# Powering a starship with solar cells

For a while now I've been building a science fiction world and I've been thinking what would be a feasible way to power a large ship capable of interstellar travel. I've always disliked the idea on a potentially explosive engine on a ship.

What I am wondering is how feasible is solar power once the ship leaves a star system and travels into deep space?

• Depends upon battery/capacitor power storage level, the duration of the trip (FTL or not) and the mass ratio of solar cells to rest of the ship. – PCSgtL Mar 28 '17 at 20:47
• Where do you store the charge? Batteries are also explosives. – Mormacil Mar 28 '17 at 20:50
• I can't hard science it enough to make an answer, but consider relocating the starship. A solar sail (better than solar cells for transportation) won't work between Sol and Alpha Centauri, but it could work in a denser area. Consider the galactic core for example. Outside the central black hole of course. – Brythan Mar 28 '17 at 22:23
• James P, please don't ask a question tagged hard-science and then accept an answer that obviously does not meet the criteria for the hard-science tag. See worldbuilding.stackexchange.com/tags/hard-science/info for information on the requirements imposed on answers by the hard-science tag. If you don't need hard science answers, then I recommend removing that tag and flagging for moderator attention so the post notices can be removed. – a CVn Mar 29 '17 at 11:15
• You should wait a day before accepting an answer, to give more people a chance to weigh in. You seem to have accepted the first post immediatly, even thougb it's not meeting the requirements of the hard-science tag, and is highly downvoted! – JDługosz Mar 30 '17 at 6:41

Solar cells would work well inside a solar system, but long distance travel will be limited by the batteries. Note that batteries are also relatively heavy and would thus take more fuel to move. Ideally, you'd want a propulsion system and its fuel to be low on mass.

Your story might be able to invent a new kind of battery which avoids this problem, or you might be able to invent a technology that takes advantage of the radiation in interstellar space.

• I think the problem is not the battery, but that the solar power from many light year distant stars is essentially zero. – Gray Sheep Mar 28 '17 at 23:53
• @MorningStar If you have super-batteries that can power a ship for years and super-efficient solar panels that can charge the batteries fast enough then your ship is still technically solar powered. – c.. Mar 29 '17 at 1:17
1. The Sun radiates around 1400 W for a single square meter of the Earth.
2. This decays quadratially with the distance.
3. 1 light year is around 60million astronomical unit (=Earth-Sun distance).
4. The nearest star is 4 ly away from us.

Thus, the solar radiation is $\frac{1400}{100000000^2}$ W from 1.5 light year distance.

You will get 0.000000000000014 W for every $m^2$ of solar panel.

It is nothing.

(Extension: it is about a Sun-sized star from 1.5 light year away. But the sky has a lot. Although they are far more away. Maybe their summed result could be even thousands times better, but I think it is still far from be enough.)

Problem #2: On Newton's second law, you can only drive the spaceship, if there is something what you can shot out from it. The solar power gives energy (quite few), but if you solve this problem somehow... for example, you use astronomical sized (i.e. million km big) solar panel foils... even in this case, you need to have some fuel.

Although I wouldn't say it a completely dead idea; with solar power it doesn't seem to meaningful, but there is a so-named Bussard ramjet:

The idea is to collect the interstellar gas with big electromagnetic fields, fuse them in fusion reactor, and shot them out with the produced energy.

Even this idea is in best case in the very far future:

• Currently, we can't fuse even deuterium-tritium mix (although it will be possible in around 20 years) (more exactly, we can fuse them long ago, but not enough efficiently to get more energy from it as we need to invest to maintain the device)
• With hydrogen is it currently totally unreachable (but seems possible)
• The interstellar gas has around 1 atom per cubic centimeter, it should be very strong, very big, very sophisticated field which can collect it into the drive of the spaceship.

• Minor quibble: Yes we can make fusion work. We just can't sustain it without putting more energy into it than we get out of it yet. I think that someone is near or reached break even but we are still a ways off from it becoming useful. – ShadoCat Mar 29 '17 at 0:43
• The first part of your answer is excellent. It answers the question as asked. Your discussion of the Bussard ramjet is irrelevant to the question. With negligible solar power it would be impossible to power an interstellar ramjet. I recommend deleting the Bussard ramjet section that will leave a first rate answer. – a4android Mar 29 '17 at 3:20
• I think your math is not quite right. If you get 14 femto watts from one star you might get a few nano watts from the billion visible stars. And since that's per square meter you might then only need solar panels with surface area equal to the earth to power your coffee maker. Of course that's using impossibly efficient collection. – user25818 Mar 29 '17 at 16:36
• @ShadoCat The currently best project, the ITER will be able nearly surely to maintain fusion. They will start D-D work in some years (in won't be yet enough, but will produce a lot of tritium), and then the D-T work with the produced tritium (it will be already enough on every reasonable calculation). Thanks, I extended the answer. – Gray Sheep Mar 30 '17 at 16:49
• @a4android Yes, you are right, the second part is not about the literally and narrowly interpreted question. It is about the more broad context, in the sense of the worldbuilding, i.e. free thinking of possibilities of alternative futures, presents and pasts. See the main structure of the post: first, I explain, why it couldn't work (without major changes on the laws of the physics). Second, I give the (for me) most similar alternative, from a worldbuilding viewpoint, what could work (although also not tomorrow). – Gray Sheep Mar 30 '17 at 16:52

The power available to a solar array decreases as it gets further from the sun. It's the same reason we can't generate power from starlight.

Currently the furthest from the sun we have sent a probe powered by solar panels is the Juno mission to Jupiter.

Further out than that we resort to radioisotope thermoelectric generators to power our probes.