# Would it be possilble to light the dark side of a tidally locked planet using fiber optics from the right side?

Assume planetary-scale cables are easily constructed, and subsequently ignored.

I am assuming these cables would run just under the surface of the planet from one side to the other for minimal maintenance from weathering. (I am also assuming minimal tectonic activity.)

Would this be able to light up the entire dark side? Only a fraction?

If it helps for quick calculations, assume an Earth-like planet.

• Attenuation: unless you use powerful light source as in really powerful one! Laser is better and try to straighten these cables will help you to feel good at best. Jan 19, 2020 at 9:57
• This is a neat question, not for the answer but for the visual it suggests. I hope the answer is yes. I love the idea of light sprouting from the ground periodically in the darkened forest. It would look like the forest in Avatar.
– SRM
Jan 19, 2020 at 15:25
• If you want to light the dark side of a tidally-locked planet, large orbital mirrors might be more useful Jan 19, 2020 at 16:24
• @Dragongeek beat me to it. You could put a large mirror in orbit at the planet-star L2 point to keep it relatively stable. It would probably be easier than embedding fiber optics under the whole crust. Think: mega-full moon. Jan 19, 2020 at 18:45

Yes theoretically, but it would create a dark patch on the bright side, as large as the area you are lightening.

Reason for this is that an optic fiber simply transport light with almost (mind this almost, it will become important in the following period you are going to read) no attenuation. But if you want to transport X amount of light from A to B, you have to take it from A. Thus, the light you cast on 100 square meter on dark side will come from 100 square meters of the bright side.

On top of that, the tenuous attenuation of those fibers will become important over the distances involved in shining the dark side. Telco usually employ amplifiers after a certain distance in order to compensate the losses. But they don't shine large surfaces. The energetic cost would be prohibitive.

• Thank you for your answer! But doesn't Telco try to maintain a data signal? I'm just looking to make it bright. Jan 19, 2020 at 16:32
• The Earth is under 13000 km diameter. “The longest un-regenerated (no repeaters) terrestrial fibre optic link is 10,358.16 km (6,436.26 mi) and was achieved by Telstra Corporation (Australia) with their link between Perth and Melbourne, Australia, as verified on 13 February 2015.“ You cannot go straight through the core, but at perimeter, you could drill shorter chords through planet.
– SRM
Jan 19, 2020 at 20:23
• As far as I could tell — no exact quote — that cable loses 25% of its light during transmission. So not full light of sun in the dark, but some light.
– SRM
Jan 19, 2020 at 20:28
• It will be easier near the terminator (light/dark boundary), where the distance between the light and the dark side is shorter. Running a cable from the middle of the day side to the middle of the night side, just under the surface, will require a 20,000 km cable (assuming Earth size). Given the light loss quoted above for 10,000 km, only 56% of the light will get through. The rest will be converted to heat along the way. Jan 20, 2020 at 8:46
• Unless you need to transport light away from the day side to reduce heating, a better solution could be orbiting mirrors. Jan 20, 2020 at 8:51