Buzz Aldrin's EKG tells the heartstopping tale of how Apollo 11 almost failed to return from the moon.

Brian Fung

The date is July 20, 1969. With minutes of fuel to spare, the astronauts of Apollo 11 are gliding across the surface of the moon, looking for a place to land. There's just one problem: boulders strewn across the landscape prevent a quick touchdown.

Back in Houston, mission control is issuing periodic readouts of the spacecraft's fuel status. The seconds tick by until Apollo 11 is running on fumes. The situation is urgent. If the astronauts can't land, they'll be stranded on the moon until they die, with millions following the broadcast live from earth.

As the astronauts frantically search for a landing zone, the health monitoring equipment linked to the men goes haywire. The chart you see above shows Buzz Aldrin's EKG readout in the final moments before touchdown, which is marked by the long vertical line about three-fourths of the way through the graph. It's a rare, alternative glimpse of a critical moment in history, preserved by TEDMED curator and Priceline.com founder Jay Walker in a vast library devoted to science, medicine, and history.

As we know, the astronauts landed safely and returned home. But for a few heartstopping seconds, anything could have happened -- and this chart tells the tale.