Hugh Pickens writes writes: "BBC recently asked physicist and Cambridge University professor Dave Ansell to draw up a balance sheet of the mass that's coming in to the earth, and the mass going out to find out if the earth is gaining or losing mass. By far the biggest contributor to the world's mass is the 40,000 tonnes of dust that is falling from space to Earth every year. "The Earth is acting like a giant vacuum cleaner powered by gravity in space, pulling in particles of dust," says Dr. Chris Smith. Another factor increasing the earth's mass is global warming which adds about 160 tonnes a year because as the temperature of the Earth goes up, energy is added to the system, so the mass must go up. On the minus side, at the very center of the Earth, within the inner core, there exists a sphere of uranium five mile in diameter which acts as a natural nuclear reactor so these nuclear reactions cause a loss of mass of about 16 tonnes per year. What about launching rockets and satellites into space, like Phobos-Grunt? Smith discounts this as the mass is negligible and most of it will fall back down to Earth again anyway. But by far the biggest factor in earth's weight loss are the 95,000 tonnes of hydrogen that escape from the atmosphere every year. "The other very light gas this is happening to is helium and there is much less of that around, so it's about 1,600 tonnes a year of helium that we lose." Taking all the factors into account, Smith reckons the Earth is getting about 50,000 tonnes lighter a year, which is just less than half the gross weight of the Costa Concordia, the Italian cruise liner, that ran aground recently."