August 5, 2015 – NASA’s Curiosity Rover, built by the Jet Propulsion Laboratory, is celebrating three years on Mars. Since landing in Gale Crater at 9:32 p.m. MDT on August 5, 2012, Curiosity has been exploring the Red Planet as part of the Mars Science Laboratory mission.
Curiosity carries the largest, most advanced suite of instruments for scientific studies ever sent to the martian surface. The rover is analyzing samples scooped from the soil and drilled from rocks to determine the planet’s climate and geological history. Its data will determine whether or not the planet could have supported life in the past. Curiosity also gives us tantalizing glimpses of another planet, using on-board cameras to send back images of the world it sees.
NASA launched the Mars Science Lab Curiosity mission on November 26, 2011. Liftoff was from Cape Canaveral Air Force Station aboard a United Launch Alliance Atlas V rocket.
The entry, descent, and landing (EDL) phase began when the spacecraft reached the Martian atmosphere, about 78 miles (125 kilometers) above the surface, and ended when the rover landed on the surface of Mars. The EDL would last seven minutes and determine the fate of the entire mission.
EDL included a combination of technologies inherited from past NASA Mars missions, as well as new technologies. The sheer size of the Mars Science Laboratory rover (over 2,000 pounds or 900 kilograms) precluded it from taking advantage of the type of airbag-assisted landing used on past Mars missions.
Mission engineers pioneered precision landing techniques that included steering the spacecraft as it flew through the atmosphere before deploying a parachute, then using a rocket-powered “sky crane” system to land the rover. The new techniques enabled a controlled landing within a drop zone four times smaller than previous missions: 4 miles by 12 miles (about 7 kilometers by 20 kilometers).
Lockheed Martin Space Systems in Denver, Colorado, designed and built the aeroshell that encapsulated the Curiosity rover during its deep space cruise to Mars, and protected it from the intense heat and friction generated as the rover roared through the Martian atmosphere at about 13,000 miles an hour. Perfect performance of the aeroshell was vital to the mission.
Lockheed Martin designed and built nearly every capsule flown by NASA for deep space exploration since Apollo, but none were as large as the Mars Science Laboratory aeroshell at about 15 feet (4.5 meters) in diameter. For comparison, the heatshields of the Spirit and Opportunity Mars Exploration Rovers measured 8.5 feet and the Apollo capsule heat shields measured less than 13 feet (at 16.4 feet or 5 meters in diameter, the Orion heat shield is now the largest heat shield ever built.)
“[A] major challenge on the MSL program was just the sheer size of the aeroshell. It was very large – it was much larger than what we had done in the past, so the logistics on handling it, the fabrication of it, all provided a challenge,” said Rich Hund, MSL Aeroshell Program Manager, Lockheed Martin.
The MSL aeroshell was comprised of two parts:
The backshell was half of the large and sophisticated aeroshell capsule. In addition to protecting the rover during cruise and descent, the backshell provided structural support for the parachute and unique sky crane, a system that lowered the rover to a soft landing on the surface of Mars.
The biconic-shaped backshell was made of an aluminum honeycomb structure sandwiched between graphite-epoxy face sheets. It was covered with a thermal protection system composed of the cork/silicone super light ablator (SLA) 561V that originated with the Mars Viking landers of the 1970s.
The heatshield was the forebody or ‘wind’ facing part of the aeroshell system during entry. Because of the unique entry trajectory profile, external temperatures reached about 3,800 degrees Fahrenheit. The heatshield used a tiled Phenolic Impregnated Carbon Ablator (PICA) thermal protection system instead of the Mars heritage (SLA)561V. This was the first time PICA had been flown on a Mars mission.
Because of its large size, the aeroshell experienced tremendous entry loads primarily as a result of dynamic pressure from the atmosphere. More specifically, the heatshield was subjected to approximately 105,000 pounds of compressive force distributed across its surface. It was important for the two parts of the MSL heatshield to work together as the front recessed, creating thousands of pounds of bending, which the back half had to react to.
The heatshield carried an intricate array of engineering sensors called the MSL Entry Descent and Landing Instrumentation (MEDLI) that measured heatshield temperatures, surface recession and atmospheric pressures. MEDLI was developed by NASA Langley Research Center and NASA Ames Research Center.
Entry, descent and landing sequence
As the MSL spacecraft approached the Martian atmosphere, an autonomous onboard computer program began conducting commands to thrusters, systems and sensors.
Just prior to atmospheric interface at Mars, the aeroshell turned so its heatshield faced forward along the direction of travel, then ejected two 178-pound weights to shift the center of mass of the capsule. The shift enabled the capsule to generate lift as it flew through the atmosphere, allowing roll control and autonomous steering to guide it to a precise landing spot.
Peak heating occurred about 75 seconds after atmospheric entry, when the heat shield temperature reached about 3,800 degrees Fahrenheit. Peak deceleration occurred about 10 seconds later, with maximum deceleration forces as high as 15 Gs.
After MSL finished its guiding entry maneuvers, and a few seconds before the parachute deployed, the back shell jettisoned another set of weights to shift the center of mass back to the axis of symmetry, rebalancing the spacecraft for the parachute portion of the descent.
At an altitude of about seven miles above the surface and a velocity of about 900 miles per hour, the 51-foot diameter parachute deployed.
Twenty-four seconds later, the heatshield separated and dropped away with the spacecraft at an altitude of about five miles and still traveling at a velocity of about 280 miles per hour.
At heatshield separation, the Mars Descent Imager began recording five images a second, looking in the direction the spacecraft was flying. The rover and its descent stage were still attached to the back shell on the parachute. Radar on the descent stage began collecting data about velocity and altitude.
About 85 seconds after heatshield separation, the back shell, with parachute attached, separated from the descent stage and the rover. Just a mile above the ground, and falling at 180 miles an hour, eight retrorockets on the descent stage began firing.
Abruptly decelerating to 1.7 miles per hour, nylon cords began to spool out to lower the rover from the descent stage in the “sky crane” maneuver. The rover’s wheels and suspension system, doubling as landing gear, rotated into place just before touchdown.
When Curiosity sensed touchdown, the connecting cords severed and the descent stage flew out of the way, coming to the surface some 500 feet from the rover’s position.
Soon after landing, Curiosity’s computer switched from EDL mode to surface mode and initiated autonomous activities for its first Martian day on the surface of Mars, Sol O.
During EDL, two other Mars spacecraft – Mars Odyssey and the Mars Reconnaissance Orbiter (MRO) – both built and operated for NASA by Lockheed Martin Space System – monitored transmissions from the Mars Science Laboratory. Odyssey received telemetry directly from MSL and sent it to Earth in near-real time (light time delay is 13.8 minutes), while MRO recorded the landing telemetry and transmitted it back to JPL an hour later.
Engineering Curiosity’s soft landing on the surface of Mars represented a huge step in Mars surface science and exploration capability because it:
Demonstrated the ability to land a very large, heavy rover on the surface of Mars, which could be used for a future Mars Sample Return mission that would collect rocks and soils and send them back to Earth for laboratory analysis.
Demonstrated the ability to land more precisely in a 20-kilometer (12.4-mile) landing circle. This high precision delivery will open up more areas of Mars for exploration and potentially allow scientists to roam “virtually” where they have not been able to before.
Demonstrated long-range mobility on the surface of the red planet (5-20 kilometers or about 3-12 miles) for the collection of more diverse samples and studies.