Apollo 13 takes a dive: how the movie crew survived the ups and downs that made gravity 'disappear.'(making the movie 'Apollo 13', includes an 8-point chart comparing real life with reel life)

Countdown to lift-off," cries the flight director. Secured aboard the spacecraft Apollo 13, the astronauts brace themselves. "Five, four, three, two, one ... liftoff." The spacecraft blasts upward, spewing flames and smoke in every direction. Minutes later, the astronauts float out of their seats as they become "weightless" and head to the Moon.

"... and cut....That's a wrap."

It's the end of another day of filming on the set of Apollo 13, the true story about three astronauts who didn't make it to the Moon--and almost didn't make it back to Earth--in 1970. For actors Tom Hanks, Kevin Bacon, and Bill Paxton, the action was almost real.

"When we did the launch sequence, in our pressure suits, with the helmets on," says Hanks, who starred as Apollo 13 Commander Jim Lovell, "I definitely felt like I was on my way [to the Moon]."

But of course the cast wasn't really launched into space. The film-makers used special effects to re-create the action.

FREE FALL

Their most awesome challenge was to create the scenes of astronauts floating "weightless" in space. On Earth, your weight equals your mass pull of gravity (Earth's downward pull on all nearby objects).

You might think astronauts in space are weightless because Earth's gravity weakens the farther you move from the planet. That's true, but you'd have to travel more than 17 times the distance to the Moon to escape Earth's gravity.

In fact, it's the presence of Earth's gravity in "nearby" space that makes astronauts weightless. What happens is this: Once the astronauts turn off the engines, the spaceship coasts. Gradually, Earth's gravity pulls the stop along a curved path back toward Earth.

Think of what happens when you launch a basketball the length of the court: It goes up and curves to trace an arc as it falls through the hoop. During the entire arc (the way up and down), gravity is the main force on the ball. That state of gravity-driven motion in any direction is called free fall.

But a spaceship takes off with much more force than a ball. So the arc it traces is much larger; it can reach all the way to the Moon. During the entire trip there and back, the spaceship, like the balls, is in free fall.

Free fall makes objects and people weightless because gravity pulls everything at the same rate. The astronauts, "falling" at the same rate as their spaceship, float like jelly beans inside a plastic jar tossed in the air (try it!). Even if the astronauts could stand on a scale, it wouldn't register any weight, because the scale would be free falling too.

ANTI-GRAVITY ACTION

Creating weightless scenes for the movie would have been no problem if the crew could have filmed in space. But that would have sent the movie's budget to the Moon! So instead, the actors did what real astronauts in training do. They flew in NASA's "zero-gravity" plane, nicknamed the Vomit Comet.

The plane climbs and dives, coasting over the top of an invisible arc, just like the spacecraft. The arc is shorter than the spacecraft's, but for a brief time, the plane is in free fall and everyone inside is weightless.

Before climbing on board the Vomit Comet, the cast and crew swallowed a potent medication that astronauts take to keep them from losing their lunch. Then, the film crew loaded mock-ups of the spacecraft, lights, cameras, and other equipment needed to film the movie.

Bob Williams, test director for NASA's zero-gravity training program, told the actors what to expect: "It's just like a roller coaster in the sky."

But the zero-gravity arcs weren't like any roller coaster the Apollo 13 actors had ever been on. "It's not a sensation you can liken to anything else," says Hanks.

"Your entire body feels funny," recalls Kevin Bacon. "You hear the change in the engines, and the wings of the plane shift, and you literally pop right up and think, `I'm upside down. I'm not--what's wrong? Oh, I'm floating.'"

When the Vomit Comet reaches the bottom of its roller-coaster-like dive, the zero-gravity environment instantly disappears. As the plane climbs again, the cast and crew hit the deck. How come? The astronauts tend to keep moving in a straight line downward even though the plane begins to climb. (Remember Newton's first law of motion? See SW/9/195, p. 12.) When gravity took hold of the Apollo 13 cast, says Hanks, the well-trained NASA crew would hold on to the actors so they wouldn't crash on top of the cameras or reach other.

Once the crew and actors had taken several test flights, they were ready to film. Each Vomit Comet zero-gravity arc lasts only 23 seconds, so the cast and crew had to work fast. These are the scenes where actor Bill Paxton, as astronaut Fred Haise, spins his sunglasses toward the camera. And actor Kevin Bacon, as astronaut John Swigert, squirts orange juice into the air.

"We definitely got better at [filming] as we went along--just knowing how to reinforce the set and stabilize the props," says the film's director, Ron Howard.

Howard admits that he expected somebody to be sick on every Vomit Comet flight. But it wasn't that bad. "If I had a chance to do more floating," he says, "I would."

Overall, the cast and crew flew 17 flights, making 609 zero-gravity arcs. "The actors playing astronauts spent more time in the zero-gravity plane than any real astronaut," says Jim Lovell, the real commander of Apollo 13. His dream of walking on the Moon was crushed during the ill-fated mission.

Like Lovell, Ron Howard has far-reaching dreams. "One of my dreams has always been to shoot location footage on the Moon," he says. "This work in the Vomit Comet was at least a baby step in that direction."