Sky Lights

There's more than one way to escape gravity

By Bob Berman|Monday, April 01, 2002

This month's income tax deadline might inspire fantasies of fleeing Earth. But the laws of physics, like the laws of the IRS, are irrevocable. Before you can set up a tax shelter on Mars, there's that little matter of achieving escape velocity.

Getting off our planet requires a textbook speed of 6.96 miles per second. That number applies only if you want to wrench free from Earth's gravity in a one-shot effort, however. If you can wait, a slow, steady thrust can also do the job. Think of Earth as lying at the bottom of a pit. If you drop a rock into the pit, it will have a certain velocity when it hits bottom. One way to get out of the hole is to jump upward at the exact same speed—the escape velocity. Another way out is to climb gradually up the wall of the pit. So long as you generate enough force to keep working your way up the wall, you'll still get out. Heading upward at even one mile per hour, a rocket could work its way out of Earth's gravitational field and travel clear across the universe if its engine fired for a long enough time.

High-speed atom fragments glow blue as they fly from an ion engine during testing. Such engines produce a weak thrust but are roughly 10 times as efficient as chemical rockets.
Photograph courtesy of NASA/JPL
Science fiction writers have long dreamed of building a space elevator, a 22,000-mile-high lift that would permit routine, gradual ascents into orbit. David Smitherman of NASA's Marshall Space Flight Center has even outlined how to create such a device (see the Web site fd02_elev. html), although he admits it is at least a century away from feasibility. For now, chemical rockets are the only practical way to ascend the steepest part of our gravitational well. But once you get past Earth's drag-producing atmosphere, a slow escape is both possible and desirable, because a tiny acceleration can produce a huge velocity if maintained long enough. Think the tortoise and the hare.

NASA's Deep Space 1 spacecraft, which shut down last December 18 after a three-year journey, shows the virtues of the leisurely approach. After a conventional launch, Deep Space 1 fired up its decidedly unconventional ion engine. Electric current from the probe's solar panels zapped an onboard supply of xenon gas and sent the resulting xenon ions racing out the back at 70,000 miles per hour. The engine generated a feeble thrust, equal to the weight of a couple of sheets of paper on one's palm, but it fired for 16,000 hours. Deep Space 1 ultimately gained 7,900 mph of extra speed, which enabled it to follow a looping path of discovery past asteroid Braille and Comet Borrelly. NASA is at work on a more powerful Next Generation Ion Engine.

This spring the privately funded Planetary Society will test an even gentler propulsion system, a solar sail. The Cosmos 1 spacecraft will blast off aboard a Russian chemical rocket. Once in orbit it will deploy eight ultrathin blades that unfurl to form a 100-foot-wide fan. The blades will capture the pressure of the sun's light, providing 1/1000 g of thrust. The force is minuscule, but solar sails use no fuel. As a result, they keep gathering speed without additional resources, steadily ascending the huge slope of the sun's gravity.

Planetary Society director Louis Friedman believes solar sailing is the only practical way to perform the great escape, digging out entirely from the sun's gravitational well for interstellar adventures. The Cosmos 1 will be a more modest affair, a mere proof-of-concept flight with no specific destination. Still, Friedman is ecstatic: "We feel like the Wright brothers during their first 12 seconds in the air. They, too, went nowhere in particular." And like Orville and Wilbur's biplane, tomorrow's interplanetary sailing ships will produce no head-snapping acceleration—hence, no seat belts required.

NASA's "Basics of Space Flight" Web site contains a tutorial about the hows and whys of getting into space:

Learn about specific space-propulsion systems at the Deep Space 1 site ( and the Planetary Society's solar sail project site (
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