The Science of Entry, Descent, and Landing in the Venus, Jupiter, and Saturn Systems

The design of the entry, descent, and landing (EDL) process is meant to bring a hypersonic spacecraft through a planet’s atmosphere and safely onto its surface. When a spacecraft probe visits a planet, such as a gas giant, without a defined surface, the ‘entry’ and ‘descent’ elements of the process that remain are no less critical for mission success. Irrespective of the destination, however, the steps of the EDL process follow the same basic approach. Following an initial period of frictional deceleration during the entry phase, which slows the spacecraft from hypersonic to supersonic speeds, a period of further deceleration on parachute—the descent phase—begins. During the descent phase, the spacecraft is slowed even further, to speeds as low as 10s m/s, at which point the final—landing—phase of EDL may begin. Depending on destination, either active or passive landing measures are employed to bring the spacecraft to a safe touchdown on the surface. With time and experience, EDL has become a reliable process, though it is still not without its challenges. A number of failures in the early years of planetary exploration of Venus by the Soviet Union led to stepwise improvements in spacecraft design, and a corresponding growth in the number of planetary targets for in situ exploration. In many instances, such as for Venus, these early probes were used as test vehicles to characterize a planet’s atmosphere for the very first time—failure was almost assured for these early missions. Slowly, though, acquisition of data from the descent probes allowed for the reconstruction of atmospheric temperature and density profiles, along with wind, composition, and cloud structure. In the outer solar system, new ground was also broken by delivery of atmospheric probes to Jupiter and Titan, offering tantalizing glimpses of the atmospheres beneath their hazy cloud layers, in many cases surprising scientists by revealing compositional or structural elements markedly different than originally believed. These differences will shed light on how these objects formed and evolved. Leveraging the opportunity to use the soon-to-be decommissioned Cassini spacecraft as an atmospheric probe in its own right gave a remarkable data return about the upper atmosphere of Saturn, providing yet another piece of the puzzle of outer planet formation and evolution. The quantity of science returned during the EDL process has greatly improved our understanding of the atmospheres of our planetary neighbors. Blended with numerical models and remote observations, we have been able to construct profiles of many of our planetary neighbors, allowing for the design of new instruments, and new spacecraft to further exploration, while reducing uncertainty about the environment into which the probes will be entering. New spacecraft have a very clear heritage to some of these earliest explorers, and draw upon the lessons learned, and experience gained, in the delivery of spacecraft into a planet’s atmosphere.