The Mars Science Laboratory Energy Descent and Landing Instrument is called MEDLI. MEDLI measured the heating and atmospheric pressure changes that occurred during the descent to help determine the effects on different parts of the spacecraft.

Tech Specs

Main Job
To collect engineering data during the spacecraft's high-speed, extremely hot entry into the Martian atmosphere.
Location Spacecraft heatshield
Mass
12.5 kilograms for the instrument suite.
Power 30 Watts during entry, descent and landing (EDL); no power during launch and cruise phases.
Size
Seven pressure ports and seven integrated sensor plugs throughout the headshield, as well as supporting electronics and wiring:

MEDLI Integrated Sensor Plug (MISP)
Each plug is 1.3 inches (33 millimeters) in diameter and 0.8 inch (20.3 millimeters) deep.

Mars Entry Atmospheric Data System (MEADS)
Port diameter is 0.1 inch (2.54 millimeters) in diameter; tube, transducer, and fittings require a volume of 3.0 by 3.4 by 7.2 inches (76 x 86 x 183 millimeters) in the interior of the heatshield.

Sensor Support Electronics (SSE)
Two electronic boards housed in a 3.0 by 9.75 by 13.25-inch (76.2 by 247.7 by 336.6 millimeter) aluminum chassis.
Data Return
0.9 MB (megabytes) of raw data.

Mars Science Laboratory Entry Descent and Landing Instrument (MEDLI)

Biggest-Ever Heat Shield Prepared for Mars Spacecraft
Biggest-Ever Heat Shield Prepared for Mars Spacecraft: The heat shield for NASA's Mars Science Laboratory is the largest ever built for a planetary mission. Technicians in the photo are installing electronics of an instrument for collecting data about temperature and pressure during descent through the atmosphere, the Mars Science Laboratory Entry, Descent and Landing Instrument (MEDLI). Credit: NASA/JPL-Caltech/Lockheed Martin
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MEDLI collected engineering data during the spacecraft's high-speed, extremely hot entry into the Martian atmosphere. MEDLI data are invaluable to engineers as they design future Mars missions. The data help them design systems for entry into the Martian atmosphere that are safer, more reliable, and lighter weight.

MEDLI was provided by NASA's Langley and Ames Research Centers. It consisted of two kinds of instruments (with seven sensors of each kind) that were installed in 14 places on the spacecraft's heat shield.

MISP (MEDLI Integrated Sensor Plugs)

Preparing Mars Science Laboratory Heat Shield
Preparing Mars Science Laboratory Heat Shield: This view shows the inner surface of the heat shield, where technicians are installing electronics of an instrument for collecting data about temperature and pressure during descent through the atmosphere, the Mars Science Laboratory Entry, Descent and Landing Instrument (MEDLI). Credit: NASA/JPL-Caltech/Lockheed Martin
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When the spacecraft faced extreme heat during entry into the Martian atmosphere, MISP measured how hot it got at different depths in the spacecraft's heat-shield material. Predicted heating levels were about three times higher than those of the Space Shuttle when it enters Earth's atmosphere. Because of the high heat, the spacecraft's thermal protection system (TPS) was designed to burn away during entry into Mars' atmosphere. MISP was designed to measure the rate of this burning, also known as "recession."

When they designed the heat shield, engineers predicted what they thought the heating rate would be as a function of time. Comparing their predictions to the actual data collected by MISP helps them learn how much heat-shield material will be needed to protect future Mars missions.

MEADS (Mars Entry Atmospheric Data System)

MEADS measured the atmospheric pressure on the heat shield at the seven MEADS locations during entry and descent through Mars' atmosphere. The MEADS pressure sensors were arranged in a special cross pattern to allow engineers to determine the spacecraft's orientation (its position and how that changed) as a function of time. Engineers use this information to see how well their models predicted the spacecraft's real trajectory (its path) and its aerodynamics (how it acted when moving through the atmosphere). That information allows them to plan future missions that will have even better performance during critical stages of entry, descent, and landing.