Quadralien

From the Back of the Box:

You are entering Jupiter Space. Your destination - Astra. The vast cybernetic nuclear-fission complex they orbited in the mid 21st century. Something strange has happened there. Astra has sealed itself to the outside, the control mechanisms will not respond, the temperature is rising. Meltdown is near.

But it is worse than that.

For in the very heart of the reactor itself, the sinister QUADRALIEN forces have established their foothold in our solar system. You have just six droids to sort it out.