If the upper atmosphere is very thin, why do space vehicles heat up as they enter the atmosphere.?

Answer 1

The Atmosphere of earth are divided into 5 layers.They are Troposphere(0-10Km), Stratosphere(10-30Km), Mesosphere(30-50Km), Thermosphere(50-400Km) and Exosphere(>400Km).
In which all the atmospheric gases like Carbon dioxide,oxygen,nitogen etc. are found. So because of the atmospheric gases the atmospheric pressure is high in the troposphere only. And the above layers contains only the ozone layerfor some height and then nothing else. So the other layers have a low pressure and temperature. The atmospheric pressure is directly proportional to the atmospheric temperature.
So eventually any aircraft in the troposphere layer would be heated by the gases and when it crosses that level to any other layer of atmosphere it won't be heated
Do you know that the columbia space shuttle lost a part of it after leaving the atmosphere somewhere. It did not show the effect until it entered back into the troposphere layer. As soon is entered the layer the tragedy happened.
So it is clear that at the troposphere level anything gets heated up including us.

Answer 2

Because their very high speed (up to around 30,000 kph) still causes friction even with the very light atmosphere. They can slow down their re-entry rate by 'skipping' into and out of the edge of the atmosphere.

They are fine until they enter the atmospheric envelope of the earth.

Answer 3

When particles in the atmosphere touch, they transfer energy to each other, creating friction/ heat. So when a space vehicle comes in, it hits the particles, which then go hit other particles, creating friction/heat around the vehicle.