I think you are discounting the enormous and enduring popularity of various sorts of space opera - think of A Fire Upon the Deep, Pandora’s Star, The Vorkosigan Saga, Hyperion, etc. not to mention such subgenres as military SF and spin-offs from popular SF in other media, like Star Trek, Babylon 5, Star Wars, etc. or SF with fantasy elements - all of which are, individually and collectively, more popular in terms of numbers that “true” hard SF (if sometimes and in some cases looked down upon by SF purists).
I find it odd that the OP calls Fred Hoyle’s 1957 book “very early sf”, as if there weren’t almost a century (depending on where you set your aritrary pointer for the beginning) of science fiction preceding it. How you define “hard sf” is an issue, too, because much science fiction mixes hard sf with less likely things, and many writers tackle both hard and “softer” SF. Hoyle is a good choice, because excerpts from this very book showed up in one of my high school science texts as an example of rigorous scientific thinking. But would you call Hoyle’s October the Third is Too Late “hard” science fiction, with different parts of the world cast into different historical epochs?
it seems pretty weird, too, that Robert heinlein isn’t even mentioned so far in this thread, yet he’s definitely among the great Hard SF writers. he himself describes doing voluminous orbital calculations for Space Cadet that weren’t strictly necessary for the plot, but he wanted to get the details right. There are plenty of examples in his work of good science, engineering, and hardcore speculation.
As has been noted (and with some additions), a good list would include
Hal Clement
Isaac Asimov
Arthur C. Clarke
Jules Verne
H.G. Wells (yes, it’s certainly defensible)
Larry Niven
Jerry Pournelle
Robert L. Forward
There are plenty of other, not-as-well known writers, too.
Poul Anderson’s “Tau Zero” comes to mind as well.
The Paradox Men by Charles L. Harness. It’s pretty much a classic of the genre.
Doesn’t he have, like, galactic zombie empires?
IMHO, he’s clearly within the “space opera” camp. 
Let’s remember than a lot of Hard SF is today published under the label Techno Triller.
You can have space opera and still try to do hard Sci-Fi, IMHO. Take a look at Alastair Reynolds’s stuff, for instance.
Part of the problem is that when the technology gets too advanced from our point of view, it starts to turn into magic, (paraphrasing Arthur C. Clarke). I personally think that magic is like omnipotent characters, in that both are too hard to handle rigorously in a plot; too many issues or conflicts disappear if the full implications of either are allowed to fully develop.
A lot of the fun for me in hard Sci-Fi is trying to see the implications of the technology or the setting and see what naturally develops from that. When characters start acting in contradiction to those natural developments—whether from plot forced stupidity, the author choosing not to explore those implications, what have you—it kills my immersion and enjoyment. (E.g. Reynolds’s Absolution Gap: God, what a wasteful, terrible story.) I’ll tolerate it more in something like Discworld, where the specifics of the mechanics of magic or of magical races, really aren’t the point of the story.
I am interested in that science fiction driven by the technical-scientific idea. That’s what I consider Hard Sci-Fi.
To read about children adventures, phantoms or vampires I better read the masters of fantasy, like Tolkien.
Perhaps you can have hard SF that is also space opera, but however you slice it, Hamilton is not particularly “hard” SF. He deals with stuff that is clearly not an extrapolation of science, unless one twists the meaning until it breaks - like the afterlife.
When you have a galactic empire of zombies led by a reincarnated Al Capone, you are not dealing with “hard” SF.
Well, by some definitions of “hard science fiction” the fact that they weren’t necessary for the plot counts against it being hard science fiction.
I agree, though, Heinlein’s works are a good example of the kind of science fiction that actually has science in it, the kind that could only be written by someone with substantial scientific knowledge, which is a necessary if not a sufficient condition for being considered “hard.”
Without a doubt, and I haven’t read anything from Peter Hamilton. I was just taking issue with the idea that space opera meant no hard science fiction. I probably just misread the earlier post.
I also disagree with the idea that the science fiction elements in a hard sci-fi story must be integral to advancing the plot. It seems to me that interpretation leads to Chekhov’s Guns sprouting up all over the place. OTOH, if you do introduce a piece of tech, and that piece of tech would have resolved a plot conflict or issue, but doesn’t, there’d better be a damn good explanation why not. Sometimes, I just like playing tourist within the author’s playground, which probably explains a lot of my love for Niven’s works.
FTR, if this is based on my comment earlier–a scientific element’s non-necessity to the plot doesn’t count against a work’s “hardness.” I just mean to say that I think of hard SF as being stories that are really about real science, or speculative extrapolations from real science, as opposed to other kinds of stories using those elements as setting.
It’s been a long time since I’ve read Heinlein, especially the “juveniles,” but to my thinking all the important themes of something like Space Cadet could have been written in a book which nobody would identify as any kind of “science fiction” at all–for example a quasi-historical adventure in which the Space Patrol is replaced by the 19th-century British Empire.
Better discount Farmer’s Riverworld stuff as not hard SF then too.
I found his novels grounded in scientific principles (observation, measurement, testing of hypotheses, etc) so didn’t mind him moving into additional dimensions and transcendent intelligences. Same way I like Vinge’s transcendent stuff.
I agree. Vinge’s “A Deepness in the Sky” is also very good and quite scientifically accurate (the other book in that series has less hard elements (but the planet-bound portion of the book is also quite hard)
I’d classify A Fire upon the Deep as “space opera”, too.
Don’t get me wrong - I love that stuff. I do not by any means equate “hard” with “good”.
It is just a different genre, one that attempts to emphasize the “science” in “science fiction” - by making science a central part of the novel-writing process.
A Mission of Gravity is a classic of this genre, because the wierd planetary properties of Mesklin - the planet on which the action was set - were (a) important to the plot, and (b) as accurate to real science as Hal Clement could make them.
That does not make A Mission of Gravity necessarily better than A Fire Upon the deep, Riverworld, or Hamilton’s Night’s Dawn. It just makes it different. In these other books, the actual accuracy of the science is more or less irrelevant to the plots, the authors have no hesitation in just making stuff up to suit the plot (the “zones of thought” is a good example; even better, the quite literal deus ex machina that ends the Night’s Dawn trilogy - a black hole that is also, well, God, and that sucks the souls of the damned possessed into something like hell).
In many ways, “hard” SF is very constraining. That’s why it is, and has always been, a minority genre.
A galactic scope, casual destruction of whole solar systems, fate of vast swathes of space dependent on the actions of less than a dozen characters - sounds like ‘space opera’ to me (but loads of fun - and the planet bound plot thread is actually pretty hard SF, as I mentioned upthread)
I’ll second everyone who suggested Greg Egan (I’m partial to the short stories) and Arthur Clarke and also Vinge’s Rainbow’s End. I’ll also side with those who believe Peter F. Hamilton doesn’t write hard sf. I love his stuff and have read most of it, but it goes well beyond our current understanding of physical laws (souls and reincarnation, psychic powers, living star-ships that produce wormholes, etc.). My one recommendation that hasn’t already been mentioned in this thread is Peter Watts, particularly Blindsight. His stuff is wildly imaginative, very well-writen and very, very grounded on science.
I’ll be mildly heretical and suggest that most Asimov isn’t actually hard sf. What fascinated Isaac were logical puzzles, not scientific ones. They’re obviously related, but not at all the same.
But Heinlein was careful in describing the science in his novels. I loved Spaceship Galileo for that.
Yes!! Vinge is one of the best in the ancient art of predicting the future through novels.
I think that Verne would have been proud of Michael Crichton, Vernor Vinge and Tom Clancy! 
(One Vinge, though is considered a SF writer)…