Copyright 2002, all rights reserved
Q: How fast do space ships cruise? William H., Albuquerque, New Mexico
A: It depends on where they're going. Let's talk about trajectories that use the least amount of rocket fuel, which is the only way we can launch. Our spacecraft now blast off from Earth.
Right: [NASA/JPL] Mars spacecraft orbiting the Sun at about 54,000 mph
So they start approximately at Earth's orbit speed: 67,100 miles (108,000 kilometers) per hour. They end up at the speed of whatever planet they intend to orbit and that speed is faster, the nearer the planet is to the Sun. Mercury hurtles around the Sun at 107,000 mph while Pluto ambles at 10,600 mph. A spacecraft cruises at orbit speed as it coasts from Earth to the target planet.
Let's illustrate the orbital mechanics with the spacecraft, Odyssey, that went into Mars orbit on October 23, 2001. On April 7, sitting on the launchpad-it zipped along at Earth's orbit speed and on Earth's orbit path (blue circle in the figure.) The plan, though, was to end up farther away on Mars' orbit (gold circle.) To go the farther distance, we had to fire rockets after lift off. This is similar to firing a cannon. If you want the cannonball to go farther, you must pack in extra powder.
Left: [NASA/JPL] How the spaceship transfers from Earth's to Mar's orbit
We launched the spacecraft on April 7. It lifted off the launch pad, rose above Earth's atmosphere, and fired its rocket to kick it the extra distance to Mars. It was then in a new orbit (red circle), called the transfer orbit. It was in "free fall" about the Sun and coasted along the path of the transfer orbit to Mars. The spacecraft slowed down the farther the orbit took it from the Sun, just as Earth does (Keppler's second law.) By the time it reached Mars, the craft was at its farthest distance from the Sun and had slowed to a little below Mars' orbital speed. Odyssey's average cruise speed was about 60,000 mph, relative to the Sun.
Spacecraft start with Earth's speed and then we either kick in extra energy to reach a planet farther from the Sun than us. Or, we decelerate to end up closer to the Sun. Space ships finish with the speed of the destination planet.
(Answered by April Holladay, science correspondent, February 6, 2002)
JPL/NASA: 2001 Mars Odyssey
JPL/NASA: Gravitation and mechanics
JPL/NASA: Interplanetary trajectories
USATODAY.com Mars Odyssey enters orbit around red planet
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