Oh, the memories. I believe that the game was actually released in 1986, by Hewson, but I can be wrong. Later on the same year, an improved version, Uridium+, was released, with new levels and enemies and a slightly different look. Then in 1993 yet Uridium 2 was released, which also was available for Amiga.

This is an excerpt from the manual:

The solar system is under fierce attack. A squadron of formidable enemy Super-Dreadnought ships, needing mineral fuel to power their interstellar unit is draining the mineral resources from all 15 planets in your galactic sector. Each Super-Dreadnought, in orbit around one planet, is mining a special metal. This irreversible mineral depletion must be stopped immediately, or it will destroy all planet life.

Your mission is to repeal the enemy attack with your special Manta fighter. Completely. Quickly. Decisively. The odds against you are enormous because you are the lone remaining defender of this planetary sector.

Sweet huh ? 

There were many different ways to make the game easier. One way was to use the lovely POKE commands before running the game: 

POKE 13205,X
SYS 2304

This would give X number of lives. Very useful when you were practicing...

These are the levels I remember: