In This Issue
Fall Issue of The Bridge on Space Exploration
September 1, 2021 Volume 51 Issue 3
Close collaboration between engineering and science has enabled marvels of space exploration over decades. Eight exemplary missions are described in this issue, conveying the excitement, challenges, and breakthroughs involved in efforts to better understand the wonders and mysteries of this solar system.

Science and Engineering Collaboration in the Design and Operation of the Curiosity Mars Rover

Wednesday, September 15, 2021

Author: Rob Manning and John Grotzinger

Communication, cooperation, and trust between scientists and engineers have ensured the success of Curiosity’s mission to Mars.

Exploration is the sine qua non of all scientific discovery and by definition must embrace the unknown and the unexpected, both good and bad. Scientific discovery also often depends on engineering design and technical development of the instruments and systems needed for observations and analysis. The robotic systems designed to explore Mars required extensive collaboration and cooperation between engineers and scientists to build, test, and use them to achieve the mission’s scientific goals.

Engineers and scientists are distinguished by vastly different objectives and practices. Success to an engineer and an engineering manager is specific: design, build, test, and deliver the machine to exacting functional requirements and interfaces on time and on budget. Success to a scientist assumes that the machine not only will operate as expected and meet all requirements but can be adapted to discoveries that may require new capabilities to test new hypotheses. Thus even if the engineers perform perfectly, a failure of their imagination to anticipate and adapt to the needs of discovery can lead to disappointment on a long-term science mission.

Here we describe collaborative interactions between engineers and scientists who worked to make the Mars Science Laboratory (MSL) Project’s Curiosity rover mission successful.


Developed at NASA/Caltech’s Jet Propulsion Laboratory in Pasadena, CA, Curiosity’s primary mission was to assess the geochemical potential for the habitability of Mars. This mission required it to acquire and process rock, soil, and air samples using many new instruments and two highly complex onboard sample analysis laboratories (figure 1). To find habitable locations, the rover had to operate for at least one Mars year (~687 Earth days) and be capable of driving up to 20 km while also collecting and processing as many as 74 rock and soil samples so that diverse sites over a wide area could be assessed.

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FIGURE 1 Mars Curiosity rover, front view. Annotated “selfie” taken October 11, 2019, using the Mars hand lens imager (MAHLI) camera on the end of the robotic arm (inset). APXS = alpha particle x-ray spectrometer; ChemCam = chemistry and camera; CheMin = chemistry and mineralogy; CHIMRA = Collection and Handling for In-Situ Martian Rock Analysis; DAN = dynamic albedo of neutrons instrument; MARDI = Mars descent imager; MMRTG = multimission radioisotope thermoelectric generator; REMS = Rover Environmental Monitoring Station; SAM= sample analysis at Mars. Image credit: NASA/JPL-Caltech/MSSS.

Initially set to launch in 2009, the engineering team had to design a Mars rover that was 1.7 times larger and 5 times more massive than its predecessors, Spirit and ­Opportunity. The larger scale and new objectives meant that the team had to develop or advance a number of technologies (table 1), all of which needed to be invented and developed in time for the 3-week October 2009 Mars launch window.

Manning and Grotzinger table 1.gif

One particular problem was the question of how to deliver a 1-ton rover ­safely to the surface of Mars. The challenges of landing heavy payloads on Mars were well known (Braun and Manning 2007) so it was appreciated early on that new technologies were needed. Technology development resources were brought to bear on some of these problems as early as 2002.


We describe three challenges that the science and engineering teams successfully navigated during MSL development and operations.

Energy Availability

How to satisfy the rover’s extraordinary energy demand was a key engineering challenge. Solar power throughout the Mars year is problematic because of low sun angles in the winter months and the continual fallout of atmospheric dust. Furthermore, several science instruments were best suited to operate at night.

The most reliable and predictable option that could guarantee a Mars year or more was a multimission radioisotope thermoelectric generator (MMRTG), which converts heat from the natural decay of ­plutonium-238 materials into a steady 100–115 W of electrical power. This would not be enough power to operate Curiosity—just sitting with only its computer turned on takes 100 W; while driving, using its drill, or using the radio, the instantaneous power use can be 350 W or higher.

The solution was to operate the rover off a rechargeable battery whose state of charge could be maintained by “trickle charging” from the MMRTG. Over the course of a 24.65-hour Mars day (called a sol), at a steady 100–115 W the MMRTG could produce 2500–2700 W-h of energy for driving, drilling, sample processing, communications, heating, and operating the science instruments. Given the large power demands when the rover was active, Curiosity was designed to “sleep” as much as possible: turn itself off, “wake” only to work a few hours (e.g., doing science, driving), and spend the rest of the sol “asleep” to allow the MMRTG to recharge the battery.

The science team reinvented
their operations plans from
scratch as the engineers worked to rewrite the
energy requirements.

The energy constraints required close collaboration between the engineering and science teams to assign power allocations. Early in the design phase, from the 2600 W-h of power the science instruments were collectively allotted about 250 W-h for use in a single sol. In 2006, provided there were no surprises, analyses indi­cated that this allocation could be met on average. Extra margin in the design could be used to handle sols when an individual instrument needed a bit more energy.

The rover uses many actuators for the wheels, arm, mast, and high gain antenna movement required to operate in the cold mornings and winter days on Mars. Actuator life testing was initiated in 2007 to ensure that the many moving parts could survive the many hundreds of hours operated at very cold temperatures (between −100°C and −55°C). To the surprise of everyone, the actuators failed catastrophically because of dry lubricant migration at low temperatures, increasing wear on the titanium gear teeth. There was no time to redesign new cold actuators, so the team decided to adapt tradi­tional actuators using “wet” lubrication and stainless steel gears. This meant that the actuators would now need to be electrically heated. Around the same time, tests of the laboratory ovens used to heat samples and release volatiles (including organic compounds) showed that operating that instrument alone would need up to 480 W-h per sol—far more than the 250 W-h allocated for all of the science instruments combined.

Suddenly the extra energy margin had gone seriously negative and, with only a year and a half to the launch date in late 2009, the team was convinced there was simply not enough energy to do the sample science. By late 2008 the team reluctantly threw in the towel for a 2009 launch date. The next launch opportunity to Mars was 22 months later. While there were other delays, the energy challenge was a contributing factor to the mission launch delay from 2009 to 2011.

With the additional 2 years, the team solved the energy problem. The battery was doubled in size to about 1200 W-h, which alleviated some of the collective power usage concern, but a bigger battery alone wasn’t enough.

To further reduce energy usage, redundant avionics were rewired so that only half the circuits needed to be on at a time. The science team also reinvented their operations plans from scratch as the engineers worked to rewrite the requirements.

In addition, the engineers and science team reimagined how each day would proceed. After a particularly energy-intensive sol, the following sol had to be allocated to minimal activities that provided ample time for the MMRTG to recharge the battery.

Selection of the Best Landing Site

Finding a site that is both scientifically promising and safe for landing is a universal challenge for the engineers and scientists of all rover missions. Scientists invariably want to land on or adjacent to rocky outcrops, which are prime targets for study but hazardous topography. Entry, descent, and landing (EDL) engineers prefer a safer flat surface.

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FIGURE 2 Gale Crater landing ellipse comparison. MSL’s first use of aerodynamic entry guidance at Mars borrowed from Apollo. Guidance can compensate for Mars’ very large atmosphere density variation, greatly reducing the (3-sigma) uncertainty from that of prior Mars missions. This new capability enabled MSL to safely land at the foot of Mount Sharp in Gale crater. Image credit: NASA/JPL-Caltech/MSSS & University of Arizona.

Uncertainties associated with terrain risk, targeting, atmospheric density, and wind together produced a 3-sigma uncertainty ellipse of about 100 km × 20 km (about the area of the state of Delaware) for previous missions (figure 2). Over decades, working closely with planetary scientists and geologists, engineers and scientists became frustrated that the wonderful diversity of Mars as discovered from orbit was unexplored because engineers could not create a landing system that could deliver a rover with sufficient safety.

In late 1999 engineering brainstorming sessions led to a series of EDL innovations that were applied to the MSL design. First, borrowing the same guided Earth entry techniques used by the Apollo missions of the 1960s to “fly out errors,” MSL significantly reduced the uncertainty ellipse to 20 km × 7 km. A second key development in early 2000 was the now famous ­heli­copter-like skycrane (Prakash et al. 2008), which eliminated the need for airbags or landing legs (Rivellini 2004). The ultraslow (0.65 m/s) touchdown velocity combined with the rover’s 0.5 m rock clearance and its highly compliant landing gear (a 6-wheel rocker-bogie mobility system; Bickler 1998) significantly improved landing success over the previous legged and airbag Mars landing systems.

This combination of innovations opened the door to many more possible safe sites that were geologically interesting—a major step forward for science exploration with a success rate better than 98 percent. For the first time, the landing site for a US Mars lander could be selected based primarily on its scientific merits, creating the wonderful problem of scientists debating with scientists rather than with engineers.

The landing site selection was eventually ­narrowed from more than 50 candidates to four finalists: ­Eberswalde crater, Gale crater, Holden crater, and Mawrth Vallis (Grotzinger et al. 2012). Each site was evaluated in detail for rock coverage, terrain shape, elevation, latitude, and other attributes. Armed with this information, the engineering team ran thousands of simulated landings at each site and counted the number of times the simulated landing exceeded one or another of the spacecraft’s specifications.

At Gale crater—the front runner and eventual ­winner—a novel challenge had to be addressed. The crater oddly has a mountain in its center that is much larger than the standard impact crater central peak. Mount Sharp (formally Aeolis Mons) rises 5.5 km (18,000 ft) above the crater floor (figures 2 and 3). It is composed of layered sedimentary rocks, similar to those seen in the US Grand Canyon, that show evidence of water in their mineral structure as observed from orbit.

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FIGURE 3 Mastcam composite image of the foothills and middle elevations of Mount Sharp (not shown), Curiosity’s prelanding primary exploration site and current location. In the foreground, about 2 miles (3 km) from the rover, is a long ridge with stores of hematite, an iron oxide. Just beyond is an undulating plain rich in clay minerals, and beyond that numerous rounded buttes are high in sulfate minerals. The different mineralogy in these layers of Mount Sharp suggests a changing environment in early Mars, with exposure to water billions of years ago. The colors are adjusted so that rocks look approximately as they would if they were on Earth, to help geologists interpret the rocks. This adjustment overcompensates for the absence of blue on Mars, making the sky appear light blue and sometimes giving dark, black rocks a blue cast. Image credit: NASA/JPL-Caltech/MSSS.

The challenge was that the science targets at Gale were located outside the safe landing ellipse and the engineers would have to ensure that the rover could safely drive the distance from the landing ellipse to the foothills of Mount Sharp, a trip that could take the better part of 2 Earth years. Could the rover make it and could the science team and NASA accept the risk to mission success that an additional 2 years and more than 20 km of driving would create? Experience driving Spirit and Opportunity did not bode well—they both encountered serious trouble at some point in their long drives (Lamarre and Kelly 2018).

To help mitigate this risk the MSL science team was assigned to groups to study the geology of Gale’s landing ellipse in more detail. What science targets could be defined in the ellipse provided by the engineers? What hypotheses could be tested at these targets? How was the science distributed spatially in each ellipse? What would be the routes to take, how long would it take to drive them, and what measurements could be made? And could the answers to these questions reveal ­whether the Gale landing ellipse contains rocks that might indicate past habitability?

The science team did something that had not been done for previous rover missions: the landing ellipse was divided into 1.5 km × 1.5 km quadrants and assigned to members with extensive experience mapping rocks on Earth. It wasn’t long before the ellipse was revealed to be full of attractive targets and, importantly, on par with the science targets identified in the ellipses at other landing sites. Most encouraging was the Peace Vallis fan and nearby high thermal inertia that were within a month’s drive of most landing locations in its ellipse. This was a critical insurance policy against a crippling anomaly.

Unlike the other landing sites, Gale crater’s most promising science lay just a few kilometers outside the ellipse in the layered foothills of Mount Sharp. Gale had earned its spot at the top of the list.

After the landing in Gale crater on August 6, 2012, the touchdown location was quickly determined to be an ancient river channel. The descent engines used during the sky crane maneuver blew the soil away to reveal ancient river ­pebbles, a great example of an engineering subsystem inadvertently enabling a science dis­covery. Furthermore, ­Curiosity ­landed only ~500 m from what appeared to be an ancient lakebed (figure 4). The MSL science team chose to explore this location before driving to Mt. Sharp and thus discovered the ancient habitability of Mars.

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FIGURE 4 Mastcam mosaic image of the lowest-lying portion of the Yellowknife Bay clay formation in Gale crater. Labeled are two drill targets, “John Klein” and “Cumberland,” in a geological region named “Sheepbed.” The rocks, exposed about 70 million years ago by removal of overlying layers due to erosion by the wind, record superimposed ancient lake and stream deposits that offered past environmental conditions favorable for microbial life. Image credit: NASA/JPL-Caltech/MSSS.

Within a few months, the rover sampled the lakebed clays and revealed that they had formed perhaps 3.5 billion years ago under aqueous conditions conducive for the habitability of Earth-like microorganisms (­Grotzinger et al. 2014, 2015; Vasavada et al. 2014). Ancient habitability had been proven, and all first-order science objectives met. This was a prime example of flexible decision making on the part of the team and its leadership.

Discovery and Mitigation of Wheel Damage

Curiosity was just a few kilometers into the long drive to the foothills of Mount Sharp when the engineering team discovered, during a routine inspection, that its wheels were slowly being destroyed. To the team’s shock, large holes and tears were visible in the aluminum wheels (figure 5). This was devastating news—it appeared that the rover wouldn’t survive the remaining 15 km or more to high-priority science targets beyond the ellipse.

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FIGURE 5 Curiosity rover left front wheel damage caused by driving over sharp, embedded rock (inset). Image credit: NASA/JPL-Caltech/MSSS.

What caused the damage? Clearly this result did not match the test result from years earlier when the rover wheel design was being validated. Or did it? It turned out that, years before launch, “Mars Yard” drive tests of a full-scale rover showed significant damage similar to that now seen on Curiosity on Mars. An investigation concluded that, under Earth’s higher gravity, the wheels had carried more than twice the weight expected at Mars. Subsequent tests under Mars conditions revealed no damage.

The question now was, What was different and what could be done to save the mission? To address this serious challenge, two “tiger teams” were formed. An engineering team sought to understand the cause and mechanics of the wheel damage, and a science team looked for alternative, less dangerous routes to drive.

The engineering team immediately noted that, unlike the testing performed on Earth, the rocks that the rover had been driving over on Mars were both sharpened by wind and tightly packed into the surface. Although the earlier tests used similarly sharp rocks, they had simply been laid on the surface, not bonded to it. But there had to be more to the explanation.

On Mars it looked like the sharp rocks had dragged themselves over the surface of the wheel, tearing it open as if by a can opener. Investigation of the wheel motion and kinematics revealed that the wheel controller design for the rover’s mobility system created the conditions for tears. The wheel controllers were designed to ensure that all the wheels turned at the same angular rate, even while driving over a rock. This induced a shearing action that tore the wheels. If Earth testing of the wheels over sharp implanted rocks had used that same wheel controller, the shear damage would have been discovered. Testing under Mars-like conditions soon validated this hypothesis, resulting in a flight software update to the wheel controller software so that wheel torque could be controlled and allow variable wheel speed (Lakdawalla 2014).

The science team studied the terrain types the rover had driven across as well as those it would encounter to find and predict terrains that would be easier on the wheels—with less bedrock, fewer large rocks, fewer sharp rocks, more gravel, and a little (but not too much) sand. Using high-resolution orbiter and rover image data, they compared terrain types and assessed the risk posed by each. This led to formulation of predictive guidance that could be implemented during future drives to limit damage.

As a result of all these studies and the implementation of predictive guidance, the Curiosity rover has ­driven over 26 kilometers with only moderate ­additional wheel damage. Over 30 drill samples have been analyzed, proving habitability in multiple environ­mental settings.

The Payoff

The Curiosity mission, a resounding success both technically and scientifically, has greatly expanded understanding of Mars at scales ranging from kilometers to millimeters and even at the molecular and atomic ­levels. Its discoveries would not be possible without the close cooperation and perseverance of engineers and scientists with diverse skills and backgrounds.

The cultural gap between engineering and science practice can be bridged
only through
interdisciplinary empathy.

After over 3130 sols on Mars, Curiosity continues to raise the bar for remote in situ exploration, proving that Mars’ watery past was once geochemically habitable for life and discovering traces of organic molecules that show that long ago the raw ingredients necessary for life were present. Other discoveries such as mysterious occasional bursts of atmospheric methane and large unexpected concentrations of silica continue to intrigue scientists worldwide.

Endeavors of this complexity are successful only if the people involved learn to trust, respect, listen, and learn from each other. The standard engineering ­formula—“specify it, design it, test it, and fly it”—is wildly oversimplified. Success comes only with constant trials, errors, new lessons, and new plans.

The resolution of the significant challenges described illustrates the need for engineers, scientists, and managers to learn, replan, and adapt quickly. The cultural gap between engineering and science practice can be bridged only through interdisciplinary empathy. Engineers must imagine the possibility that scientists might not know for certain what they need to succeed until they do. Scientists must imagine the many difficult and imperfect decisions, trade-offs, and even mistakes that engineers might make while trying to meet the launch deadline. For these imaginings to happen at all requires respectful conversation and patience.

Achieving respectful conversation and patience requires leadership that encourages teamwork and socialization between groups. Despite time pressures, throughout the project’s lifecycle efforts were made on all sides for people to get to know and understand each other both professionally and personally. There were regular science talks for engineering staff, and engineers had long discussions with the science team about their struggles to find design compromises. Open and honest communication was essential. Over time, differences between engineers and scientists faded to create a cohesive group whose ongoing communication and cooperation led to an unprecedented success.


The research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004). © 2021 ­California Institute of Technology. Government sponsorship acknowledged.


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About the Author:Rob Manning is JPL Chief Engineer at NASA Jet Propulsion Laboratory. John Grotzinger is the Fletcher Jones Professor of Geology and chair of the Division of Geological and Planetary Sciences at the California Institute of Technology.