The human race can be divided into three groups, in increasing order of savvy:
First, those who have never heard of H P Lovecraft.
Second, those who think of him just as an author of macabre, claustrophobically shuddersome horror stories.
Third, those who are aware that he was a writer of awesome science fiction.
Stid: But he didn't exactly rove much in the Old Solar System, did he? Just one of his tales is set on another planet - the Venus of In the Walls of Eryx.
Zendexor: A pity, for it's a good tale and we could have done with more such interplanetary stuff from his pen. However, you're leaving two important points out of account.
First, his three great SF tales, though set on Earth, are full of outreach, either into our deep past (At the Mountains of Madness) or into both past and future (The Shadow Out of Time) or by contact with beings from another world (The Whisperer in Darkness).
Second, he is a master of the intriguing allusion, the brief but haunting reference.
Hear the voice speaking through the deceased moron in Beyond the Wall of Sleep, telling us that we are more than we know:
"I am an entity like that which you yourself become in the freedom of dreamless sleep... It is not permitted me to tell your waking earth-self of your real self, but we are all roamers of vast spaces and travelers in many ages. Next year I may be dwelling in the Egypt which you call ancient, or in the cruel empire of Tsan Chan which is to come three thousand years hence. You and I have drifted to the worlds that reel about the red Arcturus, and dwelt in the bodies of the insect-philosophers that crawl proudly over the fourth moon of Jupiter. How little does the earth self know life and its extent! How little, indeed, ought it to know for its own tranquility!
We find another list of experiences, longer and even more fascinating, in The Shadow Out of Time. But the mind-swapped time-traveller in that tale learns yet more:
I shivered at the mysteries the past may conceal, and trembled at the menaces the future may bring forth. What was hinted in the speech of post-human entities of the fate of mankind produced such an effect on me that I will not set it down here.
After man there would be the mighty beetle civilization, the bodies of whose members the cream of the Great Race would seize when the monstrous doom overtook the elder world. Later, as the earth's span closed, the transferred minds would again migrate through time and space - to another stopping place in the bodies of the bulbous vegetable entities of Mercury. But there would be races after them, clinging pathetically to the cold planet and burrowing to its horror-filled core, before the utter end.
For more discussion of this story see our page on alien scripts.
Mercury: The Shadow Out of Time (one-line mention, quoted above)
Venus: In the Walls of Eryx
Earth: At the Mountains of Madness and The Shadow Out of Time (both involving the discovery of pre-human civilizations); The Whisperer in Darkness (sinister visitors from Pluto); The Shadow Over Innsmouth (involving a civilization of undersea creatures)
The Moon: The Dream Quest of Unknown Kadath
Callisto: Beyond the Wall of Sleep (the one-line mention, quoted above)
Pluto: native name "Yuggoth"; provenance of the aliens in The Whisperer in Darkness
H P Lovecraft, At the Mountains of Madness (written 1931; serialized in Astounding Stories, February - April 1936); "Beyond the Wall of Sleep" (Pine Cones, October 1919); "In the Walls of Eryx" (Weird Tales, October 1939); "The Shadow Out of Time" (Astounding Stories, June 1936); "The Shadow Over Innsmouth" (written Nov-Dec 1931; published as booklet April 1936); "The Whisperer in Darkness" (Weird Tales, August 1931)
For comments by John Greer on HPL and the OSS, see Lovecraft's Moon.
For rejections of HPL's tales as recounted in Lin Carter's A Look Behind the Cthhulu Mythos, see the OSS Diary for 7th October 2016.
For the link between At the Mountains of Madness and the work of yours truly, see the OSS Diary for 7th April 2017.
For the HPL-inspired The Weird of Hali - Innsmouth by John Michael Greer, see the Diary entry entitled Lovecraft Reversed with Added Belbury.
A scientific problem with Beyond the Wall of Sleep is mentioned in Lawless Science: Style and Mood.