The Design and Engineering of Curiosity

by Emily Lakdawalla

  A year after Mars Polar Lander and Mars Climate Orbiter failed, NASA announced a reformulated Mars program.1 Their goal: to search Mars’ geologic present and past for the kinds of environments that could support life. The search would require a “sustained presence in orbit around Mars and on the surface with long-duration exploration.” Joining Mars Global Surveyor in orbit would be two orbiters, 2001 Mars Odyssey (to be launched in 2001) and Mars Reconnaissance Orbiter (2005). NASA also announced two rover missions: the twin Mars Exploration Rovers (2003) and a “mobile science laboratory,” to be launched “as early as 2007,” which would eventually become Mars Science Laboratory, or MSL.

  From the start, MSL was an ambitious mission. It would deliver a Viking-sized suite of science instruments to the surface of Mars. But that huge science capability could move around the surface on wheels. NASA promised a precision landing, close to a very interesting geologic site on the surface of Mars. They also proposed a lifetime of two Earth years, much longer than the proposed one-month life for Pathfinder or three months for the Mars Exploration Rovers. Finally, the intent to carry analytical laboratory instruments that could ingest Martian rock required entirely new sample handling technology.

  MSL occupies a pivotal position in NASA’s Mars Exploration program. An advisory group stated in 2003 that MSL “both concludes the currently planned missions and…initiates the paths of exploration in the next decade.” Mindful of the number of Mars missions that would be active in the years prior to its landing, NASA tasked the project with being able to respond to discoveries made while the spacecraft was being prepared for launch.2 To be so flexible, the mission had to be able to achieve success at a wide variety of landing site locations: from equatorial sites to near-polar ones, and from sites where ancient geology and hard rocks would be the target, to sites where it might be possible to sample ice and search for recently habitable zones. This wide envelope of possibility meant that the spacecraft and landing system that were ultimately built had capabilities that were never used.

  MSL would eventually become the most complex mission ever launched beyond Earth. Its development required a gargantuan effort spanning more than a decade. Its success depended on the invention of new technologies. Challenges in the development program forced NASA to delay the launch, at great financial cost. Originally proposed for the 2007 launch opportunity, MSL would finally depart for Mars in November, 2011.

  1.2 DESIGNING A BIGGER LANDER (2000–2003)

  1.2.1 “Rover on a Rope”

  Chief engineer Rob Manning traces the origin of MSL’s landing system to the terrible failures of 1999, particularly Mars Polar Lander. “We came to realize that we did not know how to land anything on Mars reliably, let alone something large,” he wrote in a 2014 mission memoir.3 NASA’s Jet Propulsion Laboratory (JPL), which had built Mars Polar Lander, formed a team to identify the technology they needed to develop in order to be able to precisely land a large rover on Mars. They began work in early 2000.

  Mars is one of the hardest places in the solar system to land. The problem is its atmosphere: there is too much to ignore, and too little to slow a spacecraft for a safe landing. On bodies lacking atmospheres, like the Moon or an asteroid, spacecraft land using rockets alone. On Earth, Venus, or Titan, which have dense atmospheres, a spacecraft decelerates from supersonic speeds with a blunt-nosed heat shield, and then drops speed nearly to zero with a parachute. On Mars, a spacecraft needs all three: heat shield for high-speed entry, parachute for slowing during descent, and rockets for landing. The entire procedure required to land on Mars is referred to as Entry, Descent, and Landing, or “EDL” for short. (Engineers delight in abbreviating frequently-used phrases into acronyms and initialisms, turning their writing into alphabet soup. In this book I refrain from using most such abbreviations for clarity.)

  All Mars landers to date have used a capsule, also known as an aeroshell, to shelter the lander during entry; the capsule is a clamshell that consists of a heat shield and a backshell. The design is similar to the capsules used by Mercury, Gemini, and Apollo astronauts to return to Earth. Astronauts in capsules usually used maneuvering rockets to guide the capsules during entry, steering them toward a landing zone where they could be picked up quickly. Mars landers, lacking human pilots, passively fell through the Martian atmosphere on a ballistic entry, like meteors. The lack of human guidance led to large uncertainty about where the spacecraft would end up landing. Achieving a precision landing required guidance, but Mars is too far away for humans on Earth to steer in real time.

  To make a precision landing possible, Manning and his teammates advanced an idea that JPL had been developing since the 1990s: autonomous guidance for a Mars entry vehicle. The capsule could use accelerometers and gyroscopes to determine its position relative to its intended target as it flew. Software would command banking turns to fly the aeroshell closer to the target. Guided entry could dramatically shrink the size of a Mars landing ellipse, placing a rover closer to interesting geology.

  The descent phase begins when the spacecraft has been slowed to something close to twice the speed of sound. All Mars landers have deployed a parachute for descent. Supersonic parachutes for Mars were first developed in the early 1970s for Viking, with expensive high-altitude tests. As long as the mass of a Mars lander could be kept similar to or less than that of Viking, they could stick with the same parachute design for the descent phase without performing new, expensive tests.

  For the final, landing phase, JPL had successfully used two different approaches. The Vikings employed retrorockets that slowed the descent to a near-standstill, and then the spacecraft dropped to a hard landing atop three legs that crushed to absorb some of the force of the impact. Pathfinder (and, later, the Mars Exploration Rovers) worked differently (Figure 1.1). The triangular lander was folded into a tetrahedral shape and the outside of the tetrahedron fitted with airbags. This contraption dangled on a rope beneath a rocket pack that was itself connected to the parachute. At the last possible moment, a mere 100 meters above the ground, the airbags inflated, the rocket jetpack fired to zero out the downward velocity, and the rope tether cut. The lander dropped and bounced repeatedly, rolling nearly a kilometer inside its airbags, before finally coming to a rest.

  Figure 1.1. Illustration of the successful Mars Pathfinder entry, descent, and landing. Based on Golombek et al ( 1999 ) .

  Neither lander design would work for MSL. If the rover were perched atop a Viking-like lander platform, the top-heavy design would tip over in a wide variety of landing scenarios. But Pathfinder’s airbag design had a maximum payload capacity of 200 kilograms; anything larger, and the airbags would shred.4 Manning thought that elements of the two could be combined into a successful landing strategy. If a Viking-like descent stage could dangle a Pathfinder-like lander on a tether, the descent stage might be able to lower the lander close enough to the ground to enable it to make a soft touchdown.5 In fact, they might be able to make the landing so soft that they could put a rover down directly on its wheels.6 Manning called this idea the “rover on a rope.” The concept became Mars Smart Lander in late 2000, when NASA announced it as part of the reformulated Mars program, with a launch “as early as 2007.”7

  As the reformulation proceeded, Mars Global Surveyor generated a bounty of science results. Its spectrometer instrument discovered gray hematite on the surface, a mineral that probably required liquid water to form. The spectrometer also mapped dust on the surface, allowing mission planners to seek out less-dusty landing sites with good access to bedrock. Its sharp-eyed camera proved that sedimentary rocks existed on Mars, a second line of evidence to a lengthy water-rich geologic history. And the mission generated a dramatically improved global topographic map of Mars, crucial for planning safe landings.

  1.2.2 Mars Smart Lander

  NASA chartered a Science Definition Team for the planned 2007 rover in April, 2001.8 The charter identified three ways in which the Mars Smart Lander concept would improve on past
landers’ ability to explore interesting scientific sites. Most of them related to landing precision, specified in the dimensions of a “landing ellipse.”

  What is a landing ellipse? Mars missions target a specific latitude and longitude spot on Mars, but a variety of factors can cause the lander to miss the target. By modeling these factors, engineers can estimate the area within which the rover is about 99% likely to land. The region is usually shaped like an ellipse with its long axis oriented in the direction of the incoming lander’s trajectory.

  Landing ellipses for Viking were 280 kilometers long and 100 kilometers across. Pathfinder’s was smaller, but not by much, at 200-by-100 kilometers. Large landing ellipses drastically limited the locations on Mars where spacecraft could land, because there are few locations that are flat enough over such a broad area, and even fewer that are geologically interesting.

  For Mars Smart Lander, the landing ellipse would be dramatically smaller: the initial directive was for an ellipse only 6-by-3 kilometers in extent, achieved using entry guidance to steer the entry capsule along its intended path. The charter also stipulated a lander with “active terminal hazard avoidance,” meaning that it should be capable of detecting large rocks or steep slopes and steering around them. Finally, the rover would have “surface mobility commensurate with landing precision errors.” In other words, if the landing ellipse was 6 kilometers in extent, then the vehicle should be able to drive at least 6 kilometers in its lifetime.

  It’s that last requirement – a roving range of the same size as the landing ellipse – that opened up the possibilities for exciting science on the proposed rover mission. The mission would not be limited to scientific exploration of sites that were also safe for landing. They could plan to explore a site with steep topography, as long as there was a safe landing zone sufficiently close by. They called these “go-to” sites, because the rover would land away from the intended scientific goal, and then go to the site before starting its scientific investigation.

  NASA directed the Mars Smart Lander science definition team to set science goals consistent with the highest priorities of the Mars Exploration Payload Analysis Group, an advisory panel of Mars scientists. The number one goal of the Mars Exploration Payload Analysis Group was the search for present and past life on Mars, so the team debated whether the mission should attempt to search for extant life on Mars.

  In the end, the Science Definition Team argued against Viking-like attempts at direct life detection experiments. Emboldened by the recent discovery of widespread layered sedimentary rocks across Mars by the Mars Global Surveyor camera team,9 they suggested an oblique approach that avoided the challenge of defining what life on Mars is expected to look like:

  The most promising place to explore for evidence of life on Mars is in lacustrine or marine sedimentary rocks that accumulated rapidly under reducing conditions and where subsequent diagenesis did not obliterate the original textural and compositional (isotopic, organic, and mineralogic) evidence for the environment of deposition and associated biomes…[The] strategy for searching for evidence of life on Mars is to maximize the probability of landing on sedimentary deposits in which reducing conditions have been preserved, to use mobility to explore and characterize the deposits…

  Direct life detection experiments are not needed to implement this strategy for the Smart Lander Mission. Rather, positive signs of biosignatures would be used to help focus locations for sample return missions and/or follow-on missions with direct life detection experiments.10

  Through all of the twists and turns of the development of the mission that followed, this strategy would remain constant. The strategy has two parts: first, search for habitable environments, places where life could thrive (now or in the past). Second, seek out rocks that have a high potential to preserve carbon-containing materials trapped within them.

  The Science Definition Team responded to the charter in October 2001. Mars Smart Lander would take one of two forms. It would either be a Mobile Geobiology Explorer – a large rover that could carry a heavy instrument package beyond the confines of its landing ellipse – or a Multidisciplinary Platform with a deep drill and a small rover that could explore the site and return samples to the stationary lander.

  As initially conceived, the Mobile Geobiology Explorer would carry a 100-kilogram science payload, powered either by solar panels or a radioisotope power supply, although the team argued strenuously for the latter. They suggested that in a 180-sol11 primary mission, the rover should be able to traverse at least 5 kilometers and preferably 9 kilometers, to perform in-situ science at 3 locations, sampling multiple geologic units. (In hindsight, this list is comically optimistic.) The team proposed a payload consisting of up to 14 different science instruments:A descent imaging system.

  A mast-based remote sensing system including color cameras, infrared spectrometer, and a laser-induced breakdown spectrometer.

  Ground-penetrating radar.

  Arm-based contact science package with rock abrasion tool, elemental and mineralogical analyzers, and microscope.

  Long-duration radiation experiments (relevant to future human exploration).

  Drill/corer and sample acquisition system.

  Sample preparation and delivery system (for grinding and partitioning sample cores).

  Laboratory instruments to determine inorganic and organic chemistry, oxidation state, mineralogy, and high-resolution images of samples.

  If possible: seismology package.

  If possible: climatology package.

  Meanwhile, JPL was in the throes of preparing the Mars Exploration Rovers for launch. To cope with the ever-increasing mass of the twin rovers, JPL added throttleable rockets to their backshells, and cameras that would take one or two pairs of images and analyze them to detect the horizontal velocity of the lander. Both of these innovations made the “rover-on-a-rope” idea more feasible.12

  Even though it was still on the drawing board, Mars Smart Lander rapidly ran into budget problems. “The Science Definition Team had defined a mission larger than NASA could afford,” recalls Mark Dahl, who was NASA Program Executive for the mission from 2002 until 2007. In order to fit this large rover into NASA’s budget, they would need to postpone it to a 2009 launch. The mission also drifted toward a name change. When Scott Hubbard developed NASA’s “follow the water” policy in 2002, he referred to the mission in different places as Mars Smart Lander; Mobile Surface Laboratory; and Smart Mobile Lab. Eventually, NASA decided that the name of the mission should describe its goals rather than its technology, and by 2003 it was being called Mars Science Laboratory. (Conveniently, its initials, MSL, remained the same through the name change.)

  1.2.3 Nuclear power

  In 2002, NASA determined that MSL would be able to do better science, accessing a wider band of latitudes and surviving longer, if it were nuclear-powered. That required a radioisotope thermoelectric generator (RTG), like the ones that powered Voyager, Viking, and more recently, Galileo and Cassini. A nuclear-powered rover would have lots of advantages over the solar-powered Spirit and Opportunity. It would be able to explore a much wider range of latitudes, and it would be able to operate year-round, rather than resting through the winter. However, the nuclear power design available in 2002 – the General Purpose Heat Source RTG used for Galileo, Ulysses, Cassini, and New Horizons – was not suitable for a Mars rover. It was too massive (more than a meter long and weighing 57 kilograms). It produced more power than needed (285 watts). Most importantly, its electricity-generating thermocouples would fail if carbon dioxide from Mars’ atmosphere were to infiltrate its container.

  Anticipating these problems, the Department of Energy and NASA were already in discussions to develop a new type of radioisotope power supply that would be appropriately sized for the lower mass and power of modern spacecraft, one that could also function in an atmosphere. The Department of Energy considered several designs and determined to develop two. One was the Multi-Mission Radioisotope Thermoele
ctric Generator (MMRTG), whose design would be based upon the RTG used on Viking lander and Pioneer missions. It would require 4.8 kilograms of plutonium dioxide fuel. The other proposed power source was a Stirling generator requiring only 1.2 kilograms of fuel. Either would deliver about 100 watts of power when first fueled. An MMRTG would throw off about 2000 watts of heat; the more efficient Stirling generator would produce about 500 watts.

  NASA considered both options for MSL. They chose the MMRTG because of concern over the reliability of the Stirling generator’s moving parts. Also, the relatively inefficient design of the MMRTG would benefit Mars surface operations: the waste heat could be collected and put to use to maintain the temperature of the rover against the extreme swings of the Martian environment. On June 30, 2003, Boeing Rocketdyne Propulsion and Teledyne Energy Systems announced their partnership with the Department of Energy to develop the new MMRTG, specifically naming MSL as the first mission that would use the new technology. “An MMRTG-powered rover will be able to land and go anywhere on the surface of Mars, from the polar caps to deep, dark canyons, and will safely provide full power during night and day under all types of environmental conditions,” Boeing stated in a press release.