When you heat the air inside a hot air balloon, it rises. why does this occur? ?

The hot air inside the balloon becomes less dense than the air outside the balloon.

the only real purpose of sputnik 1 was to beat the us into space. we were getting close to launching a useful satellite, explorer 1 - which launched on january 31, 1958. the ussr had to beat that, so they launched sputnik 1 on october 4, 19