Between 35.8 million and 249.1 million miles from Earth, on a dusty, wind-swept, airless planet, a robot explorer will complete its latest drill campaign and deliver its sample for analysis. The Curiosity rover’s robotic arm will drop a small lump of rock through a hatch on its topside, the doorway to a tiny automated laboratory located in it’s own belly. This is the Sample Analysis at Mars instrument, or SAM and the process is known as EGA (evolved gas analysis). As the rock is vaporized in a thousand degree micro furnace, the escaping gases pass by a mass spectrometer, gas chromatograph, and laser spectrometer scanning for vital elements: Carbon, Hydrogen, Oxygen, within compounds essential for life as we know it: atmospheric gases such as methane, water vapor and carbon dioxide.
Curiosity touched down at the Aeolis Palus plain inside the Gale crater, beween its northern wall and its central peak, Aeolis Mons on August 5th 2012 at 10:32 p.m. Pacific Daylight Time. At the time of writing (September 21st 2021) the Curiosity rover has spent 3239 sols exploring the alien world and sending its findings back to Earth. Sol is the name for the entire Martian day night cycle, which is approximately 40 minutes longer than an Earth day. In that time (9 years, 1 Month and 16 days in Earth time) Curisoity has travelled 16.39 miles, collected 33 rock samples and sent over 842,335 photographs of Mars via the Deep Space Network satellites and antennas to the Jet Propulsion Laboratory back on Earth.
The weight of this 1,982lb rover, including 180lb of scientific instruments, is borne by 6 20-inch diameter wheels made from a single piece of flight grade aluminium attached to a rocker-bogie suspension arrangement. This springless design allows the rover to drive over rocks up to twice the diameter of the wheels whilst keeping all 6 wheels on the ground. Each wheel has 19 grousers, ridged tracks that provide grip and an odometry feature – a pattern of small holes in the wheel. This feature allows the rover’s software to visually track progress by pointing it’s camera back on the tracks it makes in the Martian sand and calculate distance travelled based on how many times the odometry pattern appears: once every revolution of the wheel’s 62 inch circumference. For fun, the odometry holes spell out the name of the rover’s creator in morse code: J P L.
Bearing the brunt of the rover’s contact with this hostile planet continues to take a toll on the wheels and the damage has been visible since the selfie Curiosity sent home on Sol 411. Dents, perforations and wear in the 0.75mm thick aluminum are now present on all 6 wheels and is now effecting the decision making process of the Strategic Route Planning Group back on Earth, who are now adjusting course to aim for the smoothest route possible and extend the life of the wheels.
If a wheel deteriorates to the point where it’s hanging off its axle, it could stop turning and create drag rather than propulsion. If a wheel loosens to the point where it’s spinning off axis, it poses a risk to the electrical cabling that connects the motor inside the wheel with the motor controller on board the rover, which could be catastrophic to the control of the other wheel motors. To prevent this critical failure JPL assigned a Wheel Wear Tiger Team to solve the problem. Tiger Teams have a long history at Nasa, as far back as the Apollo 13 lunar mission. They consist of a group of uninhibited technical specialists assigned to relentlessly track down every possible source of failure within complex mission critical systems and propose, design and build solutions and work arounds.
In the case of Curiosity’s wheels, the solution they advanced builds acceptance of the eventual failure of the wheel into its design and construction. A wheel approaching catastophic levels of damage can be removed using a wheel shedding manoeuvre. Each wheel has a stronger outer third where it connects to its internal motor hub, spindle and a weaker inner two thirds, vulnerable to off-axis motion following severe degradation. The manoeuvre involves finding a sharp rock at just the right height to catch the damaged wheel. The rover then individually rotates the wheel 90 degrees to the axis of travel and drives it into the rock with a forward/reverse motion, using metal fatigue to eventually shed the broken component like bending a paper clip back and forth until it snaps. The rover can then continue its extended mission unhindered.
Although showing signs of wear, Curiosity’s wheels are nowhere near the state of disrepair that requires such drastic action. It has exceeded its initial one Martian year mission plan (687 Earth days) by 4 Martian Years (2,598 Earth days). Thanks in part to the switch from solar panels that scuppered it’s sybling rover Opportunity after it was caught in a sandstorm in 2018, to a radioisotope power source known as the Multi-Mission Radioisotope Thermoelectric Generator (MMRTG). Heat from a decaying radioactive pellet of plutonium is converted into electricity for instant consumption by the rover or stored in 2 rechargeable lithium-ion batteries for moments of peak power usage that exceed the generator’s steady output.
2309 miles away, inside the Jezero Crater, Curiosity’s younger sybling rover Perseverance is performing many of the same activities in another promising location. Jezero crater, site of a 40km wide lake that dried up billions of years ago is prime hunting ground for biosignatures of long extinct Martian life.
Perseverance, a state of the art car-sized rover that touched down in 2020 has a few other tricks up its sleeve over its sybling. Instead of vapourising drill samples and analysing the fumes in a tiny onboard laboratory, Perseverance uses a sample caching system that deposits and seals rock samples in metal tubes that it leaves behind on the martian surface for later collection and return to Earth by future mission vehicles and robots currently in development. Sample caching is a complex system comprised of 3 seperate robots: the first one is a 7 foot extendable arm at the front that wields a rotary percussive drill with interchangeable bits designed for abrasion (to remove the rock’s surface), regolith (loose rock fragments) and a hollow coring bit for solid rock core sample extraction. Inside this hollow bit is a metal sample tube that the drilled out piece of rock slots into.
Other scientific tools located longside the drill at the “hand” end of the arm includes SHERLOC (Scanning Habitable Environments with Raman & Luminescence for Organics & Chemicals), a laser spectrometer and macro camera combo that likes to get up close to the fine details of the Martian surface, like the eponymous detective’s magnifying glass. Accompanying SHERLOC is WATSON (Wide Angle Topographic Sensor for Operations and eNgineering), a camera with a wider field of view that helps locate the next target for the arm. The hand also includes PIXL (Planetary Instrument for X-ray Lithochemistry), an X-ray spectrometer and camera used to identify chemical elements in terrain features as small as a grain of salt, and a ground contact sensor that tells the rover to stop when the arm touches the ground as a damage prevention measure.
Once a pencil sized sample of solid rock has been drilled out using the hollow coring bit, the rover manoeuvres its arm towards it’s main imaging system, the Mastcam-Z, which acts as the rover’s head with 2 zoomable cameras for eyes, to inspect the sample. Once the presence of the sample inside the the sample tube inside the bit is confirmed the rover begins its “percuss to ingest” procedure. The bit is vibrated for 1 second five times to clear the lip of the sample tube of any debris and to more firmly seat the sample further inside the tube. The Mastcam-Z double checks the sample is still in the tube and a volume probe is inserted to test to see if the expected resistance is present. When the check is complete, the sample is delivered to the second robot in the sequence, the bit carousel. This flying saucer shaped device acts as a 2-way gateway to the belly of the rover, passing out one of the 9 stored drill bits, or ingesting a completed sample tube so it can be collected by the next robot in the sequence. Found inside the rover, the adaptive caching assembly consists of a sample handling robotic arm, 7 motors and over 3000 parts. The interior robotic arm collects the docked sample tube from the bit carousel and runs it through a second set of checks, similar to the first set outside the rover but with a different set of samller instruments inside the rover. Once inspection is passed, the sample tube is inserted into a sealing station where the tube is hermetically sealed, so nothing can get in or out, and then placed in the caching assembly, a storage rack for the sample tubes. The entire process takes hours – Martian rovers move at a snail’s pace to preserve energy and to allow for checks to made back on Earth over a communication relay that has a delay of between 3 and 22 minutes depending on the changing distance of the 2 planets.
At a future point in the rover’s mission, the 30 sample tubes will be deposited at a strategic location on the Martian surface for collection by a future mission robot yet to be designed and constructed. This robot will not only travel to and deploy onto the surface of Mars, it will also collect the sample tubes and utilise a launch vehicle in order to leave Mars and voyage back to Earth where the Martian samples can be studied by human scientists first hand for the first time in history.
The Mars 2020 mission has a minimum projected mission length of one Martian year (approximately 687 Earth days) and in order to preserve it’s mobile state the Perseverance rover has a different wheel design and preservation strategy to the Curiosity rover’s wheel shedding manoeuvre. Constructed from the same flight grade aluminium as Curiosity’s wheels, Perserverance’s wheels sport a more aggressive tread consisting of 48 gently curved grousers that grip the Martian surface for traction. That’s double the number of Curiosity’s 24 chevron-shaped grousers. Extensive testing in the “Mars Yard” rover testing site at JPL has shown these treads can withstand sharp rocks and grip just as well or better than Curiosity’s when driving on sand.
Route planning is a vital wheel preservation strategy in order to keep the rover on soft ground, away from sharp wheel wearing rocks as much as possible. Satellite photography from the Mars Reconnaissance Orbiter spacecraft, circling Mars at a height of between 250km and 316km since 2006, provides the Strategic Route Planning Group with images at 0.3 square meters per pixel resolution. These images allow the team to make decisions about potential water features worth investigating but are not clear enough to analyze terrain viability for the rover. Instead, Perseverance has an aerial scouting capability in the form of a small remote controlled helicopter named Ingenuity. Armed with its own on board computers, navigation sensors and cameras, this solar powered drone sends recon images of potential paths of exploration back to the JPL HQ on Earth. This small autonomous explorer arrived on Mars strapped to the belly of Perseverance and was deployed for its pioneering Martian flight on April 3rd 2021, 74 Earth days after Perseverance landed in the Jezero crater.
The Mars helicopter Ingenuity has flown 2883m during 15 flights in an atmosphere less than 1% dense as Earth’s, captured 83 13-megapixel colour images and 1,772 black and white navigation images and is now operated as a seperate vehicle to Perseverance since it’s deployment, with its own team of engineers and pilots back at JPL.
Since landing 256 sols ago, Perserverance has gathered, sealed and stored 2 samples, published over 161,000 images to the mission blog website (https://mars.nasa.gov/mars2020/), delivered the first Martian weather report using the onboard Mars Environmental Dynamics Analyzer (MEDA) instrument and continues to do so on a daily basis. Despite an earlier hiccup when a drill sample disappeared from it’s tube likely due to the rock disintegrating during collection, Perseverance continues to successfully meet all its mission goals.
Source: “NASA’s Mars Exploration Program.” NASA, https://mars.nasa.gov/
