Why is earth’s atmosphere not blown away into space?

The primary reason is gravity, which acts like an invisible tether. Every molecule of gas in our atmosphere, nitrogen, oxygen, carbon dioxide, etc., has mass. Earth’s core exerts a gravitational pull that attracts these molecules towards the center of the planet. Even though earth travels at 1,07,000 kmph around the sun, the atmosphere is trapped inside earth’s ‘gravity well’.

Also, the atmosphere possesses the same momentum as the solid earth. Since the gases formed alongside the planet or were released from it, they were already moving at that speed. In space, there’s no friction to brush the atmosphere away. Imagine being inside a fast aeroplane: the air inside moves with the plane and you don’t feel a 900 kmph wind hitting your face: because you and the air share the same motion.

While gravity holds the air down, the magnetosphere protects it from being stripped away by the solar wind, a stream of charged particles from the sun that could otherwise blow the atmosphere away.