Mechanics
Asteroid mining
Where asteroid fields appear, which fleet can mine them and how much ore a stay brings home.
Asteroid fields are temporary rocks of metal and crystal. Each galaxy keeps one field for every 0 systems. A field lasts 3 days or until it is mined out, and a new one then appears elsewhere. A fresh field holds 600,000 metal and 400,000 crystal.
Finding them
You see a field while one of your planets or moons in its galaxy lies within 50 systems of it. The Asteroids page lists them.
Mining
A mining fleet needs at least one Recycler and may carry only recyclers and cargo ships: the Light Cargo, the Heavy Cargo, the Battle Transporter and the Battle Recycler. Pick a stay of up to 8 hours when you send it.
At the end of the stay every Recycler has mined 2,000 for each hour, as much as the fleet's free cargo space and the field's reserve allow, split like the reserve. The ore reaches your planet when the fleet is back.
Questions
What happens if I recall a mining fleet?
It stops at once, keeps what it mined for the time it stayed and flies home.
Can two fleets mine the same field?
Yes. They take their ore from the same reserve, in the order their stays end, so the field runs out sooner.