I finally found the UFO plant and completed the Gardening collection. There are so many collectibles that require the Rocket Ship adventures, and this is one of them. Do you need to have a specific skill level in Rocket Science before you get access to the correct adventure that produces the UFO plant? Do you have to be in the Astronaut career? Do you have to answer the correct way on the adventure questions? Well, all I can say is that I kept going on adventures with Sims who were not in the astronaut career and had up to level 6 skill, and never got the UFO. But my Sim who IS in the astronaut career gained level 7 Rocket Science; only level 5 of the career; and then got the adventure of the Green Man. The correct answers to the questions are: the Jungle, Take the Weapon. You will defeat the Nega-Sim and as a reward get a UFO plant and Cowberry plant.