Atmosphere diagram showing the mesosphere and other layers. The layers are not to scale.
The mesosphere is a part of the atmosphere. It is between the stratosphere and the thermosphere. On Earth, the mesosphere starts at 50 kilometers above the planet to about 90 km above the planet.
The mesosphere is the most poorly understood part of the atmosphere. The mesosphere lies above the highest point for flying a plane, but below the lowest point for flying a spaceship. It also has a layer of electrically charged particles called the ionosphere.