What is the ruling on oaths and vows concerning the performance of impossibilities?

Swearing to do something impossible, like ascending to the sky, immediately necessitates expiation, due to the certainty of the action's impossibility. As for vowing to do something impossible, nothing is required, because it is an impossible vow that cannot be made binding, and because a vow is only binding for acts of obedience to God. However, if one makes a vow contingent upon an impossible matter, then he must fulfill what he vowed, because the breach is certain, and the ruling on a contingent vow is like the ruling on an oath.