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2003 Mars Rovers NASA’s next mission to Mars Ever since the first fly-by of Mars in 1965, the red planet has captured the imagination of scientists and explorers worldwide. The 1997 landing of the Pathfinder Mission further inspired the world with footage from the Rover as it traversed the rocky surface of Mars. Six years on, NASA/JPL are set to launch two bigger and smarter Rovers to continue the exploration. T he Mars Exploration Rover Mission is part of NASA’s longterm series of missions to undertake robotic exploration of the surface of the planet. This month’s launch of the mission will take advantage of the periodic alignment of various planets which occurs every 26 months. This mission will have numerous scientific instruments but it primarily seeks answers about water on Mars. This fits into the four objectives of the long term Mars Exploration Program: 8  Silicon Chip (a) to determine if life ever existed on Mars; (b) Characterisation of the climate of Mars; (c) Characterisation of the surface of Mars and (d) Preparation of scientific knowledge for potential future human exploration of Mars. Two separate Boeing Delta II launch rockets, each carrying a Mars Rover By Sammy Isreb exploration vehicle, will be launched from Cape Canaveral, Florida, between 30th May and 12th July 2003. The spacecraft will arrive at Mars during January 2004. Rover A, to be launched between 30th May and 16th June, is currently planned to arrive on 4th January 2004. Rover B, to be launched shortly afterwards (between 25th June and 12th July), is set to arrive on 25th January 2004. The Rover vehicles will land on two separate sites on the Martian surface. www.siliconchip.com.au of the craft will be altered by thruster burns to ensure the appropriate spin rate and that the antennas are directed towards the Earth and the solar panels towards the sun. The communications system has several modes to enable reception and transmission of data to Earth. The Deep Space Network is used on Earth to communicate with both craft and, later on, the Rovers. A low gain antenna is used in the early part of the mission near Earth. As the distance increases, a switch will be made to a medium gain antenna. Arrival at Mars Ahh. . . it’s good to see the boffins at the JPL have a sense of humour . . . or is that face to scare the Martians? Here’s one of two Rovers being packaged ready for blast-off on a Delta II launch vehicle, planned for this month. (NASA/JPL) After leaving the Earth’s gravitational pull, the spacecraft will separate from the Boeing Delta II launch vehicle. The craft measures around 2.65m in dia-meter and 1.6m in height, with a mass of 1063kg. The structure is comprised primarily of aluminium ribs, covered by solar panels. The panels generate around 600W of power shortly after leaving Earth, dropping to around 300W on approach to Mars. A complex system is used to regulate the temperature of vital components inside the craft during the cruise stage. Heaters and multi-layer insulation are employed in order to keep the spacecraft electronics warm, with a Freon system used to pump heat from the core of the flight computer and telecommunications equipment. Like all spacecraft, an onboard navigation system and a compensatory propulsion system are used for numerous trajectory correction manoeuvres. In order to determine when a trajectory correction is necessary, the Star Scanner and Sun Sensor is used. This allows the spacecraft’s flight computer to determine its location by using the sun and various stars as references. If a corrective burn is required, various thrusters use a hydrazine propellant. This is carried in two tanks with a total capacity of 31kg. During flight, the craft is spin-stabilised at around 2rpm. Occasionally, the orientation Stowed in the nose cone of a Delta II rocket, the two Mars Exploration Rovers blast off this month from the Kennedy Space Center in Florida. www.siliconchip.com.au As the craft enters the Martian atmosphere the Aero Shell and Retrorocket assembly will slow it from over 16,000km/h to around 1600km/h within one minute. Central to the survival of the descent is the heat shield portion of the Aero Shell, which is primarily an aluminium honeycomb structure sandwiched between graphite epoxy sheets. The shield is coated with a phenolic compound impregnated with corkwood and tiny silica glass spheres, which react with the Martian atmosphere to dissipate heat from the structure, leaving a wake of hot gas. After the initial atmospheric braking, about 10km above the surface of Mars a parachute is deployed and the heat shield jettisoned. Because the Martian atmosphere is only about 1% as dense as that on Earth, the parachute assembly does not slow the craft down enough to permit a safe landing. For this reason, The nose cone of the rocket separates during the launch phase and the Mars Exploration Rover is sent on an eight-month journey to Mars. June 2003  9 a technique known as Rocket Assisted Descent (RAD) is used. Three RAD motors (solid state rockets) provide over one tonne of reverse thrust for around two seconds. These are fired to bring the craft to a stop about 10 or so metres above the surface. The craft then drops but just before it hits the surface, numerous airbags encasing the Lander will be inflated. The inflated Lander structure will then bounce along the surface, rolling to a stop. A radar altimeter unit is used to determine when to deploy the parachute, when to release the chute, when to fire the RAD rockets and finally, when to deploy the airbags. Shortly after landing, the airbags will be deflated, the Rover will emerge, unfurl its petal-like solar panels and commence the ground-based portion of the mission. Rover deployment Inside the protective airbags is the Lander structure which houses the Rover. The structure consists of a tetrahedron base, with three “petals” folded up to create a pyramid. These petals are hinged, with a motor driving each hinge, so that the pyramid will be unfolded upon landing (to form a flat structure). Each motor is strong enough to lift the entire assembly, so that the Lander will be unfolded to its desired position, irrespective of which side it initially falls on. The Rover is secured in the Lander with special bolts, which contain explosive charges to unshackle it from its storage position. The Rover will then The Rover emerging from its lander structure – a tetrahedron base with three petals which fold up to create a pyramid. No matter which way up it lands, strong motors on the petals will turn it right-side-up. (NASA/JPL) roll down specially built ramps on the petals, which protect it from getting tangled up in the remains of the airbags, or falling and being damaged. It is estimated the time taken from when the Lander touches down to the time the Rover rolls onto the Martian soil will be about three hours. The Rover The Rover has six 25cm wheels, each driven by its own motor. The front and back two wheels have their own steering motors, to allow the Rover to turn a full 360° on the spot. This is designed to allow the Rover to escape any tight situations it may find itself in. The suspension setup is known as a “rocker-bogie” system which can swivel its wheels to arc around The aeroshell protects the Rover from fiery temperatures as it enters the Martian atmosphere. The craft are scheduled to arrive in January 2004. 10  Silicon Chip corners. Rocker suspension systems prevent the vehicle from moving up or down a great deal whilst traversing rocky terrain and even out the weight distribution across all wheels. Through careful weight distribution and the advanced suspension system, the Rover can withstand being tilted to 45° in any direction without overturning. As a safety mechanism, however, the control software will avoid getting the vehicle into any position where the tilt exceeds 30°. On flat ground the Rover has a maximum speed of 5cm per second. However, in order to avoid getting stuck, the control software causes the Rover to stop and assess its location every few seconds. This results in an average speed of around 1cm/sec or With the parachute deployed, three retrorockets fire their engines, suspending the lander 10-15m above the Martian surface. It then drops onto its own deployed airbags. www.siliconchip.com.au The Rover undergoing testing on a simulation of the Mars surface. The real mission is scheduled to run for 90 Martian days. Conversion factor: 1 DayM = 24HE, 38ME and 22SE, (about 92-and-a-bit DE!) (NASA/JPL) 36 metres/hour. (It won’t be able to escape any war-like Martians!) Driving the Rover consumes around 100W. This is supplied by solar panels generating about 140W while they are illuminated for the four hours of each (Martian) day. For the rest of the time two rechargeable batteries provide power to the Rover. The mission is scheduled to run for 90 Martian days, during which time the solar panels will become increasingly coated with dust. By the end of the mission, their generating capacity will be reduced to 50W. This phenomenon was initially observed during the 1997 Pathfinder mission and is one of the factors which will ultimately end the mission. The Rover Electronics Module (REM) processes information from the various sensors, power systems and communications links to control the Rover and send data back to Earth. The REM contains 128Mb of DRAM and 3Mb of EEPROM. This does not sound like a great amount but specialised memory chips must be used to safeguard against data loss from the extreme radiation encountered in space, as well as the possibility of power outages. To put it into context, these Rovers will have around 1000 times the memory capacity of the Rover aboard the Pathfinder mission. A fair proportion of the computing power is dedicated to running the IMU, or Inertial Measurement Unit. This provides triaxial information www.siliconchip.com.au on its position, allowing the Rover to make precise vertical, horizontal and yaw movements. Another function of the Rover software is to perform constant system health checks, ensuring that the temperature is regulated and that the power systems are functioning, for example. Communications systems The Rovers employ a complex communications system to send data back to Earth. Direct communication to Earth, via either low gain (omnidirectional) or high-gain (directional) antennas, is one option. In addition, the Rovers can communicate directly with Mars-orbiting craft, such as 2001 Mars Odyssey and the Mars Global Surveyor satellites. These satellites can then relay information back to Earth. Using a combination of these two techniques, the Rover can maximise possible transmission times (as the relative positions of Earth, Mars and the satellites will affect which is the suitable means of transmission). The data rate between the Rover and Earth varies between 3,500 bits/ second and 12,000bps, depending on various environmental factors. The data rate to the orbiting satellites, on the other hand, is 128,000bps, so this form of relayed communications is used wherever possible. Visual systems Rovers carry nine onboard cameras. Four are for hazard avoidance while two are used for navigation. The four hazard-avoidance cameras are mounted on the bottom at the front and rear of the Rover. They operate, in black and white, to build a three dimensional map of the surrounding terrain extending 4m around the vehicle. Onboard image processing software allows Rover to think for itself, in addition to commands issued to it from Earth, in order to provide an additional safeguard to avoid obstacles. The two navigational cameras are mounted atop the Rover’s mast, to provide a stereoscopic 45° view of the terrain in front of the cameras. These images are used to support navigational planning by scientists and engineers back on Earth. Motors within the mast assembly allow the cameras to rotate. Head and neck Giving the Rover its distinct appearance, the 1.4m Pancam Mast Assembly serves two functions. It acts as a periscope for the Mini-TES scientific instrument which must be housed within the Rover body for thermal reasons. Secondly, the mast provides a high vantage point for the cameras. Built into the mast assembly is a motor which can turn the cameras and Mini-TES 360° in the horizontal plane. A second motor, responsible for elevation, can point the cameras 90° above and below the horizon. A third motor, dedicated to moving the mini-TES, can rotate this instrument from 30° above and 50° below the horizon. Robotic arm More than just another gadget to convey human-like characteristics, the Rover arm, also called the Instrument Deployment Device (IDD), manoeuvres the geological instruments for examining the Martian rocks and soils. IDD has three joints, a wrist, elbow and shoulder joint. At the end of the arm is cross-shaped turret which rotates to whichever of the four scientific instruments is needed at the time. When the Rover is moving on to its next destination, the arm is folded onto itself around the elbow and rests in the front of the Rover body, safe from harm. When it is needed it simply extends, selects the appropriate tool and goes to work. The four instruments are as follows: June 2003  11 Rock abrasion tool (RAT) The RAT is a powerful grinder weighing just 720g, able to create a hole 45mm in diameter and 5mm in depth into solid rock. Three motors drive the abrasive grinding head. When a fresh rock surface is exposed by RAT, it can be examined by Rover’s other scientific tools. Microscopic imager (MI) The MI is a combination of a microscope and a CCD camera (1024 x 1024 pixels) which will provide close-up views of the surface details of soils or rocks, especially rocks previously operated upon by the RAT. Mossbauer spectrometer (MB) The MB is a spectrometer which is designed to provide the specific compositions of iron-rich minerals which predominate on Mars. The measurement head of the MB resides on the end of the robotic arm, with the associated electronics taking shelter in the Rover’s Warm Electronics Box (WEB – insulated using gold sheeting and very precisely temperature regulated). To take a measurement, the sensor head is pressed against the rock or soil sample for a 12-hour period. Alpha Particle X-Ray Spectrometer (APXS) Another tool designed to determine the chemical composition of the surface of Mars, the APXS measures emitted alpha and X-ray particles from rock and soil samples. At the end of the robotic arm is the RAT, a powerful grinder which can make a hole 45mm in diameter, 5mm deep. The grindings can then be analysed using a range of on-board scientific equipment. (NASA/JPL) Alpha rays are emitted by radioactive decay, indicating the presence of various isotopes. X-rays will be reflected, like light or microwaves, from the surface in amounts depending on composition. Like the other instruments in the arm, the APXS electronics reside in the WEB. A single APXS measurement will take several hours at least, in order to gather enough useful data. Mast instruments In addition to these four instruments residing in the Rover’s arm, there is other scientific apparatus in the Pancam Mast Assembly, as follows. Miniature Spectrometer (Mini-TES) The Mini-TES is a standard spectrographic device which is used to determine the composition of rocks and soils. It does this by analysing their patterns of reflected thermal radiation, which vary based upon the composition of the material. A goal of the Mini-TES is to search for materials which owe their existence to a presence of water such as clays and carbonates. The body of the Mini-TES is in the chassis of the Rover, where the mast meets the base. At the top of the mast is a periscope which moves around in various directions and focuses light down through the mast towards the Mini-TES apparatus. Pancam Mounted atop the mast, the Pancam is an ultra-high resolution CCD imaging system. Weighing just 270g, it can produce image mosaics with resolutions as high as 4000 pixels high and 24,000 pixels around. A filter wheel sitting in line with the Pancam lens provides imaging within various wavelength bands. Seven months to go! All told, when the Rovers arrive at Mars during January 2004, they are set to provide the most amazing insight SC into the planet to date. While the Rover can communicate directly with Earth, it will usually use the Odyssey spacecraft (in orbit around Mars) as a repeater, with data transmission rates to Earth up to 50 times faster compared to direct transmission. (NASA/JPL) 12  Silicon Chip Acknowledgement: Thanks to NASA and the Jet Propulsion Laboratory for the information and photographs used in this feature. www.siliconchip.com.au