This is thinking big - no doubt. If anyone thought pumping oil from ANWR was environmentally unfriendly, consider the future (humorously). The Human Race has been consuming energy at an ever increasing rate. Russian astronomer Nikolai Kardashev said this is the measure of advanced civilizations (Kardashev Scale) At some point in the future, we will want to harness the entire energy output of the Sun. To do that, a Dyson sphere would be constructed enveloping the Sun with a shell just a little larger than Earth's orbit. By then our world will be an entire city of McMansions so it makes sense to wall the neighborhood. What about the view of the stars and the light that bounces of the sphere to illuminate the nightime? Light pollution ruined that back in the 21st century. Isn't it going to be expensive to pay for it? Just levy a toll for anyone that goes into and out of the sphere in their PUV (Planetary Utility Vehicle). Of course the settlers on Mars might complain about this...they might compromise and allow a midrift along the orbital plane of the sphere so most comets, asteriods, etc get thru.