When spacecraft return to Earth, they don't need to shed all their velocity by firing retro-rockets. Instead, they use the atmosphere as a brake to slow down for a soft landing. Every planet in the solar system except Mercury has enough of an atmosphere to allow aerobraking maneuvers, and could allow high-speed exploration missions. A new paper looks at the different worlds and how a spacecraft must fly to take advantage of this "free lunch" to slow down at the destination.

Aerocapture is an orbital transfer maneuver in which a spacecraft makes a single pass through a planetary atmosphere to decelerate and achieve orbit insertion. On the other hand, aerobraking uses a propulsive burn plus repeated dips into the atmosphere—i.e., atmospheric drag—to gradually slow the spacecraft and reduce the size of the orbit to achieve orbit insertion.

The new paper posted to the arXiv preprint server, by Athul Pradeepkumar Girija from the School of Aeronautics and Astronautics at Purdue University, notes that one of the significant risks associated with aerocapture is the uncertainty in the atmospheric density.