NASA's Mars 2020 Rover: New Landing Technique | JPL
The Mars 2020 rover mission has major new technologies that improve entry, descent, and landing (EDL): Range Trigger, Terrain-Relative Navigation, MEDLI2, and its EDL cameras and microphone.
Terrain-Relative Navigation helps us land safely on Mars—especially when the land below is full of hazards like steep slopes and large rocks!
HOW TERRAIN-RELATIVE NAVIGATION WORKS
Orbiters create a map of the landing site, including known hazards.
The rover stores this map in its computer "brain."
Descending on its parachute, the rover takes pictures of the fast approaching surface.
To figure out where it's headed, the rover quickly compares the landmarks it "sees" in the images to its onboard map.
If it's heading toward dangerous ground up to about 985 feet (300 meters) in diameter (about the size of two professional baseball fields side by side), the rover can change direction and divert itself toward safer ground.
WHY TERRAIN-RELATIVE NAVIGATION IS IMPORTANT
Terrain-Relative Navigation is critical for Mars exploration. Some of the most interesting places to explore lie in tricky terrain. These places have special rocks and soils that might preserve signs of past microbial life on Mars!
Until now, many of these potential landing sites have been off-limits. The risks of landing in challenging terrain were much too great. For past Mars missions, 99% of the potential landing area (the landing ellipse) had to be free of hazardous slopes and rocks to help ensure a safe landing. Using terrain relative navigation, the Mars 2020 rover can land in more—and more interesting!—landing sites with far less risk.
HOW TERRAIN-RELATIVE NAVIGATION IMPROVES ENTRY, DESCENT, & LANDING
Terrain-Relative Navigation significantly improves estimates of the rover's position relative to the ground. Improvements in accuracy have a lot to do with when the estimates are made.
In prior missions, the spacecraft carrying the rover estimated its location relative to the ground before entering the Martian atmosphere, as well as during entry, based on an initial guess from radiometric data provided through the Deep Space Network. That technique had an estimation error prior to EDL of about 0.6 - 1.2 miles (about 1-2 kilometers), which grows to about (2 - 3 kilometers) during entry.
Using Terrain-Relative Navigation, the Mars 2020 rover will estimates its location while descending through the Martian atmosphere on its parachute. That allows the rover to determine its position relative to the ground with an accuracy of about 200 feet (60 meters) or less.
It takes two things to reduce the risks of entry, descent, and landing: accurately knowing where the rover is headed and an ability to divert to a safer place when headed toward tricky terrain.
IMPROVING MODELS OF THE MARTIAN ATMOSPHERE FOR ROBOTIC AND FUTURE HUMAN MISSIONS TO MARS.
MEDLI2 is a next-generation sensor suite for entry, descent, and landing (EDL). MEDLI2 collects temperature and pressure measurements on the heat shield and afterbody during EDL.
MEDLI2 is based on an instrument flown on NASA's Mars Science Laboratory (MSL) mission. MEDLI stands for "MSL Entry, Descent, and Landing Instrumentation." The original only collected data from the heat shield. MEDLI2 can collect data from the heat shield and from the afterbody as well.
This data helps engineers validate their models for designing future entry, descent, and landing systems. Entry, descent, and landing is one of the most challenging times in any landed Mars mission. Atmospheric data from MEDLI2 and MEDA, the rover's surface weather station, can help scientists and engineers understand atmospheric density and winds. The studies are critical for reducing risks to both robotic and future human missions to Mars.
ENTRY, DESCENT, AND LANDING (EDL) CAMERAS AND MICROPHONE
UNPRECEDENTED VISIBILITY INTO MARS LANDINGS
Mars 2020 has a suite of cameras that can help engineers understand what is happening during one of the riskiest parts of the mission: entry, descent, and landing. The Mars 2020 rover is based heavily on Curiosity's successful mission design, but Mars 2020 adds multiple descent cameras to the spacecraft design.
The camera suite includes: parachute "up look" cameras, a descent-stage "down look" camera, a rover "up look" camera, and a rover "down look" camera. The Mars 2020 EDL system also includes a microphone to capture sounds during EDL, such as the firing of descent engines.
A FIRST-PERSON VIEW OF LANDING ON MARS
In addition to providing engineering data, the cameras and microphone can be considered "public engagement payloads." They are likely to give people on Earth a good and dramatic sense of the ride down to the surface! Memorable videos depicting EDL's "Seven Minutes of Terror for the 2012 landing of NASA's Curiosity Mars rover went viral online, but used computer-generated animations. No one has ever seen a parachute opening in the Martian atmosphere, the rover being lowered down to the surface of Mars on a tether from its descent stage, the bridle between the two being cut, and the descent stage flying away after rover touchdown!Engineering Constraints for Mars 2020 Mission Landing Site
Engineering constraints on potential 2020 landing sites are based on those derived for the MSL “sky crane” landing system, with some important exceptions.
Below -0.5 km MOLA elevation, with respect to the MOLA geoid.
Within ±30° of the equator./
Like MSL, the 2020 mission has a nominal landing ellipse of about 25 km by 20 km, oriented roughly east-west. A potential improvement under investigation, called range trigger, would allow landing within a 18 km long by 14 km wide ellipse. It may be possible in the future that the range trigger ellipse could become as small as 13 km by 7 km.
Terrain Relief and Slopes:
Less than ~100 m of relief at baseline lengths of 1-1,000 m to ensure proper control authority and fuel consumption during powered descent.
Less than 25°-30° slopes at length scales of 2-5 m to ensure stability and trafficability of the rover during and after landing.
The probability that a rock taller than 0.55 m high occurs in a random sampled area of 4 m2 (the area of the belly pan and area out to the inside of the wheels) should be less than 0.5% for the proposed sites. This corresponds broadly to 7% rock abundance, which is near the mode in the rock abundance for Mars as estimated from thermal differencing techniques. Subsequent analysis indicates the most critical area is just the belly pan of the rover, which covers ~2.7 m2 and can tolerate 0.6 m high rocks, which corresponds to about 12% rock abundance. Because rocks will eventually be counted in HiRISE images, rock abundance could locally be up to 20% provided that the overall risk for the ellipse does not exceed the 0.5% probability level.
The Ka band radar backscatter cross-section must be > -20 dB and < +15 dB at Ka band to ensure proper measurement of altitude and velocity by the radar velocimeter/altimeter of the descent vehicle.
Load Bearing Surface:
Surfaces with thermal inertias greater than 100 J m-2 s-0.5 K-1 and albedo lower than 0.25 and radar reflectivities >0.01 to avoid surfaces dominated by dust that may have extremely low bulk density and may not be load bearing. Surfaces with thermal inertias less than ~150 J m-2 s-0.5 K-1 with high albedo may also be dusty and so should flagged for further investigation.
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