The mesosphere is the third layer of Earth's atmosphere and resides from 50 to 85 km above the surface. It is characterized by very low air density and extremely cold temperatures, reaching down to -90 degrees Celsius. These conditions make it impossible for conventional airplanes to fly in the mesosphere.

As for satellites, while some do operate in the upper part of the mesosphere, they are specifically designed to withstand the harsh environmental conditions. These satellites are typically small and lightweight, and use specialized materials and technologies to protect them from the cold temperatures and radiation present in the mesosphere.

Overall, the vast majority of satellites and airplanes operate in the lower layers of the atmosphere, such as the troposphere and stratosphere, where conditions are more favorable for flight.