On Friday, a retired software program engineer named Martin C. Martin announced that he lately found a bug within the unique 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, nevertheless it set the stage for future variations to come back.
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 regulate a lunar module’s descent onto the Moon’s floor. Gamers should rigorously handle their gasoline utilization to attain a delicate touchdown, making crucial choices each ten seconds to burn the correct amount of gasoline.
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 generally known as a graphical sport, because of the graphical version from 1974 and a 1979 Atari arcade title. After I found that Storer created the oldest identified 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.
![A scan of printed teletype output from the original <em>Lunar Lander</em> game, provided by Jim Storer.](https://cdn.arstechnica.net/wp-content/uploads/2024/06/lunar_lander_teletype_output-640x524.jpg)
Jim Storer
Quick ahead to 2024, when Martin—an AI knowledgeable, 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 gasoline effectivity—a method identified amongst Kerbal Space Program fans because the “suicide burn.” This technique entails falling freely to construct up pace after which igniting the engines on the final potential second to decelerate simply sufficient to the touch down safely. He additionally tried one other strategy—a extra mild touchdown.
“I lately explored the optimum gasoline burn schedule to land as gently as potential and with most remaining gasoline,” Martin wrote on his weblog. “Surprisingly, the theoretical finest technique didn’t work. The sport falsely thinks the lander doesn’t contact down on the floor when actually it does. Digging in, I used to be amazed by the subtle physics and numerical computing within the sport. Finally I discovered a bug: a lacking ‘divide by two’ that had seemingly gone unnoticed for almost 55 years.”
A matter of division
![Diagram of launch escape system on top of the Apollo capsule.](https://cdn.arstechnica.net/wp-content/uploads/2019/03/Apollo_Spacecraft_diagram-640x408.jpg)
NASA
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 fully. Intrigued by the anomaly, Martin dug into the sport’s supply code and found that the touchdown algorithm was primarily based on extremely subtle physics for its time, together with the Tsiolkovsky rocket equation and a Taylor series enlargement.
As talked about within the quote above, the basis of the issue was a easy computational oversight—a lacking division by two within the formulation used to calculate the lander’s trajectory. This seemingly minor error had huge 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 continues to be spectacular even by at present’s requirements. Martin reached out to Storer himself, and the Lunar Lander writer advised Martin that his father was a physicist who helped him derive the equations used within the sport simulation.
Whereas folks performed and loved Storer’s sport for years with the bug in place, it goes to point out that realism is not at all times a very powerful 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’ll be able to learn extra about Martin’s thrilling debugging journey over on his blog.