The Alignment of Planets

Heavy-lifting chemical rockets like the ones that propel the space shuttle have trouble escaping Earth's clingy gravity. Pictured: STS-101.

Such checks are rarely done in science fiction. When Star Trek's Captain Kirk wants to go someplace he never waits for a propitious alignment -- he just points the Enterprise in the right direction and cries "Warp Speed, Sulu!" Or in Star Wars, when Han Solo wants to travel to the Alderaan District, he simply pushes a few buttons and off he goes.

Unlike the mighty vessels of Kirk and Solo, however, our present-day space ships harbour limited power. Even the awesome Saturn V rocket, which carried 45,000 kg to lunar orbit during the Apollo program, didn't completely escape the pull of Earth's gravity. (Remember, the Moon is trapped by our planet's gravitational field and that's as far as the Saturn V went.) Nowadays the space shuttle can haul about 25,000 kg into low Earth orbit. Without extra propulsion built in, however, those payloads are still tightly bound to Earth's gravitational field.

Of course, some real-life spacecraft can reach escape velocity and travel to other worlds. Delta 2 rockets -- often used to send missions to Mars -- can loft about 700 kilograms free of Earth's gravity. But we can't send those 700 kg anywhere we want, for two reasons. First, such payloads remain bound to the Sun's gravitational field. Even after escaping Earth, they are still trapped within the solar system! Second, once the rocket engine exhausts its fuel, which happens quickly for chemical rockets, the payload can do little but coast in the direction it was slung.

Interplanetary coasting can take a long time. The recently-launched 2001 Mars Odyssey, for instance, will reach the Red Planet fully six months after it left Earth. During that interval Mars will have moved one-quarter of the way around its orbit. Clearly, it's vital that we understand not only where the target is at launch, but also where it will be when the spacecraft arrives. Present-day astronomers and mission planners find themselves calculating planetary motions and alignments much as their ancient ancestors did!"