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The Mars 2020 mission: Perseverance & Ingenuity Source: https://mars.nasa.gov/resources/25640/mastcam-zs-first-360-degree-panorama/ “ A re we alone? We came here to look for signs of life, and to collect samples of Mars for study on Earth. To those who follow, we wish a safe journey and the joy of discovery.” These words are written on the Perseverance rover as a message for future human explorers, or other intelligent lifeforms that might find the machine in the future. The Mars 2020 mission involved landing the Perseverance rover vehicle and the Ingenuity helicopter on Mars. Planning for the mission started in 2012, and the Atlas V rocket launched Fig.1: the Mars 2020 launch on an Atlas V rocket at 11:50am UTC on July 30th, 2020. 12 Silicon Chip on July 30th 2020 (see Fig.1). Touchdown occurred on February 18th 2021. The mission has a strong astrobiological emphasis, looking for evidence of past or present conditions suitable for lifeforms on Mars, or the actual lifeforms themselves. The landing is in an area thought to have once had conditions suited to life. Great care was taken to ensure no lifeforms from Earth were accidentally transferred to Mars. The lander will also collect and cache samples for a later Earth return mission, planned for 2031, for further analysis. It will also demonstrate technologies for future robotic missions (such as the helicopter), and future manned exploration such as oxygen production from the CO2 atmosphere of Mars. Perseverance is the fifth NASA rover to land on Mars after Sojourner (1997), Opportunity (2004), Spirit (2004) and Curiosity (2012). The first three were solar-powered and no longer function, while Curiosity is nuclear-powered via a radioisotope thermoelectric generator (RTG). Curiosity, which landed on August 6th 2012, is still operational, having travelled more than 25km so far. Perseverance is based on Curiosity Australia’s electronics magazine and is also powered by an RTG. The spaceflight and landing Launch dates and times are chosen carefully to fulfil numerous requirements such as: • Earth and Mars being in suitable locations within their orbits to minimise travel time. • an existing Mars orbiter be over the proposed landing site to relay data to Earth during the Mars entry and landing. • suitable weather conditions at the launch site. There were seventeen days over which the launch could have occurred, with available launch windows on each day from 30 minutes to two hours long (see siliconchip.com.au/link/ab8f for details). After launch (Fig.2), the next phase was interplanetary cruise (Fig.3), which started as soon as the spacecraft separated from the launch vehicle. During this time, checks were run on various spacecraft systems and several trajectory correction manoeuvres were made, especially on the final approach to Mars. The final phase was the entry, descent and landing (EDL) – see Fig.4. Ten minutes before this happened, siliconchip.com.au Mars is currently the only planet we know of occupied only by robots. This article is about NASA’s latest robotic visitors to Mars, the nuclear-powered Perseverance rover and the groundbreaking Ingenuity helicopter. Shown in the background is Perseverance’s first 360° panorama, taken by the Mastcam-Z instrument. This panorama was stitched together from 142 individual images. The rover looks distorted because of the 360° view. By Dr David Maddison the cruise stage was jettisoned. EDL began when the spacecraft, protected by an “aeroshell” heat shield, entered the top of the Martian atmosphere at 19,500km/h. During entry, small thrusters on the aeroshell were used to manoeuvre the spacecraft to its target landing location. Peak heating occurred 80 seconds into the entry, with parts of the craft reaching about 1300°C. Four minutes after entry, a parachute was deployed. The parachute is 21.5m in diameter and deployed at an altitude of 9-13km and a speed of 1512km/h. Twenty seconds after parachute deployment, the heat shield separated from the underside of the spacecraft. Another 30 seconds after that, the radar and Lander Vision System were activated at an altitude of about 7-8km. At 4km and 6m30s, the Terrain Relative Navigation (TRN) system, using inputs from the Lander Vision System (LVS), had determined the spacecraft position and the desired landing target. More on the TRN and LVS later. This was followed by back-shell and parachute separation at 6m50s, at an altitude of 2.1km and speed of 320km/h, followed by a powered descent. The descent vehicle, with the rover attached, used manoeuvring siliconchip.com.au Fig.2: the launch profile for Mars 2020 - SRB stands for solid rocket booster and PLF for payload fairing. These events occupy the first two hours; from launch to separation was just under one hour. Fig.3: the route to Mars. TCM stands for trajectory correction manoeuvre. Some of these TCMs were cancelled due to the high level of navigational accuracy achieved. Australia’s electronics magazine July 2021 13 Fig.4: the Mars 2020 entry, descent and landing sequence. thrusters to fly the vehicle to the landing target. The next phase of the landing was rover separation from the descent stage for the Sky Crane manoeuvre at an altitude of 21m. The powered descent stage becomes the Sky Crane, which uses its thrusters to remain stationary and lowers the rover on cables (Figs.5 & 6). As soon as the rover touchdown was confirmed, the Sky Crane flew away to a safe distance and landed about 700m away from the rover. The Sky Crane concept was used because the rover was too heavy to permit an airbag type of landing, as used for some past Mars missions. A retrorocket landing, as used for Viking 1 and 2, was deemed unsuitable as the rockets would have thrown up debris that could have affected the rover’s sensors. Note that the entire landing sequence was autonomous; due to speed-of-light limitations, the radio delay at the time of landing was over 11 minutes. Seven minutes after first atmospheric entry, the rover and its payload Ingenuity were safely on the surface. This period of seven minutes is known as “The Seven Minutes of Terror” because so many things can go wrong, and nobody on Earth knows what has happened until it is all over. There is a video of the landing with imagery looking down from the descent vehicle and up from the rover, titled “Perseverance Rover’s Descent and Touchdown on Mars (Official NASA Video)”, viewable at https:// youtu.be/4czjS9h4Fpg 14 Silicon Chip For a commentary on that video, see the video titled “Landing On Mars Like You’ve Never Seen It Before” at https://youtu.be/mfgzTfw_J6o Fig.5: a rendering of the final landing stage, with the rover being lowered beneath the Sky Crane. A hidden message Embedded on Perseverance’s parachute was a binary code that stated “Dare Mighty Things”, which is both a quote from a speech from President Roosevelt and the motto of the NASA JPL Laboratory (see Fig.7). The GPS coordinates for the California JPL Laboratory are also on it. Navigating from Earth to Mars For most of its journey, Mars 2020 received navigational signals from Earth. Remarkably, the spacecraft entered the Martian atmosphere within 200m of the desired entry point. This high level of accuracy made two planned correction manoeuvres unnecessary (see siliconchip.com.au/ link/ab8m). This was achieved partly by knowing the spacecraft thruster exhaust velocity exactly, to within millimetres per second. Even thermal radiation and solar radiation pressure, which were incredibly insignificant forces (about one-billionth of the force of gravity on Earth) had to be taken into account, or the spacecraft could deviate up to 3.7km in the final ten days. Importantly, antennas in NASA’s Deep Space Network, some of which are in Australia, were used to determine the spacecraft’s exact position. The location of these antennas on the Earth’s surface had to be known precisely, because an antenna location Australia’s electronics magazine Fig.6: an actual image of the Perseverance rover being lowered to the ground by the Sky Crane, as seen from the descent stage. Fig.7: a photograph of the descent stage’s parachute, showing the decoded binary message. siliconchip.com.au Fig.8: how spacecraftquasar delta differential one-way ranging works. The angle between the spacecraft and quasar should be less than 10° for good accuracy. error of 5cm would result in a 500m error over the 150 million kilometres to Mars. Also, the speed of rotation of the Earth had to be known within 0.2m/s, and the exact location of Mars, as determined by Mars Global Surveyor and Mars Odyssey, had to be known within about 800m or less. Navigators even had to take into account the wobble of the Earth and how solar plasma affected the speed of navigational radio signals from Earth. Additionally, a technique known as spacecraft-quasar delta differential one-way range or DDOR (pronounced “delta door”) was used to help locate the spacecraft (see Fig.8). A location in space can, in principle, be determined by trigonometry. That is, using the distance between it and two antennas, the angle between the antennas and the spacecraft and the baseline between the antennas. But inaccuracies are introduced due to variations in the speed of light/radio waves in the atmosphere and solar plasma, and clock instabilities in the ground station. An additional radio source is used to compensate for these variations, which comes from the same approximate direction as the spacecraft. The radio source used is that from quasars, which result from gases falling into supermassive black holes at the centre of some galaxies. Since radio signals from both the spacecraft and quasar follow the same path, the radio delay time from atmospheric effects and clock variations can be determined and compensated for. The spacecraft’s location is compared to previously-established maps with the planets in the positions as they appear during the spacecraft’s journey. Taking into account the gravitational 1 2 1 2 3 3 0 effects of nearby moons and planets, signals are sent to the spacecraft to fire thrusters to correct the course. Once close to Mars, Earth-based navigation can no longer be used due to the 11+ minute radio signal delay (the exact delay varies depending upon the relative position of Mars and Earth). It was desired to land within 40m of the target area; the final landing position was determined visually with reference to ground features, just like the Apollo astronauts did. But in the case of Mars 2020, it had to be done by computer alone. Terrain images previously acquired by Mars-orbiting spacecraft were stored in the spacecraft computers. The lander’s radar and visual landing (Lander Vision System, LVS) took over at an altitude of 4.2km. The Lander Vision System is the camera and computer system used to provide data for Terrain-Relative Navigation. Starting at an altitude of 4200m, the LVS has to process live visual imagery and compare it with stored visual imagery, taking an initial navigational position error that could be as much as 3.2km before entry (but it turned out to be 200m). It determined the precise spacecraft location with reference to that stored imagery, reducing the position error to a desired 40m or less for landing, all within 10 seconds. For details on the LVS, see siliconchip.com.au/link/ab8g Using the position established by the LVS, the Guidance, Navigation and Control (GNC) system selected a suitable landing position that was reachable with the available fuel for the eight thrusters on the descent vehicle (see Fig.9). Fig.9: matches between the stored navigational map and a simulated descent image from the spacecraft, as used in Terrain-Relative Navigation. Note how the matches are made despite the different orientations and resolutions of the two images. 4 4 siliconchip.com.au Australia’s electronics magazine July 2021 15 Fig.10: safe landing areas from the Safe Targets Map, within and near the target landing zone that avoid hazardous terrain and unfavourable slopes. The thrusters on the descent vehicle ignited at an altitude of 2100m. To manoeuvre to the selected landing site, it could alter the landing position of the rover by up to 600m. There is a Safe Targets Map covering a 20km x 20km area, and each pixel in the map is assigned a landing risk level and information on whether that area has a favourable slope or not (see Fig.10). The objective of the GNC system was to fly to the most favourable target that was reachable. For further details of the GNC, see the PDF at siliconchip. com.au/link/ab8h Mars 2020 is regarded as the most accurately navigated space mission ever. Jezero crater Jezero crater was chosen as the landing site for Perseverance because it was once thought to be filled with water, and thus a possible location for life in the past. There is also evidence of two ancient river deltas (see Figs.11 & 12). It is possible that deposits washed down by the river would also contain evidence of ancient life. Apart from the ancient river deltas, it was determined that there must be extensive sedimentation, perhaps up to 1km thick, because the crater is much shallower than expected. There are also clay minerals present and cracking of the surface, both suggestive of the past presence of water. Fig.11: a geological survey map of part of the Jezero crater landing region, showing ancient river delta, dunes, shoreline, ash and other deposits. This map includes the Perseverance landing site and a possible exploration route (the yellow line). You can see an interactive and larger version of this map at https:// planetarymapping.wr.usgs.gov/interactive/sim3464 Source: Wikimedia user Hargitai. Parachute & Back Shell Descent Stage Heat shield Perseverance Fig.12: an image taken from the Mars Reconnaissance Orbiter of the Perseverance landing site, showing the lander plus various components jettisoned during landing. 16 Silicon Chip Australia’s electronics magazine The Perseverance rover The Perseverance rover (Figs.13-16) is an upgraded version of the previous Mars rover, Curiosity. The rover weighs 1025kg, which happens to be exactly the weight of an Australian spec Toyota Yaris, unladen. The rover is 3m long, 2.7m wide and 2.2m tall. The rover consists of an enclosed box called the Warm Electronics Box (WEB), in which sensitive electronics and other equipment is kept warm by surplus heat from the nuclear power source. Six wheels are attached to the WEB via a suspension system. On top of the WEB is an Equipment Deck, with the following accessories attached: • the camera mast • a primary 2.1m-long robotic arm • a secondary robotic arm to assist with sample storage • three telemetry antennas • the nuclear power source siliconchip.com.au Navcam Rear Hazcams SuperCam Navcam SHERLOC (WATSON) Mastcam-Z Front Hazcams PIXEL (Micro-Context Camera) Fig.14: a comparison of the wheels from the older Curiosity rover with Perseverance. The tread pattern enhances traction. Fig.13: the location of some of the cameras on the Perseverance rover. There are a total of 23 cameras – 9 for engineering, 7 for science, and 7 for entry, descent and landing. Note that the MEDA SkyCam is not shown. • various sensors for dust, wind, noise, air pressure and radiation • other cameras and miscellaneous items The Ingenuity helicopter was stored beneath the rover. Some key differences between Perseverance and Curiosity are: • Perseverance is heavier by 100kg+ • a larger robotic arm with a bigger turret • more cameras and new science instruments • it will collect rock samples and cache them for later collection by an Earth return mission • improved wheels • the software has greater autonomy Perseverance wheels, suspension and motors The Perseverance wheels are attached to the body by titanium tubing. The “rocker-bogie” suspension is designed so the rover can drive over rocks up to 40cm tall, or into depressions up to the size of the wheels. The six wheels are made of aluminium with titanium spokes and are 52.5cm in diameter. They have a reduced width, larger diameter and improved design compared to the Curiosity wheels, due to those wheels having sustained some damage in the previous mission (see Fig.14). A separate motor drives each wheel, and the front and rear sets of wheels can be steered, meaning the rover can perform a 360° turn on the spot. The rover can tilt as much as 45°, but for safety, the tilt angle is kept under 30°. The top speed of the rover is 0.152km/h (~4.2cm/s). For the science mission, no greater speed is necessary. The drive system uses less than 200W peak; 110W or less from the nuclear power source, plus auxiliary power from batteries when necessary. Mars Relay Network, which relays data from Perseverance, Curiosity and the InSight lander to the Deep Space Network (DSN). Perseverance antennas Perseverance is equipped with three antennas. These are a UHF antenna for about 400MHz, a high gain X-band and a low gain X-band antenna for communications in the 7GHz to 8GHz range. The UHF antenna is used to communicate with Mars orbiters which relay the message to Earth. Data can be transmitted from the rover to the orbiter at up to two megabits per second (2Mb/s). This is the main communication system. For redundancy, the X-band highgain antenna is steerable and can transmit data directly to Earth, and also receive data. The antenna is 30cm in diameter and can transmit or receive data to or from Earth at 160 or 500 bits per second, or faster from the DSN’s 34m antennas, or at 800 or 3000 bits per second with the DSN’s 70m antennas. Mars Relay Network Two Mars orbiting spacecraft, the Mars Reconnaissance Orbiter (MRO) and the Mars Atmospheric and Volatile EvolutioN (MAVEN), form the Fig.15: the locations of various instruments on Perseverance. ► Fig.16: a depiction of the Perseverance rover operating on Mars. siliconchip.com.au Australia’s electronics magazine July 2021 17 Fig.17: the layout of a RAD750 3U CompactPCI singleboard computer used on the Mars Curiosity rover and similar to the one used on Perseverance. The version used on Perseverance has more memory and a higher clock speed. Fig.18: the Mastcam-Z cameras before being mounted on the rover, with a pocket knife for scale. an earlier RAD6000 computer). The computer has 2GB of flash memory (about eight times as much as Spirit and Opportunity), 256MB of DRAM (dynamic random access memory) and 256KB of EEPROM (electrically erasable programmable read-only memory). There is a second copy of the main computer for backup, plus another one for image processing. The computer might be ‘old tech’, but it is super-reliable and has ample power for the job. A modern CPU with smaller feature sizes would be more prone to errors in the high-radiation environment in space and on Mars. The operating system used on Perseverance is VxWorks by Wind River Systems. It is designed for embedded systems, operates in real-time with minimal processing delays and supports the PowerPC architecture. Perseverance cameras The low gain X-band antenna is used to back up the X-band high gain antenna and communicate with the DSN. It is not steerable, so the data rate is much lower at 10 bits per second with the 34m DSN antennas and 30 bits per second with the 70m antennas. Perseverance microphones There have been three prior attempts to send microphones to Mars, but they all failed. Perseverance carries two microphones. One was a commercial off-the-shelf microphone to record the sounds of the entry, descent and landing. That one failed to work during entry, but it recorded the sounds of the nuclear power source cooling pump and other sounds during spaceflight and a system check. To listen to the spaceflight sounds, visit siliconchip.com.au/link/ab8i Since landing, it has functioned and has recorded other sounds. 18 Silicon Chip The other microphone is attached to the SuperCam Mast. It is used to make recordings on Mars and listen to the laser’s sounds interacting with rock specimens; the popping sounds giving off clues about rock density. To listen to some more sounds recorded by the rover, visit siliconchip.com. au/link/ab8j Perseverance computer Perseverance uses a PowerPC 750 chip which is radiation-hardened. It is the BAE RAD 750 processor and associated single-board computer (see Fig.17). This is essentially the same processor as used on the 1998 “Bondi blue” iMac G3, although the version with radiation-hardening costs over US$200,000 (that’s the 2002 price, but it is still in production and is used in over 100 spacecraft). It operates at up to 200MHz, ten times faster than those on Mars rovers Spirit and Opportunity (which used Australia’s electronics magazine Perseverance has a total of 23 cameras, as shown in Fig.13. This is an unprecedented number for any space mission. The cameras can be divided by purpose into three categories: entry, descent and landing; engineering cameras; and science cameras. An emphasis was placed on using commercially-available hardware when possible. For details of the cameras, see the PDF file at siliconchip. com.au/link/ab8k Entry, descent and landing cameras Seven cameras were used for entry, descent and landing: • three on the back shell looking up at the parachute • one on the descent stage looking down at the rover while the Sky Crane lowered it • another down-looking camera on the descent stage, used by the Lander Vision System (1024x1024 pixels) for use in Terrain Relative Navigation • one on the rover looking up to watch the Sky Crane manoeuvre • one on the rover looking down to watch the landing (with a microphone) Engineering cameras Nine engineering cameras are divided into three sub-categories: six hazard avoidance cameras (HazCams), two stereo navigation cameras siliconchip.com.au (Navcams) and one CacheCam. These are mounted in various locations. Each has a 5,120 x 3,840 pixel sensor (20MP). They use the same camera body but different lenses according to their task. The HazCams are mounted three at each end. They are used both for rover navigation and by engineers when directing the robotic arm. The two mast-mounted stereo Navcams are designed for autonomous rover navigation, without decisions being made by controllers on Earth. The CacheCam is for taking pictures of collected samples before they are placed inside sample tubes, sealed and deposited for later pickup by an Earth return mission. Science cameras There are seven science cameras, as follows: Mastcam-Z (Fig.18) comprises a pair of mast-mounted stereo zoom cameras that can rotate in all directions. It captures colour images and video at up to four frames per second at 1600 x 1200 pixels and can generate a 3D image. The zoom range is 28-100mm and the image sensor is a Kodak Truesense KAI-2020 CM interline transfer CCD. The resolution is about 1mm close to the rover and 3-4cm at 100m distance. It is equipped with several bandpass optical filters to help identify or distinguish various minerals, plus solar filters to image the sun. The main purposes of Mastcam-Z are to characterise the Martian landscape, observe atmospheric phenomena such as clouds and dust devils, assist in rover navigation, sample collection and sample caching. The SuperCam is a mast-mounted instrument that uses a laser to either reflect off or vaporise soil, rock and dust samples beyond the reach of the rover’s robotic arm, up to 12m away. One of two lasers is fired at a sample of interest, and then one or more of four spectrometers are used to determine the sample composition. The red laser is used to vaporise samples of interest up to 7m away, with three spectrometers determining the sample’s elemental composition. The green laser is directed at samples up to 12m away but does not vaporise them. The identities of minerals or organic compounds can be determined by analysing the reflected beam using spectrometers. siliconchip.com.au Fig.19: a plot of the relative number of counts at different energies to identify elements with the PIXL X-ray fluorescence instrument. The infrared spectrometer, one of the four spectrometers, can see out to the horizon. SuperCam also incorporates a high-resolution colour camera, a Remote Microscopic Imager (RMI) to take pictures of distant samples using a telescope and one of the two microphones, a Knowles Corp EK Series. SuperCam was a collaboration between the Los Alamos National Laboratory (LANL) and the IRAP Astrophysics and Planetology Research Institute (France), with a contribution from the University of Valladolid (Spain). PIXL (Planetary Instrument for X-ray Lithochemistry) is an X-ray fluorescence instrument for elemental chemical analysis mounted on the robot arm. In X-ray fluorescence, an X-ray beam is directed at a material of interest. The energy of the X-ray removes one or more electrons from an atom by ionisation, and other electrons in higher energy orbitals within the atom move down in energy level to replace the ionised electron. When an electron or electrons move to a lower energy orbital, they emit radiation of a wavelength equivalent to the energy difference. This wavelength is characteristic and unique for each element and can be used for identification. The instrument can look at structures in soil or rock at a sub-millimetre level with a 0.12mm beam width, and operates at high speed. It can detect the following chemical elements: Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Br, Rb, Sr, Y, and Zr. That includes most of the elements from atomic number 11 to 40. Australia’s electronics magazine PIXL uses a Micro Context Camera (MCC) to acquire images of the test areas – see Fig.19. The SHERLOC (Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals) Context imager is an ultraviolet Raman spectrometer that uses a UV laser to look at mineral samples at a fine scale, to detect organic compounds, including biosignatures (see Fig.20). It is mounted on the robot arm. The rover is equipped with small pieces of spacesuit material, which it tests for accuracy and to see how they degrade with time. SHERLOC has a monochrome camera for context, attached to the robotic arm. SHERLOC can image an area of 2.3cm x 1.5cm with the camera (the Advanced Context Imager, ACI) and performs spectroscopy on a 7mm x 7mm area. Also associated with SHERLOC is a “context imager” camera named WATSON (Wide Angle Topographic Sensor for Operations and eNgineering), which takes extreme close-up photographs of the sample areas tested by SHERLOC. Fig.20: the SHERLOC ultraviolet spectrometer engineering model. July 2021 19 Apart from working with SHERLOC, WATSON bridges the resolution gap between the very fine detail obtained from SHERLOC and the much larger scale from Mastcam-Z and SuperCam (see Fig.15). WATSON is attached to the robotic arm and is mainly concerned with details of rock textures, fine debris, dust and structures. MEDA (Mars Environmental Dynamics Analyzer; see later) has a SkyCam camera to take images of the Martian sky. Power source Fig.21: a photo of the rover upside-down, showing the MMRTG unit in the centre. It is surrounded by eight cooling fins; the curved panels on each side are heat exchangers connected to the core by yellow coolant tubes. Bimetal ring Seal weld cover Surface emissivity change Min-K insulation Isolation bellows T/E getter assembly Isolation liner assembly Heat distribution block Mica Cooling tube General purpose heat source Microtherm insulation Thermoelectric couple assembly New TE technology Microtherm insulation Module bar Power out receptacle Fig.22: a cutaway view of the Enhanced Multi-Mission Radioisotope Thermoelectric Generator, similar to the one on Perseverance. Navigating with the Deep Space Network (DSN) Spacecraft can navigate using the radio telescopes of the DSN. The distance from Earth is established when a precise time-coded radio signal is sent from the DSN and returned. The time taken is used to calculate the distance, while the dish antennas can determine the angular position of the spacecraft compared to Earth. More precise measurements can be made using two DSN telescopes at the same time. This gives the spacecraft distance to each telescope. The distance between each telescope is also known precisely, so triangulation can be used to calculate the distance. Further accuracy can be obtained using the signals from a star type known as a quasar, with a known position as a reference, as explained in the main text. What is a sol? A sol is a solar day on Mars. It is slightly longer than an Earth day at 24 hours, 39 minutes, 35 seconds. There are 668 sols in a Martian year (about 687 Earth days). 20 Silicon Chip Australia’s electronics magazine The rover uses a Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) for electrical power – see Figs.21 & 22. It was designed by Teledyne Energy Systems and is based on the design previously used by Pioneer 10 (1972) and 11 (1973), Viking 1 (1975) and 2 (1975). It converts heat from radioactive decay directly into electricity using thermoelectric couples connected in series as thermopiles. The MMRTG produces about 110W at launch, but due to radioactive decay and degradation of the thermocouples, that reduces over time. The Perseverance rover has a design lifetime of three of our Earth years, but it is expected that the MMRTG will produce sufficient power for its design life of 14 years; it will likely last much longer than that. The RTGs on the Voyager spacecraft (described in the December 2018 issue; siliconchip.com.au/Article/11329) are still going 44 years after launch (since 1977). To meet brief periods of peak electrical demand, the MMRTG charges two Li-ion batteries which provide supplemental power. Excess heat is dissipated with a heat exchanger that uses trichlorofluoromethane (CFC-11) fluid. Some of this heat is used to keep the rover systems warm during interplanetary cruise and on Mars’ surface. The MMRTG is a cylinder 64cm in diameter and 66cm long, weighing 45kg. It uses 4.8kg of plutonium dioxide as fuel, containing the isotope Pu-238. The radioactive heat source is contained within multiple layers to remain safe and survive the worst possible launch accident. Perseverance instruments apart from cameras MEDA is an instrument located at siliconchip.com.au siliconchip.com.au ► Fig.23: the process by which MOXIE converts Martian CO2 to O2. C&DH stands for Command and Data Handling systems, RCE is Rover Compute Element, RPAM is Rover Power and Analog Assembly and RAMP is Rover Avionics Mounting Panel (or Plate according to some sources). ► various places on the robot body to analyse airborne Martian dust and also make weather measurements – see Fig.15. It measures wind speed and direction, temperature and humidity, quantity and size of dust particles, and radiation from the sun and space. The instrument was developed and provided by the Spanish Astrobiology Center at the Spanish National Research Council in Madrid. You can view the latest Martian weather report at https://mars.nasa.gov/mars2020/ weather/ to see whether you need an umbrella on your Martian vacation. MOXIE, the Mars OXygen In-situ resource utilization Experiment, is a device inside the rover which is designed to test the technology of turning carbon dioxide (CO2), the dominant gas in the atmosphere of Mars, into oxygen (O2) – see Figs.2325. This technology could be used on later manned missions to produce breathable oxygen for Martian explorers to breathe. It is a 1:200 scale model of a plant that might be used for a manned mission. Oxygen can also be used as one component of rocket propellant. The reaction of 2CO2 → O2 + 2CO is a solid-state electrolysis reaction conducted within a ceramic reaction cell at high pressure and temperature (800°C). The carbon monoxide, CO, produced from this reaction can be used as a low-grade fuel when oxidised with the O2. Alternatively, it can be combined with hydrogen (H2) from the electrolysis of water (H2O), believed to be present on Mars in numerous locations, to produce methane (CH4) via the reaction CO + 3H2 → CH4 + H2O. H2 is a high-grade rocket fuel when used with O2 as the oxidiser. CO2 can also be converted to CH4 (methane) by the reaction CO2 + 4H2 → CH4 + 2H2O. Producing oxygen for breathing and propulsion and methane for propulsion is important because the large quantities required would be unfeasible to bring from Earth. Nuclear power would be the power source for these reactions. R I M FA X ( R a d a r I m a g e r f o r Mars’ Subsurface Experiment) is a ground-penetrating radar to probe the ground beneath the rover, looking at subsurface geological features Fig.24: a top view of MOXIE. It is designed to operate at very low Martian atmospheric pressures, 1% or less than Earth’s at sea level. Fig.25: the MOXIE device being lowered into the belly of the rover. The rover is upside-down to give better access for the installation. The unit measures 24 x 24 x 31cm, weighs 15kg and consumes 300W. Australia’s electronics magazine July 2021 21 – see Fig.29. It operates at 150MHz to 1200MHz, has a vertical resolution of 15cm to 30cm and a penetration depth up to 10m, depending on conditions. It can detect water, ice or salty brines, important in the search for water, and will operate as the rover drives along. It was developed and built by the Norwegian Defence Research Establishment (FFI). Ingenuity helicopter Fig.26: the locations of various systems on the Ingenuity helicopter, see https://w.wiki/3LWt Fig.27: technicians preparing Ingenuity, the actual vehicle that went to Mars, for flight tests inside the NASA/JPL 25-foot Space Simulator. The gold tubes are a support structure, not part of the helicopter. The stainless steel Simulator chamber is 26m high with an 8.2m diameter, and can be pumped down to the vacuum of space, or in this case, it can be pressurised to be the same as the Martian atmosphere. The facility has been in use since 1961. Fig.28: a selfie taken by Perseverance, along with the Ingenuity helicopter it carried as payload on April 5th 2021. Note the rover tracks. 22 Silicon Chip Australia’s electronics magazine The Perseverance rover carried with it a small helicopter which was the first powered aircraft to fly on another planet (see Figs.26 & 27). It is a technology demonstrator to prove whether a helicopter can fly on Mars. Photographs from a helicopter would have about ten times the resolution of orbital images, and could assist with route planning and mapping on future missions. The helicopter could fly ahead of a rover as a scout (see Fig.28), or it could pick up samples and bring them back to a central point for analysis. It could go to places a rover could not reach, such as to take close-up images of the sides of cliffs. Note that while this is the first powered aircraft on another planet, it is not the first aircraft. In 1985, the Soviet Vega missions deployed two helium balloons (“aerobots”) on Venus. Ingenuity was planned to have a 30-day program of test flights. A typical flight lasts up to 90 seconds, and it can go as far as 300 metres from the “airstrip” and as high as 3-5 metres. Images are taken during the flight. The helicopter communicates with Earth via a datalink with the rover or Martian orbiters. Once the flight test program is complete, the rover will drive off, leaving the helicopter behind, and it is not planned to be used again. Flying a helicopter on Mars has many challenges. The atmospheric pressure is extremely low; about 1% of that on Earth. This is eased somewhat by the lower gravity on Mars, about 38% that of Earth. According to Bob Balaram, Chief Engineer of JPL Mars Helicopter, flying a helicopter near the surface of Mars is equivalent to flying one on Earth at an altitude of 30,000m. The highest altitude ever achieved on Earth by a helicopter was 12,954m on March 23rd, 2002 by Fred North in a Eurocopter AS350 B2 (view the video at www. fred-north.com/record). siliconchip.com.au siliconchip.com.au Leg assembly Upper sensor assembly ► To fly on Mars, the helicopter’s coaxial rotors have to spin at about 2400rpm, compared to about 500rpm of a full-size Earth-based helicopter. However, this is not as fast as the rotors on a small quadcopter, which can reach about 6000rpm. The helicopter weighs 1.8kg on Earth or 684g on Mars (comparable to a DJI Phantom 4 at 1.38kg on Earth). Ingenuity’s rotors have a diameter of 1.2m, weigh 35g each, and are made of foam-cored carbon fibre. Their tip speed is restricted to Mach 0.7, as there are lots of undesirable effects at higher tip speeds. The rotor size was dictated by the available accommodation space on the rover. A further detail for aviation buffs is that the cyclic and collective are on the lower rotor, with just a collective on the upper rotor. A solar panel charges a six-cell Li-ion battery to allow one 90-second flight per day. The power required for flight is 350W. At night, energy is also consumed to keep the battery and other electronics warm and functional despite outside temperatures of -18°C to -100°C. Two-thirds of the battery energy is used to keep the batteries and electronics warmed to a temperature of at least -15°C, with only one-third of the battery energy used for flight operations. The cells used are commercially available Sony units, US18650VTC4 Li-ion cells of nominal 2.1Ah capacity each (2.0Ah rated capacity), which anyone can buy off the shelf! Some sensors on the aircraft include: • a solar tracker • gyros • inertial measurement unit (IMU) • a visual navigation camera (to keep track of flight by feature comparison with previous video frames) • a 13-megapixel Sony colour camera for photography • tilt sensors • laser altimeter (Garmin LIDARLite v3) • hazard detectors The helicopter runs Linux with multiple processors. The main one is a Qualcomm Snapdragon 801 2.26GHz ARM processor with 2GB RAM and 32GB of flash memory for high-level functions; this was also used in some smartphones. Two Texas Instruments Hercules TMS570LC43x automotive safety microcontrollers at 300MHz with Fig.29: an example of what a RIMFAX subsurface image might look like showing sedimentary layers. Avionics boards Battery Lower sensor assembly Fig.30: the arrangement of the avionics ► boards and other items around the six-cell battery assembly. This way, the heat generated to keep the battery warm also keeps the other parts warm. 512KB RAM and 4MB flash are used for flight control – see Fig.31. They run in synchrony, and if an error is detected in one, the other takes over and the one with the error is power cycled to reset it. A MicroSemi ProASIC3L FPGA (field-programmable gate array) is the heart of the helicopter, providing functions not implemented in software due to resource limitations such as processing time or bandwidth. It provides high-level flight control, including: • attitude control • motor control • waypoint guidance • sensor I/O from the inertial measurement unit (IMU) • altimeter and inclinometer interface • current monitoring and temperature sensing • fault monitoring • system time management (eg, waking up the helicopter at a particular time) It does this using 25 separate serial interfaces. The FPGA functions are implemented using configurable logic gates rather than software. The FPGA and the battery management system are the only two systems on the machine powered at all times. Communications uses the lowpower Zigbee protocol (COTS 802.15.4) with 900MHz SiFLEX02 chipsets relaying data at up to 250kbps with a range of up to 1000m. The ‘copter was test flown in a large vacuum chamber at JPL, the “25-foot Space Simulator” pumped out and back-filled with a carbon dioxide atmosphere at Mars pressure. Lower gravity was simulated by partially supporting the craft on a fishing line connected to a constant-force linear motor to offset part of the weight. The helicopter cannot fly freely on Earth without this offset. The reason for using a coaxial helicopter design rather than a quadcopter design, as is commonly used for drones, is that the blades would have Fig.31: the layout of the avionics boards on Ingenuity. They are wrapped into five sides of a cube around the battery pack as shown in Fig.30. Australia’s electronics magazine July 2021 23 Fig.32: NASA’s proposed Kilopower concept, with four individual reactors (umbrella-like objects) of 10kW each, plus a nuclear-powered crewed vehicle. to be so large that the aircraft would not fit on the rover. Coaxial rotors are also an efficient arrangement for providing thrust, although they are mechanically more complex than a traditional helicopter arrangement using a tail rotor. The helicopter’s software, like the rover, can be remotely updated from Earth. During the first high-speed rotor spin test of Ingenuity on Mars, a problem was identified: it “did not transition from a pre-flight check-out mode to its flight mode as expected... The onboard logic did not recognize the flight control computers as healthy and functional, even though it was confirmed they were.” A software update was developed and validated, then sent via the DSN to a Mars orbiting satellite, transferred to Perseverance, then to Perseverance’s Helicopter Base Station (HBS). The HBS is a “dedicated controller in the rover which collects, stores, and configures data communications between the rover and the helicopter”. The software was then relayed to the helicopter. Ingenuity had its first successful flight on April 19th, 2021. It lasted 39.1 seconds. See the video titled “First Video of NASA’s Ingenuity Mars Helicopter in Flight, Includes Takeoff and Landing (High-Res)” at https://youtu. be/wMnOo2zcjXA For further details on the Ingenuity helicopter, see the PDF file at http:// siliconchip.com.au/link/ab8l Power sources for future Mars settlements This mission partly relates to gathering information in preparation for a human landing on Mars, including converting atmospheric CO2 to O2. So it is worth considering what power sources could be used for such a settlement. Solar energy is too weak on Mars for serious use (sunlight is about 40% as intense as on Earth). Large amounts of power would be needed for atmospheric processing and other functions; therefore, nuclear power would likely need to be used. NASA has developed the Kilopower concept for nuclear power on Mars (see Figs.32 & 33). It uses a Uranium-235 core and can run for 10 years without maintenance. It uses a Stirling engine to convert heat to mechanical force, to power a generator producing electricity. It also uses a titanium radiator to dispose of excess heat, beryllium as a neutron reflector and a boron carbide rod to control the reactor’s output or shut it down. For more information on the Mars 2020 mission visit: https://mars.nasa. SC gov/mars2020/ Stirling engines and balancers Titanium radiator Stirling converters Sodium heat pipes Lithium hydride shielding Sodium heat pipes Beryllium shield and uranium core Fig.33: a highly simplified diagram of the NASA Kilopower nuclear reactor. Some of the internal detail is shown on the right. A Stirling radioisotope generator is about four times more efficient than a radioisotope thermoelectric generator (RTG), as used on the Perseverance rover and Voyage spacecraft. 24 Silicon Chip Australia’s electronics magazine Beryllium oxide reflectors Reactor core Boron carbide control rod siliconchip.com.au