I am slowly getting to the point where I can finally build an explorer. But it takes lots of time and patience.
The Coal and Ore mine are at L11, while the copper and Uranium are at level 10.
The depots around 6 and 7.

The shipyard skill is also at L11, while the explorer ship skill is only at 5.
Just like in the real space out there, it takes time, lots of time :)

Cheers,
Peter