A rigid designator is a name which describes, refers to a thing in all the possible existing worlds. In addition it is important to underline that the designator does not refer to any other thing which exists in those other possible worlds where we know for sure that the thing under discussion does not exist. The regular proper names are a relevant examples to illustrate the concept of rigid designators. The debate which takes place between Russell and Kripke has to do with the power of description that the names of the objects have as far as the objects they are associated to are concerned. As far as Russell is concerned, the names are nothing but disguised descriptions of the things. According to Kripke this is far from being true. A chair might be a called something else and still be a chair. The name in itself does not provide any indication about the object, it is a mare tag or convention which has been established in order for us to have references. Discussing the argument regarding the process through which these references are fixed it is safe to say that it is rather random. In other words I for example could decide to give a certain name to a certain object even I have no idea about how that thing looks like. If I manage to get the name accepted by the community then the object and the name of the object will pass as being one and the same thing despite the fact that their bringing together was only a formality and not a decision based on the characteristics of the object I was meaning to speak about.

Once a name is associated with an object in a very powerful manner then we have another issue rising, namely the one of perception. There is a famous quote that...

In saying that one is the other one then we say that the heat is the motion of the molecules. Naturally we need to test this statement in order to verify its value of truth. The experiments will lead to an a posteriori type of knowledge since we will discover that heat indeed causes the motion of the molecules. Viceversa the movement of the molecules implies heat. So far it is clear that the proposition is a posteriori and also necessary.
We also know that the names refer to the same reality, that is Venus, only in different hypostases. They are different descriptions of the same entity in different states. From this point-of-view it is safe to state that they are mere tags. The names could very well differ and so could the hypostases of the entity but fundamentally it remains the same, its substance remains constant. Therefore, the fact that from an etymological point-of-view it is quite possible for Hesperus not to be Phosphorus, from a pragmatic point-of-view logic prevents us from believing that.

What may be contingent is the fact that the sensation of heat is caused by the movement of the molecules. However the intrinsic process has nothing contingent about it as the empiric testing has demonstrated. The heat exists outside of our perception and the presence or absence of our perception does not change its nature. Therefore, just as Kripke says, the only thing that might cause us to believe that contingency is present here would eb the confusion between the thing itself and its perception by us.