Hard SF is the category of science fiction that pays at least as much attention to scientific accuaracy as to the storytelling. That might seem like a redundant classification, but genuine hard SF is actually massively outnumbered by other, softer varieties.
That is why The sentinel, a collection of short stories by Arthur Clark brought so much a joy. Among its pages you will find the stories that later grew into novels 2001: A space odyssey and Childhood's End, both of which I had enjoyed tremendously (short stories growing into novels is rather the norm for SF). And there are others which are no less impressive. Wind from the Sun describes a race of spaceships using solar sails, something which is slowly turning into reality. Meeting with the Medusa explores the fantastic possibility of floating life in Jovian atmosphere. And just to give an example of 'hardness quotient' (to coin a phrase) of the stories, Clark mentions in the introduction to Jupiter V, a story which describes a journey to the inner Jovian moons, that he did 20-30 pages of orbital calculations to get the details right. How many SF writers can beat that?
In the introduction to the book Clark calls science fiction 'the only genuine consciousness expanding drug'. This book is surely a concentrated dose.