Hero coder finds bug in Lunar Lander after 55 years—'I suspect everybody else was just happy to play the game and have fun'

GamesSimHero coder finds bug in Lunar Lander after 55 years—‘I suspect everybody else was just happy to play the game and have fun’When you purchase through links on our site, we may earn an affiliate commission.Here’s how it works.

When you purchase through links on our site, we may earn an affiliate commission.Here’s how it works.

(Image credit: Bettmann via Getty Images)

Neil Armstrong pilots the lunar lander training vehicle for the Apollo 11 mission.

Apollo 11 touched down on the moon’s surface on July 20, 1969, an achievement for all humankind that put a full stop on the first iteration of the space race. Neil Armstrong stepped into history as the globe watched and among the millions in awe was 17 year-old schoolboy Jim Storer.

Text-based and written in the early programming language FOCAL, Lunar Landing Game would ask the player for instructions on each turn, with the height, velocity, and remaining fuel of the lander calculated over each in-game second of the descent. Essentially it’s a game about using your fuel supply to slow and control the craft’s descent, and when the craft reaches the surface the player gets a report on their landing speed and remaining fuel.

The whole game is less than 50 lines of code, but its apparent simplicity belied the calculations underlying the simulation, and over the years the game became widely popular as a programming challenge. Even later, various popular commercial versions would add a visual layer (to give an idea of its longevity, Atari is currently in the process ofdeveloping an official sequel).

Suicide burn

The biggest gaming news, reviews and hardware deals

“With some trial and error and a bit of (manual) binary search, you can find the schedule that just barely has you landing. You burn nothing for 70 seconds, then 164.31426784 lbs/sec for the next 10 seconds, then the max 200 lbs/sec after that,” writes Martin.

“The game considers a perfect landing to be less than 1 MPH, but here we land at over 3.5 MPH and are told we ‘could be better.’ Yet burn even 0.00000001 more lbs/sec, and you miss the surface entirely, ascending at 114 MPH:

“How did we go from a hard landing to not landing at all, without a soft landing in between?”

When the coder started looking into why, he found that rather than “the simple Euler integration that’s common in video games even today”, underpinning the game’s logic were more sophisticated equations: “Jim [Storer] used the exact solution, the Tsiolkovsky rocket equation, with a Taylor series expansion for the logarithm,” writes Martin. “He also used some algebra to simplify it and reduce the amount of round off error. Very impressive for a high school senior in 1969.”

Martin contacted Storer to confirm what he’d found, and ask about where this sophisticated math had come from. “I was skilled at calculus at the time and familiar with concepts like a Taylor series,” says Storer, “but also my recollection is that my father, who was a physicist, helped me in the derivation of the equations.”

Either way, the the rocket equation is what should make the suicide burn optimal, and the accuracy of the Taylor series suggests that’s not the issue either. Instead the problem was contact: the moment where the Lander lands.

As Martin goes on to explain, the error creeps in when the game is trying to approximate the bottom of the trajectory. The coder presents his full working outin the blog postbut, essentially, it comes down to Storer using an alternate form of a quadratic formula where for whatever reason “he’s missing the 2 in the denominator inside the square root! It was probably a simple error, either when deriving the formula or typing it into the computer.” As Martin is careful to point out, Storer wouldn’t have had access to algebra software and was doing this all with pencil and paper.

Martin goes on to explain the effect of fixing the bug, and spitball about some of the reasoning. Though he does note sadly that, even with the bug fixed, there are some optimal landing strategies but “the theoretical full-thrust-on-landing suicide burn, that takes around 148 seconds, eludes us.”

As for a bug surviving in a piece of popular software for 55 years?

“That’s probably because, even with the bug, it was a fun game, both difficult and possible to land gently,“writes Martin.“The quest to not just win, but find the optimal strategy, can certainly lead to trying to understand small discrepancies. I suspect everybody else was just happy to play the game and have fun.”

If you’re interested in the history of Lunar Lander,this 40th anniversary retrospective speaks to Storer himselfabout the game’s creation. And if you just like old bugs, here’s a doozy: the developerwho returned to his game after four decades, then found and fixed a typo so it actually worked.

More about simThe Factorio ‘God Factory’ was one of this year’s greatest gaming feats: ‘Most players will never find themselves hitting the limits of the game. We are.‘Rule the talking movie industry in tycoon game Hollywood AnimalLatestToday’s Wordle answer for Saturday, January 11See more latest►

More about sim

The Factorio ‘God Factory’ was one of this year’s greatest gaming feats: ‘Most players will never find themselves hitting the limits of the game. We are.‘Rule the talking movie industry in tycoon game Hollywood Animal

Factorio base screenshot with photo of Oppenheimer actor Cillian Murphy in front

The Factorio ‘God Factory’ was one of this year’s greatest gaming feats: ‘Most players will never find themselves hitting the limits of the game. We are.’

Screenshot of a movie-making compound from game Hollywood Animal

Rule the talking movie industry in tycoon game Hollywood Animal

LatestToday’s Wordle answer for Saturday, January 11See more latest►

Latest

Today’s Wordle answer for Saturday, January 11

Wordle answers

Today’s Wordle answer for Saturday, January 11

See more latest►

Most PopularPiranha Games will lay off employees after Mechwarrior 5: Clans ‘performed below projections’Marvel Rivals’ latest update quietly killed the game’s burgeoning mod sceneSorry everyone, Minecraft 2 is cancelled, and it might be our faultTeam Fortress 2 just had the single biggest trade in its history, with ‘the absolute best hat you can get’ going for around $40KIs the new RTX 5070 really as fast as Nvidia’s previous flagship RTX 4090 GPU? Turns out the answer is yes. Kinda.Elden Ring: Nightreign is getting a closed network test, and I’m totally not fuming because PC players aren’t invited to the party, or anythingIf you’re a content creator and gamer looking for a CPU upgrade, this Ryzen 9 9900X deal will get you $90 offAMD is now reportedly making all-American Ryzen 9000 CPU dies at TSMC’s Arizona fabI tried a sim racing rig that generates ‘wind’ at CES 2025 and it’s claimed to help keep VR nausea at bayFinal Fantasy 14 communities panic as it turns out change to blacklisting, meant to help reduce stalking, also lets players use mods to track their altsTikTok’s time in the US may be ticking to a close as it makes a last stand in the Supreme Court, though Trump now says he ‘opposes banning’ it

Most Popular

Piranha Games will lay off employees after Mechwarrior 5: Clans ‘performed below projections’

Marvel Rivals’ latest update quietly killed the game’s burgeoning mod scene

Sorry everyone, Minecraft 2 is cancelled, and it might be our fault

Team Fortress 2 just had the single biggest trade in its history, with ‘the absolute best hat you can get’ going for around $40K

Is the new RTX 5070 really as fast as Nvidia’s previous flagship RTX 4090 GPU? Turns out the answer is yes. Kinda.

Elden Ring: Nightreign is getting a closed network test, and I’m totally not fuming because PC players aren’t invited to the party, or anything

If you’re a content creator and gamer looking for a CPU upgrade, this Ryzen 9 9900X deal will get you $90 off

AMD is now reportedly making all-American Ryzen 9000 CPU dies at TSMC’s Arizona fab

I tried a sim racing rig that generates ‘wind’ at CES 2025 and it’s claimed to help keep VR nausea at bay

Final Fantasy 14 communities panic as it turns out change to blacklisting, meant to help reduce stalking, also lets players use mods to track their alts

TikTok’s time in the US may be ticking to a close as it makes a last stand in the Supreme Court, though Trump now says he ‘opposes banning’ it

HARDWARE BUYING GUIDESLATEST GAME REVIEWS1Best Steam Deck accessories in Australia for 2025: Our favorite docks, powerbanks and gamepads2Best graphics card for laptops: the mobile GPUs I’d want in my next gaming laptop3Best mini PCs in 2025: The compact computers I love the most4Best 14-inch gaming laptop: The top compact gaming laptops I’ve held in these hands5Best Mini-ITX motherboards in 2025: My pick from all the mini mobo marvels I’ve tested1Thank Goodness You’re Here! review2Shiren the Wanderer: The Mystery Dungeon of Serpentcoil Island review3WD Black SN850X 8 TB NVMe SSD review4Ikea Utespelare desk review5Asus ROG Harpe Ace Mini wireless mouse review

HARDWARE BUYING GUIDESLATEST GAME REVIEWS

1Best Steam Deck accessories in Australia for 2025: Our favorite docks, powerbanks and gamepads

A bunch of the best Steam Deck accessories on a blue background.

1Best Steam Deck accessories in Australia for 2025: Our favorite docks, powerbanks and gamepads

Best Steam Deck accessories in Australia for 2025: Our favorite docks, powerbanks and gamepads

2Best graphics card for laptops: the mobile GPUs I’d want in my next gaming laptop

Best graphics card for laptops: the mobile GPUs I’d want in my next gaming laptop

3Best mini PCs in 2025: The compact computers I love the most

Best mini PCs in 2025: The compact computers I love the most

4Best 14-inch gaming laptop: The top compact gaming laptops I’ve held in these hands

Best 14-inch gaming laptop: The top compact gaming laptops I’ve held in these hands

5Best Mini-ITX motherboards in 2025: My pick from all the mini mobo marvels I’ve tested

Best Mini-ITX motherboards in 2025: My pick from all the mini mobo marvels I’ve tested

1Thank Goodness You’re Here! review

Thank Goodness You’re Here review

1Thank Goodness You’re Here! review

Thank Goodness You’re Here! review

2Shiren the Wanderer: The Mystery Dungeon of Serpentcoil Island review

Shiren the Wanderer: The Mystery Dungeon of Serpentcoil Island review

3WD Black SN850X 8 TB NVMe SSD review

WD Black SN850X 8 TB NVMe SSD review

4Ikea Utespelare desk review

Ikea Utespelare desk review

5Asus ROG Harpe Ace Mini wireless mouse review

Asus ROG Harpe Ace Mini wireless mouse review

Suicide burn#

The biggest gaming news, reviews and hardware deals#

Suicide burn

The biggest gaming news, reviews and hardware deals