American Scientist

It had all gone precisely according to script. On landing day, August 6, 2012, the Mars Science Laboratory rover (named Curiosity in a public contest of U.S. students in 2009) became the most complex mission ever launched beyond Earth. But 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, it finally departed for Mars in November 2011.

Curiosity began in the wreckage of NASA’s Mars hopes. Two spacecraft launched to Mars in 1998. Neither survived arrival. The twin disasters could have doomed NASA’s Mars program—again. But the American public enthusiastically supported a NASA search for Martian life following the announcement of possible fossils in a Mars meteorite recovered from Antarctica.

NASA had enjoyed early success on Mars with the Mariners and Vikings, though the Viking landers’ powerful (and expensive) life-detection experiments had failed to reveal signs of biologic activity on Mars. A lengthy hiatus in Mars exploration followed Viking in the 1980s, and the 1990s were mostly cruel to Mars missions. NASA’s Mars Observer, launched in 1992, failed just days before arrival. Mars 96, a Russian mission, failed to leave Earth Parking Orbit. But things had been looking up at the end of the decade. Mars Global Surveyor successfully entered orbit in 1997 and began its mapping mission in 1999. And the world fell in love with a little six-wheeled robot named Sojourner that had trundled around NASA’s Pathfinder lander for three months in the summer of 1997, sharing daily reports and Mars photos on the new medium of the internet. The American public was willing to support another try at Mars.

A year after Mars Polar Lander and Mars Climate Orbiter failed, NASA announced a reformulated Mars program. 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.”

Sample Handling

From the start, it was an ambitious mission. This mobile science laboratory 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.

NASA selected a total of 10 instruments for the mission payload. This was a huge and exciting instrument package. Some, the remote sensing instruments, would study the landscape from a distance, mostly from the top of the remote sensing mast. Others, the in situ instruments, would study rocks and soil from a turret at the end of the robotic arm, or measure the environment that the rover experienced. Finally, there were two analytical laboratory instruments buried within the body of the rover that would accept samples of rock, soil, and atmospheric gas for detailed study.

Geologists salivated over the prospect of performing x-ray diffraction and x-ray fluorescence on Mars. All previous methods of mineral identification on the surface of Mars were indirect; these measurements are diagnostic, as long as the samples contain crystals. The instruments would sensitively study atmospheric gas isotopes, could follow up on the possible discovery of methane, and would be capable of detecting organics, dangling the possibility of finding direct evidence for Martian life.

One group was both excited and dismayed by the list of instruments: the engineers, who would have to find space, mass, and power to accommodate them all in their rover, never mind operating a machine with so many capabilities.

After the Preliminary Design Review, the sample handling approach changed. A drill would simultaneously penetrate into and powder the rock, augering it into a sample chamber. Then the drill would transfer the material from the sample chamber into a device on the arm that could sieve, portion, and deliver the right kind of sample to the waiting science instruments. The addition of sample handling hardware to the end of the arm increased the weight of the turret from 15 to 34 kilograms. The 2-meter-long arm and its 5 motors would need to be much more robust than planned to support all of that weight.

On May 25, 2008, the Phoenix mission landed in Mars’s high northern latitudes. Although the landing went perfectly and NASA heralded it as a success, during its short, 5-month mission Phoenix would have frustrating problems attempting to sample Martian soil and ice and deliver it to laboratory instruments. Puffs of wind blew the samples away from the instrument doors. When sampled material did fall onto the instrument, the Martian soil tended to clump and stick, failing to fall through sieves that protected the instruments from large particles, even when the sieves were vibrated. The difficulties on Phoenix were sobering news for Curiosity’s team.

The sample handling team mounted prongs and other tools to the front of the rover to allow it to poke out stubborn gunk, and a “sample playground” with a tray, funnel, and other devices, where they could dump samples for visual inspection. They added wind baffles around the sample inlets and across the sample portioning device. They modified the sample portioner from a straight tube to an inverted funnel shape, to make sure sample material would not get stuck as it had on Phoenix.

Touchdown

Curiosity landed in the northern floor of Gale crater, at an elevation of 4,501 meters below the Martian datum. One of the deepest holes on Mars, Gale displays clear evidence of water having once flowed from the highlands surrounding the crater through gaps in the rim and then depositing overlapping alluvial fans of sediment on the crater floor.

At the center of Gale crater is a 5- kilometer-tall central mound of layered sediments formally named Aeolis Mons. The science team refers to the mountain as Mount Sharp, after Robert Sharp, a pioneering Caltech planetary geologist. NASA’s Mars Reconnaissance Orbiter had spotted spectral signs of clays, sulfates, and hematite in Gale’s lowermost layered rocks, all of which form in different kinds of wet environments. Reaching the lowermost slopes of the mountain to study those rocks was a major goal for the science team. Curiosity faced a lengthy drive—more than 9 kilometers as the orbiter flies, much longer for a wheeled rover dodging obstacles.

Curiosity acquired its first drilled sample on sol 176. (One Mars solar day is called a sol, a term coined during the Viking missions, and is 24 hours, 39 minutes, 35.244 seconds long, on average.) Analyses of the first drill samples were interrupted by a major anomaly. After recovering, rover operations almost immediately stood down again because of solar conjunction. When the Sun is within 3 degrees of Mars in Earth’s sky, radio communications can be affected by solar radio emissions. After conjunction, the rover drilled for a second time at a nearby site named Cumberland, on sol 279.

Early impressions of the drilled material suggested that Curiosity had accomplished its science objectives: The mission had successfully explored the biological potential of at least one target environment and had gathered the data needed to conclude that the environment was a biologically relevant one (the still water of a lake bottom). The mission had characterized the regional geology of the landing site before landing, and followed that up with successful chemical, mineralogic, and isotopic analyses with the science instruments. The isotopic measurements of water in the ancient Mars rocks had corroborated orbital science results indicating that Mars has lost much of its atmosphere. And Curiosity’s last goal hit was characterizing surface radiation. With all the crucial first-time activities complete and minimum mission success achieved, the science team could go on their driving adventure.

By 2015 Curiosity had traveled more than 10 kilometers across Mars, and still the full depth of its scientific promise hadn’t been realized. As the rover drove into its second Mars year, it had only just reached the base of the mountain its science team hoped to study. Curiosity arrived at basal Mount Sharp rocks on sol 751. As of sol 1800, Curiosity had attempted sampling in 20 locations, of which 17 resulted in the successful acquisition of sample and subsequent delivery. Even if the mission were to end tomorrow, scientists would be working on interpreting Curiosity’s data for decades. Of course, the mission hopes for much longer survival than that.

The Hardware

Curiosity has unprecedented capability for interacting with the Martian surface using a collection of hardware called the Sample Acquisition, Processing, and Handling (SA/SPaH, pronounced “saw-spaw”) system. SA/SPaH includes the robotic arm and turret, the drill, and the sample scooping/sieving/portioning apparatus called Collection and Handling for In situ Martian Rock Analysis (CHIMRA, pronounced “chimera”). Also included in SA/SPaH are the Dust Removal Tool (DRT, but usually just called the “brush”), a variety of immobile hardware bolted to the front of the rover that supports sampling and drilling activities called the “sample playground,” and motorized inlet covers and springloaded wind guards for the instruments.

Curiosity’s arm is huge. It measures 2.2 meters long from its base to the center of the turret. The arm weighs 101 kilograms; the turret alone is 34 of that. The turret is about 60 centimeters in diameter. The centerpiece of the turret is the drill. The science instruments are separated from the drill by vibration isolator mounts to mitigate the effects of vibration from CHIMRA and drill percussion. The arm functions slowly and deliberately, its tip moving at a maximum speed of 1 centimeter per second. Curiosity’s drill is a percussion instrument that hammers its rotating bit, boring holes 1.6 centimeters wide and up to 6.5 centimeters deep. The drill has four motors: A drill feed mechanism for moving the drill bit up and down; a drill spindle mechanism to rotate the bit; a percussion mechanism; and a drill chuck mechanism that can release the drill bit assembly and exchange it for a new one from one of two bit boxes located on the front of the rover. (Curiosity is still using the original drill bit; although it is not as shiny as it once was, it has not dulled dramatically.)

A Change of Plans

Several issues have affected the drill both before and after launch. One was the potential contamination of the drill bit that caused the reclassification of Curiosity’s planetary protection status. In addition, shortly before launch in November 2011, engineers doing testing of drilling operations found that seals inside the engineering model of the drill bit assembly were slipping during drilling, which generated Teflon debris that mixed with the drilled rock powder. This was a potentially serious source of contamination that could compromise the rover’s ability to detect organic materials within Mars’s rocks. But the amount of contamination was small enough that it would not likely affect results, and no sign of Teflon contamination has been noticed in drilled samples since landing.

Another potentially serious problem was discovered during Earth testing of a test bed version of the drill mechanism in 2011. A broken bushing caused a short circuit in the test drill that could have fried the rover’s motor controller if engineers had not acted swiftly. The consequences of such an event happening on Mars would be dire. It was too late to make any changes to the flight drill. Engineers in Florida opened the belly pan of the rover to install a “battle short” that would route half of the excess current to ground if such a short circuit developed in flight. On sol 911, sensors detected current flowing through the battle short as Curiosity was using drill percussion to transfer a sample from the drill to CHIMRA, halting the operation. There is no way to know if the cause is the same as the problem discovered on Earth, but the effect is similar. The shorts have recurred since sol 911, but are intermittent and extremely brief. If they remain that way, the battle short adequately protects the electronics.

At the same time, the mission has shifted to avoid using drill percussion for sample transfer, relying on CHIMRA vibration. They have also changed the way they operate the drill: Originally, they began drilling with a medium percussion level and made adjustments according to the penetration rate, but they now begin with very light percussion and only increase the rate as needed.

On sol 1536, the engineers attempted rotary-only drilling at a site called Precipice. The operation did not complete, because the drill feed mechanism stalled immediately. Current flowed to the drill feed motor, but the motor produced no motion. Like the problem with the percussion mechanism, it is intermittent, so has been difficult to troubleshoot, but it appears to reside in the drill feed brake mechanism. Since this incident, the rover hasn’t done any drilling.

The drill feed motor has a power-off brake: When no electricity is flowing to the brake, a disk (the moveable brake) is pressed against another disk (the fixed brake) by a set of springs. The pressure holds the drill feed firmly in position even when percussion, vibration, and rotation mechanisms are operating. Energizing a solenoid pulls the moveable brake away from the fixed brake, allowing the drill feed motor to spin a worm drive that slowly translates the drill feed out or in. The brake has two solenoids for redundancy.

Engineers troubleshooting the issue found that energizing either solenoid with the normally commanded current failed to produce any feed motion. Commanding with tweaked parameters (such as higher current, energizing both solenoids instead of one, multiple attempts to disengage the brake, and so on) produced some motion, but not reliably. The team strongly suspects that a displaced component or piece of foreign debris is interfering with motion of the movable brake, preventing it from fully disengaging when commanded.

From December 2016 through March 2017, engineers tested and performed diagnostics in an attempt to recover the full use of the drill feed. After developing several innovative techniques, they achieved the full range of feed motion, albeit at speeds too high to drill into rocks. However, after using CHIMRA to sieve a sand sample at Ogunquit Beach on sol 1651 (March 29, 2017), engineers found that the behavior of the drill feed had deteriorated.

The engineering team is pursuing a new drilling and sample delivery approach that does not require using the drill feed. They successfully extended the feed to its full 110-millimeter distance on sol 1780. On Earth, they are working on developing the ability to perform feed-extended drilling, using arm motion instead of feed motion to advance the drill bit into the rock. Initial testing of feed-extended drilling began on Mars on sol 1848. Although this mechanism can recover the ability to drill, not using the feed also prevents transfer of sample material to CHIMRA. Future feed-extended sample transfer may involve reverse-augering material from the sample chamber out through the bit and directly into the instruments. The only other way to transfer material from the drill to CHIMRA will be by dumping the drilled material somewhere and picking it up again with the scoop, a difficult or perhaps impossible proposition.

Rolling Along

That Curiosity is still operating on Mars more than 5 years after landing is testament to the dedication and focus of a huge human team that keeps it safe and productive. The science team has more than 500 members scattered around the world. Over the course of the rover’s development, launch, cruise, landing, and surface operation, more than 7,000 people from 12 countries have been involved in the mission. As of October 2017, the mission counts 250 peer-reviewed publications by team members and 157 by nonteam members using mission data.

Many of the people who were key to development are now working on the mission’s descendant, currently known as Mars 2020, which will reuse the designs of the cruise stage and entry, descent, and landing architecture to deliver a Curiosity-like rover (though with a different science package) to collect samples on Mars for a hypothetical future sample return mission.

In late 2017, the rover climbed onto Vera Rubin Ridge, seeing for the first time into the valley beyond. It paused to take a self-portrait on sol 1943. The ridge and valley represent new rocks and new history for Curiosity, embodying a 500-member science team, to explore.

This article is excerpted and adapted from The Design and Engineering of Curiosity: How the Mars Rover Performs Its Job (March 2018), © Emily Lakdawalla, with permission of Springer-Praxis.