In The Halley Project, the player is a star pilot who is given missions to travel between various objects within the solar system. The player starts out from a base on Halley's Comet and must first make his way to Earth, and then back to the comet. As the missions are completed, the player's rank increases. After completing all missions, a code is provided which could be mailed back to the software publisher, who would send back a letter with instructions to access a secret mission.
The game was published to capitalise on Halley's Comet pass by Earth in 1986 which was generating a lot of public interest in astronomy. Although the game was released in 1985 before the Comet was visible, the publishers did not mail out the code for the final mission until January 1986 which caused some consternation with players who were forced to wait to complete the game.