This is obviously somewhat subjective, and I'd say that all four examples you list are correct and, most importantly, would be understood. The only one I think I generally wouldn't use is "Mars is near" in the context you've given. For me, "near" gives the impression that something is approaching and will arrive at the speaker's location, or at least that it can be reached from the speaker's location. In the sense that you're viewing Mars from Earth and it will never arrive at your location, this sounds a little strange in my mind. However, if I were in a space ship and Mars was approaching to within a short distance, I would say something like "Mars is near" or "We are near Mars". "The enemy is close" and "The enemy is near" both sound the same to me, probably because the enemy will eventually arrive at your location.