Mars Exploration Rover

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NASA's Mars Exploration Rover (MER) Mission (since 2003) is a crewless 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 three previous successful landers: the two Viking landers in 1976 and Pathfinder in 1997.

Artist's illustration of the MER Rover
Artist's illustration of the MER,

NASA (larger version)

Timeline

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 landed in the 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.

On January 21st, the Deep Space Network lost contact with the Spirit rover, for reasons originally thought to be related to a thunderstorm over Australia. The rover transmitted a message with no data on Wednesday the 21st, but the Spirit rover missed another communications session with the Mars Global Surveyor later that day. JPL succeeded on Thursday the 22nd in receiving a beep from the rover, indicating that it is in fault mode. On the 23rd, the flight team succeeded in getting the rover to send back data.

On March 23rd, a news conference has been held in the NASA Headquarter, revealing what have been announced to be "major discoveries", in the seek for hints of past water. A delegation of the science team showed pictures and data revealing a stratification pattern and crossbedding within the rocks in the outcrop inside the crater in Meridiani Planum, landing site of the MER-B, Opportunity Rover, suggesting a history of flowing water in the region. The irregular distribution of chlorine and bromine also suggests that the rover sat in a place that once have been the shoreline of a salty sea, now evaporated.

Spacecraft design

The Mars Exploration Rover is stowed in the nose of a Delta II rocket. Each spacecraft consists of several components. These are:

  • Rover - 185 kg (408 lb)
  • Lander - 348 kg (767 lb)
  • Backshell / Parachute - 209 kg (742 lb)
  • Heat Shield - 78 kg (172 lb)
  • Cruise Stage - 193 kg (425 lb)
  • Propellant - 50 kg (110 lb)

For a total mass of 1,063 kg (2,343 lb).

Cruise stage

The cruise stage is the component of the spacecraft used for travel from Earth to 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 m (5.2 ft) tall including the entry vehicle (see below).

The primary structure is aluminium with an outer ring of ribs covered by the solar panels, which are about 2.65 m (8.7 ft) in diameter. Divided into five sections, the solar arrays can provide up to 600 Watts of power near Earth and 300 W 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.

Cruise stage navigation components

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 (500 million km) 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 kg (about 68 lb) 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:

  • an axial burn uses pairs of thrusters to change spacecraft velocity
  • a lateral burn uses two "thruster clusters" (four thrusters per cluster) to move the spacecraft "sideways" through seconds-long pulses
  • pulse mode firing uses coupled thruster pairs for spacecraft precession maneuvers (turns)

Cruise stage communication components

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 antennae 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.

Aeroshell

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.

Parts of the aeroshell

The aeroshell is made of two principle parts:

  • the heat shield (flat, brownish half)
  • the backshell (large, white-painted, cone-shaped half)

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.

The backshell carries the parachute and several components used during later stages of entry, descent, and landing, including:

  • a parachute (stowed at the top of the backshell)
  • the backshell electronics and batteries that fire off pyrotechnic devices like separation nuts, rockets and the parachute mortar
  • a Litton LN-200 Inertial Measurement Unit (IMU), which monitors and reports the orientation of the backshell as it swings under the parachute
  • three large solid rocket motors called RAD rockets (Rocket Assisted Descent), each providing about a ton of force for over 2 seconds
  • three small solid rockets called TIRS (mounted so that they aim horizontally out the sides of the backshell) that provide a small horizontal kick to the backshell to help orient the backshell more vertically during the main RAD rocket burn

Composition

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 (12.7 mm) 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.

Parachute

The parachute will help slow the spacecraft down during entry, descent, and landing. It is located in the backshell.

Parachute design

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 lbf) when the parachute fully inflates. By comparison, Pathfinder's inflation loads were approximately 35,600 N (about 8,000 lbf). The parachute was designed and constructed in South Windsor, Connecticut by Pioneer Aerospace.

Parachute composition

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.

Parts that work in tandem with the parachute

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 m (65 ft) 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 m (30-50 ft) 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

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 (10 mg/m) layers of the light but tough Vectran protecting one or two inner bladders of the same material in 200 denier (20 mg/m). Using the 100 denier (10 mg/m) 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.

Lander

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.

Turning the rover upright

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 the entire lander. The Rover plus Lander has a mass of about 533 kilograms (1175 pounds), but the lower gravity on Mars means it weighs only 201 kgf (442 lbf). The Rover alone masses about 185 kg (408 lb), 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.

Moving the rover safely onto Martian surface

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.

Rover design

Drive system

Each rover has six wheels mounted on a rocker bogie suspension system that ensures all six wheels will remain on the ground while driving over rough terrain. The rocker design ensures that the rover body only goes through half of the range of motion that the "legs" and wheels could potentially experience without this suspension system. 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 (250 mm or 10 in) in size. Each wheel also has cleats, providing grip for climbing in soft sand and scrambling over rocks. Each wheel has 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. 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.

Each rover has the ability to spin one of its front wheels in place to grind deep into the terrain. The rover is designed to remain motionless while the digging wheel is spinning.

The rover has a top speed on flat hard ground of 50 mm/s (2 in/s). 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 10 mm/s. 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.

Power and electronic systems

When fully illuminated, the rover solar arrays generate about 140 watts for up to four hours per sol. The rover needs about 100 watts to drive. 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 well as the change in season.

The rovers run a VxWorks embedded operating system on a radiation-hardened 20 MHz RAD6000 CPU with 128 MB of DRAM with error detection and correction and 3 MB of EEPROM. Also, the rovers each have 256 MB of flash memory. To survive during all of the various mission phases, the rover's "vital organs" must not exceed extreme temperatures of -40 ºC to +40 ºC (-40 ºF to 104 ºF). At night the rovers are heated by eight RHUs which each continuously generate 1 W of thermal energy from the decay of radioisotopes, along with electrical heaters that operate only when necessary. A sputtered gold film and a layer of silica aerogel are used for insulation.

Communication

The rover has both a low-gain and high-gain antenna. The low-gain antenna is omnidirectional, and transmits data at a low rate to Deep Space Network (DSN) antennas on Earth. The high-gain antenna is directional and steerable, and can transmit data at a much higher rate.

The rovers are also able to uplink information to other spacecraft orbiting Mars, utilizing the 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 shorter range than the low- and high-gain antennas. One UHF antenna is on the rover and one is on a petal of the lander to aid in gaining information during the critical landing event.

The images are stored and sent to Earth using a software of ICER for all lossy image compression. All MER cameras produce 1024-pixel by 1024-pixel images at 12 bits per pixel. [1]

Navigation, thumbnail, and many other image types are compressed to approximately 1 bit/pixel, and lower bit rates (less than 0.5 bit/pixel) will be used for certain wavelengths of multi-color panoramic images.

Scientific instrumentation


Image from Miniature Thermal Emission Spectrometer (Mini-TES)

Located on the rover's Pancam Mast Assembly are:

  • Panoramic Camera (Pancam), for determining the mineralogy, texture, and structure of the local terrain.
  • Miniature Thermal Emission Spectrometer (Mini-TES), from Arizona State University, for identifying promising rocks and soils for closer examination, and to determine the processes that formed Martian rocks. The instrument will also look skyward to provide temperature profiles of the Martian atmosphere.

The mast-mounted cameras are mounted 1.5 metre high. 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.

The mast also carries two monochrome navigation cameras, and four monochrome hazard cameras are mounted on the rover's body (two in front and two to the rear).

The rover arm (also called the instrument deployment device, or IDD) holds the following:

  • Mössbauer Spectrometer (MB) MIMOS II, developed by Dr. Göstar Klingelhöfer at the Johannes Gutenberg University in Mainz, Germany, is used for close-up investigations of the mineralogy of iron-bearing rocks and soils.
  • Alpha Particle X-Ray Spectrometer (APXS), developed by the Max Planck Institute for Chemistry in Mainz, Germany, is used for close-up analysis of the abundances of elements that make up rocks and soils.
  • Magnets, for collecting magnetic dust particles. The Mössbauer Spectrometer and the Alpha Particle X-ray Spectrometer will analyze the particles collected, and help determine the ratio of magnetic particles to non-magnetic particles and composition of magnetic minerals in airborne dust and rocks that have been ground by the Rock Abrasion Tool.
  • Microscopic Imager (MI), for obtaining close-up, high-resolution images of rocks and soils.
  • Rock Abrasion Tool (RAT), for removing dusty and weathered rock surfaces and exposing fresh material for examination by instruments onboard.

The robotic arm will be able to place instruments directly up against rock and soil targets of interest.

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

NASA websites

Other sites

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