# How much light is there on the way from Earth to Proxima Centauri?

I want to know how much light is there for a spacecraft that flies from earth to Proxima Centauri. I know there's the sun but what happens after the spacecraft leaves the solar system? Are there stars that provide enough light for solar panels to work?

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• I deeply resent at least 9 of those 12 significant digits. May 12, 2017 at 1:00
• If there would be stars that would provide enough light for solar panels, you would easily see them at night as extreamly bright stars, much brighter than any other star. These stars should be very close to the spacecraft on its way to Proxima Centauri. But as Proxima Centauri is the closest star to earth, there are no such stars.
– Uwe
May 12, 2017 at 8:22
• You can get some information here: en.wikipedia.org/wiki/Lux Full sunlight is 100,000 Lux, the starlight at night without moon about 0.002 Lux. 8 orders of magnitude less, as written by Russel Borogove.
– Uwe
May 14, 2017 at 16:08

No, the power collected by solar panels is reduced by the square of the distance from the light source.

At the Earth's distance from the sun, the energy of sunlight is about 1300 watts per square meter, of which something like ~30% can be converted to electricity by solar panels. Once the sun is far enough away to be "just another star", the total starlight is about 8 orders of magnitude dimmer, something like a microwatt per square meter. If I've done my math right, powering the ISS from interstellar starlight would require solar panels about the size of California.

Sunlight doesn't provide enough power for solar panels to be useful even in the outer solar system, let alone interstellar space, so probes going beyond Mars orbit more often use radioisotope thermoelectric generators (RTGs) for power, which work for decades and do not depend on light.

For interstellar journeys which will take hundreds of years, nuclear fission or fusion reactors would probably be necessary.

• Still, an estimate in W/m^2 of what starshine of the dark space provides would be nice.
– SF.
May 11, 2017 at 15:52
• I've proposed (informally) that spacecraft circle around the Sun, collect coronal matter, convert it to energy, and, once they're going at the "right speed", stop circling and shoot off in the direction of the destination.
– user7073
May 11, 2017 at 17:10
• @SF. Concur, If a solar panel is 100% effective, how much surface would it need to provide to power something like the ISS needs? Would it need to be pointed towards the galactic center? Space is mostly empty but still full of stuff, would that make travel towards the galactic center a net loss of energy? In relationship to the galactic center, what direction does travel from Earth to Proxima Centauri go? May 11, 2017 at 18:05
• Starlight isn't vastly brighter in the direction of galactic center, as a good look at the night sky can tell you; you can point your notional starlight collection panels any direction you want relative to your travel direction, anyway. May 11, 2017 at 19:42
• I'm picturing a solar sail powered craft, with a concave, reflective solar sail. During the initial part of the trip, it operates as a common solar sail; later it serves as a mirror focusing a good square kilometer of light stream of our distant Sun onto relatively small solar panel.
– SF.
Jun 22, 2018 at 23:18