Learn Spanish (Latin America): El globo está por ascender.

While va a ascender simply means it is going to ascend at some point in the future, está por ascender highlights that the action is pending and imminent. It translates perfectly to "is about to ascend."

Could I also use está a punto de ascender here?

Yes, estar a punto de is interchangeable with estar por in this context. Both express that an event is on the verge of happening. Estar por is a very common, slightly shorter alternative for imminent actions in Latin America.

Does estar por mean the same thing in all Spanish-speaking regions?

No, this is a great regional distinction to know! In Latin America, estar por means "to be about to" do something. In Spain, estar por often implies being in the mood for or in favor of something, and they would strictly use estar a punto de for the balloon's imminent departure instead.

Why use ascender instead of the much more common subir?

You can absolutely use subir. Ascender is a direct cognate to the English "ascend" and carries a similar formal or technical tone. It is commonly used for aircraft, hot air balloons, or promotions at work, but subir works perfectly for everyday speech.