I believe it was Russian probes skirmishing Venus.
No person has ever 'explored' any planet, besides Earth. Several people have been to the moon. Unmanned vehicles have been to Mars. Mankind has only 'explored' other planets visually.
I'm not sure what "first" means in this context. Venus can be closer to Earth than any other planet, and Venus can be brighter than any other planet, but it's not so all the time.
Venus
If the "first an" is meant to be "first man", then the answer is that no man has been on any planet other than the earth.
All planets other than Earth could be considered alien. Earth is the only planet known to harbor life.
.How do you know that earth isn't the other planet?
Other planets do have oxygen, but not in as high of concentration as we have here on Earth.
5th largest planet out of the eight in our solar system.
Well, no. It has not been explored by anyone. It is currently the most inhospitable planet in the solar system. We can't land on it successfully.
Venus was "hit" by the soviet Venera 3 in 1966.
No planet that we know of other than Earth is capable of supporting life.
The only planet that can't be seen from earth is earth itself. Other than earth itself other planet can be seen, however sometimes it might requires use of high technology!