This “open source” simulator was writen around 1972 and published in 1977 - long before open source movement came to existance and a little bit before it’s author got his first computer. And it runs (flies?) on a scientific calculator with a segment-display.
Paul Lutus has become a legend among computer enthusiasts thanks to his brilliant career from a self-taught electronics technician to a NASA engineer. He is also a programmer who was the first one to work remotely in the 1970s. Without Teams, Slack or GIT, he worked as a developer for Apple in his woods cabin on the 8-bit Apple II.
In June 1977, Lutus published an article in Popular Electronics listing his first computer game. Space Flight was a short (49 commands) program for HP-65 scientific calculator. The player was to draw two planets and the flight path on graph paper. Gameplay consists in entering (or not entering) the speed parameter, reading the X and Y coordinates and plotting them on the graph paper. If we do not interact with the program, our spaceship will go around both planets indefinitely. When the player enters a negative value, the landing process will begin and the careful manipulation of the values will be necessary to not crash the spaceship.
Paul Lutus posted a scan of the article along with the program listing on his website. There are HP-65 calculator emulators available on the Internet, as well as user manuals and tutorials. Persistent enthusiasts can recreate the game of Lotus.