Technique to reduce a spacecraft's speed and change its orbit from highly
elliptical to circular.
Aerobraking involves allowing the
spacecraft to dip into the
atmosphere. This creates frictional
drag on the spacecraft and results in a slower, lower and more circular orbit.
The lower circular orbit is better for taking scientific measurements - but it does not come without risks. The heat generated by friction with the atmosphere could damage the ship or its instruments. The benefit is that far less fuel is required to insert the craft into orbit - allowing for smaller, cheaper spacecraft and larger scientific payloads.
The first use of aerobraking in planetary exploration was by the Magellan spacecraft settling into an orbit around Venus in 1989.