NASA's Mars Exploration Rover (MER) Mission is an unmanned Mars exploration mission that includes sending two Rovers (robots) to explore the Martian surface and geology.
Primary among the mission's scientific goals is to search for and characterize a wide range of rocks and soils that hold clues to past water activity on Mars. The Mars Exploration Rover mission is part of NASA's Mars Exploration Program which includes the previous successful landers Viking in 1975 and Pathfinder in 1996.
The MER-A rover, Spirit, was launched on June 10, 2003 at 1:59 p.m. EDT, and MER-B, Opportunity, on July 7, 2003 at 11:18 p.m. EDT. Both launches were successful. Spirit landed in Gusev crater, and Opportunity is targeted to land in Meridiani Planum. In the week following Spirit's landing, NASA's website recorded 1.7 billion hits and 34.6 terabytes of data transferred, eclipsing records set following previous NASA missions.
The Mars Exploration Rover is stowed in the nose of a Delta II rocket.
Each spacecraft consists of several components. These are:
The cruise stage is the component of the spacecraft used for travel between Earth and Mars. The cruise stage is very similar to the Mars Pathfinder design and is approximately 2.65 meters (8.7 feet) in diameter and 1.6 meters (5.2 feet) tall when attached to the aeroshell.
The primary structure is aluminium with an outer ring of ribs covered by the solar panels, which are about 2.65m (8.7 feet) in diameter.
Divided into five sections, the solar arrays can provide up to 600 Watts of power near Earth and 300 Watts at Mars.
Heaters and multi-layer insulation keep the spacecraft electronics "warm." There is also a freon system used to remove heat from the flight computer and telecommunications hardware inside the rover so they don't get overheated. Cruise avionics systems allow the flight computer in the rover to interface with other electronics such as the sun sensors, the star scanner, and the heaters.
Star scanner and sun sensor: The star scanner (with a backup system) and sun sensor allow the spacecraft to know where it is in space by analyzing the position of the Sun and other stars in relation to itself. Sometimes the spacecraft can be slightly off course, a situation that is expected given the 320 million mile journey the spacecraft will make. Navigators thus plan up to six trajectory correction maneuvers, along with health checks.
Propellant tanks: To ensure the spacecraft arrives at Mars in the right place for its planned landing, two light-weight, aluminium-lined tanks carry a maximum capacity of about 31 kilograms (about 68 pounds) of hydrazine propellant. Along with cruise guidance and control systems, these tanks of propellant allow navigators to keep the spacecraft closely on course during cruise. Through burns and pulse firings, the propellant enables three different types of trajectory correction maneuvers:
The spacecraft uses a high-frequency X-band radio wavelength that allows spacecraft communications with less power and smaller antennas than many older spacecraft, which used S-band. Navigators send the commands through two X-band antennaee on the cruise stage:
Cruise Low-gain Antenna: The cruise low-gain antenna is mounted inside the inner ring and the cruise medium-gain antenna is mounted in the outer ring. During flight, the spacecraft is spin-stabilized with a spin rate of 2 rpm. Periodic spin axis pointing updates will make sure the antenna stays pointed toward Earth and that the solar panels stay pointed toward the Sun. The spacecraft will use the low-gain antenna early in cruise when the spacecraft is close to Earth. The low-gain antenna is omnidirectional, so the transmission power that reaches Earth falls off rapidly with increasing distance.
Cruise Medium-Gain Antenna: As the spacecraft moves farther from Earth and closer to Mars, the Sun comes into the same area of the sky as viewed from the spacecraft and not as much energy falls on the Earth alone. Therefore, the spacecraft switches to a medium-gain antenna, which can direct the same amount of transmission power into a tighter beam to reach Earth.
The aeroshell forms a protective covering for the lander during the seven month voyage to Mars. The aeroshell, together with the lander and the rover, constitute what engineers call the "entry vehicle." The aeroshell's main purpose is to protect the lander with the rover stowed safely inside from the intense heating of entry into the thin Martian atmosphere on landing day.
The aeroshell for the Mars Exploration Rovers is based on the Mars Pathfinder and Mars Viking designs.
The aeroshell is made of two principle parts:
The backshell carries the parachute and several components used during later stages of entry, descent, and landing, including:
Built by the Lockheed Martin Astronautics Co. in Denver, Colorado, the aeroshell is made out of an aluminium honeycomb structure sandwiched between graphite-epoxy face sheets. The outside of the aeroshell is covered with a layer of phenolic honeycomb. This phenolic honeycomb is filled with an ablative material (also called an "ablator"), that dissipates heat generated by atmospheric friction.
The ablator itself is a unique blend of cork wood, binder and many tiny silica glass spheres. It was invented for the heat shields flown on the Viking Mars lander missions 25 years ago. A similar technology was used in the first US manned space missions Mercury, Gemini and Apollo. It is specially formulated to react chemically with the Martian atmosphere during entry and carry heat away, leaving a hot wake of gas behind the vehicle. The vehicle will slow from 19,000 km/h (about 12,000 mph) to about 1600 km/h (1000 mph) in about a minute, producing about 6 G of acceleration on the lander and rover.
Both the backshell and heat shield are made of the same materials, but the heat shield has a thicker (1/2 in) layer of the ablator. Also, instead of being painted, the backshell will be covered with a very thin aluminized mylar blanket to protect it from the cold of deep space. The blanket will vaporize during Mars atmospheric entry.
The parachute will help slow the spacecraft down during entry, descent, and landing. It is located in the backshell.
The 2003 parachute design is part of a long-term Mars parachute technology development effort and is based on the designs and experience of the Viking and Pathfinder missions. The parachute for this mission is 40% larger than Pathfinder's because the largest load for the Mars Exploration Rover is between 80,100 - 84,600 N (18,000 and 19,000 pounds) when the parachute fully inflates. By comparison, Pathfinder's inflation loads were approximately 35,600 N (about 8,000 pounds).
The parachute is made out of two durable, lightweight fabrics: polyester and nylon. The parachute has a triple bridle (the tethers that connect the parachute to the backshell) made of Kevlar.
The amount of space available on the spacecraft for the parachute is so small that the parachute must be pressure packed. Before launch, a team must tightly fold together the 48 suspension lines, three bridle lines, and the parachute. The parachute team loads the parachute in a special structure that then applies a heavy weight to the parachute package several times. Before placing the parachute into the backshell, the parachute is heat set to sterilize it.
Zylon Bridles: After the parachute is deployed at an altitude of about 10 km (6 miles) above the surface, the heatshield is released using 6 separation nuts and push-off springs. The lander then separates from the backshell and "rappels" down a metal tape on a centrifugal braking system built into one of the lander petals. The slow descent down the metal tape places the lander in position at the end of another bridle (tether), which is made of a nearly 20-meter-long (65-foot-long) braided Zylon.
Zylon is an advanced fiber material similar to Kevlar that is sewn in a webbing pattern (like shoelace material) to make it stronger. The Zylon bridle provides space for airbag deployment, distance from the solid rocket motor exhaust stream, and increased stability. The bridle incorporates an electrical harness that allows the firing of the solid rockets from the backshell as well as provides data from the backshell inertial measurement unit (which measures rate and tilt of the spacecraft) to the flight computer in the rover.
Rocket assisted descent (RAD): motors. Because the atmospheric density of Mars is less than 1% of Earth's, the parachute alone cannot slow down the Mars Exploration Rover enough to ensure a safe, low landing speed.
The spacecraft descent is assisted by rockets that bring the spacecraft to a dead stop 10-15 meters (30-50 feet) above the Martian surface.
Radar altimeter unit: A radar altimeter unit is used to determine the distance to the Martian surface. The radar's antenna is mounted at one of the lower corners of the lander tetrahedron. When the radar measurement shows the lander is the correct distance above the surface, the Zylon bridle will be cut, releasing the lander from the parachute and backshell so that it is free and clear for landing. The radar data will also enable the timing sequence on airbag inflation and backshell RAD rocket firing.
Airbags used in the Mars Exploration Rover mission are the same type that Mars Pathfinder used in 1997. Airbags must be strong enough to cushion the spacecraft if it lands on rocks or rough terrain and allow it to bounce across Mars' surface at freeway speeds after landing. To add to the complexity, the airbags must be inflated seconds before touchdown and deflated once safely on the ground.
The fabric being used for the new Mars airbags is a synthetic material called Vectran that was also used on Mars Pathfinder. Vectran has almost twice the strength of other synthetic materials, such as Kevlar, and performs better at cold temperatures.
There will be six 100-denier layers of the light but tough Vectran protecting one or two inner bladders of the same material in 200-denier. Using the 100-denier means there is more actual fabric in the outer layers where it is needed, because there are more threads in the weave.
Each rover uses four airbags with six lobes each, which are all connected. Connection is important, since it helps abate some of the landing forces by keeping the bag system flexible and responsive to ground pressure. The fabric of the airbags is not attached directly to the rover; ropes that crisscross the bags hold the fabric to the rover. The ropes give the bags shape, which makes inflation easier. While in flight, the bags are stowed along with three gas generators that are used for inflation.
The spacecraft lander is a protective "shell" that houses the rover and protects it, along with the airbags, from the forces of impact.
The lander is a strong, lightweight structure, consisting of a base and three sides "petals" in the shape of a tetrahedron. The Lander structure consists of beams and sheets that are made from a composite material. The lander beams are made out of carbon-based layers of graphite fiber woven into a fabric, creating a material that is lighter than aluminium and more rigid than steel. Titanium fittings are bonded (glued and fitted) onto the lander beams to allow it to be bolted together. The Rover is held inside the lander with bolts and special nuts that are released after landing with small explosives.
The three petals are connected to the base of the tetrahedron with hinges. Each petal hinge has a powerful motor that is strong enough to lift entire lander.
The Rover plus Lander has a mass of about 533 kilograms (1175 pounds), but the lower gravity on Mars means it weighss only 201 kgf (442 lbf). The Rover alone masses about 185 kilograms (408 pounds), weighing about 70 kgf (154 lbf) on Mars. Having a motor on each petal ensures that the lander can place the rover in an upright position no matter which side the lander comes to rest on after the bouncing and rolling subsides on the surface of Mars.
The Rover contains accelerometers that can detect which way is down (toward the surface of Mars) by measuring the pull of gravity. The Rover computer, knowing which way is down, commands the correct lander petal to open to place the rover upright. Once the base petal is down and the rover is upright, the other two petals are opened.
The petals will initially open to an equally flat position, so all sides of the lander are straight and level. The petal motors are strong enough so that if two of the petals come to rest on rocks, the base with the rover will be held in place like a bridge above the surface of Mars. The base will hold at a level even with the height of the petals resting on rocks, making a straight flat surface throughout the length of the open, flattened lander. The flight team on Earth may then send commands to the rover to adjust the petals to create a better pathway for the rover to drive off of the lander and safely move onto the Martian surface without dropping off a steep rock.
The process of the rover moving off of the lander is called the egress phase of the mission. The rover must be able to safely drive off of the lander without getting its wheels caught up in the airbag material or without dropping off a sharp incline.
To aid in the egress process, the lander petals contain a retraction system that will slowly drag the airbags toward the lander to get them out of the path of the rover (this step is performed before the Lander petals are opened.) Small ramps or "ramplets" are also connected to the petals, which fan out and create "driving surfaces" that fill in large spaces between the lander petals. These ramplets, nicknamed "Batwings," are made out of Vectran cloth. The "batwings" help cover dangerous, uneven terrain, rock obstacles, and leftover airbag material that could get entangled in the rover wheels. These Vectran cloth surfaces make a circular area from which the rover can roll off the lander, providing additional directions the rover can leave the lander. The ramplets also lower the height of the "step" that the rover must take off of the lander, preventing possible death of the rover. If the rover banged its belly on a rock or smashed into the ground as it was moving off the lander, the entire mission could be lost.
About 3 hours is allotted to retract the airbags and deploy the lander petals.
With their relative freedom of movements, the rovers will perform on-site geological investigations. The mast-mounted cameras are mounted 1.5 metre high and will provide 360-degree, stereoscopic, views of the terrain. The robotic arm will be able to place instruments directly up against rock and soil targets of interest. In the mechanical "fist" of the arm is a microscopic camera that will serve the same purpose as a geologist's handheld magnifying lens. The Rock Abrasion Tool serves the purpose of a geologist's rock hammer to expose the insides of rocks.
Spacecraft design
For a total mass of 1,063 kg (2,343 lbs).Cruise stage
Cruise stage navigation components
Cruise stage communication components
Aeroshell
Parts of the aeroshell
The heat shield protects the lander and rover from the intense heat from entry into the Martian atmosphere and acts as the first aerobrake for the spacecraft.Composition
Parachute
Parachute Design
Parachute composition
Parts that work in tandem with the parachute
Airbags
Lander
Lander Design for Turning the Rover Upright
Lander Design for Moving the Rover Safely Onto the Martian Surface
Scientific instruments carried by the Rovers
Miniature Thermal Emission Spectrometer (Mini-TES)
The rover is designed to drive up to 100 meters each Martian day. Its top speed is 5 centimeters per second on a flat surface; however, on average, it will only move at 1 centimeter per second for a maximum of 4 hours each day.Maestro
The NASA team uses a software application called Maestro to view images collected from the rover. Maestro is written in Java so it will run on many different platforms including Microsoft Windows, Macintosh, Solaris, and Linux.References
External links