Gravity Boost

Space probes sent across the solar sys­tem sometimes use the gravity of other planets to help them on their way. As a space probe flies toward a planet, the planet’s gravity pulls the probe and speeds it up. When it flies past the plan­et, gravity acts like a brake and slows it down again. If a planet stood still in space, this is all that would happen, and the probe would not gain or lose any speed. Planets do not stand still, how­ever, they move.

Jupiter flies around the Sun at about

30,0 miles per hour (about 48,000 kilometers per hour). If a space probe is flying in the same direction as Jupiter, it is swept along by the giant planet’s gravity. It speeds up by the amount of Jupiter’s speed, therefore gaining 30,000 miles per hour (48,000 kilometers per hour) without having to use any fuel. A planet’s gravity also can slow a probe

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GRAVITY ON OTHER PLANETS

PLANET GRAVITY

Mercury	0.38 x Earth gravity
Venus	0.91 x Earth gravity
Earth	1.00 x Earth gravity
Mars	0.38 x Earth gravity
Jupiter	2.36 x Earth gravity
Saturn	0.92 x Earth gravity
Uranus	0.89 x Earth gravity
Neptune	1.12 x Earth gravity

down or bend its flight path to steer it in a different direction. These maneuvers are called “gravity assist.”

Gravity assist enables a small space probe to visit distant planets without having to carry a huge amount of fuel. Space probes going to Mercury use Venus’s gravity to slow them down and set them on course for their destination. Space probes going to the outer planets use planets that they pass on the way to pick up the extra speed they need.

A planet has to be in the right place at the right time to give gravity assist to a space probe exactly when it is needed. For this reason, space probes often have to be launched within a certain period of time, called the launch window.

О Spanish astronaut Pedro Duque watches a water bubble float between himself and the camera on board the International Space Station. The bubble, like a lens, shows the astronaut’s miniature image.

О Neap tides and spring tides are the names given to the highest and lowest tides produced when the Sun’s gravity either combines with or opposes the Moon’s pull on Earth’s oceans.