A Russian Soyuz rocket carrying two crew members to the International Space Station malfunctioned shortly after launch early Thursday morning, forcing the space flyers to make an emergency return to Earth — what’s known in aerospace circles as a ballistic re-entry.

NASA astronaut Nick Hague and Russian cosmonaut Alexey Ovchinin are said to be in good condition after their capsule landed safely near the city of Dzhezkazgan in Kazakhstan. But their unexpected trip home was a tumultuous one marked by periods of intense G-forces.

“What ‘ballistic’ means is basically an unguided, uncontrolled free fall,” said Scott Kelly, a former NASA astronaut who made four trips to space, including two aboard the Soyuz.

In a ballistic re-entry, the capsule falls at a steeper angle than a spacecraft normally takes when returning to Earth. That unusual trajectory causes the capsule to decelerate rapidly as a result of increased friction with the atmosphere — but rapid deceleration means the crew is subjected to unusually strong G-forces.

In today’s emergency, Hague and Ovchinin experienced G-forces six to seven times Earth’s gravity, Reid Wiseman, NASA’s deputy chief astronaut, told reporters Thursday at a press briefing.

In addition to decelerating rapidly, the space flyers’ capsule spun slowly as it fell, Wiseman said, giving it a measure of aerodynamic stability as it plunged through the atmosphere.

“It’s like shooting a bullet out of a rifle barrel,” he said, before deadpanning that the forces the astronauts experienced were “not insignificant.” At seven Gs, a 180-pound astronaut weighs 1,260 pounds.

There has been no word yet from Hague or Ovchinin about what the experience was like. But Peggy Whitson, a retired NASA astronaut who survived a ballistic re-entry in a Soyuz capsule in 2008, described the experience as a harrowing one in an interview with The Houston Chronicle later that year.

“I felt my face getting pulled back,” she said of the re-entry, in which she was subjected to 8 Gs for about 60 seconds while her capsule plummeted 400,000 feet. “It was hard to breathe, and you kind of have to breathe through your stomach, using your diaphragm instead of expanding your chest.”

Under normal conditions, astronauts aboard Soyuz capsules experience about 4.5 Gs during re-entry, Kelly said — and even that is a notoriously violent event. “The way I describe it,” he said, “it’s like going over Niagara Falls in a barrel while you’re on fire.”

The emergency Thursday began about two minutes into the flight, when computers onboard the Soyuz capsule detected a problem with the rocket’s second stage. That automatically triggered the launch abort, beginning with the separation of the capsule from the rocket to get the crew away from the booster, which, of course, is laden with explosive rocket fuel.

Wiseman said 34 minutes passed from that point to the moment the capsule landed under its enormous parachute.

The incident was the fourth time a Soyuz crew has been forced to make a ballistic re-entry. It’s not yet known what caused the malfunction, but the Russian space agency Roscosmos said it has launched an investigation into the incident.

Additional reporting by Shoshana Wodinsky.

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