Earth's moon destined to disintegrate

By David Powell

updated 2:12 p.m. ET Jan. 22, 2007

The sun is midway through its stable hydrogen burning phase known as the main sequence. But when the sun enters the red giant phase in around 5 billion years things are going to get a lot rougher in the Earth-moon system.

During the red giant phase the sun will swell until its distended atmosphere reaches out to envelop the Earth and moon, which will both begin to be affected by gas drag — the space through which they orbit will contain more molecules.

The moon is now moving away from Earth and by then will be in an orbit that's about 40 percent larger than today. It will be the first to warp under the sun’s influence.

“The Moon's actual path is a wiggly line around the sun, with it moving faster when it is slightly farther out (at full moon) and more slowly when it is slightly closer (at new moon),” said Lee Anne Willson of Iowa State University. “So the gas drag is more effective at the farther part of the orbit and this will put the Moon into an orbit where the new moon is closer to Earth than the full moon.”

Willson's idea about the moon's demise, explained recently to Space.com, is an unpublished byproduct of her research into Earth's fate in the face of an expanding sun.

Moving away
Today, the moon is on average 239,000 miles (385,000 kilometres) away and has reached this point after a long and dramatic journey.

Earth’s moon was born around 4.5 billion years ago in a titanic collision between our planet and a Mars-sized sibling, according to the leading theory. The enormous impact threw debris into orbit around the young Earth and from this maelstrom the moon coalesced.

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For the last few billion years the Moon’s gravity has been raising tides in Earth’s oceanswhich the fast spinning Earth attempts to drag ahead of the sluggishly orbiting moon. The result is that the moon is being pushed away from Earth by 1.6 inches (4 centimeters) per year and our planet’s rotation is slowing.

If left unabated the Moon would continue in its retreat until it would take bout 47 days to orbit the Earth. Both Earth and Moon would then keep the same faces permanently turned toward one another as Earth’s spin would also have slowed to one rotation every 47 days.

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