NASA's Perseverance rover successfully landed on mars, on February 18, 2021, starting its robotic exploration of the Red Planet. MEDLI2 was one of the crucial technologies in the rover's protective aeroshell that helped document the spacecraft's entry, descent and landing (EDL). All MEDLI2 data was stored in Perseverance for transmission to Earth after a successful landing.
MEDLI2's role was to collect critical data about the hostile environment during Perseverance's entry into the planet's atmosphere. It included three types of sensors – thermocouples, heat flow sensors and pressure transducers – that measured extreme heat and pressure during entry. It also contained electronics and hardware to record the thermal and pressure loads experienced during entry and through parachute deployment.
MEDLI2 used its measurements to determine the heating and atmospheric forces that occurred in the heat shield and posterior shell. Together, these two components form the aeroshell, which housed and protected the Perseverance rover during its cruise to Mars and EDL.
MEDLI2 was turned on five hours before the “7 Minutes of Terror” or the final descent of 7 minutes to Mars. This allowed time for the MEDLI2 electronics to stabilize the temperature and measure the initial conditions prior to entry. The team was relieved to receive an indication that MEDLI2 was successfully activated. The team continued to monitor inbound data throughout this five-hour onshore phase prior to entry.
Most of the MEDLI2 sensors and primary electronics were mounted on the Mars 2020 heat shield. About 10 seconds after the supersonic parachute was deployed, the MEDLI2 shut down for the last time as it had completed its job. As it is critical that the heat shield be detached to allow the Perseverance rover to be extracted from the incoming vehicle, it was necessary for the MEDLI2 to be turned off a few seconds before separation to avoid electrical power issues. The harness connecting the heat shield and the rear shield was then severed by firing a fire cutter, and the heat shield was brought down.
“We didn’t find any problem with the separation”, said Henry Wright, MEDLI2 project manager at NASA's Langley Research Center in Hampton, Virginia. “The heat shield separated cleanly from the incoming Mars 2020 vehicle. The MEDLI2 hardware was clearly visible on the heat shield as it fell towards the surface of Mars. Good job! ”
Perseverance also returned"critical event data” in real time during the EDL. It included a subset of the MEDLI2 data that allowed observations about what the entry vehicle was experiencing while the entry was taking place. Three days after Perseverance's successful landing, the remaining data from MEDLI2 was transmitted back to Earth and the next phase of the project began: data analysis and performance reconstruction.
“The data returned is fascinating. It's like getting a bird's-eye view of what's happening with the aeroshell as it flies through the skies of Mars. The signals from the MEDLI2 sensor are so clear that we can immediately detect interesting phenomena and crucial events”, said Todd White, MEDLI2 principal investigator at NASA's Ames Research Center in Silicon Valley, Calif.
Data collected from MEDLI2 also provides measurements that will be used to determine the properties of the atmosphere through which the Mars 2020 entry vehicle flew. MEDLI2 provides essential EDL observation data to understand how much margin remained in the Perseverance entry along with data that will be used to improve forecasting models and tools for future missions.
Digging deeper into the data
Heat shield insulation temperatures were recorded throughout the entry phase and were consistent with entry predictions. The maximum temperature measured in the heat shield during entry was 1.830 degrees Fahrenheit, or 1.000 degrees Celsius. This correlates with an estimated peak external heat shield temperature of around 2.550 degrees Fahrenheit, or 1.400 degrees Celsius.
MEDLI2 also used its built-in thermocouples to determine how much of the heat shield's protective insulation may have burned or ablated. All thermocouples survived the input heating pulse, indicating that the heat shield ablation was too low. This observation can be used to reassess the amount of insulation that is needed in a heat shield to potentially reduce the overall mass of the incoming vehicle.
Surface pressures were also measured over the same phase with a peak heat shield surface pressure corresponding to the team's input predictions. MEDLI2 chose different sensors to focus on accurately capturing different flight regimes. One sensor covered the full range of maximum surface pressures. The remaining six heat shield pressure measurements had a range to more accurately capture conditions during the supersonic flight regime (from about five times the speed of sound on Mars to supersonic parachute deployment). These sensors, combined with onboard inertial measurements, allow additional insight into the performance of the incoming vehicle when the impacts of atmospheric density variations and winds are most pronounced. MEDLI2 pressure data will be used to improve the modeling approach for future EDL missions.
MEDLI2 included sensors in the rear hull of the Mars 2020 entry vehicle, an area that has so far had limited observations. Surface pressure, insulation temperature and direct surface heat measurements made up the back cover sensor suite. Knowledge of surface pressure at the rear of the incoming vehicle contributes to reducing the size of the landing footprint. The back layer insulation temperature data were within initial predictions, which can be used to reduce modeling uncertainty. As with thermal insulation, understanding the temperature performance of back insulation can lead to a reduction in back insulation mass. Direct heating measurements of the back shell surface also contribute to reducing uncertainty in predictive models.
The MEDLI2 data also included a range of “maintenance” measurements. This included a series of temperature compensation measurements from essential scientific sensors. Much of this maintenance data is also of great interest to the Mars 2020 team to help with their own EDL reconstruction efforts. Part of the maintenance measurements included sensors internal to the MEDLI2 ing electronics (voltages, internal temperatures, MEDLI2 heart rate/clock, etc.).
The MEDLI2 team will continue to analyze the data over the next six months, refining NASA's understanding of Mars' atmosphere, extreme entry conditions, and how the Mars 2020 aeroshell protected the rover. These lessons will be immediately useful for missions to Mars, and even missions targeting Titan.
Langley led MEDLI2 instrument development and project management. NASA's Ames Research Center in California's Silicon Valley and NASA's Jet Propulsion Laboratory in Southern California contributed to MEDLI2. NASA's Game Changing Development program within the Space Technology Mission Directorate funded the technology's development.
Source: NASA
