The following answer has been selected and edited by New Scientist staffThe mass of the Earth (M) is 6 x 10 to the 24 kilograms and its radius (R) is 6.6 x 10 to the 6 metres. Assuming it to be a solid homogeneous sphere, its moment of inertia (J) is given by 0.4 x M x R squared. It works out at 1 x 10 to the 38 kg m squared.The planet spins once in 24 hours (86,400 seconds) so its angular velocity is 4.16 x 10 to the -3 degrees per second, or, more properly, 7 x 10 to the -5 radians per second.Earth's angular momentum (h) is the product of the moment of inertia and angular velocity, which gives 7 x 10 to the 33 newton metre (Nm) seconds. This is the momentum the shuttle engines will have to counter.The thrust (F) of the shuttle engines on take-off is around 4 x 10 to the 7 newtons and, if acting tangentially at the surface of the Earth, the torque (T) - or rotational force - about the Earth's centre is F x R, which gives 3 x 10 to the 14 Nm.This torque acting over time (t) will change the Earth's angular momentum by an amount T x t. The time needed to reduce it to zero is h/T or 3 x 10 to the 19 seconds, or 840 billion years.This is some 60 times the age of the universe, and by the time the shuttle had done its job there would be no weather or tides worth having. There is one other wrinkle: if the fuel needed comes from the Earth, the planet will get lighter and lighter. The whole of the Earth's mass will be expended as fuel long before the Earth stops spinning.Hugh Hunt, Senior lecturer in engineering, Trinity College, Cambridge, UK

The following comment has been selected and edited by New Scientist staffQuite apart from the shuttle engines' puny thrust, there is a fundamental problem with using them at ground level to exert the required torque. A rocket engine works by ejecting mass at high speed in the opposite direction to the required thrust. But at ground level, the material ejected would be rapidly slowed by the atmosphere, which would acquire its momentum in the process. So the initial effect of firing the engine would not only be to slow down the Earth's rotation but also to speed up the rotation of the atmosphere. Eventually, friction between the Earth and its atmosphere would tend to slow the atmosphere and speed up the Earth again, so the net effect would be zero.To avoid this, the rocket motor would have to be tethered high enough for the material it ejected to escape from the atmosphere. The net effect would be to transfer a little of the Earth's angular momentum to the rocket exhaust.It is fortunate that stopping the Earth's rotation is no easy task, because without it the Earth would be a much less pleasant place. There would be no day and night as we know it: each point on the Earth would be in permanent sunshine for six months of the year and then darkness for six months.There would still be tides as the moon continued to orbit the Earth, but instead of two tides each day there would be two tides every 28 days. On the positive side, there would be no hurricanes, because without the Coriolis forces provided by the Earth's rotation air would flow straight from high pressure to low without any swirling.Ian Vickers, Harrison, ACT, Australia

The following answer has been selected and edited by New Scientist staffIf each day were to last a year, there would be six months of darkness and cold which would kill most plants and wipe out higher life. The dark side of the Earth would get much colder and the light side hotter.Without rotation, the dynamics of the atmosphere and oceans would alter profoundly. There could be no hurricanes or revolving weather systems, and ocean currents would change too. Once the Earth had stopped turning, the pull of the moon would very slowly spin it back up until its rate of rotation matched the moon's orbital period.Because the equilibrium of the atmosphere depends on many biological and geochemical feedbacks, what happened next would depend upon how much land and ocean was left facing the sun. There may even be limits on day length beyond which a complex ecosystem and atmosphere could not exist.Don't worry though, the amount of energy needed to stop the Earth turning is so vast that nothing humans can do will ever stop it.J. McIntyre, Balsham, Cambridgeshire, UK

Just to further the question a little, once the earth stopped spinning, would it start spinning of its own accord and what factors actually make it spin the first place?

I was intrigued by the first answer given here, more precisely by the statement that "The whole mass of the Earth will be expended as [rocket] fuel long before the Earth stops spinning". I knew that the exhaust velocity of a typical rocket is about 10 times larger than the rotational speed of the Earth (at the equator); then a very rough calculation shows that less than 1/10 of the Earth's mass equivalent of fuel is needed. Indeed, the equatorial speed is 465 m/s while a typical rocket exhaust velocity would be 4.5 km/s, which gives a ratio of 9.68; if we assume (very conservatively) that the Earth's mass is concentrated at the equator, then the momentum of the Earth would be canceled using the equivalent of 1/9.68 of the Earth's mass as fuel. In fact, the mass of the Earth being distributed inside of a sphere rather than concentrated at the equator, the momentum to be canceled (and the fuel to be used) is only 2/5 of this conservative estimate, i.e. only about 1/25 or 4% of the Earth's mass is used as fuel.This does not contradict the results in the first answer - the burn time of 3x10^19 seconds for the Space Shuttle. It is known that the engines of the Space Shuttle burn 1000 tons of solid fuel in 124sec (8.06 tons/sec) and 736 tons of liquid fuel in 480 sec (1.53 tons/sec), thus a total of 9.6 tons/sec. Then the total amount of fuel needed would be 2.9 x 10^20 tons which is less than 5% of the Earth's mass of 6x10^24 kg.