For more than six years, the Mars Entry, Descent, and Landing Instrumentation 2 (MEDLI2) team have waited to hear and experience the successful landing of NASA’s Perseverance rover. Their patience was finally paid when NASA’s Perseverance rover successfully landed on Feb. 18, 2021, beginning its robotic exploration of the Red Planet.
A little about MEDLI2 and its role in the mission
MEDLI2 was one of the crucial technologies on the rover’s protective aeroshell that helped document the entry, descent, and landing (EDL) of the spacecraft. All of the MEDLI2 data was stored on Perseverance for transmission to Earth after a successful landing.
The role of MEDLI2 was to collect critical data about the harsh environment during Perseverance’s entry through the planet’s atmosphere. It included three types of sensors: thermocouples, heat flux sensors, and pressure transducers, which measured extreme heat and pressure during entry. Furthermore, it contained electronics and hardware for recording the thermal and pressure loads experienced during entry and during the parachute deployment.
What exactly is Aeroshell?
The measurements of MEDLI2 were used to determine the heating and atmospheric forces that occurred on the heat shield and back shell. These two components – the heat shield and the back shell, together make up the aeroshell, which housed and protected the Perseverance rover during the cruise to Mars and EDL.
MEDLI2 was powered on five hours before the final 7-minute descent to Mars, better known as the “7 Minutes of Terror.” and the team continued to monitor the incoming data throughout this five-hour coast phase prior to entry. Those five hours provided time for MEDLI2’s electronics to stabilize temperature and measure the initial conditions before entry. It was a big relief when the indication of successful activation of MEDLI2 was received.
More details on how did the separation phase go?
Since MEDLI2 had completed its job, it was powered down for one last time after about 10 seconds of supersonic parachute deployment. Majority of the MEDLI2 sensors and the primary electronics were mounted on the Mars 2020 heat shield. Separation of the heat shield is a critical step to allow the Perseverance rover to be extracted from the entry vehicle. Hence, it was required for the MEDLI2 to be turned off a few seconds before the separation to prevent any electrical power issues. Finally the harness connecting the heat shield and the back shell was severed by firing the pyro-cutter.
Henry Wright, MEDLI2 project manager at NASA’s Langley Research Center in Hampton, Virginia said, “We didn’t find any issues with the separation.” “The heat shield cleanly separated from the Mars 2020 entry vehicle. The MEDLI2 hardware was clearly visible on the heat shield as it fell toward the surface of Mars. Job well done!”
Data transmission and the next phases that follow
“critical event data” was returned by perseverance in real time during the EDL. It included a subset of the MEDLI2 data which allowed observations into what the entry vehicle was experiencing as the entry was happening. The remaining MEDLI2 data was transmitted back to Earth three days after Perseverance’s successful landing. The next phase of the project – data analysis and performance reconstruction has begun.
Todd White, MEDLI2 principal investigator at NASA’s Ames Research Center in California’s Silicon Valley said, ““The returned data is fascinating. It’s like having a bird’s eye view of what’s happening to the aeroshell as it flies through Martian skies. The MEDLI2 sensor signals are so clear we could immediately pick out interesting phenomenon and crucial events.”
Taking a Deeper Look Into the Data from Perseverance
Throughout the complete entry phase the heat shield insulation temperatures were recorded and were found to be consistent with the entry predictions. the peak temperature of the heat shield measured during the entry was 1,830 degrees Fahrenheit, or 1,000 degrees Celsius, which correlated to the estimated peak heat shield outer temperature of about 2,550 degrees Fahrenheit, or 1,400 degrees Celsius.
To know how much of the heat shield insulation might have burned away, MEDLI2 also used its embedded thermocouples. The fact that all the thermocouples survived indicated that the heat shield ablation was very low. Similarly, surface pressures were also measured throughout the same phase with a peak surface pressure of the heat shield matching the team’s entry predictions. The MEDLI2 pressure data will be used to improve the modeling approach for future EDL missions.
The back shell is an area of the Mars 2020 entry vehicle which until now, has had limited observations. MEDLI2 included sensors on it this time. Things like surface pressure, insulation temperature, and direct surface heating measurements comprised the back shell sensor suite.
More details on MEDLI2 data
Knowledge of the surface pressure on the back of the entry vehicle contributes to a reduction in the size of the landing footprint. Back shell insulation temperature data was within the initial predictions, which can be used to reduce the modeling uncertainty. As with the heat shield insulation, understanding the back shell insulation temperature performance could lead to a reduction in the back shell insulation mass. Direct surface heating measurements of the back shell also contribute to reducing the uncertainty in the predictive models.
MEDLI2 data also included a range of “housekeeping” measurements. These included a number of compensating temperature measurements of critical science sensors. Much of this housekeeping data is also of keen interest to the Mars 2020 team to aid in their own EDL reconstruction efforts. Part of the housekeeping measurements included sensors internal to the MEDLI2 support electronics (voltages, internal temperatures, MEDLI2 heartbeat/clock, etc.).
Next steps with the data
The MEDLI2 team will continue to analyze the data for the next six months, refining NASA’s understanding of the Mars atmosphere, the extreme conditions of entry, and how well the Mars 2020 aeroshell protected the rover. These lessons will be immediately useful for the next Mars missions, and even missions headed for Titan.
Langley led the 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 development.
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