History was made at 1:31am EDT (05:31 UTC) Monday on the fourth planet in this solar system as NASA’s Mars Science Laboratory rover Curiosity descended to a picture-perfect rocket-guided and -slowed descent to a gentle, wheels-first, sky crane touchdown on the surface of the Red Planet. NASA is 14 for 20 in their attempted missions to Mars (a 70 percent success rate) and 4 for 4 in their attempts to land a rover on Mars (a stunning 100% success rate).
History of the Martian Challenge:
From the 1600s to the 1960s, ground-based observations of Mars from Earth provided all the data humanity had on the Red Planet – information provided solely via the use of ground-based telescopes.
It was not until the 1960s and the development of space-based technologies and rockets powerful enough to launch space probes onto trajectories to other planets that up-close exploration of Mars – named after the Roman god of war – began.
Since the advent of space rocket technology, the exploration of Mars has figured prominently in the space programs of the United States, Russia (formerly the Union of Soviet Socialist Republics – USSR), and Europe.
And in 1960, on October 10, the Earth’s first space probe to Mars was launched by the USSR. Named the Mars 1M No.1, the mission ended shortly after liftoff due to a launch failure.
And thus the reality of up-close exploration of Mars was realized.
Since 10 October 1960, 43 dedicated missions (some with more than one mission element) to Mars – be they fly-bys, orbiters, landers, rovers, or sample return missions – have been launched by NASA, Russia/USSR, Japan, ESA (European Space Agency) UK, and China (hitching a ride with a Russian probe).
Of those 43 attempts, 42 of the missions’ fates are known: 15 successes, 4 partial successes, and 23 failures.
One mission’s fate, the Curiosity mission, will be known in large part Monday morning – with final analysis in one Martian year (two Earth years).
Despite a straight-up, world-wide Mars mission success rate of only 34.8 percent, a percentage which increases to 44 percent if partial successes are included, there is great optimism that Curiosity will make it safely to the Martian surface.
First, portions of its landing systems are derived in part from previous NASA missions to Mars and from the Apollo Program. Second, NASA itself has an impressive achievement rate of 13 successes out of 19 attempts when it comes to Mars endeavors.
Overall, NASA’s Martian success rate is an impressive 68 percent, with accomplishments including Earth’s first successful mission to Mars with Mariner 4, the first successful Martian orbiter with Mariner 9, and first completely successful Martian landing with Viking 1, the first successful Martian rover with the Mars Pathfinder mission, and the longest surviving Human technology on another world with the Mars Exploration Rover Opportunity (which has been actively roving across the surface of Mars since 25 January 2004).
Moreover, humankind has attempted to land, to date, five rovers on Mars – two from Russia, and three from the United States.
Of those five rovers, only three have successfully made it to the Martian surface in a manner that allowed them to perform their missions: Mars Pathfinder’s Sojourner, the Mars Exploration Rover Spirit, and the Mars Exploration Rover Opportunity.
All of the successful rovers have been from NASA.
The two Russian/USSR rovers (Mars 2 and Mars 3) in May 1971 encountered a crash landing on Mars and a lander failure – the lander failed 15 seconds after reaching the surface of Mars – respectively.
The United Kingdom’s Beagle 2 mission – which arrived at Mars on 25 December 2003 with ESA’s Mars Express orbiter – was not a rover, but encountered a landing failure on 6 February 2004. Its ultimate fate, aside from the fact that something went wrong, is unknown.
Thus, the Mars Science Laboratory’s rover Curiosity will be the sixth rover to attempt to land on Mars – and NASA’s fourth overall rover sent to the Red Planet.
Curiosity’s arrival at Mars – Entry, Descent, and Landing – Preview:
Curiosity’s arrival at Mars will be one for the record books, not only in terms of the information the rover will be capable of sending back, but also for the simple fact that the rover’s landing at Gale Crater will be one of pin-point accuracy.
For landing, Curiosity will target a landing ellipse area at Gale Crater that is 20km by 7km (roughly 12miles by 4.3miles). This is in striking contrast to the much larger 150 by 20km (93 by 12mile) landing ellipses used for the Mars Exploration Rovers Spirit and Opportunity back in 2004.
To accomplish such a feat, Mars Science Laboratory engineers designed a new high-precision Entry, Descent, and Landing (EDL) system – one that requires six different spacecraft configurations, 76 pyrotechnic devices, the largest supersonic parachute ever designed and manufactured, and more than 500,000 lines of code to execute the required maneuvers to safely ease Curiosity to the surface.
Since Mars’s atmosphere is much thinner than Earth’s, it presents several unique challenges for landing a spacecraft on the surface of the planet. Mars’s atmosphere is too thin for parachutes and aerobraking alone to slow landing spacecraft to a safe speed.
Nonetheless, Mars’s atmosphere is still thick enough to induce instability to a landing spacecraft when deceleration rockets are used (like they will be for Curiosity) to slow a spacecraft’s approach to Mars’s surface.
Thus, Curiosity’s EDL sequence will be challenging. For simplicity’s sake, Curiosity’s EDL sequence can be broken down into four phases: Guided Entry, Parachute Descent, Powered Descent, and Sky Crane landing.
1. GUIDED ENTRY:
Tucked safely inside its aeroshell, Curiosity will begin its descent to the Martian surface folded up inside the protective casing it has been in during its entire cruise to the Red Planet.
To accomplish a stable entry into Mars’s atmosphere, the aeroshell (with Curiosity inside) will separate from the main cruise stage that has been with the rover/aeroshell duo since their launch on an Atlas V rocket on 26 November 2011 10 minutes before atmospheric entry over Mars.
The cruise stage was used during the interplanetary cruise to provide power, communications with Earth, and propulsion for course correction burns.
One minute after cruise stage/aeroshell separation (9mins before atmospheric entry), thrusters on the aeroshell will fire to nullify the aeroshell’s 2-rpm (rotations per minute) rotation that helped stabilize the vehicle during its interplanetary cruise stage.
This thruster firing will also properly orient the aeroshell to place its heat shield into the direction of travel.
Just minutes prior to atmospheric entry, two 165 lb tungsten ballast weights will jettison from the aeroshell, offsetting the aeroshell’s center of mass from the axial centerline and allowing the aeroshell to have lift as it enters the Martian atmosphere.
Made from a phenolic impregnated carbon ablator, the 4.5m (15 ft) diameter heat shield is the largest single-piece heat shield ever flown in space.
For the first four minutes of atmospheric entry, the heat shield will reduce the aeroshell’s velocity through an ablation process.
Ablative heat shields work on the principle of lifting the hot shock layer gas (created by friction from a hypersonic spacecraft’s interaction with the relatively stationary atmosphere) away from the heat shield’s outer wall.
In turn, this creates a cooler boundary layer that provides solid protection against high heat flux through a process called blowing.
The ablation process consequently causes the TPS (Thermal Protection System) layer to char, melt, and sublime through the process of pyrolysis: the thermochemical decomposition of organic material at elevated temperatures without the participation of oxygen.
The gas produced by pyrolysis is what drives blowing and causes blockage of convective and catalytic heat flux – thus protecting the spacecraft’s payload from atmospheric entry heating.
For Curiosity, the aeroshell’s ablative heat shield will provide protection for the rover from the moment of Martian atmospheric entry at a velocity of 5.8 km/s (3.6 miles/s) through an approximate speed of 470 m/s (1,500 ft/s).
The aeroshell’s heat shield will experience peak heating of 3,800 degrees F approximately 1minute 15seconds after entering Mars’s atmosphere.
A maximum deceleration force of 15 times the force of Earth sea-level gravity is expected approximately 10 seconds after peak heating.
Derived from an algorithm from the Apollo Command Modules of the Apollo Program, the reduction in landing precision error for Curiosity is made possible by the use of real-time data of the lifting forces experienced by the aeroshell.
This real-time data will allow the aeroshell’s guidance system to detect dispersions and discrepancies in the entry profile and correct, or “fly out,” those errors to keep Curiosity pointed toward its pin-point landing zone at Gale Crater.
The ability to “fly out” these discrepancies relates to the just-prior-to-atmospheric-entry jettison of the two 165 lb tungsten weights and the resultant off-center trim angle of the aeroshell during atmospheric flight.
Each pair of RCS thrusters will be capable of generating 110 lbf of thrust. Pulsing/Firing of the thrusters will allow the aeroshell to change the direction of its lift and thus steer toward its landing zone.
Toward the very end of the entry sequence plunge through the atmosphere, six 55 lb tungsten weights will be jettisoned from the aeroshell to eliminate the center of mass offset in preparation for supersonic parachute deployment.
The entire atmospheric entry portion of the EDL sequence will span approximately 4 minutes, after which phase 2 of the EDL process will begin.
2. PARACHUTE DESCENT:
Once the aeroshell capsule has slowed to 470 m/s, or 1500 ft/s (roughly Mach 2), Curiosity will be approximately 10 km (6.2 miles) above the surface of Mars.
The use of a supersonic parachute during Martian landings is not new. The Viking landers, Mars Pathfinder’s Sojourner rover, and the Mars Exploration Rovers Spirit and Opportunity all used supersonic parachutes during their respective EDL sequences.
The supersonic parachute that will be used by Curiosity’s aeroshell contains 80 suspension lines and is 160ft long and 52ft in diameter.
It will generate approximately 65,000 lbf of drag on the aeroshell as it descends through the lower portion of the Martian atmosphere.
With the heat shield gone, a camera on the underside of the Curiosity rover will begin acquiring images of the Martian surface at an altitude of 3.7km (2.3 miles) and at a rate of 5 frames per second. The camera’s resolution is 1600×1200 pixels.
This acquisition will continue until Curiosity is safely on the ground.
After 5.1 miles altitude of parachute descent, Curiosity will be just 1.8km (1.1 miles) off the ground of Mars.
At this point, the rover will be descending at a velocity of 100 m/s (220 mph). It is at this point that the third phase of the EDL sequence will begin.
3. POWERED DESCENT:
At an altitude of 1.1 miles and a velocity of 220 mph, the Curiosity rover and its descent stage will literally drop out of the bottom of the aeroshell.
The descent stage, a platform located above the rover (when viewed from the side with the rover’s wheels oriented toward the ground), will provide the final breaking thrust to gently lower Curiosity to the ground of Gale Crater.
This will be accomplished via the use of eight of Aerojet’s variable thrust mono propellant hydrazine rocket thrusters located on the descent stage.
These rocket thrusters, called Mars Lander Engines (MLEs), will produce 700 lbf of thrust each. They were derived from the thrusters used on the Viking landers in the 1970s, again developed by Aerojet, with the Californian company having a long history with NASA’s Mars missions.
The MLEs’ firing sequence will initially veer Curiosity away from the still-descending parachute/aeroshell duo before reorienting Curiosity and quickly slowing its downward, vertical velocity to 4 m/s and nullifying all horizontal movement relative to the ground.
Attitude rates will also be leveled out to properly level Curiosity for landing.
Once the slowing and leveling process is complete, the fourth and final stage of the EDL sequence begins.
4. SKY CRANE:
Once leveling with the ground and achieving a continually decreasing descent rate, Curiosity itself will undergo an operation that no other space vehicle has: A sky crane will lower it from the descent stage to the Martian surface.
Three bridles will control Curiosity’s lowering from the descent stage while an umbilical line will ensure complete communication between the rover and the descent stage.
Once the sky crane lowering process begins, Curiosity will begin transforming from its stowed, interplanetary cruise configuration into its landing configuration
As the bridles come to full extension (25ft below the descent stage), all loads relating to Curiosity will be transferred to the bridles as the descent stage slows itself and Curiosity to a relative velocity to the ground of just 0.75 m/s.
At an altitude of 15m (45ft), Curiosity’s Touchdown Logic software will be initiated and the touchdown velocity of just 0.75 m/s will be reached.
This will begin the constant velocity descent sequence.
The Touchdown Logic will wait two (2) seconds to confirm full weight on wheels for Curiosity while instructing the descent stage to throttle the MLEs to maintain a 0.75 m/s descent rate (even though Curiosity will already be on the ground. This will be done to ensure that all proper backups and safeties are in place and that erroneous touchdown indicators do not result in a loss of the mission).
Once touchdown is confirmed across multiple platforms, control of the landing sequence will be handed from Curiosity to the descent stage, the umbilical lines “dead-faced and cut,” and the bridles pyrotechnically detached from Curiosity.
The descent stage will then assume command of itself, quickly ascend to an altitude of 15m (45ft), pitch over 45-degrees, perform a “fly away” maneuver to clear Curiosity’s vicinity, and then crash land at least 500ft away.
Curiosity is expected to land at Gale Crater at 05:31 UTC (01:31 EDT) – 15:19 LMST (Local Mean Solar Time) at Gale Crater.
EDIT: Amazingly, all the events proved to be successful. Refer to the live threads.
(Images: L2 Content, NASA, JPL)
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