on the moon, using them to capture asteroids, stabilizing them, mounting more lasers on the rocks that don't have a good mineral value but a good mass. Rinse and repeat on other bodies. I get that the stabilizing part is nontrivial. Its a lot of mass, thats why they're desirable anchors too. – Chris W Oct 9 '18 at 14:41
• Each sight wouldn't necessarily have to produce a laser pulse. A site could be as simple as a reflector that is capable of bouncing the laser from an outside source to the destination. – Skek Tek Oct 11 '18 at 14:02