I feel the earth move under my feet...

In the wake of hysterical articles about the bulk of our non-renewable resources having been two-thirds used up, there have been very few articles about the fact that our renewable resources are half used up. The Earth's supply of radioactive material – what powers geothermal energy – is half gone. And the energy source for all our other renewable energy – the Sun – is halfway through its store of usable nuclear fuel.

In five billion years, the Sun will enter the giant phase, and swell up to a diameter that completely engulfs the orbit of Mars. But within only one billion years, the Sun will have warmed up to the point where life on Earth is no longer possible. The extra heat will have triggered a runaway greenhouse effect, no matter what we do the inventory of greenhouse gases.

There are ways to deal with this problem, of course. One is to move the Earth out to a larger orbit.

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The approach discussed here is to use the "slingshot effect" to move the Earth. Send an astroid past the Earth, in from the orbit of Jupiter, and let the gravitational pull nudge Earth out to a higher orbit. Taking a 100-kilometer wide rock (some six times larger than the one thought to have wiped out the dinosaurs)...

Here's what you do: * Using humans or robotic spacecraft, attach retrorockets -- like those that maneuver spacecraft -- to the rock. Alter its course of so that it passes near Earth. The planet then steals some of the space rock's orbital energy and uses it to move into an orbit slightly farther from the Sun. (NASA employs a similar technique to propel spacecraft, sending them around a planet in order to boost them into new trajectories at higher speeds.) * Send the comet or asteroid back out around Jupiter and Saturn, where it will regain orbital energy by robbing it from the giant planets. (In effect, Earth is ultimately getting its orbital boost from Jupiter and Saturn.) Make the rock continue on a long, elliptical orbit that goes way the heck out there -- 325 times the distance from Earth to the Sun. * Bring the rock back around Earth every 6,000 years or so, and each time the planet will creep outward a few more miles. The goal: An ultimate retreat of several million miles (kilometers).

Repeat as necessary.

One of the risks has to do with dinosaur-killer asteroids passing that close to the Earth. A little too close, and...

However, there's no need to use gigantic rocks. Instead of one gigantic rock every 6000 years, you could send past a much smaller rock (5.5 Km diameter) every year. Or you could send past a 2.4 Km diameter rock past the Earth every month. Or a 760-meter diameter rock every day. Or a 17-meter rock every second. Or a six-meter rock twenty times a second.

Higher frequency events using smaller rocks would translate to less damage if one rock goes astray. And the more it looks like a stream, the easier it is to tell when one of the rocks is swerving too far from its assigned path. At one per second, we're looking at rocks a two or three dozen miles apart at most. If one of them gets out of line, it's a lot more obvious.

Perhaps in the future, we'll have major industries set up to project matter streams past various planets in order to move them around the solar system.