Jason Jupiter was one of the first games that I can remember playing - it came from the coverdisk of a 1989 issue of PC Plus, the IT magazine that my dad used to buy. It's been on the family Amstrad (now under the desk in my former bedroom at my parents' house, still working without a replacement part since the mid-80s) ever since.
After a while away from game making, I remembered about the game, realized it was nowhere to be found on the Internet, and tried out MMF's Java export to make it an online applet (imitating the original game's ASCII characters graphically). You can also download the original game from the applet page - it will run under DOSBox.
From the original README file:
Jason Jupiter has been called in on the deadly mission of decontaminating the planet Shufti-Arquar III by the Galactic Federation. The fragments of radioactove borxidium naturally form the shape of a golden heart when open to the air.
Unfortunately Jason couldn't get to the planet quick enough to stop some of the creatures on Shufti-Arquar III becoming terribly mutated and ferocious. The main offender is called the Glutanous-Green-Groted-Gronge. This wasn't the most friendly creature in the first place, but now its too ferocious to describe. (By the way don't let its name deceive you, it's Purple). Another mutation that will cause Jason a problem is a plant called the Horrible-Herbaceous-Headeating-Honkhider. One touch from this and it gets the taste of your flesh.
This translates into getting all the hearts on the screen and then touching the E. Use O and P to move left and right, and hold down Q and a direction to jump.