Slingshot Around the Moon: How Apollo 13 Returned Home

Apollo 13 was supposed to be NASA’s third crewed mission to the surface of the moon, but nearly 56 hours into the flight, command module pilot John “Jack” Swigert radioed a troubling message to Mission Control: "OK, Houston, we've had a problem."

An explosion in one of the ship’s oxygen tanks knocked out power to the command and service module (CSM), the main part of the spacecraft. Without the CSM’s powerful engines, it was unclear how the crippled ship and its three astronauts—James "Jim" Lovell, Fred Haise and Swigert—were going to get home.

Public interest in the space program had waned since the historic Apollo 11 moon landing, but the plight of the Apollo 13 astronauts captured the world’s attention. Pope Paul VI led an audience of 10,000 in prayer, and 13 countries—including the Soviet Union—contacted President Richard Nixon to offer naval support for a rescue.

At Mission Control, NASA's operations team kept cool heads and decided that the best chance of getting the astronauts home safely was to slingshot them around the moon using what’s known as a “free-return trajectory.”

“The beauty of the free-return trajectory is that the moon's gravity will bend your flight path around the moon in a kind of a figure-8 loop, and send you back to Earth,” says space historian Andrew Chaikin. “It’s an insurance policy. If you're on the way to the moon and something goes wrong and need to abort the flight, you have a free ticket back to Earth.”

The Apollo 13 astronauts were 200,000 miles from Earth when trouble struck on April 13, 1970. It started with a routine order from Mission Control to “stir” the cryogenic oxygen tanks. Soon after lunar module pilot Swigert flipped the switch, the astronauts heard a bang and they started losing power and oxygen in the command and service module.

"Lovell looked out the window and saw that there was some kind of gas streaming into space,” says Chaikin, author of A Man on the Moon. “And that's when everybody put two and two together and realized that something really bad had happened.”

The command and service module was supposed to be the astronauts’ home for the 10-day mission, but without power or life support systems, they were forced to squeeze into the smaller lunar module (LM) and use it as a lifeboat.

The LM was designed to carry two astronauts on a short trip to the lunar surface, not ferry three astronauts hundreds of thousands of miles through space. Not only was it cramped, but the LM only had two small engines. Would those descent and ascent engines be enough to navigate a safe course home?

The Apollo 13 flight crew had a backup plan for nearly every possible contingency, says Charles Deiterich, a key member of the Apollo 13 mission operations team in Houston.

“I was a retro fire officer on Apollo 13, and my job was to come up with a trajectory plan—all the maneuvers and stuff to get the crew back,” says Deiterich, who worked for NASA for 30 years. “I’d say 80 to 90 percent of our work [at NASA] was figuring out how to do things if they weren’t nominal.”

Training simulations are one thing, but in the nine years of the Apollo program, this was the first time that a retro team had to execute a real-world abort scenario.

The first option if something went wrong on an Apollo mission was a so-called “direct abort.” For that emergency maneuver, the astronauts would spin the command service module 180 degrees so that its tail was pointing forward. Then they would blast the spacecraft’s engines at full power, rapidly decelerating from 25,000 mph to a standstill before rocketing in the opposite direction.

With the command and service module’s engines down, however, Deiterich and his team dismissed the direct abort option.

The next-best plan was to catch a “free ride” home using the moon’s gravity as a slingshot. Starting with Apollo 8—the first manned space flight to orbit the moon—every Apollo crew had followed the free-return trajectory on its way to the moon. If something went wrong with the spacecraft’s engines, the free-return trajectory would whip them around the back of the moon and send them on a safe path back to Earth.

Apollo 13 was the first crewed mission to experience a catastrophic spacecraft failure. Not only was Apollo 13 the first time that a free-return trajectory was used in an abort scenario, but unlike those earlier Apollo missions, Apollo 13 wasn’t on a free-return trajectory when the explosion happened. Instead, the spacecraft was on a “hybrid” trajectory that positioned the astronauts to land on the moon’s Fra Mauro Highlands.

If the astronauts stayed on that hybrid trajectory, they would still slingshot around the moon, but they would miss the Earth by 2,600 miles on their return. It would take 39 days, and two more slingshots around the Earth and moon, before the astronauts made it home. Their oxygen supply would run out long before then.

Thankfully, the Apollo 13 astronauts weren’t stuck on that hybrid trajectory. They still had the two engines of the Lunar Module to work with. It was now Deiterich and the rest of the retro fire team’s responsibility to figure out how to use the LM’s engines to nudge the wounded spacecraft onto a free-return trajectory. The lives of three astronauts absolutely depended on it.

Deiterich and his team knew that the window was closing fast for the Apollo 13 astronauts to correct their course, but a maneuver as precise as this couldn’t be done manually. It required the ship’s computer-controlled autopilot. It was Deiterich’s job to calculate the exact sequence of “burns”—NASA speak for firing an engine—that would propel the spacecraft onto the correct free-return trajectory.

“We had an IBM 360, which was a big computer,” says Deiterich. “It would take 10 minutes for that large mainframe computer to calculate the maneuvers to come back from the moon. There's no way you could do that with a calculator.”

Once the NASA computer spit out the burn sequence, the Apollo 13 astronauts needed to manually input the instructions and coordinates into the LM’s guidance system. The burn sequence was tested on the Apollo simulators in Houston but there was still a chance that a single input error could send the spaceship careening in the wrong direction.

As everyone at Mission Control held their breath, Jim Lovell executed the burn at 61 hours 30 minutes. It worked exactly as Deiterich’s calculations said it should. The Apollo 13 astronauts were officially on a free-return trajectory. But their ordeal was far from over.

On earlier Apollo missions, the astronauts would decelerate the spacecraft as it approached the moon so they could be pulled into lunar orbit. But that wasn’t the case with Apollo 13. The explosion on Apollo 13 made a moon landing impossible, so there was no reason to go into lunar orbit. That meant the astronauts were traveling faster and farther as they whipped around the back of the moon.

As of this writing, the three Apollo 13 astronauts—Jim Lovell, Jack Swigert and Fred Haise—hold the distinction of traveling farther from the surface of the Earth than any other human beings. On April 15, 1970, they swung around the far side of the moon 158 miles above the lunar surface. That put them 248,655 miles from Earth.

The Apollo 13 astronauts were likely in no mood to celebrate their record-setting achievement. The journey home in the cramped LM was a grueling physical and psychological trial. The astronauts relied on the spaceship’s onboard equipment to generate heat, but with electricity usage cut by 80 percent to conserve power, the temperature inside the LM dropped to 38 degrees Fahrenheit.

The men also had to ration food and water—just six ounces of liquid a day—causing severe dehydration. Collectively, the three men lost a total of 31.5 pounds during their harrowing six days in space.

For his part, Deiterich never doubted that NASA had the right contingency plans in place to get the astronauts home safely.

“Dave Reid was a flight dynamics officer on our team, and he and I never thought we weren't going to get the crew back,” says Deiterich. “Although we didn't really get relieved until the crew was on the carrier.”

The Apollo 13 capsule splashed down safely in the Pacific Ocean on April 17, 1970, to the tremendous relief of millions of people watching and praying the world over. On April 18, Richard Nixon awarded Deiterich and the rest of the Apollo 13 Mission Operations Team the Presidential Medal of Freedom for their successful rescue of the three astronauts.

“The free-return trajectory was the first critical step in getting those guys home alive, and it worked,” says Chaikin.

More than half a century later, NASA's Artemis II mission plans to send humans on a similar path around the moon. This time, the astronauts could fly even deeper into space—reaching about 250,000 miles from Earth—some 1,345 miles farther than the Apollo 13 crew.