High Velocity Impact of Planets - Atomsphere question

[Nexus555]

I have a question relating to astroid collisions with terrestrial planets.

I was reading the Venus wiki, and linked to the wiki about colonizing Venus. One method for reducing the massive amounts of CO2 was a big impact on the planet, making the CO2 eject into space (if I remember correctly.) But there was critism to this, something along the lines that gravity would pull the CO2 back into the planet.

My question is why? Let's say an astroid/comet impacted earth, or any planet with an atmosphere. If the Oxygen, CO2, etc molecules were ejected into space, that would mean they were traveling faster than the planets escape velocity correct? How would the gases get back into the earth, if they're traveling faster than the escape velocity.

It just seems to me if something is flying away from the planet, faster than it's pull, then between it's direction and speed in space vs the planet continually revolving around the sun that the gas would simply keep traveling in that direction..

Or would the gravity of the planet slow the gases traveling in space and eventually pull them back to the planet. Please enlighten me on this matter! Thanks!

 

[russ_watters]

It is probably just the usage of the word "ejected". I don't think the word necessarily implies a speed above the escape velocity. You can send an object pretty far away (theoretically, the limit is essentially infinite) on a ballistic trajectory and still have it come back.