MEDLI
Initialism of MSL Entry, Descent, and Landing Instrumentation, a suite of engineering sensors in the Mars Science Laboratory entry vehicle heat shield involved in the landing of the Curiosity rover on Mars.
MEDLI: the Curiosity rover's heat shield thermometer
MEDLI is a package of thermocouples, pressure transducers, and radiometers mounted inside and on the back of the Curiosity rover's entry vehicle heat shield. During atmospheric entry in August 2012, it measured the extreme thermal and aerodynamic environment the spacecraft experienced during descent through the Martian atmosphere, collecting data that engineers on Earth could not obtain any other way.
The heat shield itself protected the rover during the hypersonic phase of entry, when friction with the thin Martian atmosphere raised surface temperatures to around 2,000 Kelvin. MEDLI's thermocouples were embedded at different depths in the 65-centimetre ablator material; pressure sensors measured stagnation point pressures and shock layer conditions; radiometers on the aft side recorded radiative heating. This instrumentation served no purpose for the landing itself, only for ground-based validation of computational models.
MEDLI instruments were mounted on a wedge-shaped plug inserted into the heat shield material during manufacturing. Most sensors remained in the shield even after landing, becoming a permanent record frozen into the surface of Mars. The data transmission window lasted only a few minutes during the descent phase, after which the heat shield was jettisoned at low altitude.
Because entry vehicle heat shields are one-time-use, destructive aeroshell components, direct measurement during entry has always been rare. The comparable Stardust mission carried similar instrumentation during Earth re-entry in 2006. MEDLI's measurements confirmed that ablation models and radiation models used in entry system design perform reliably at Mars, where the lower atmospheric density and entry velocity profiles differ significantly from Earth or Venus missions.
The name reflects its narrowly focused mission: MEDLI was not a rover science instrument like the radiometer or the drill. It existed solely to characterize the mechanical and thermal event that delivered Curiosity safely to the surface, a single-purpose engineering package that answered fundamental questions about how well we understand planetary entry environments.