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Slingshots in space

NASA's New Horizons probe is zooming past Jupiter to pick up speed on its way to Pluto, becoming just the latest spacecraft to get a slingshot boost from a planetary flyby. It won't be the last. Mission planners are increasingly taking advantage of celestial mechanics to speed up interplanetary trips.

This year is prime time for gravitational slingshotting: Just last Sunday, the European Space Agency's Rosetta probe got a gravity assist from Mars on its way to a comet encounter in 2014. New Horizons' encounter, which reaches its climax on Wednesday, will speed the probe toward a visit to Pluto in 2015. In June, NASA's Messenger probe to Mercury gets a boost from Venus, while the Dawn probe will be launched on a Mars-assisted mission to the asteroids Ceres and Vesta.

The reason for such assists is obvious: If a well-chosen course through the planets saves on a spacecraft's fuel requirements, that precious mass can instead be used for things like a bigger rover or an extra camera. The gravity-assist technique was developed back in the 1960s by Michael Minovitch, a student working at NASA's Jet Propulsion Laboratory, and has been used extensively since then. The Cassini orbiter, for example, went through four gravity assists during its seven-year journey to Saturn (two with Venus, and one each with Earth and Jupiter).

NBC News space analyst James Oberg says a similar strategy could someday be used to carry human crews on a "cycling" trajectory to Mars and back. "Buzz Aldrin and I published an article in Scientific American about that in March 2000," he wrote in an e-mail.

How does it work? It turns out that a slingshot isn't the best analogy. In a sense, the spacecraft is stealing some of the angular momentum from the planet. As the probe zooms toward the planet from behind, the two bodies exert gravitational force on each other. The planet, which is much larger, is slowed ever so slightly in its orbit - while the probe is pulled ahead, benefiting from the transfer of momentum.

Some have compared the phenomenon to lobbing a ping-pong ball at the blades of an electric fan. When the ball hits the fan, the blade slows down ever so slightly, while the ball careens off at a much faster speed.

Oberg took his own stab at an explanation:

"The best analogy to explaining 'gravity assist' - how a probe can pass near a planet and exchange momentum with it, either speeding up or slowing down - is the 'ball-off-a-wall' model. Like a planetary flyby, the bounce is 'elastic': Total energy is conserved. Bounce a ball off a wall and it comes back more or less at the same speed (as long as you ignore friction).

"But bounce a ball off a wall that is moving toward or away from you, and the ball comes back either faster or slower - it either gains energy or loses it, and the wall does the opposite. And if the wall is also moving sideways, the direction of the resulting bounce is also changed, just as with interplanetary spacecraft.

"It's not magic, but it sure seems like it. And this year, it's going to become very, very 'ordinary.'"

Oberg wrote a much more detailed explanation for Astronomy magazine back in 1999, when it was swing time for Cassini. Wikipedia also delves into the gravity-assist concept - noting that the technique played a part in multiple "Star Trek" plots, usually with a time-travel twist.

Although that last part may seem weird, gravity-assist maneuvers have been proposed as a test of general relativity - and theoretically, slingshotting around a massive object should slow down your spaceship's clock. Am I right on that? Feel free to set me straight in the comments section.