Scene Context: The crew approaches landing after losing the primary Martian orbital network, relying on the emergency-deployed backup relay.
Essence of the Phenomenon: During peak heating, the lander cannot reliably depend on an external radio channel through the plasma cocoon. However, once the plasma dissipates—and especially after heat shield jettison—communication with the orbital node becomes critical again: it provides external telemetry, synchronization, and orbital context until the module's own sensors take over for altitude and local landing geometry.
Scientific Basis: In Martian EDL architectures, an orbital UHF relay is genuinely utilized to receive descent telemetry. NASA explicitly considers the orbiter a key communication node during EDL, primarily because it is convenient to pull data "from above" when the direct-to-Earth link is constrained by delay and, during the peak entry phase, by plasma effects. At the same time, the lander's own radar and lidar systems become decisive only after heat shield separation, when altitude, vertical velocity, and local terrain must be measured.