* It's a gas giant: Jupiter is made mostly of gas (hydrogen and helium) and does not have a solid surface.

* No spacecraft have landed: While we've sent many spacecraft to explore Jupiter, they have only flown by or orbited the planet.

* Extreme conditions: The extreme pressure and temperatures on Jupiter would make it impossible for humans to survive.

So, while we've learned a lot about Jupiter, no human has ever set foot on it!