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Why Do the Planets Orbit the Sun?

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Narrator: Welcome to Space Place Musings, where an expert answers questions from our Space Place museum partners across the nation. I'm Diane Fisher, of the New Millennium Program. Our expert is Dr. Marc Rayman, a scientist at the Jet Propulsion Laboratory.

Marc, we have a question from Gheens Science Hall and Rauch Planetarium at the University of Louisville in Kentucky. They ask "Why do the planets orbit the Sun?"

Rayman: A planet orbits the Sun just as the moon or a satellite orbits Earth. Isaac Newton, one of the most brilliant scientists of all time, provided the first good explanation of orbits. Newton realized that the reason the planets orbit the Sun is related to why objects fall to Earth when we drop them. The Sun's gravity pulls on the planets, just as we now understand Earth's gravity pulls down anything that isn't held up by some other force. More massive objects produce a bigger gravitational pull than less massive ones, so as the heavyweight in our solar system, the Sun exerts the strongest gravitational pull.

Narrator: But if the Sun is pulling the planets, why don't they just fall into the Sun and burn up?

Rayman: Well, in addition to falling toward the Sun, the planets are moving sideways, around the Sun. This is the same as if you have a weight on the end of a string and swing it around. You are constantly pulling it toward you, but the motion sideways keeps it swinging around. Like the string, the Sun's gravity pulls on the planets, but the planets have enough sideways motion to keep them in their orbits.

Narrator: So is this sideways motion partly why we need rockets to launch a spacecraft into orbit?

Rayman: Yes, it is. The rocket has to do much more than boost the spacecraft above the atmosphere—it also has to provide enough speed for the spacecraft to keep swinging around Earth and not just fall back to the surface. Now if we give the spacecraft a really big boost, it can travel fast enough to leave Earth orbit and enter its own orbit around the Sun, eventually meeting up with its target elsewhere in the solar system.

Narrator: Why do some spacecraft fly close to Jupiter when it isn't even close to their destination?

Rayman: If the spacecraft is headed, say, to Saturn, in some cases we can get it there faster if we fly it close enough to Jupiter, with its tremendous gravity, to take a little of the planet's orbital energy and fling the spacecraft even faster. If the spacecraft is going fast enough past a planet it won't get captured into orbit, but the planet's gravity will bend the spacecraft's path and make it go faster, helping propel it along its way through the solar system.

Narrator: So isn't there any other way to make spacecraft go faster? Can't we just "step on the gas" a little more?

Rayman: Well, not really. As a rule, spacecraft get a big initial boost from a rocket and then coast to their destinations. Most spacecraft can carry only enough fuel to fire their engines for a few minutes to make course changes or slow down just enough to be captured into orbit around another planet. We do have a different kind of propulsion system, though, that outperforms conventional systems It's known as ion propulsion, which many science fiction fans have heard of. This engine is so efficient that it can thrust for years instead of minutes, which means its speed gradually increases until the spacecraft is going extremely fast indeed. NASA's New Millennium Program tested ion propulsion on the Deep Space 1 mission a few years ago and it worked fantastically. In 2007 we will use it on the Dawn mission to the asteroid belt, and I'm sure other exciting missions of the future will use it as well.

Narrator: I think you would have to understand an awful lot about all these gravitational effects from the Sun and the planets in order to reach a moving target in the solar system. It's very amazing.

Rayman: Yes, we try to use the gravitational forces in the solar system to our advantage, rather than fighting them. There's a great introduction to orbits and gravity at spaceplace.nasa.gov. Type "borrowing energy" into the search field. The site also explains how we use planetary gravity assists for flying through the solar system and the secrets of our ion engine's amazing fuel economy and how it achieves such high speed.

Narrator: Thanks, Marc. And thanks to you out there for listening. We hope you'll come back soon for another session of Space Place Musings.

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