Why there is a key at B2 of The Talos Principle?

There is a key at the puzzle "Man on the Moon" of world B2 of The Talos Principle. You can "use" it and it disappears immediately. What does this key open?

Solution 1:

It's part of the Portal 2 easter egg. From this guide:

On the left side of the moon you can see a connector (Bind the zoom, it help).
Power from the "Moonshot" puzzle using the connector and the red laser. Surprise !!

For the telescope, get Inside "Man on the Moon" puzzle. First you need to get the key in the far right area of the puzzle, it's easy to find. The telescope is just above the key.
To get there, use the left fan to jump on the platform on the left. To manage this you must hold left and bounce on the column during the jump.

Look into the telescope... Spaaaaaaaaaaaaace!