Langley's Role in the Mars Science Laboratory Mission
NASA's Langley Research Center in Hampton, Va., has two critical roles in the Mars Science Laboratory (MSL) mission.
- Entry, descent and landing -- or EDL -- the so-called "seven minutes of terror" when the spacecraft enters the atmosphere of Mars and descends through it.
- MSL Entry, Descent and Landing Instrumentation, or MEDLI -- which will record heat and atmospheric pressure during entry and descent for the first time ever.
NASA engineers Michelle Munk and David Way explain the MEDLI -- Mars Science Laboratory (MSL) Entry, Descent and Landing Instrumentation -- package on the MSL aeroshell that will measure the heating and pressure the Curiosity rover experiences as it flies through the atmosphere of Mars to its landing site.
› More on MEDLI
NASA's Mars Science Laboratory is expected to improve not only our knowledge of Mars, but also the science of out-of-this-world hypersonic flight -- or more than five times the speed of sound.
That's something of great interest to NASA researchers who study flight through all atmospheres so they can design better air and spacecraft.
The MSL Curiosity rover is protected by an encapsulating aeroshell made up of a heat shield and a back shell. Embedded in the heat shield is a set of sensors designed to record the heat and atmospheric pressure experienced during the spacecraft's high-speed, extremely hot entry into the Martian atmosphere. The sensor suite is called MEDLI for MSL Entry, Descent and Landing Instrumentation.
This is the first time NASA has ever had sensors to collect accurate, high fidelity data of atmospheric entry at another planet. Having that knowledge is important to spacecraft designers -- especially when it comes to developing future Mars entry systems that are safer, more reliable and lighter.
The MEDLI suite includes two kinds of instruments (with seven sensors of each kind) in 14 places on MSL's heat shield, all powered by and feeding data to a black box, the Sensor Support Electronics Unit.
One set of sensors, the Mars Entry Atmospheric Data System (MEADS), required that seven one-tenth-inch diameter holes be drilled into the heat shield in a cross pattern. The holes are ports for pressure sensors that will measure the atmospheric pressure on the heat shield at the seven MEADS locations during entry and descent through Mars' atmosphere.
NASA did extensive testing to make sure the MEADS pressure points can withstand the heat of entry. The sensors' cross pattern will allow engineers to determine the orientation of the MSL aeroshell and how it changes during the less than 10 minutes it takes to go from the top of the atmosphere to the surface.
Engineers will use this information to see how well computer models predicted the spacecraft's path and aerodynamics, as well as to determine the atmospheric density and winds it encountered.
The other set of seven sensors, MEDLI Integrated Sensor Plugs -- or MISP, will measure how hot it gets at different depths in the spacecraft's heat shield material.
Predicted heating levels are about three times higher than those of the space shuttle when it re-entered Earth's atmosphere. The heating levels are so high that the spacecraft's thermal protection system is designed to burn away during entry into Mars' atmosphere.
MISP will measure the rate of this burning, so researchers can compare real data to their predictions.
MEDLI will collect data in the last seven minutes of the MSL's flight. That's about how long it takes to slow the spacecraft from 13,000 miles per hour (21,000 kilometers per hour) to just under two miles per hour (0.9 meters per second). Instruments will record the heat and atmospheric pressure experienced during entry, descent and landing, then turn off before ejection of the heat shield and landing.
The data will be stored on the Curiosity rover, then sent back to Earth so engineers can study it and gain a better understanding of spacecraft performance during hypersonic flight into a planetary atmosphere.
MEDLI is led by Langley in partnership with NASA's Ames Research Center at Moffett Field, Calif. It is the first Technology Demonstration Mission from NASA's Space Technology Program to go into space.
NASA's Aeronautics Research Mission Directorate, Science Directorate and Human Exploration and Operations Mission Directorate have also supported the $28-million MEDLI system research, development and data analysis.
For more about MEDLI, go to www.nasa.gov/mission_pages/tdm/medli/
Entry, Descent and Landing
Team members share the challenges of Curiosity's final minutes to landing on the surface of Mars.
› High-Res Video (download)
A narrated play-by-play of Curiosity's entry, descent, and landing on Mars!
The intense period called the entry, descent and landing (EDL) phase of the MSL mission begins when the spacecraft reaches Mars' atmosphere traveling about about 13,200 miles per hour (5,900 meters per second).
It ends about seven minutes later with the rover and descent stage stationary on the surface in Gale Crater -- one about to begin it's mission and the other with its job completed.
NASA Langley leads the mission's EDL research and computer simulation effort. The team has practiced millions of computer landings on Mars, trying to include all the variables that could affect the spacecraft landing.
"When we get to Mars, we'll have 7,000 pounds (3,175 kilograms) of spacecraft traveling at 13,000 miles per hour (20,921 kilometers per hour)," said David Way, Langley entry, descent and landing team lead. "In just about seven minutes, we'll slow the spacecraft all the way down to just under two miles an hour (0.9 meters per second), gently landing Curiosity right on her wheels."
To do that, the onboard computer will autonomously execute a complex sequence of events, first using atmospheric drag, then a parachute and, finally, rocket engines to slow down.
The atmospheric entry point is 81.46 miles (131.1 kilometers) above Gale Crater. Ten minutes before the spacecraft enters the atmosphere, it will exit the cruise stage. Then, the MSL MEDLI suite begins taking measurements.
Starting a minute later, small thrusters on the back shell will halt the spin the spacecraft maintained during cruise and approach phases. Nine minutes before entry, the back shell thrusters orient the spacecraft so the heat shield faces forward, a maneuver called "turn to entry."
After the turn to entry, the back shell jettisons two solid-tungsten weights, called the "cruise balance mass devices." Ejecting these devices, which weigh about 165 pounds (75 kilograms) each, shifts the center of mass of the spacecraft.
Offsetting the center of mass when the spacecraft is in Mars' atmosphere allows MSL to fly through the atmosphere at an angle, generating lift. That lift is used for "guided entry" to steer out of unpredictable variations in the atmosphere and improve landing precision.
As the spacecraft interacts with the upper atmosphere, thrusters on the back shell can adjust the angle and direction of tilt, letting the spacecraft fly a series of "S" curves. Those curves reduce the horizontal distances the spacecraft covers as it descends. Using fewer of them prevents undershooting the target area; using more of them prevents overshooting the target area. The guided entry maneuvers can also correct for drift to the left or right due to winds.
More than 90 percent of the deceleration before landing results from friction with the atmosphere of Mars before the parachute opens. Peak heating occurs about 80 seconds after atmospheric entry, when the temperature at the external surface of the heat shield will be about 3,800 degrees Fahrenheit (about 2,100 degrees Celsius). Peak deceleration occurs about 10 seconds later.
After the spacecraft finishes its entry maneuvers, a few seconds before the parachute is deployed, the back shell jettisons more tungsten weights to shift the center of mass back to the axis of symmetry. This set of six weights, the "entry balance mass devices," each has a mass of about 55 pounds (25 kilograms).
The parachute deploys about 255 seconds after entry, at about 7 miles (11 kilometers) high and a velocity of about 900 miles per hour (402 meters per second). After about 24 more seconds, the heat shield separates and drops away when the spacecraft is at an altitude of about 5 miles (8 kilometers) and traveling about 280 miles per hour (125 meters per second).
The Mars Descent Imager begins recording video of the ground beneath the spacecraft. The rover, with its descent-stage "rocket backpack," is still attached to the back shell on the parachute. The terminal descent sensor, a radar system mounted on the descent stage, begins collecting data about velocity and altitude. The back shell, with parachute attached, separates from the descent stage and rover about 80 seconds after heat shield separation. At this point, the spacecraft is about 4,800 feet (1,450 meters) above the surface dropping at about 180 miles per hour (80 meters per second). All eight throttleable retrorockets on the descent stage -- called Mars landing engines -- begin firing for the powered descent phase.
After the engines have decelerated the spacecraft to about 1.7 miles per hour (0.75 meters per second), the descent stage maintains that velocity until touchdown. Four of the eight engines shut off just before nylon cords begin to spool out to lower the rover from the descent stage in the "sky crane" maneuver.
The rover separates its hard attachment to the descent stage, though still attached by the sky crane bridle, at an altitude of about 66 feet (20 meters), with about 12 seconds to go before touchdown.