Curiosity on Mars
[The Landing: Another Small Step for Man - A Giant Leap for Robot Kind]    [The Landing Site: Geological Jackpot at Gale Crater]   
[Curiosity: The Escalade of Mars Rovers]    [The Science Payload]    [References and Further Reading]

The Landing
Another Small Step for Man - A Giant Leap for Robot Kind
Written and edited by Bob Keller

Gale Crater

Jubilant Mars Science Laboratory Team Members Celebrate Curiosity's Touchdown Thirty-six weeks after take-off and a 352 million mile voyage from Earth, NASA's Curiosity Mars Rover has successfully landed at Gale Crater on Mars. Curiosity first touched wheels upon a gravelly plain of the Red Planet on Monday, August 6th at 01:32 AM EST after autonomously executing the most complex and unprecedented landing procedure ever attempted by any Mars spacecraft.

Even some mission engineers candidly characterized Curiosity's novel landing procedure as "crazy". The size of a small SUV and nuclear powered, the Curiosity Rover, along with its sample acquisition, processing, and distribution system, and package of science instruments are collectively named the "Mars Science Laboratory" (MSL). During its prime mission lasting one Martian year - nearly two Earth years - Curiosity will examine geologic evidence in Gale Crater for environmental conditions favorable for supporting life and for preserving clues about whether life has ever existed on Mars.

Curiosity traveled to Mars inside its aeroshell, a blunt-nosed cone designed to encapsulate and protect the rover during its deep space cruise, and from the intense heat generated by atmospheric friction as it descended through the Martian atmosphere. The aeroshell capsule separated from the vehicle cruise stage ten minutes before hitting the Martian atmosphere and fired maneuvering thrusters to de-spin from its cruise rate of two revolutions per minute to a three-axis stabilized condition with its heat shield oriented towards its direction of travel.

Mars Science Laboratory Flight System

Six minutes prior to entering the atmosphere, two external, 170 pound tungsten, spin-stabilizing cruise balance masses were ejected from one side of the aeroshell, off-setting its center of mass. It was traveling at 13,000 miles per hour when it hit atmosphere and began decelerating 80 miles above the Martian surface. The effect off-setting its center of mass was to cause the cone-shaped heat shield to assume an approximate 15° angle of attack relative to the aeroshell's flight path, generating aerodynamic lift.

As on Earth, the weather on Mars is not perfectly predictable. A gyroscopic inertial guidance system was employed to detect deviations from the aeroshell's optimum flight path through the atmosphere. A closed loop, feedback-driven control system used the aeroshell's eight 68-pound thrust Aerojet MR-107U maneuvering thrusters to bank it left and right through S-shaped curves to modulate and vector the lift, executing course corrections and additionally slowing it to the targeted velocity and position for parachute deployment. This guided flight capability distinguishes Curiosity from its predecessors and enables it to "fly out" small imperfections in its entry angle and aerodynamic performance, and to compensate for atmospheric variables working to tug the descent path off target, such as unpredictable variations in winds and fluctuations in atmospheric density.

Guided hypersonic flight significantly improved the accuracy of Curiosity's 3-sigma (99% probability) ellipse-shaped landing footprint over that of its predecessors to only 12 miles by 4 miles. By comparison, the 3-sigma landing ellipse for the 2008 Phoenix Mars Lander's unsteered, ballistic entry was 62 miles by 12 miles. Curiosity's greatly increased landing accuracy allowed scientists and mission planners to consider exploring geologic features embedded in hazardous terrains presenting unacceptable risk to less accurate predecessors, and was a key factor in the selection of Gale Crater as Curiosity's mission site, and Curiosity's chances of making a successful landing there. Comparison with Other Mars Lander Footprints

The Phenolic Impregnated Carbon Ablator covering the aeroshell's heat shield had to insulate Curiosity and its descent stage from the hottest temperatures and heating ever experienced by a Mars mission payload. 85 seconds after entering the Martian atmosphere, temperatures on the aeroshell's heat shield peaked at 3800 °F - much hotter than the melting point of basalt lava or steel. Peak deceleration, exceeding 11 g, occurred ten seconds later. Dynamic pressure from the atmosphere subjected the 15 foot diameter heat shield - the largest to ever travel to another planet - to over 100,000 pounds of compressive force.

Four minutes after entering the atmosphere, the aeroshell performed a "Straighten Up and Fly Right" (SUFR) maneuver by sequentially ejecting six internal, 55 pound tungsten entry balance masses over a period of several seconds from the side opposite the previously ejected ballast, restoring a symmetrical center of gravity and gradually returning the heat shield to a zero degree angle of attack. 15 seconds later at six miles above the Martian surface and a velocity of 1000 miles per hour, the largest supersonic parachute ever designed by NASA was mortar deployed to further slow the aeroshell's rate of descent. The parachute survived a 65,000 pound peak load during its deployment shock.

The parachute rapidly slowed the aeroshell from supersonic to subsonic velocities. 20 seconds after parachute deployment at five miles above the surface and a velocity of 500 miles per hour, the descent imaging camera aboard Curiosity was activated and pyrotechnic devices fired, releasing springs that separated and pushed the heat shield free of the aeroshell to fall away to the Martian surface. The heat shield was allowed to fall away for eight seconds before the no longer obscured landing radar was activated to begin making critical measurements of the range to the Martian surface. The parachute-braked descent phase lasted for only 2 minutes, but during this interval the parachute and Martian atmosphere dissipated 95% of the remaining kinetic energy.

Ten seconds prior to separating from the backshell and parachute, the descent stage primed its eight 800-pound thrust Aerojet MR-80B hydrazine fueled descent engines. At an altitude of one mile, a velocity of 225 miles per hour, and one minute from touchdown, pyrotechnics fired, separating the descent stage from the backshell and parachute. The descent stage carrying Curiosity then fell free under Martian gravity while purchasing separation from the still overhead backshell and parachute. While falling, it warmed the descent engines for 0.2 seconds at reduced throttle. Once warmed, the descent engines were powered up to full throttle and they began burning through the descent stage's precious 860 pound supply of hydrazine fuel in earnest.

About 30 seconds and half its fuel were required for the descent stage to finally halt its remaining horizontal motion and reduce its vertical descent rate to about 45 miles per hour, during which time it also executed an avoidance maneuver to gain a position more safely away from the still descending backshell and parachute. After arresting its horizontal motion, the descent stage continued to descend at 45 miles per hour while ranging the surface with six independent beams from its pulse Doppler radar refining its altitude measurements.

Upon reaching an altitude of 150 feet the descent stage increased descent engine power and smoothly decelerated to a descent rate of about 1.5 miles per hour, the speed of a relaxed walk. 20 seconds prior to touchdown at an altitude of 60 feet, the descent stage idled four of its eight descent engines and adjusted power on the four still thrusting engines, continuing its 1.5 mile per hour descent toward the surface.

Because Curiosity is too heavy for its landing to be cushioned with air bags as were used on previous Mars rover missions, a novel and unprecedented sky crane type maneuver was employed to deliver it onto the Martian surface. A fully rocket-braked descent could not be used all the way to the surface because the engines would have raised a massive, mechanism and instrument damaging cloud of dust.

Descent Stage Bridle Device Assembly for Deploying Curiosity's Landing Tether

The photo at right details the Bridle Umbilical and Descent Rate Limiter (BUD) on the descent stage by which Curiosity was lowered to the Martian surface. The assembly is about two feet from top to bottom and has two main components. The cylinder on the top is the descent brake. The conical-shaped mechanism below it is the bridle assembly.

Three nylon cables of the bridle, attached to the rover at three points, are spooled around the BUD, with enough length to lower the rover about 25 feet below the descent stage during the sky crane maneuver. Also spooled around the bridle assembly was a slightly longer umbilical, carrying data and power connections between the rover and the descent stage. Curiosity's main computer controlled the aeroshell and descent stage throughout the entry, descent and landing through the umbilical cable.

12 seconds prior to touchdown at an altitude of 40 feet, pyrotechnic devices fired to sever the rigid connection between Curiosity and the descent stage, allowing gravity to pull Curiosity down as the tether unspooled. The rotation rate of the spool and descent rate of the rover were governed by gear boxes and banks of mechanical resistors inside the descent brake.

Curiosity deployed its six wheels as it was gently lowered and deposited upon the Martian surface at the end of the tethering bridle and umbilical cable. Upon touching down, pyrotechnic devices separated Curiosity first from its tether and then the umbilical cable. Once the tether and unbiblical were severed, springs inside the bridle assembly quickly retracted them and the descent stage then powered away to impact at a safe distance from the rover.

Curiosity's novel sky crane maneuver could not be fully tested and vetted on Earth, primarily due to the significant difference in density between Earth's and Mar's atmospheres. Due to the distance between Earth and Mars at the time of Curiosity's landing, radio signals required nearly fourteen minutes to traverse the void one way.

The entire landing procedure had to precisely execute in just over seven minutes from the time the lander hit Martian atmosphere to the moment Curiosity's wheels contacted Martian soil. Curiosity had actually touched down seven minutes before MSL mission controllers on Earth received radioed telemetry reporting it was entering the outer atmosphere of Mars.

Landing Positions of Curiosity, Heatshield, Backshell, Parachute and Descent Stage

Because the extremely complex and critical sequence of events required to successfully land Curiosity on Mars could not be controlled from Earth in real time, the landing was of necessity under autonomous control of Curiosity's on-board computers and software.

The landing involved six different vehicle configurations, the largest aeroshell and heat shield to ever enter the Martian atmosphere, the first ever guided lifting entry at Mars, the largest supersonic parachute ever deployed on Mars, a daring, never-before employed sky crane maneuver, 76 pyrotechnic devices, and over 500,000 lines of computer code. There were thousands of ways for it to catastrophically fail and just one way to get it right.

Curiosity soft-landed itself along with the largest, most powerful scientific payload to ever reach the surface of Mars, and it did so with flawless text-book precision.

[The Landing: Another Small Step for Man - A Giant Leap for Robot Kind]    [The Landing Site: Geological Jackpot at Gale Crater]   
[Curiosity: The Escalade of Mars Rovers]    [The Science Payload]    [References and Further Reading]


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Bob Keller