Re: How could Earth's gravity be turned off?

Hi Leonard,

I believe that you meant "sped up to 11 km/s." If you slow down the rotation of the Earth, then the gravity will actually do a better job of holding things onto the planet, and nothing would disintegrate. Right now, Earth's rotation tends to slightly counteract the affect of gravity, which would show up as things weighing slightly less at the equator than at the poles.

I was a bit loose with my numbers in my posting 9 years ago. In order for somebody at the equator to feel no gravity, they would basically have to be in orbit. They would still be experiencing gravity, but would feel weightless, because orbit is a kind of freefall. The only difference between orbiting and straight freefall is that, in orbit, you have a sideways velocity that is just sufficient to keep you at the same distance from Earth as you "fall" towards it.

The orbital speed at Earth's surface is 8 km/s. So, if Earth's equator were spinning at that speed, objects there would experience weightless conditions. Any faster, and they would be moving too quickly to remain in a circular orbit, and would start to move onto larger orbits, ie. they would move away from Earth's surface. At 11 km/s, they would no longer be bound to Earth at all, and would escape completely.

As for how to accelerate Earth's rotation up to that speed, it would be difficult. It would be almost impossible to conceive of a way by which humans could do the job. No known, or even dreamt-of rockets would accelerate something as large as our planet up to the required speeds.

There might be a way in which it could happen naturally, though we wouldn't survive the process. Earth is already a fairly rapidly rotating planet. We believe that the current rapid rotation is due to the same process that led to the formation of the Moon. That is believe to have been caused by an off-center collision with an object about the size of Mars early very early in the history of Earth. The collision sped up the rotation of Earth, and flung material into orbit that eventually coalesced into the Moon. Immediately after the collision, the young Earth was rotating much more rapidly than now (it has since been slowed down due to tidal interactions with the Moon). A similar collision with a massive object today could accelerate Earth's rotation significantly.

A (somewhat) slightly less violent means of speed up Earth's rotation would be for it to accrete matter more gradually, via a disk of material. Say that a large object didn't quite collide with Earth, but wandered close enough to be broken apart by the Earth's tidal field. It's debris would form a spinning disk around Earth, slowly spiralling down to Earth. As the material collided with Earth, the angular momentum that it had due to its orbit would be given to Earth, and would go into speeding up Earth's rotation. That is, in fact, essentially the process by which we believe the so-called millisecond pulsars are sped up to their high rotation speeds (though the process involves the accretion of gas, not broken up asteroids).

Because the material wouldn't fall in at faster than orbital speed, the gradual accretion process could only accelerate Earth's equator up to 8 km/ s, not all the way to escape velocity. Even then, an enormous amount of mass would be required, in order to have enough angular momentum to give to Earth. That would mean that we'd have to either break up a very large object (comparable in size to Earth), or encounter a very large field of smaller debris (which doesn't exist right now in the Solar System).