Life on the surface of the Earth is entirely dependent on the sun, our only external source of energy, writes Dr William Reville. The sun is an enormous nuclear fusion reactor but the reactor fuel will eventually run out, energy production will cease and life will no longer be sustainable on Earth.
However, well before that inevitable end point arrives, conditions on Earth will become quite difficult for living things, as the sun slowly grows more luminous. To prevent things from getting unbearably hot, it may be necessary to move the Earth further away from the sun.
A team led by D.G. Korycansky of the University of California has developed a plan that would allow life to continue comfortably on Earth until close to the time our sun dies - about six billion years from now. It is outlined in the March 2001 edition of Astrophysics and Space Science.
In the sun, hydrogen, the lightest element in nature, is fused into helium the second lightest element. In this process, some mass is converted into energy and this accounts for the huge energy generation in the star. In another five to six billion years, the sun will have used up all the hydrogen in its core, which will then be composed almost entirely of helium. There will still be plenty of hydrogen in the outer layers of the star.
At this stage, the core will shrink and get even hotter, allowing helium to be fused into carbon. The extra heat in the core will make the outer layers swell up, turning the sun into a red giant star. The sun will engulf the innermost planet Mercury, and then Venus. It will boil away the Earth's oceans and destroy any life that has not already fled to another planet. Then the sun will engulf the Earth itself.
AFTER a further billion years or so, when its supply of helium is exhausted, the sun will stop generating energy and will fade into a cooling cinder no bigger than the Earth, called a white dwarf, circled forlornly by the charred remnants of its remaining planets. (Let us pause here for a stiff whiskey!)
Our sun is about half way through the main phase of its life, a phase called the main sequence. This phase lasts about 10 billion years in a star such as the sun, so we have five or six billion years to go.
During the main sequence, the sun generates energy at a more or less steady rate, but it does gradually grow brighter. Already the sun shines 30 to 40 per cent brighter than when it first entered the main sequence. In another billion years, the sun will be 10 per cent brighter again, which will make land-based life on Earth difficult, or even impossible.
Korycansky's plan could extend bearable conditions for life by a further five billion years. It relies on the well-known gravitational slingshot used to redirect satellites through space with added energy.
In a slingshot, you accelerate a projectile by whirling it around your head in a sling. At the appropriate time you release one arm of the sling and the projectile speeds away towards its target powered by the built up energy.
In the same way as a spacecraft nears a planet, it is accelerated by gravitational attraction. A redirection of the craft by the firing of a booster rocket at the right time will shoot it away in the new desired direction with the added energy delivered by the gravitational pull.
The interaction between the spacecraft and the planet is obvious in the behaviour of the spacecraft, but not in the behaviour of the planet which remains apparently unaffected. However the planet suffers an equal and opposite change in energy and momentum to the spacecraft - it isn't noticed because of the huge mass of the planet. But, if the spacecraft was replaced by a large asteroid, the effect on the planet would become noticeable.
Korycansky's paper demonstrates how Earth's orbit around the sun can be increased slightly if an asteroid about 100 km across and weighing about 1016 tonnes can be manoeuvred to fly in front of the Earth as it moves in its orbit.
The orbit of the asteroid would be adjusted so that it then heads outwards towards Jupiter or Saturn where in a reverse process to its manoeuvre near Earth it picks up the orbital energy it lost to Earth. The asteroid continues on its course and when it reaches its farthest distance from the sun a slight course correction is applied (by firing booster rockets) to send it back again towards Earth to tug our planet a little further from the sun.
In order for Earth to continue to enjoy the current intensity of sunlight it will be necessary to push our planet outwards from the sun a little bit more every 6,000 years. Over the approximately six billion years remaining before our sun dies, this would push Earth's orbit just beyond the current orbit of Mars. The technology necessary to achieve this will be available within several decades.
There are risks associated with this plan. If the asteroid hit the Earth rather than flying past it would have catastrophic consequences. Also, assuming the plan worked, the change in the Earth's orbit might disturb the orbits of the other planets.
Finally, jiggling the Earth's orbit as described would probably strip away the Moon. The Moon helps to stabilise the tilt of the Earth's axis and its absence could drastically affect the climate of our planet.
William Reville is Associate Professor of Biochemistry and Director of Microscopy at UCC