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Is it right?

'Humans haven't walked on Neptune but maybe, in the twenty first century, it might happen.'

Alternatively, should I say "Humans didn't walk.."

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    Grammatically okay. The chances of anyone walking on Neptune this century? Highly unlikely. – Helmar Sep 27 '16 at 20:20
  • My mistake."on Mars" but what if I use past simple (didn't walk) – J.meet Sep 27 '16 at 20:25
  • There's no past reference in the context, so the past would be odd. "When didn't they walk on Mars?" But haven't walked means haven't ever walked, so there's no past reference needed. – John Lawler Sep 27 '16 at 20:39
  • Since Neptune is a gas giant, it is impossible to walk on it. There is no solid part (at least near the surface), so all that would happen is that you would fall through the surface, and continue falling until the pressure crushed you into goo. – David Handelman Sep 27 '16 at 22:07
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    Are you asking specifically about American English? There isn't an [american-english] tag here, but using didn't in these circumstances is absolutely and categorically wrong in British English. – Andrew Leach Sep 27 '16 at 22:56
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The difference between your two examples is the difference between the perfect and preterite(simple past) tenses.
The main semantic difference is, as detailed by the following article, one of time-frame.

"Both [sentences] refer to an action that was finished in the past. But there is one important difference: “I saw the movie” suggests that you saw the movie at a specific time in the past. “I have seen the movie” suggests that you saw the movie at an unknown time in the past.

If you swap their examples for yours it all fits into place; humans didn't walk infers that at a specific time in the past humans did not walk on Neptune, while have not walked denotes that from the start of humanity to now humans have not walked on Neptune.

In other words, the present perfect also implies some temporal connection to the present, now.
In contrast, the preterite refers to a specific designated time-frame e.g. last week humans did not walk on Neptune.

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"Humans haven't walked on Neptune" means that humans walking on Neptune has never happened, from the distant past up to now. It's fine as a complete sentence.

"Humans didn't walk on Neptune" means that humans walking on Neptune did not happen at a specific point in the past. If that point is clear from context, it's fine as a complete sentence; otherwise, you have to indicate it, e.g. "Humans didn't walk on Neptune in 1843".

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You definitely want "haven't walked" in this case. The reason is that given by BladorthinTheGray. It is clear from your sentence that it's the "have never walked" meaning that you intend.

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