The planet was Jupiter, not Venus. The story is ["Desertion"](http://www.isfdb.org/cgi-bin/title.cgi?47576) by [Clifford D. Simak](http://en.wikipedia.org/wiki/Clifford_D._Simak) (rhymes with "gimmick"), which was also the subject of [this question](https://scifi.stackexchange.com/questions/8095/looking-for-an-old-story-about-genetically-engineering-humans-to-live-on-a-metha). Reprinted many times; you might recognize one of [these covers](http://www.isfdb.org/cgi-bin/titlecovers.cgi?47576). You can listen to the [*Mind Webs*](http://www.otrplotspot.com/mindwebs.html) reading of this story at the [Internet Archive](https://archive.org/details/MindWebs_201410/Desertion+-+Clifford+Simak.mp3).

**They want to see if humans can survive in harsh planet conditions with a view to future colonisation. Scientist guy invents a machine which can transform humans into beings which are adapted to conditions on planet. They send folks in but none ever report back.**

> Four men, two by two, had gone into the howling maelstrom that was Jupiter and had not returned. They had walked into the keening gale—or rather, they had loped, bellies low against the ground, wet sides gleaming in the rain.  
<br>For they did not go in the shape of men.

**Scientist, in despair, sends his old dog in. Dog doesn't return either so scientist finally goes himself.**

Not exactly; the man and his dog go together:

> "Just wanted to tell you, Miss Stanley, to get ready for two more."  
<br>"Aren't you afraid," asked Miss Stanley, "that you'll run out of them? Sending out one at a time, they'd last longer, give you more satisfaction."  
<br>"One of them," said Fowler, "will be a dog."  
<br>"A dog!"  
<br>"Yes, Towser."  
<br>He heard the quick, cold rage that iced her voice. "Your own dog! He's been with you all these years—"  
<br>"That's the point," said Fowler. "Towser would be unhappy if I left him behind."

**Gets to planet and, in his new state, finds himself strong and perfectly adapted to life on new planet. Meets his old dog who is also fine, they can now communicate and end up going off to discover the new world together.**

The ending:

> !"We can't let them down," said Fowler.  
> !<br>Fowler took a step or two, back toward the dome, then stopped.  
> !<br>Back to the dome. Back to that aching, poison-laden body he had left. It hadn't seemed aching before, but now he knew it was.  
> !<br>Back to the fuzzy brain. Back to muddled thinking. Back to the flapping mouths that formed signals others understood. Back to eyes that now would be worse than no sight at all. Back to squalor, back to crawling, back to ignorance.  
> !<br>"Perhaps some day," he said, muttering to himself.  
> !<br>"We got a lot to do and a lot to see," said Towser. "We got a lot to learn. We'll find things—"  
> !<br>Yes, they could find things. Civilizations, perhaps. Civilizations that would make the civilization of Man seem puny by comparison. Beauty and, more important, an understanding of that beauty. And a comradeship no one had ever known before—that no man, no dog had ever known before.  
> !<br>And life. The quickness of life after what seemed a drugged existence.  
> !<br>"I can't go back," said Towser.  
> !<br>"Nor I," said Fowler.  
> !<br>"They would turn me back into a dog," said Towser.  
> !<br>"And me," said Fowler, "back into a man."