Will a hot air balloon still rise if the surrounding air is the same temperature?
A balloon filled with hot air rises because of the force of buoyancy. That means the density of air inside is lesser than the surrounding air (because hot air expands thus density decreases) and thus there is an upthrust acting on the balloon. Now, if the surrounding air is at the same temperature then its density is same as that inside the balloon and so the balloon will not rise. But the balloon will not not remain at the same point. it will fall because of the weight of the rubber in it.
In case of exploration on other planets, firstly other planets have very low density of air in the atmosphere (most of them do not have any air) and secondly even then is one wants to use this one has to heat up the balloon to a temperature for which the density decreass lower than the surrounding. Hope this answers your question.