Movies about space travel enjoy a huge following. Just about every year, audiences are sure to be offered at least one major motion picture depicting the glories of spaceflight. Usually, the actual space travel is incidental to the plot. Among those films that aim for some kind of scientific accuracy, one effect stands out above all others in creating the unique feel of actual space travel—weightlessness.
As a visual effect, weightlessness is hard to beat. To have half a dozen A-list actors bouncing lines off each other in the graceful slow motion that the public has come to know from the real-world space program is a gift to performance-driven directors. To have the light glint off the surface of a drop of liquid—artfully suspended and rotating in midair—is just as big a gift to talented cinematographers. It's such a good trick for a movie, frankly, that it's a wonder more films aren't set in space, if only for the visuals.
It can be a pricey gift, however. There are several methods at filmmakers' disposal for simulating microgravity, and none of them are especially easy. The first, and in many ways most obvious, way to get the desired effect is just to fake it with computers. Ever since the development of new techniques for rendering columns of water for "The Abyss," the technology has existed to render believable zero-g effects. The spectacle of a shimmering blob of water, held together by nothing but its own surface tension, has been a staple of science fiction ever since.
Unfortunately, the CGI approach is time-consuming and expensive. Virtual armies of visual artists have to be pulled in from multiple disciplines to make the effect work, and the slightest error is enough to utterly devastate the impact of the sequence. Rendering any surface in a graphics program is a multistage effort that calls for dozens of separate layers to work together seamlessly. Throw in the fact that such sequences usually call for chaotic movement of actors, props, and rendered graphics, and the difficulty of composing each frame multiplies exponentially, as does the price tag. When it isn't worth it to hire an entire visual effects studio, especially if the script only calls for floating actors, the effects department will often fall back on that old sci-fi standby—invisible wires.
Wire techniques have advanced considerably since the old days when a common "special" effect was to hang a flying saucer from a fishing pole and swing it in front of a model of the US Capitol. Today's wire effects usually entail a concealed harness for the actor, clever camera angles, and even small-scale CGI to rub out any wires that might have been caught on film. Often, the actors will be suspended in front of a green screen that will allow the easy addition of matte effects later at a fraction of the cost of a total CGI rendering.
Another common green screen technique for rendering weightlessness involves filming objects suspended by the airflow of a large, high-power fan concealed out of frame. This method is comparatively rare, as the airflow can't easily be disguised when the suspended object has loose or moving parts. This is especially true of a human actor, and so the fan is usually reserved for scenes involving sky diving or other high-airflow sequences.
When everything has to be suspended in still air, and extensive CGI isn't an option for whatever reason, film makers have another trick up their sleeves—the Vomit Comet. The Vomit Comet is a large, comparatively roomy, KC-135A jet aircraft that's been modified for use in the training of astronauts. The Comet follows a unique flight profile that's designed to take advantage of the fact that microgravity is indistinguishable from free fall.
Once the plane reaches its cruising altitude, the pilot dumps power into the engines and pulls back on the stick. This pulls the aircraft into a steep climb that will level off abruptly, leaving the Comet to nose forward into an equally steep dive that will last up to thirty seconds. During the terminal dive, everyone and everything aboard is maintained in free fall, which accurately simulates the total lack of gravity.
It's during these thirty-second windows that filming can be done under actual conditions of zero-g. While thirty seconds doesn't sound like much, it's considerably longer than most film sequences, and a well-planned shoot can usually be pulled off in less time. In the event that the shot isn't just right, a second attempt isn't long in coming, as the plane repeats its parabolic arc over and over until either fuel is short or everyone aboard is crippled by motion sickness—hence the "Vomit Comet" moniker. Many of the scenes from "Apollo 13" were shot this way.
Weightlessness is a stunning effect to pull off successfully. Done well, it can transport an audience into space, under water, or through the excitement of a falling sequence. Done poorly, it breaks the fourth wall and ruins the suspension of disbelief. It's well worth a film maker's time to develop more than one method for simulating microgravity, and to choose the method that will work best for a given sequence.