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Summer, despite being the desert, this will be the least expensive time to travel and you will encounter fewer tourists.
This isn't just the set for the corny, 1996 movie, Bio-Dome. This is one of the world's most fascinating facilities dedicated to, "The research and understanding of global scientific issues" ... as their website describes. Biosphere 2 was named giving a head nod to Earth (Biosphere 1 so to speak). http://biosphere2.org/
Personally, the most interesting aspect of exploring the Biosphere was witnessing the two massive 'lungs' connected via underground air tunnels used to maintain the pressure gradient inside Biosphere 2. It is crucial for this structure to have these two variable expansion chambers.
Tickets are about $20 with tours daily, but make sure to check out their website ahead of time for exact info. http://biosphere2.org/visit/tour...
And since the days are scorchers, the Biosphere will allow for comfortable indoor exploration, then when the sun sets, be amazed by Tuscon's famous starry skies.
If you head to Oracle State Park you can get to the American Avenue trailhead parking lot that is open seven-days-a-week to access the National Scenic Arizona Trail, this accommodates after hours use by stargazers.