| 摘 要： | 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' II still get out. Heading upward at even one mile per hour, a rocket could work its way out of Earth' s gravitational field travel clear across the universe if its engine fired for a long enough time.
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