The debris always consists of only metal and crystal resources and represents 20% to 40% of the construction costs of the total ships destroyed. When lots of debris is generated there is a rare chance for a satellite to be formed.
Debris floating around any planet can be recovered with one or more recovery vessels. When only recovery vessels are sent on a recovery mission without other types of ships, their recovery process will not be detectable (unlike when sent as part of an attack).
The amount of debris recovered is limited by the total cargo size of all the recovery ships sent and the resources will always be collected in the same proportions as the debris floating around the planet (i.e. if there is twice as much metal as crystal in the debris, then the collected resources will have the same proportions).