The recent crash landing of Russian spacecraft Phobos-Grunt has focused attention on the increasing amount of space junk orbiting the planet. So does this mean the Earth has been getting lighter? The BBC's Radio 4 programme More or Less turned to a group of Cambridge University academics for the answer.

There are factors that are causing Earth to both gain and lose mass over time, according to Dr Chris Smith, a medical microbiologist and broadcaster who tries to improve the public understanding of science.

Using some back-of-the-envelope-style calculations, Dr Smith, with help from physicist and Cambridge University colleague Dave Ansell, drew up a balance sheet of what's coming in, and what's going out. All figures are estimated.

By far the biggest contributor to the world's mass is the 40,000 tonnes of dust that is falling from space to Earth, says Dr Smith.

"[The dust] is basically the vestiges of the solar system that spawned us, either asteroids that broke up or things that never formed into a planet, and it's drifting around.

"The Earth is acting like a giant vacuum cleaner powered by gravity in space, pulling in particles of dust," says Dr Smith.

Another much less significant reason the planet is gaining mass is because of global warming.

The answer It's getting lighter, by about 50,000 tonnes in mass each year, but not due to space dust

Some factors include:

Gains: Mostly dust (like an asteroid, above) falling from space, plus increased energy from increases in the planet's temperature

Mostly dust (like an asteroid, above) falling from space, plus increased energy from increases in the planet's temperature Losses: Mostly hydrogen, plus some helium and a tiny amount of lost energy

Nasa has calculated that the Earth is gaining energy due to rising temperatures. Dr Smith and his colleague Mr Ansell estimate this added energy increases the mass of Earth by a tiny amount - 160 tonnes.

This means that in total between 40,000 and 41,000 tonnes is being added to the mass of the planet each year.

Population growth and new buildings are not a factor, he says, because both of these are actually made up of existing matter on the planet.

But overall, Dr Smith has calculated that the Earth - including the sea and the atmosphere - is losing mass. He points to a handful of reasons.

For instance, the Earth's core is like a giant nuclear reactor that is gradually losing energy over time, and that loss in energy translates into a loss of mass.

But this is a tiny amount - he estimates no more than 16 tonnes a year.

And what about launching rockets and satellites into space, like Phobos-Grunt? Dr Smith discounts this as most of it will fall back down to Earth again.

Finding space dust Micro meteors and particles falling from space land on roof

They are washed with rain water into water butt

Run a magnet round sludge at bottom of butt

Any iron rich particles in there are probably from space Dr Chris Smith, Cambridge University

But there is something else that is making the planet lose mass. Gases such as hydrogen are so light, they are escaping from the atmosphere.

"Physicists have shown that the Earth is losing about three kilograms of hydrogen gas every second. It's about 95,000 tonnes of hydrogen that the planet is losing 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."

So taking into account the gains and the losses, Dr 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.

Clearly, compared to the immense size of the world, this is a tiny difference, a loss of just 0.000000000000001%.

More or Less: Behind the Stats You can listen to More or Less on BBC Radio 4 and the BBC World Service or by downloading the free BBC podcast Download the More or Less podcast

Should we be worried?

It may seem a small amount, but is the world in danger of running out of hydrogen? Dr Smith is not worried.

"It would take trillions of years to empty the earth's oceans and since the planet is only about 5 billion years old, probably a hell of a lot longer than we have been here already."

Dr Chris Smith and David Ansell are part of Naked Scientists, a group of academics from Cambridge University who broadcast on BBC Radio about science issues