4. The Great wall of China is not visible from space with the naked eye even in a low Earth orbit. However, many other manmade objects are (such as bridges and dams).
5. The Alpha Centauri system is the closest star system to our own, but neither Alpha Centauri A or B (it is a double star system) is the closest star – that honor goes to Proxima Centauri, which is about 0.2 light years away from the other two stars.