Back to earth

4000 years ago, earth become unhabitble for humans to sustainably survive on Earth. As the air became poisoned. So they built 3 space stations as a last effort to save humanity. One on the moon and the others circling Earth. Once the project was done, 35 people from each country were chosen to move into the space stations. With the rest of the human race staying, and 15 years later, all humans on earth went extinct.

Year 8327. With the help of robots, they have discovered that the air on Earth has become purified and is once again suitable for human life. So it was decided for humans to go back and recolonize Earth once more. So after 7 years of preparations, they decided to send their first human squad to undergo this mission. So they sent a team of soldiers, scientists, as well as some prisoners and volunteers. Back down to earth.