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Moon and Jupiter

November 23, 2016

Jupiter is the biggest gas station in the solar system. The planet has provided the energy to propel a half-dozen spacecraft to destinations beyond Jupiter.

The planet itself is a big envelope of gas wrapped around a small solid core. Most of the gas consists of hydrogen and helium. The hydrogen would make a good energy source for rockets. But it’s so far below the clouds that it would take more energy to go down and get it than you’d gain from the hydrogen itself.

Instead, spacecraft get their “gas” in the form of gravitational energy. As a craft approaches Jupiter, it’s accelerated by the giant planet’s gravity. How much acceleration depends on how close it gets to the planet — a closer approach provides a more powerful “kick.” Craft have used that kick to reach Saturn, Uranus, Neptune, and Pluto, and to loop above the Sun’s poles.

These craft “stole” momentum from Jupiter, giving Jupiter an equal kick in the opposite direction. But since Jupiter is so much heavier than any spacecraft, no one will ever notice the difference.

Jupiter itself is quite noticeable at first light tomorrow. It looks like a brilliant star, close to the lower left of the Moon. It’s the brightest pinpoint of light in the sky at that hour, so it’s hard to miss. And the bright star Spica, the leading light of the constellation Virgo, stands almost directly below it, completing a beautiful celestial trio.

More about the Moon and Spica tomorrow.


Script by Damond Benningfield

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