KitBash3D, a popular 3d model kit vendor, have released free sci-fi assets in a pack called Mission to Minerva. KitBash3D packs are available for a variety of game engines and content creation tools including Unreal, Unity, Mac, Maya and Blender and can be easily used in any 3D game engine.
The Minerva Kit is described as:
From cargo ships, terrain rovers and terraformers to community centers and endlessly modular habitats, this Kit has everything a spacefaring explorer needs.
You will need to create an account to “purchase” the assets, but you do not need to give any payment information or credit card details to check out. Once yours, the assets are the same as any assets purchased from the KitBash3D store (license link below).
In addition to the free sci-fi content pack, they are also running a competition using those assets:
Once your world is built, share it using #KB3Dchallenge on Facebook, Instagram, Twitter or Artstation to enter the challenge and contribute your piece to the Minerva Galaxy.
You can enter into either of these 2 categories:
Concept Art Awards for still image renders
In-Motion Awards for animations
October 11: Challenge Begins
December 2: Submission Deadline
December 20: Winners Announced
Additionally they have created a pair of tutorials showing how to utilize the assets, one for Blender and the other for Unreal Engine 5.
You can learn more about the KitBash3D free assets in the Mission to Minerva pack in the video below.