With this essay by Ray Villard, news director for the Hubble Space Telescope, Astrobiology Magazine presents another in our series of 'Gedanken', or thought, experiments - musings by noted scientists on scientific mysteries in a series of "what if" scenarios. Gedanken experiments, which have been used for hundreds of years by scientists and philosophers to ponder thorny problems, rely on the power of one's imagination to project these scenarios to logical conclusions.

They do not involve lab equipment or, often, even experimental data. They can be thought of as focused daydreams. Yet, as in the famous case of Einstein's Gedanken experiments about what it would be like to hitch a ride on a light wave, they have often led to important scientific breakthroughs.

Earth's ultimate fate 5 billion years from now is a death spiral into the Sun. As the Sun ages, its gravitational pull will weaken, and Earth will briefly migrate out to the distance of Mars' orbit. At this distance, however, the Earth still will be close enough to generate a tidal bulge in the now-bloated Sun.

The gravitational tug from the solar bulge will slow Earth's orbital velocity, eventually causing our planet to spiral in toward the Sun. Friction from the tenuous gases in the Sun's ballooning atmosphere will speed up this process, dragging us irrevocably inward.

But pondering the far future demise of Earth may be a moot point for humans, who won't be able to survive changes in the Sun a mere 1 billion years from now, 4 billion years before the end. The Sun will bake Earth with enough energy at this time to cause the oceans to evaporate away and carbon dioxide to be unleashed from sedimentary rocks. That's because the Sun's core is getting hotter as fusion reactions migrate to an ever-growing shell around a nucleus of helium "ash."

In 1 billion years Earth will become more like Venus with a thick carbon dioxide atmosphere. Topographically, Earth's surface will resemble that of Titan today. There will be great equatorial dune fields and a smattering of evaporating lakes near the poles. The first homesteaders on Earth, the extremeophiles, will also be its last surviving inhabitants. The meek truly will inherit the Earth.

So we've got about 1 billion years to get out of town. But it's difficult to imagine humans surviving for a billion years regardless of the Sun's evolution. Any number of manmade or celestial events could bring on Armageddon. Probably the highest probability for our destruction would be a comet collision, because comets can come and go unpredictably.

Asteroids would be civilization killers too if we don't develop the technological wherewithal to deflect them. Manmade disasters could include nanotechnology run amok, plagues brought on by terrorist-engineered super-organisms, or extinction by intelligent machines - among many other man made disasters yet to be imagined.

The absence of any evidence for intelligent life in space, commonly known as the Fermi Paradox, would suggest that extraterrestrial civilizations are short-lived because they easily succumb to natural or technology-induced catastrophes, otherwise they would have stopped by and visited us by now. The vast age of our Milky Way allows more than enough time to star-hop across the galaxy at a fraction of the speed of light.

But let's be wildly optimistic for a moment and assume that humanity will have the stability, cultural tenacity, and technological prowess to hold onto our planet for the next billion years.

Knowing that our world will inevitably succumb to the Sun's evolution, a far advanced civilization on Earth could undertake an extraordinary engineering project to keep Earth inhabitable for the next 5 billion years.

A study led by D. G. Korycansky of the University of California at Santa Cruz describes a high-stakes, high reward, mother-of-all astro-engineering project: transforming Earth into a spaceship. His basic idea uses a comet or asteroid to transfer orbital energy from Jupiter to Earth on a life-and-death interplanetary billiard ball game.

An asteroid's orbit would be modified to swing very close to Earth. Our planet would gain energy from an asteroid swing-by if the asteroid would need to be at least 100 kilometers across, and pass within 10,000 kilometers of Earth. This energy transfer could slightly increase the diameter of Earth's orbit, nudging it farther from the Sun.

On an outbound trajectory, the asteroid swings by Jupiter and robs energy from Jupiter's orbital momentum to make up for energy lost to Earth. This slightly shrinks Jupiter's orbit.

For Earth to maintain the "Goldilocks" distance where the amount of solar energy remains constant, the asteroid must swing by Earth for another momentum transfer once every 6,000 years, according to Korycansky.

But this mega-project could go awry if the asteroid went off course and plowed into Earth. Imagine filing an environmental impact statement (no pun intended) for this project, much less convincing world governments to collectively support it.

Another challenge is that the time span between asteroid encounters is equal to a good-sized chunk of current recorded human history. Civilizations could forget about it, or they might even view the incoming asteroid as a threat to Earth and destroy it!

The project's original engineers would need to outfit the asteroid with a transponder, broadcasting an anti-encrypted message that explains its origin and purpose to far future societies watching it barrel back toward Earth.

Just imagine a future Bruce Willis (as in the 1998 science fiction film Armageddon) landing on the asteroid to place a nuclear bomb. His crews come upon a 2001-type monolith that is a time capsule, perhaps with an image of Earth etched onto it showing the continent's positions at the time the project was started. The crew has a launch window of a few hours to decide how to proceed. What would they do? The future of Earth hangs in balance!

However, twiddling with the gravitational stability of the solar system could bring on interplanetary chaos. For starters, changes in Jupiter's orbit could disrupt asteroid orbits, hurtling them into the inner solar system.

One computer simulation of the dynamical evolution of the solar system by Jacques Laskar of the Paris Observatory predicts there is a small chance the solar system could become chaotic in about 3 billion years, even without our tinkering with it.

He ran 2,501 numerical simulations of the dynamical evolution of the solar system over the next 5 billion years. In one simulation Mercury's orbit becomes so eccentric that the planet falls into the Sun or collides with Venus. In another simulation run, Mercury's eccentricity causes angular momentum to be transferred from the giant outer planets. This destabilizes all the terrestrial planets 3.34 billion from now. In the Mother-of-All Apocalypses, Mercury, Mars or Venus smashes into Earth.

In another simulation there is a close encounter where Mars passes within 500 miles of Earth! Such a sideswipe would probably obliterate all higher life forms on the Earth. Mars could be tidally ripped apart on approach and the pieces would carpet-bomb Earth. What's left of Mars might form a ring around our lifeless planet - a mocking tiara for an Earth thrown backward in time to the Hadean era.

It is possible to test this scenario by doing an infrared survey of very mature Sun-like stars that would have formed planets billions of years ago. Infrared excess indicting the glow of a dusty debris disk near the star could be explained by the disintegration of a planet. Roughly one out of 100 systems surveyed should show this debris field, if the simulations reflect reality.

However, we could never be certain if the planet disintegrated by natural collision, or it was the result of a planet migration experiment run amok.

Despite the future dangers we face, it is ludicrous for some folks to think the "end-of-world" is right around the corner.

We are the only intelligent species capable of taking possession of the solar system. Our civilization might come up with a strategy to build artificial mini-planets - essentially flying city-states - that would modify their orbits to migrate along with the petulant Sun's expanding and shrinking habitable zone. As the white dwarf cools, the wagon train of space habitats would move inward. Raw materials would be harvested from in-falling comets and asteroids.

Explorers would be free to travel outward to visit surviving planets and moons. Given our passion for survival, bolstered by super-technology, the future for mankind could truly stretch on indefinitely, beyond even the life of the Sun.