Arthur C. Clarke's "A Meeting with Medusa" (1971).
Adding a summary, as requested.
This story is centered around Howard Falcon, a human test pilot, and is told partly in a series of flashbacks while he is being briefed for a hot-hydrogen balloon descent into the atmosphere of Jupiter.
The first flashback takes him into an enormous dirigible, making its maiden flight through the Grand Canyon (Southwest USA). Owing to a design flaw, whereby the control signals for the dirigible are inadvertently routed through a longer-than-normal connection, causing delays that amplify the errors in course correction, the dirigible crashes, and Falcon barely escapes with his life. In the process, he encounters a "super chimp" (modified life form) that was part of the ship's crew. This foreshadowing is paid off by the end of the story.
Falcon uses this experience to convince the planners of the Jupiter mission that they cannot rely on a remote pilot, and successfully pilots the mission. In another flashback, he recalls discovering a whole range of life-forms living in the Jovian atmosphere. These life-forms were re-visited by Clarke in his novel 2010: Odyssey Two, and were an influence on the second episode of Carl Sagan's Cosmos, where he explained the makeup of Jupiter and mused about the possible biosphere of that planet and other "gas giants".
At the end of the story, Falcon demonstrates why he was the best person for the job, by performing a test of reaction time for his interviewer. With his fingers barely apart across the thickness of a playing card, he asks the interviewer (who is holding the card) to drop it at random. His reaction time is so fast that he is able to grasp the card before it leaves his hand. And then, in a novelistic "wide shot", we discover that Falcon is now a cyborg, with a mechanical body and superhuman reflexes and resistance to radiation.