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Willson's idea about the Moon's demise, explained recently to SPACE.com, is an unpublished byproduct of her research into Earth's fate in the face of an expanding Sun.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

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.

Solar influence

The Sun's mutation into a red giant provides a huge stumbling block to the Moon's getaway and is likely to ensure the Moon ends its days the way it began; as a ring of Earth-girdling debris.

"The density and temperature both increase rapidly near the apparent surface (photosphere) of the future giant Sun," Willson explained. As the Earth and Moon near this blistering hot region, the drag caused by the Sun's extended atmosphere will cause the Moon's orbit to decay. The Moon will swing ever closer to Earth until it reaches a point 11,470 miles (18,470 kilometers) above our planet, a point termed the Roche limit.

"Reaching the Roche limit means that the gravity holding it [the Moon] together is weaker than the tidal forces acting to pull it apart," Willson said.

The Moon will be torn to pieces and every crater, mountain, valley, footprint and flag will be scattered to form a spectacular 23,000-mile-diameter (37,000-kilometer) Saturn-like ring of debris above Earth's equator. The new rings will be short-lived. Theory dictates they'll eventually rain down onto Earth's surface.

"Particles of different masses will have different survival times; the smaller particles will be removed first, and the biggest ones last. Most of the ring particles would be gone by the time the Earth reaches the stellar photosphere," Willson said.

If the Sun's photosphere reaches Earth, our planet too will experience drag and spiral into the Sun to be incinerated.

Possible out

There are possible natural alternatives, however.

If the Sun as a red giant sloughs off enough material before Earth evaporates, our planet will be revealed from its stellar cocoon in a Moon-less guise. Earth, robbed of its companion, would undertake a lonely vigil as the Sun turns eventually into a stellar corpse called a white dwarf, fading to black over the ensuing trillions of years.

Alternatively, if the swelling Sun loses 20 percent of its mass prior to it reaching our vicinity, both Earth and Moon could be spared incineration and remain together facing each other for eternity. The actual outcome remains a theoretical uncertainty because no red giant star has been observed during this crucial phase.

This article is part of SPACE.com's weekly Mystery Monday series.