Spacecraft: Surface Operations: Rover

The Mars Exploration Rovers act as robot geologists while they are on the surface of Mars.

You can explore the various parts of the rover by clicking on the image to the left.

In some senses, the rovers´ parts are similar to what any living creature would need to keep it "alive" and able to explore.

The rover has a: 

• The rover´s "body" :-   a structure that protects the rovers´ "vital organs".
  The rover body is called the warm electronics box, or "WEB" for short. Like a car body, the rover body is a strong, outer layer that protects the rover´s computer, electronics, and batteries (which are basically the equivalent of the rover´s brains and heart). The rover body thus keeps the rover´s vital organs protected and temperature-controlled.
  
  The warm electronics box is closed on the top by a triangular piece called the Rover Equipment Deck (RED). The Rover Equipment Deck makes the rover like a convertible car, allowing a place for the rover mast and cameras to sit out in the Martian air, taking pictures and clearly observing the Martian terrain as it travels.
  The gold-painted, insulated walls of the rover body also keep heat in when the night temperatures on Mars can drop to -96 degrees Celsius (-140 degrees Fahrenheit). [More on the rovers'  temperature controls].                                                                                         
• The rover´s "brains"  computers to process information.Unlike people and animals, the rover brains are in its body. The rover computer (its "brains") is inside a module called "The Rover Electronics Module" (REM) inside the rover body. The communication interface that enables the main computer to exchange data with the rover´s instruments and sensors is called a "bus" (a VME or Versa Module Europa bus to be exact). This VME bus is an industry standard interface bus to communicate with and control all of the rover motors, science instruments, and communication functions.
  
  Better memory than ever
  The computer is composed of equipment comparable to a high-end, powerful laptop computer. It contains special memory to tolerate the extreme radiation environment from space and to safeguard against power-off cycles so the programs and data will remain and will not accidentally erase when the rover shuts down at night.
  On-board memory includes 128 MB of DRAM with error detection and correction and 3 MB of EEPROM. That´s roughly the equivalent memory of a standard home computer. This onboard memory is roughly 1000 more than the Sojourner rover from the Pathfinder mission had.
  Better "nerves" for balance and position
  The rover carries an Inertial Measurement Unit (IMU) that provides 3-axis information on its position, which enables the rover to make precise vertical, horizontal, and side-to-side (yaw) movements. The device is used in rover navigation to support safe traverses and to estimate the degree of tilt the rover is experiencing on the surface of Mars.
  Monitoring its "health"
  Just like the human brain, the rover computers register signs of health, temperature, and other features that keep the rovers "alive."
  The software in the main computer of the rover changes modes once the cruise portion of the mission is complete and the spacecraft begins to enter the Martian atmosphere. Upon entry into the Martian atmosphere, the software executes a control loop that monitors the "health" and status of the vehicle. It checks for the presence of commands to execute, performs communication functions, and checks the overall status of the rover. The software does similar health checks in a third mode once the rover emerges from the lander.
  This main control loop essentially keeps the rover "alive" by constantly checking itself to ensure that it is both able to communicate throughout the surface mission and that it remains thermally stable (not too hot or too cold) at all times. It does so by periodically checking temperatures, particularly in the rover body, and responding to potential overheating conditions, recording power generation and power storage data throughout the Mars sol (a Martian day), and scheduling and preparing for communication sessions.
  Using its "computer brains" for communications
  Activities such as taking pictures, driving, and operating the instruments are performed under commands transmitted in a command sequence to the rover from the flight team.
  The rover generates constant engineering, housekeeping and analysis telemetry and periodic event reports that are stored for eventual transmission once the flight team requests the information from the rover.
  To find out more about how the rovers "talk" to engineers back on Earth, see the Communications section.                                                                                                                                                           
• The rover´s temperature controls :-  internal heaters, a layer of insulation, and more.
  Like the human body, the Mars Exploration Rover cannot function well under excessively hot or cold temperatures. In order to survive during all of the various mission phases, the rover´s "vital organs" must not exceed extreme temperatures of -40º Celsius to +40º Celsius (-40º Fahrenheit to 104º Fahrenheit).
  The rover´s essentials, such as the batteries, electronics, and computer, which are basically the rover´s heart and brains, stay safe inside a Warm Electronics Box (WEB), commonly called the "rover body." Heaters are packed inside the rover body, and like a warm coat, the WEB walls help keep heat in when the night temperatures on Mars can drop to -96º Celsius (-140º Fahrenheit). Just as an athlete sweats to release heat after an intense workout, the rover's body can also release excess heat through its radiators, similar to ones used in car engines.
  There are several methods engineers used to keep the rover at the right temperature:
  • Preventing heat escape through gold paint
  • Preventing heat escape through insulation called "aerogel"
  • Keeping the rover warm through heaters
  • Making sure the rover is not too hot or cold through thermostats and heat switches
  • Making sure the rover doesn't get too hot through the heat rejection system
  Why the rover needs temperature control systems on Mars
  Many of these methods are very important to making sure the rover doesn´t "freeze to death" in the cold of deep space or on Mars. Many people often assume that Mars is hot, but it is farther away from the sun and has a much thinner atmosphere than Earth, so any heat it does get during the day dissipates at night. In fact, the ground temperatures at the rover landing sites swing up during the day and down again during the night, varying by up to 113 degrees Celsius (or 235 degrees Fahrenheit) per Mars day. That´s quite a temperature swing, when you consider that Earth temperatures typically vary by tens of degrees on average between night and day.
  At the landing sites, an expected daytime high on the ground might be around 22º Celsius (71º Fahrenheit). An expected nighttime lows might be -99º Celsius (-146º Fahrenheit). Atmospheric temperatures, by contrast, can vary up to 83º Celsius (181º Fahrenheit). An atmospheric daytime high might be -3º Celsius (26º Fahrenheit), while a nighttime low might be -96º Celsius (-140º Fahrenheit).                                                                                                                                           
•  The rover´s "neck and head"
  
  What looks like the rover "neck and head" is called the Pancam Mast Assembly. It stands from the base of the rover wheel 1.4 meters tall (about 5 feet). This height gives the cameras a special "human geologist´s" perspective and wide field of view.
  The pancam mast assembly serves two purposes:
  
  • to act as a periscope for the Mini-TES science instrument that is housed inside the rover body for thermal reasons
  • to provide height and a better point of view for the Pancams and the Navcams
  Essentially, the pancam mast assembly enables the rover to see in the distance. The higher one stands, the more one can see. You can test this out for yourself by lying on the ground and observing as much as possible, then standing up and seeing the difference in the amount of greater detail you can observe about the world with a broader field of view.
  One motor for the entire Pancam Mast Assembly head turns the cameras and Mini-TES 360º in the horizontal plane. A separate elevation motor can point the cameras 90º above the horizon and 90º below the horizon. A third motor for the Mini-TES elevation, enables the Mini-TES to point up to 30º over the horizon and 50º below the horizon.
  During cruise, the Pancam Mast Assembly lays flat against the top of the rover equipment deck in a stowed configuration. After the lander opens on the surface of Mars, pyros release the bolts holding it down . Pyros are solid mechanical devices that contain as much power as a bullet release mechanism in a gun. Pyros were designed in the 1960s for the Apollo mission as a safe way to release bolts and strong devices on spacecraft while humans were in the vicinity.
  The Pancam Mast Assembly rises from the rover equipment deck by driving a motor that moves the Pancam upward, in the shape of a helix. It sweeps out in a cone-like manner as it deploys. Once the Pancam Mast Assembly is in its full-upright position, it does not stow again, but stays upright for the entire duration of the mission.                                                                                                         
• The rover's "eyes" and other "senses"
  
  Each rover has nine "eyes."
  
  Six engineering cameras aid in rover navigation and three cameras perform science investigations.
  
  Each camera has an application-specific set of optics.
  

  Four Engineering Hazcams (Hazard Avoidance Cameras):

  Mounted on the lower portion of the front and rear of the rover, these black-and-white cameras use visible light to capture three-dimensional (3-D) imagery. This imagery safeguards against the rover getting lost or inadvertently crashing into unexpected obstacles, and works in tandem with software that allows the rover make its own safety choices and to "think on its own."

  The cameras each have a wide field of view of about 120 degrees. The rover uses pairs of Hazcam images to map out the shape of the terrain as far as 3 meters (10 feet) in front of it, in a "wedge" shape that is over 4 meters wide at the farthest distance. It needs to see far to either side because unlike human eyes, the Hazcam cameras cannot move independently; they¹re mounted directly to the rover body.

  Two Engineering Navcams (Navigation Cameras):

  Mounted on the mast (the rover "neck and head), these black-and-white cameras use visible light to gather panoramic, three-dimensional (3D) imagery. The Navcam is a stereo pair of cameras, each with a 45-degree field of view to support ground navigation planning by scientists and engineers. They work in cooperation with the Hazcams by providing a complementary view of the terrain.

  Two Science Pancams (Panoramic Cameras):

  This color, stereo pair of cameras is mounted on the rover mast and delivers three-dimensional panoramas of the Martian surface. As well as science panoramas, the narrow field of view and height of the cameras basically mimic the resolution of the human eye (0.3 milliradians), giving the world a view similar to what a human geologist might see if she or he were standing on the surface of Mars. Also, the Pancam detectors have 8 filters per "eye" and between the two "eyes" there are 11 total unique color filters plus two-color, solar-imaging filters to take multispectral images. The Pancam is also part of the rover's navigation system. With the solar filter in place, the Pancam can be pointed at the Sun and used as an absolute heading sensor. Like a sophisticated compass, the direction of the Sun combined with the time of day tells the flight team exactly which way the rover is facing. [More on the Pancams in the science instrument section.]

  One Science Microscopic Imager:

  This monochromatic science camera is mounted on the robotic arm to take extreme close-up pictures of rocks and soil. Some of its studies of the rocks and soil help engineers understand the properties of the smaller rocks soil that can impact rover mobility (how much resistance it has against the rover wheels, how far they'll sink) . [More on the Microscopic Imager in the science instrumentsection.]

  For more information on how the rover uses its cameras to move on the surface of Mars, please see Rover Navigation in the Surface Operations section of the mission timeline.
  For more information on how the rover uses its cameras to conduct science investigations, please see Science Investigations in the Surface Operations section of the mission timeline.                            
•  The rover´s "arm"
  The rover arm (also called the instrument deployment device, or IDD) holds and maneuvers the instruments that help scientists get up-close and personal with Martian rocks and soil.
  Much like a human arm, the robotic arm has flexibility through three joints: the rover's shoulder, elbow, and wrist. The arm enables a tool belt of scientists´ instruments to extend, bend, and angle precisely against a rock to work as a human geologist would: grinding away layers, taking microscopic images, and analyzing the elemental composition of the rocks and soil.
  At the end of the arm is a turret, shaped like a cross. This turret, a hand-like structure, holds various tools that can spin through a 350-degree turning range.
  The four tools, or science instruments, on the robotic arm are:
  

  The Microscopic Imager provides close-up images of rocks and soil The Mössbauer Spectrometer analyzes the iron in rocks and soil The Alpha Particle X-Ray Spectrometer  analyzes the elemental composition of rocks and soil The Rock Abrasion Tool (RAT) grinds away the outer surface of rock to expose fresh material

  The forearm also holds a small brush so that the Rock Abrasion Tool can spin against it to "brush its teeth" and rid the grinding tool of any leftover pieces of rock before its next bite.
  Thirty percent of the mass of the titanium robotic arm comes from the four instruments it holds at the end of the arm. This weight makes maneuvering the lightweight arm a bit of a challenge -- like controlling a bowling ball at the end of a fishing rod. The arm must be as lightweight as possible for the overall health of the mission, and holes are even cut out in places where there is no need for solid titanium.
  
  Protecting the arm
  Once the arm and instruments have succeeded in one location but before the rover begins another traverse, the arm stows itself underneath the "front porch" of the rover body. The elbow hooks itself back onto a pin, and the turret has a T-bar that slides back into a slotted ramp. The fit is almost as tight as a necklace clasp, and it can withstand shocks of 6 G´s while roving along the rocky terrain. "Six G´s" is roughly equivalent to dropping a box onto a hard floor from a height of 20 centimeters (almost 8 inches). During launch and landing, the arm is restrained by a retractable pin restraint, and can withstand even higher loads of 42 G´s.                                                                                           
• The rover´s wheels "legs"
  The Mars Exploration Rover has six wheels, each with its own individual motor.
  The two front and two rear wheels also have individual steering motors (1 each). This steering capability allows the vehicle to turn in place, a full 360 degrees. The 4-wheel steering also allows the rover to swerve and curve, making arching turns.
  
  How the Wheels Move
  The design of the suspension system for the wheels is similar to the Sojourner rover "rocker-bogie" system on the Pathfinder mission. The suspension system is how the wheels are connected to and interact with the rover body.
  The term "bogie" comes from old railroad systems. A bogie is a train undercarriage with six wheels that can swivel to curve along a track.
  The term "rocker" comes from the design of the differential, which keeps the rover body balanced, enabling it to "rock" up or down depending on the various positions of the multiple wheels. Of most importance when creating a suspension system is how to prevent the rover from suddenly and dramatically changing positions while cruising over rocky terrain. If one side of the rover were to travel over a rock, the rover body would go out of balance without a "differential" or "rocker", which helps balance the angle the rover is in at any given time. When one side of the rover goes up, the differential or rocker in the rover suspension system automatically makes the other side go down to even out the weight load on the six wheels. This system causes the rover body to go through only half of the range of motion that the "legs" and wheels could potentially experience without a "rocker-bogie" suspension system.
  The rover is designed to withstand a tilt of 45 degrees in any direction without overturning. However, the rover is programmed through its "fault protection limits" in its hazard avoidance software to avoid exceeding tilts of 30 degrees during its traverses.
  The rover rocker-bogie design allows the rover to go over obstacles (such as rocks) or through holes that are more than a wheel diameter (25 centimeters or 10 inches) in size. Each wheel also has cleats, providing grip for climbing in soft sand and scrambling over rocks.
  
  Rover Speed
  The rover has a top speed on flat hard ground of 5 centimeters (2 inches) per second. However, in order to ensure a safe drive, the rover is equipped with hazard avoidance software that causes the rover to stop and reassess its location every few seconds. So, over time, the vehicle achieves an average speed of 1 centimeter per second. The rover is programmed to drive for roughly 10 seconds, then stop to observe and understand the terrain it has driven into for 20 seconds, before moving safely onward for another 10 seconds.                                                                                           
•  The rover´s energy
  The rover requires power to operate. Without power, it cannot move, use its science instruments, or communicate with Earth. The main source of power for each rover comes from a multi-panel solar array. They look almost like "wings," but their purpose is to provide energy, not fly.
  When fully illuminated, the rover solar arrays generate about 140 watts of power for up to four hours per sol (a Martian day). The rover needs about 100 watts (equivalent to a standard light bulb in a home) to drive. Comparatively, the Sojourner rover´s solar arrays provided the 1997 Pathfinder mission with around 16 watts of power at noon on Mars. That´s equivalent to the power used by an oven light. This extra power will potentially enable the rovers to conduct more science.
  The power system for the Mars Exploration Rover includes two rechargeable batteries that provide energy for the rover when the sun is not shining, especially at night. Over time, the batteries will degrade and will not be able to recharge to full power capacity. Also, by the end of the 90-sol mission, the capability of the solar arrays to generate power will likely be reduced to about 50 watts of power due to anticipated dust coverage on the solar arrays (as seen on Sojourner/Mars Pathfinder), as well as the change in season. Mars will drift farther from the sun as it continues on its yearly elliptical orbit, and because of the distance, the sun will not shine as brightly onto the solar arrays. Additionally, Mars is tilted on its axis just like Earth is, giving Mars seasonal changes. Later in the mission, the seasonal changes at the landing site and the lower position of the Sun in the sky at noon than in the beginning of the mission will mean less energy on the solar panels.        
• The rover´s antennas
  The rover has both a low-gain and high-gain antenna that serve as both its "voice" and its "ears". They are located on the rover equipment deck (its "back").
  The low-gain antenna sends and receives information in every direction; that is, it is "omni-directional." The antenna transmits radio waves at a low rate to the Deep Space Network (DSN) antennas on Earth. The high-gain antenna can send a "beam" of information in a specific direction and it is steerable, so the antenna can move to point itself directly to any antenna on Earth. The benefit of having a steerable antenna is that the entire rover doesn´t necessarily have to change positions to talk to Earth. Like turning your neck to talk to someone beside you rather than turning your entire body, the rover can save energy by moving only the antenna.
  Not only can the rovers send messages directly to Earth, but they can uplink information to other spacecraft orbiting Mars, utilizing the 2001 Mars Odyssey and Mars Global Surveyor orbiters as messengers who can pass along news to Earth for the rovers. The orbiters can also send messages to the rovers.
  The benefits of using the orbiting spacecraft are that the orbiters are closer to the rovers than the Deep Space Network antennas on Earth and the orbiters have Earth in their field of view for much longer time periods than the rovers on the ground.
  The radio waves to and from the rover are sent through the orbiters using UHF antennas, which are close-range antennas which are like walky-talkies compared to the long range of the low-gain and high-gain antennas. One UHF antenna is on the rover and one is on the petal of the lander to aid in gaining information during the critical landing event. The Mars Global Surveyor will be in the appropriate location above Mars to track the landing process. (2001 Mars Odyssey will not be in the vicinity.)
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