Well first of all, there's been no mission to the moon because it would be completely useless. There's no way any nation or institution would spend hundreds of millions just because it would be cool to visit the moon. There are a lot of plans to create a station on the moon, the chinese are planning for 2020. Helium-3 is the only reason to ever go there and harvesting it is not easy, less alone shipping to Earth.
What comes to asteroids, there actually are a few projects going on right now. Rosetta is following 67P/Churyumov-Gerasimenko as we speak. Also the Near probe photographed Eros for a year in late 1990's and even landed on it.
The reason not to send people to Mars is because we do not have the sufficient technology nor the funds. It would take a massive ship to deliver water and food (it's over a 6 month trip one way) and it would take billions of dollars. The biggest issue tho is the radiation, Mars has pretty much no atmosphere and radiation is brutal. But having rovers out there ain't nothing. It's pretty huge we have a kickass robot there roaming about and we can control it from Earth.
What comes to going further out, especially Europa, that's where most often people don't actually really understand the size of the solar system. While Mars in only 1,5AU away from the Sun, Jupiter is 5,2AU away (astronimical unit which is the distance between Earth and Sun, approx 150 million km).
In the quest of sending people out the first step is the moon of course and helium-3. If we ever manage to harvest it we would have fuel compact enough (1kg of helium-3 equals thousands of tons of oil) to send people on their way.
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