You need to beat the boss on Mars - this chart is really useful:

Derelict are high-level Infested missions, most of which have a chance of gaining Corrupted mods (if you are carrying the correct key to unlock the vault - so don't do these solo). You need nav coordinates to craft a starchart key to go to the derelict. Just as a note, derelict is a terrible way to farm resources, people mainly go for the corrupted mods and for the Nekros warframe.

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