to happen and basically makes all of life on Earth possible. Except... that precious

atmosphere of ours is actually leaking, every second, into outer space.

Luckily, it's a really slow leak, since for any object--be it a molecule of gas, a rocket,

or a cat -- to break the tether of Earth's gravity and escape, it has to hightail it

out of here at 34 times the speed of sound.

It takes the energy of a metric tonne of TNT to boost a person to that speed, and less

energy for lighter objects: 1/10th of that for a cat, for example. Barring a large asteroid

impact that can eject large swaths of atmosphere into space, the only gases that regularly

escape Earth's atmosphere today are Hydrogen and Helium - the lightest elements in the

universe.

There are a few ways hydrogen and helium molecules can wind up on a one-way mission to space.

Some, near the top of the atmosphere simply get enough energy from the sun's heat to escape.

Others are high-energy charged particles that would usually be stopped from escaping by

the Earth's magnetic field. Occasionally, though, these speedy electron-lacking particles

crash into a neutral molecule with enough force to knock loose--and steal--one of its

electrons. Now neutral itself, the speeding particle is free of the earth's magnetic field,

and if the collision happens to set it on a course for the stars, that's where it goes.

Finally, some of Earth's magnetic field lines are weakened and pushed away from the earth

by the solar wind, a violent stream of plasma emanating from our sun. Charged particles

guided by these magnetic fields can simply fly off the weak ends like sparks off a live

wire.

But if our planet didn't have a magnetic field at all, things could be a lot worse. Mars,

for example, has no protective magnetic field, and so what little atmosphere it has is constantly

buffeted and ripped away by the solar wind. Even with its protective bubble, Earth is

losing enough hydrogen to fill a meter-wide balloon every second. No need to worry - it'll

take a few billion years before we lose all of our hydrogen this way, but maybe sometime

in the distant future, someone will look at Earth and ask, just as we do of Mars now:

Was there ever life on this chunk of rock?