Mars Atmospheric Entry, Descent, and Landing: An Atmospheric Perspective

Abstract

Beginning in the very earliest years of the space age, a flotilla of robotic explorers have been sent to study Mars—first simply to fly by, then to orbit, and, later, to attempt landing on the surface. For these landers, separating the rapidly approaching spacecraft from the surface is little but a tenuous carbon dioxide atmosphere, too thin to be useful but too thick to ignore. The purpose of the entry, descent, and landing (EDL) process is to take these hypersonic spacecraft through the approximately 6 mb atmosphere and place them safely on the Martian surface. The sequence of steps required to progressively slow and control this descending spacecraft has been honed throughout the decades but follows the same basic approach. A period of frictional deceleration during the entry phase of EDL first slows the spacecraft to a point where a supersonic parachute can be deployed to further slow the spacecraft during its descent phase. Whether a spacecraft is following a ballistic or a guided entry determines the need to control the downrange motion of the spacecraft during the entry phase, providing more or less targeting accuracy, at the expense of EDL complexity. The third and terminal EDL phase, consisting of a powered or semi-powered landing, brings the spacecraft to the surface. Over the years, a range of different powered landing approaches have been employed, from basic retropropulsion, to airbags to the SkyCrane, as spacecraft size has grown and landing sites have become more challenging. Despite this seemingly straightforward description, EDL at Mars is an exceptionally intricate process, with numerous failures over the decades; as of 2023, four space agencies have attempted, with varying degrees of success, to land on Mars. Environmental uncertainties during the EDL process typically remain a large mission concern. The process of characterizing the Martian atmosphere at the time, season, and location of touchdown has advanced incrementally from the earliest landings that relied on coarse orbital or flyby measurements of surface temperature and pressure to more modern efforts that incorporate sophisticated numerical models with high spatial and temporal resolution, pinpointing the most likely conditions that a spacecraft will experience during its traverse through the atmosphere and providing comprehensive uncertainty measurements to statistically bound the range of possible conditions. As spacecraft become more complex, it has become possible to add in situ sensors to the descending spacecraft to directly measure the local environment. Combined with numerical modeling and information provided by other spacecraft, these data have helped increase knowledge of the local environment to a substantial degree, reducing environmental uncertainty from being a major risk to a manageable concern.