Celest

Celest is a 1997 orbital flight simulation by Mike Ash in which the player pilots a small craft through scenario-based missions, reaching objectives while burning as little fuel as possible. Its appeal is the precision of its physics: gravity, thrust, and timestep all matter.

The premise

Each scenario sets a goal -- reach a planet, achieve an orbit, rendezvous with a target -- and challenges the player to do so with the smallest possible fuel expenditure. The author's own framing is direct: "do what the scenario says, or any other objective you wish, preferably with the smallest possible amount of fuel expended."

Controls and display

Basic flight is handled by left/right rotation and a thrust input. The display draws the player ship in green with a blue trail, planets in light blue, and target bodies in yellow. Twin fuel bars track the cheaper low-thrust reserve (green) and the heavier high-thrust reserve (red).

Simulation

The player can zoom, center on any object, and adjust the simulation timestep manually. The engine also auto-scales its timestep when the ship draws close to a body, trading speed for accuracy so that close passes and capture orbits resolve cleanly instead of tunneling through gravity wells.

Author

Mike Ash released Celest as freeware in 1997. It is small, focused, and very much in the lineage of hobbyist Mac orbital sims that prized realism over spectacle.