Originally posted by spy66
How could the astronauts jump up and land again on the Moon if the Moon dont produce a atmosphere?
But in space they float freely.
If the Moon dont produce a atmosphere it would mean the moons surface is the same matter as space it self. But then if a astronaut jumps on the moon
he shouldn't be able to land again?
If you go out side and pic up a rock and drop it... i bet it will hit the ground because of the atmosphere between it and the ground.
An atmosphere is caused by gravity holding gases against the surface of a planet or moon so atmosphere is a product of gravity, not the reverse
An astronaut in orbit is weightless because his orbital velocity is producing a centrifugal force equal to the centripetal force of gravity causing
the 2 to cancel out. He still has mass and if he slows down he'll descend (centifugal force less than gravity) and if he speeds up he'll move away
from the planet (centrifugal force exceeds gravity). That gravitational attraction will be basically the same regardless of whether the planet has an
atmosphere or not and I say 'basically the same' because the atmosphere itself has mass which adds an infinitesimal amount to the planet's total
mass and therefore increases the gravity effect of the total mass by an unmeasurably small amount. The earth would have the same gravitational
acceleration on the surface (9.8m/sec^2) with or without atmosphere so a rock would drop at the same acceleration in both cases (yes I know it would
reach terminal velocity in an atmosphere because of friction losses (drag) which increase as the square of velocity). With no atmosphere the
acceleration would continue until hit the surface - so the impact velocity in a vacuum is higher than in atmosphere.
The upper reaches of the atmosphere are highly energised by solar wind, a variety of radiation from lots of sources etc and molecules can reach
extremely high velocities, sufficient to exceed escape velocity and cause a constant 'bleeding' of the atmosphere into space where it's lost. In
the earth's case, the abundance of life and moisture combined with solar warming is producing gasses at a high enough rate to compensate for the
losses. IE it's a delicate balance between gas loss, gas production and gravity that maintains an apparently constant atmosphere.
If there's no process going on in or over the planet like active lifeforms, volcanic activity, evaporation or inorganic gas producing chemical
processes then the atmosphere is eventually depleted leaving a barren airless body like the moon behind. Mars appears to be in the latter stages of
totally losing its atmosphere because there seems to be nothing happening on the planet to replenish it.
[edit on 30/6/2009 by Pilgrum]
