Retired engineer discovers 55-year-old bug in Lunar Lander computer game code

On Friday, a retired software program engineer named Martin C. Martin announced that he just lately found a bug within the authentic Lunar Lander pc sport’s physics code whereas tinkering with the software program. Created by a 17-year-old high school student named Jim Storer in 1969, this primordial sport rendered the motion solely as textual content standing updates on a teletype, however it set the stage for future variations to return.

The legendary sport—which Storer developed on a PDP-8 minicomputer in a programming language referred to as FOCAL simply months after Neil Armstrong and Buzz Aldrin made their historic moonwalks—permits gamers to manage a lunar module’s descent onto the Moon’s floor. Gamers should rigorously handle their gas utilization to realize a delicate touchdown, making vital selections each ten seconds to burn the correct amount of gas.

In 2009, simply wanting the fortieth anniversary of the primary Moon touchdown, I got down to discover the writer of the unique Lunar Lander sport, which was then primarily often called a graphical sport, because of the graphical version from 1974 and a 1979 Atari arcade title. Once I found that Storer created the oldest recognized model as a teletype sport, I interviewed him and wrote up a history of the game. Storer later released the source code to the unique sport, written in FOCAL, on his web site.

Quick ahead to 2024, when Martin—an AI skilled, sport developer, and former postdoctoral affiliate at MIT—stumbled upon a bug in Storer’s highschool code whereas exploring what he believed was the optimum technique for touchdown the module with most gas effectivity—a way recognized amongst Kerbal Space Program lovers because the “suicide burn.” This methodology includes falling freely to construct up pace after which igniting the engines on the final doable second to decelerate simply sufficient to the touch down safely. He additionally tried one other method—a extra light touchdown.

“I just lately explored the optimum gas burn schedule to land as gently as doable and with most remaining gas,” Martin wrote on his weblog. “Surprisingly, the theoretical greatest technique didn’t work. The sport falsely thinks the lander doesn’t contact down on the floor when the truth is it does. Digging in, I used to be amazed by the delicate physics and numerical computing within the sport. Ultimately I discovered a bug: a lacking ‘divide by two’ that had seemingly gone unnoticed for practically 55 years.”

A matter of division

Regardless of making use of what ought to have been a textbook touchdown technique, Martin discovered that the sport inconsistently reported that the lander had missed the Moon’s floor solely. Intrigued by the anomaly, Martin dug into the sport’s supply code and found that the touchdown algorithm was based mostly on extremely refined physics for its time, together with the Tsiolkovsky rocket equation and a Taylor series growth.

As talked about within the quote above, the foundation of the issue was a easy computational oversight—a lacking division by two within the components used to calculate the lander’s trajectory. This seemingly minor error had large penalties, inflicting the simulation to underestimate the time till the lander reached its lowest trajectory level and miscalculate the touchdown.

Regardless of the bug, Martin was impressed that Storer, then a highschool senior, managed to include superior mathematical ideas into his sport, a feat that is still spectacular even by immediately’s requirements. Martin reached out to Storer himself, and the Lunar Lander writer instructed Martin that his father was a physicist who helped him derive the equations used within the sport simulation.

Whereas individuals performed and loved Storer’s sport for years with the bug in place, it goes to indicate that realism is not at all times crucial a part of a compelling interactive expertise. And fortunately for Aldrin and Armstrong, the actual Apollo lunar touchdown expertise did not endure from the identical subject.

You possibly can learn extra about Martin’s thrilling debugging journey over on his blog.