Here's why:
* Prepositional phrases start with a preposition (in this case, "to") and are followed by an object (in this case, "the launch pad").
* They function as adverbs or adjectives modifying other words in the sentence. For example:
* "The rocket traveled to the launch pad." (adverb modifying "traveled")
* "The engineers were busy at the launch pad." (adjective modifying "engineers")