Artist's conception of MER rovers on Mars
NASA's Mars Exploration Rover (MER) mission was a robotic space mission involving two Mars rovers, Spirit and Opportunity, exploring the planet Mars. It began in 2003 with the launch of the two rovers: MER-A Spirit and MER-B Opportunity—to explore the Martian surface and geology; both landed on Mars at separate locations in January 2004. Both rovers far outlived their planned missions of 90 Martian solar days: MER-A Spirit was active until March 22, 2010, while MER-B Opportunity was active until June 10, 2018 and holds the record for the longest distance driven by any off-Earth wheeled vehicle.
Objectives
The mission's scientific objective was to search for and characterize a wide range of rocks and soils that hold clues to past water activity on Mars. The mission is part of NASA's Mars Exploration Program, which includes three previous successful landers: the two Viking program landers in 1976 and Mars Pathfinder probe in 1997.
The total cost of building, launching, landing and operating the rovers on the surface for the initial 90-sol primary mission was US$820 million.
Since the rovers have continued to function beyond their initial 90 sol
primary mission, they have each received five mission extensions. The
fifth mission extension was granted in October 2007, and ran to the end
of 2009.
The total cost of the first four mission extensions was $104 million,
and the fifth mission extension is expected to cost at least
$20 million.
In July 2007, during the fourth mission extension, Martian dust
storms blocked sunlight to the rovers and threatened the ability of the
craft to gather energy through their solar panels,
causing engineers to fear that one or both of them might be permanently
disabled. However, the dust storms lifted, allowing them to resume
operations.
On May 1, 2009, during its fifth mission extension, Spirit became stuck in soft soil on Mars.
After nearly nine months of attempts to get the rover back on track,
including using test rovers on Earth, NASA announced on January 26, 2010
that Spirit was being retasked as a stationary science platform. This mode would enable Spirit to assist scientists in ways that a mobile platform could not, such as detecting "wobbles" in the planet's rotation that would indicate a liquid core.
Jet Propulsion Laboratory (JPL) lost contact with Spirit after last
hearing from the rover on March 22, 2010 and continued attempts to
regain communications lasted until May 25, 2011, bringing the elapsed
mission time to 6 years 2 months 19 days, or over 25 times the original
planned mission duration.
In recognition of the vast amount of scientific information amassed by both rovers, two asteroids have been named in their honor: 37452 Spirit and 39382 Opportunity. The mission is managed for NASA by the Jet Propulsion Laboratory, which designed, built, and is operating the rovers.
On January 24, 2014, NASA reported that current studies by the remaining rover Opportunity as well as by the newer Mars Science Laboratory rover Curiosity will now be searching for evidence of ancient life, including a biosphere based on autotrophic, chemotrophic and/or chemolithoautotrophic microorganisms, as well as ancient water, including fluvio-lacustrine environments (plains related to ancient rivers or lakes) that may have been habitable. The search for evidence of habitability, taphonomy (related to fossils), and organic carbon on the planet Mars is now a primary NASA objective.
The scientific objectives of the Mars Exploration Rover mission are to:
- Search for and characterize a variety of rocks and soils that hold clues to past water activity. In particular, samples sought include those that have minerals deposited by water-related processes such as precipitation, evaporation, sedimentary cementation, or hydrothermal activity.
- Determine the distribution and composition of minerals, rocks, and soils surrounding the landing sites.
- Determine what geologic processes have shaped the local terrain and influenced the chemistry. Such processes could include water or wind erosion, sedimentation, hydrothermal mechanisms, volcanism, and cratering.
- Perform calibration and validation of surface observations made by Mars Reconnaissance Orbiter instruments. This will help determine the accuracy and effectiveness of various instruments that survey Martian geology from orbit.
- Search for iron-containing minerals, and to identify and quantify relative amounts of specific mineral types that contain water or were formed in water, such as iron-bearing carbonates.
- Characterize the mineralogy and textures of rocks and soils to determine the processes that created them.
- Search for geological clues to the environmental conditions that existed when liquid water was present.
- Assess whether those environments were conducive to life.
History
Launch of MER-A Spirit
Launch of MER-B Opportunity
The MER-A and MER-B probes were launched on June 10, 2003 and July 7, 2003, respectively. Though both probes launched on Boeing Delta II 7925-9.5 rockets from Cape Canaveral Air Force Station Space Launch Complex 17 (CCAFS SLC-17), MER-B was on the heavy version of that launch vehicle, needing the extra energy for Trans-Mars injection. The launch vehicles were integrated onto pads right next to each other,
with MER-A on CCAFS SLC-17A and MER-B on CCAFS SLC-17B. The dual pads
allowed for working the 15- and 21-day planetary launch periods close
together; the last possible launch day for MER-A was June 19, 2003 and
the first day for MER-B was June 25, 2003. NASA's Launch Services Program managed the launch of both spacecraft.
The probes landed in January 2004 in widely separated equatorial locations on Mars.
Timeline of the Mars Exploration mission
On January 21, 2004, the Deep Space Network lost contact with Spirit, for reasons originally thought to be related to a thunderstorm over Australia. The rover transmitted a message with no data, but later that day missed another communications session with the Mars Global Surveyor. The next day, JPL
received a beep from the rover, indicating that it was in fault mode.
On January 23, the flight team succeeded in making the rover send. The
fault was believed to have been caused by an error in the rover's flash memory
subsystem. The rover did not perform any scientific activities for ten
days, while engineers updated its software and ran tests. The problem
was corrected by reformatting Spirit's flash memory and using a software patch to avoid memory overload; Opportunity was also upgraded with the patch as a precaution. Spirit returned to full scientific operations by February 5.
On March 23, 2004, a news conference was held announcing "major
discoveries" of evidence of past liquid water on the Martian surface. A
delegation of scientists showed pictures and data revealing a stratified
pattern and cross bedding in the rocks of the outcrop inside a crater in Meridiani Planum, landing site of MER-B, Opportunity. This suggested that water once flowed in the region. The irregular distribution of chlorine and bromine also suggests that the place was once the shoreline of a salty sea, now evaporated.
On April 8, 2004, NASA announced that it was extending the
mission life of the rovers from three to eight months. It immediately
provided additional funding of US $15 million through September, and
$2.8 million per month for continuing operations. Later that month, Opportunity arrived at Endurance crater,
taking about five days to drive the 200 meters. NASA announced on
September 22 that it was extending the mission life of the rovers for
another six months. Opportunity was to leave Endurance crater, visit its discarded heat shield, and proceed to Victoria crater. Spirit was to attempt to climb to the top of the Columbia Hills.
With the two rovers still functioning well, NASA later announced another 18-month extension of the mission to September 2006. Opportunity was to visit the "Etched Terrain" and Spirit was to climb a rocky slope toward the top of Husband Hill. On August 21, 2005, Spirit reached the summit of Husband Hill after 581 sols and a journey of 4.81 kilometers (2.99 mi).
Spirit celebrated its one Martian year anniversary (669 sols or 687 Earth days) on November 20, 2005. Opportunity
celebrated its anniversary on December 12, 2005. At the beginning of
the mission, it was expected that the rovers would not survive much
longer than 90 Martian days. The Columbia Hills were "just a dream",
according to rover driver Chris Leger. Spirit explored the semicircular rock formation known as Home Plate. It is a layered rock outcrop that puzzles and excites scientists.
It is thought that its rocks are explosive volcanic deposits, though
other possibilities exist, including impact deposits or sediment borne
by wind or water.
Spirit's front right wheel ceased working on March 13, 2006, while the rover was moving itself to McCool Hill.
Its drivers attempted to drag the dead wheel behind Spirit, but this
only worked until reaching an impassable sandy area on the lower slopes.
Drivers directed Spirit to a smaller sloped feature, dubbed "Low
Ridge Haven", where it spent the long Martian winter, waiting for
spring and increased solar power levels suitable for driving. That
September, Opportunity reached the rim of Victoria crater, and Spaceflight Now reported that NASA had extended mission for the two rovers through September 2007. On February 6, 2007, Opportunity became the first spacecraft to traverse ten kilometers (6.2 miles) on the surface of Mars.
Opportunity was poised to enter Victoria Crater from its perch on the rim of Duck Bay on June 28, 2007, but due to extensive dust storms, it was delayed until the dust had cleared and power returned to safe levels. Two months later, Spirit and Opportunity
resumed driving after hunkering down during raging dust storms that
limited solar power to a level that nearly caused the permanent failure
of both rovers.
On October 1, 2007, both Spirit and Opportunity entered their fifth mission extension that extended operations into 2009, allowing the rovers to have spent five years exploring the Martian surface, pending their continued survival.
On August 26, 2008, Opportunity began its three-day climb out of Victoria crater amidst concerns that power spikes, similar to those seen on Spirit
before the failure of its right-front wheel, might prevent it from ever
being able to leave the crater if a wheel failed. Project scientist
Bruce Banerdt also said, "We've done everything we entered Victoria
Crater to do and more." Opportunity will return to the plains in
order to characterize Meridiani Planum's vast diversity of rocks—some
of which may have been blasted out of craters such as Victoria. The
rover had been exploring Victoria Crater since September 11, 2007. As of January 2009, the two rovers had collectively sent back 250,000 images and traveled over 21 kilometers (13 mi).
After driving about 3.2 kilometers (2.0 mi) since it left Victoria crater, Opportunity first saw the rim of Endeavor crater on March 7, 2009. It passed the 16 km (9.9 mi) mark along the way on sol 1897. Meanwhile, at Gusev crater, Spirit was dug in deep into the Martian sand, much as Opportunity was at Purgatory Dune in 2005.
On January 3 and 24, 2010, Spirit and Opportunity marked six years on Mars, respectively. On January 26, NASA announced that Spirit will be used as a stationary research platform after several months of unsuccessful attempts to free the rover from soft sand.
NASA announced on March 24, 2010, that Opportunity, which
has an estimated remaining drive distance of 12 km to Endeavor Crater,
has traveled over 20 km since the start of its mission. Each rover was designed with a mission driving distance goal of just 600 meters. One week later, they announced that Spirit may have gone into hibernation for the Martian winter and might not wake up again for months.
On September 8, 2010, it was announced that Opportunity had reached the halfway point of the 19-kilometer journey between Victoria crater and Endeavor crater.
On May 22, 2011, NASA announced that it will cease attempts to contact Spirit,
which has been stuck in a sand trap for two years. The last successful
communication with the rover was on March 22, 2010. The final
transmission to the rover was on May 25, 2011.
In April 2013, a photo sent back by one of the rovers became widely circulated on social networking and news sites such as Reddit that appeared to depict a human penis carved into the Martian dirt.
On May 16, 2013, NASA announced that Opportunity had driven further than any other NASA vehicle on a world other than Earth. After Opportunity's total odometry went over 35.744 km (22.210 mi), the rover surpassed the total distance driven by the Apollo 17 Lunar Roving Vehicle.
On July 28, 2014, NASA announced that Opportunity had driven further than any other vehicle on a world other than Earth. Opportunity covered over 40 km (25 mi), surpassing the total distance of 39 km (24 mi) driven by the Lunokhod 2 lunar rover, the previous record-holder.
On March 23, 2015, NASA announced that Opportunity had driven the full 42.2 km (26.2 mi) distance of a marathon, with a finish time of roughly 11 years, 2 months.
On June 2018, Opportunity was caught in a global-scale
dust storm and the rover's solar panels were not able to generate enough
power, with the last contact on June 10, 2018. NASA resumed sending
commands after the dust storm subsided but the rover remained silent,
possibly due to a catastrophic failure or a layer of dust covered its
solar panels.
A press conference was held on February 13, 2019, that after numerous attempts to obtain contact with Opportunity with no response since June 2018, NASA declared
Opportunity mission over, which also draws the 16-year long Mars Exploration Rover mission to a close.
Spacecraft design
MER launch configuration, break apart illustration
The Mars Exploration Rover was designed to be stowed atop a Delta II rocket. Each spacecraft consists of several components:
- Rover: 185 kg (408 lb)
- Lander: 348 kg (767 lb)
- Backshell / Parachute: 209 kg (461 lb)
- Heat Shield: 78 kg (172 lb)
- Cruise Stage: 193 kg (425 lb)
- Propellant: 50 kg (110 lb)
- Instruments: 5 kg (11 lb)
Total mass is 1,063 kg (2,344 lb).
Cruise stage
Cruise stage of Opportunity rover
MER cruise stage diagram
The cruise stage is the component of the spacecraft that is used for
travel from Earth to Mars. It is very similar to the Mars Pathfinder in
design and is approximately 2.65 meters (8.7 ft) in diameter and 1.6 m (5.2 ft) tall, including the entry vehicle (see below).
The primary structure is aluminum 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 electronics "warm". A freon
system removes heat from the flight computer and communications
hardware inside the rover so they do not overheat. Cruise avionics
systems allow the flight computer to interface with other electronics,
such as the sun sensors, star scanner and heaters.
The star scanner (without a backup system) and sun sensor
allowed the spacecraft to know its orientation in space by analyzing
the position of the Sun and other stars in relation to itself. Sometimes
the craft could be slightly off course; this was expected, given the
500-million-kilometer (320 million mile) journey. Thus navigators
planned up to six trajectory correction maneuvers, along with health
checks.
To ensure the spacecraft arrived at Mars in the right place for
its landing, two light-weight, aluminum-lined tanks carried about 31 kg
(about 68 lb) of hydrazine propellant.
Along with cruise guidance and control systems, the propellant allowed
navigators to keep the spacecraft on course. Burns and pulse firings of
the propellant allowed three types of 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).
Communication
The spacecraft used a high-frequency X band radio wavelength to communicate, which allowed for less power and smaller antennas than many older craft, which used S band.
Navigators sent commands through two antennas on the cruise stage: a cruise low-gain antenna
mounted inside the inner ring, and a cruise medium-gain antenna in the
outer ring. The low-gain antenna was used close to Earth. It is
omni-directional, so the transmission power that reached Earth fell
faster with increasing distance. As the craft moved closer to Mars, the
Sun and Earth moved closer in the sky as viewed from the craft, so less
energy reached Earth. The spacecraft then switched to the medium-gain
antenna, which directed the same amount of transmission power into a
tighter beam toward Earth.
During flight, the spacecraft was spin-stabilized with a spin rate of two revolutions per minute (rpm). Periodic updates kept antennas pointed toward Earth and solar panels toward the Sun.
Aeroshell
Overview of the Mars Exploration Rover aeroshell
The aeroshell maintained a protective covering for the lander during
the seven-month voyage to Mars. Together with the lander and the rover,
it constituted the "entry vehicle". Its main purpose was to protect the
lander and the rover inside it from the intense heat of entry into the
thin Martian atmosphere. It was based on the Mars Pathfinder and Mars
Viking designs.
Parts
The aeroshell was made of two main parts: a heat shield
and a backshell. The heat shield was flat and brownish, and protected
the lander and rover during entry into the Martian atmosphere and acted
as the first aerobrake for the spacecraft. The backshell was large, cone-shaped and painted white. It carried the parachute and several components used in later stages of entry, descent, and landing, including:
- A parachute (stowed at the bottom 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 (10 kilonewtons) for about 60 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 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 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. A similar technology was used in the first US manned space missions Mercury, Gemini and Apollo. It was 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 slowed from 19,000 to 1,600 km/h (5,300 to 440 m/s) in about a
minute, producing about 60 m/s2 (6 g) of acceleration on the lander and rover.
The backshell and heat shield are made of the same materials, but the heat shield has a thicker, 13 mm (1⁄2 in), layer of the ablator. Instead of being painted, the backshell was covered with a very thin aluminized PET film blanket to protect it from the cold of deep space. The blanket vaporized during entry into the Martian atmosphere.
Parachute
Mars Exploration Rover's parachute test
The parachute helped slow the spacecraft during entry, descent, and landing. It is located in the backshell.
Design
The 2003
parachute design was 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 80 to 85 kilonewtons
(kN) or 80 to 85 kN (18,000 to 19,000 lbf) when the parachute fully
inflates. By comparison, Pathfinder's inflation loads were approximately
35 kN (about 8,000 lbf). The parachute was designed and constructed in South Windsor, Connecticut by Pioneer Aerospace, the company that also designed the parachute for the Stardust mission.
Composition
The parachute is made of two durable, lightweight fabrics: polyester and nylon. A triple bridle made of Kevlar connects the parachute to the backshell.
The amount of space available on the spacecraft for the parachute
is so small that the parachute had to be pressure-packed. Before
launch, a team tightly folded the 48 suspension lines, three bridle
lines, and the parachute. The parachute team loaded the parachute in a
special structure that then applied a heavy weight to the parachute
package several times. Before placing the parachute into the backshell,
the parachute was heat set to sterilize it.
Connected systems
Descent is halted by retrorockets and lander is dropped 10 m (33 ft) to the surface in this computer generated impression.
Zylon Bridles: After the parachute was deployed at an altitude
of about 10 km (6.2 mi) above the surface, the heatshield was released
using 6 separation nuts and push-off springs. The lander then separated
from the backshell and "rappelled" down a metal tape on a centrifugal braking system
built into one of the lander petals. The slow descent down the metal
tape placed the lander in position at the end of another bridle
(tether), made of a nearly 20 m (66 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 could not slow down the Mars Exploration Rover enough to ensure a
safe, low landing speed. The spacecraft descent was assisted by rockets
that brought the spacecraft to a dead stop 10–15 m (33–49 ft) above the
Martian surface.
Radar altimeter unit: A radar altimeter
unit was 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 showed the lander was the
correct distance above the surface, the Zylon bridle was cut, releasing
the lander from the parachute and backshell so that it was free and
clear for landing. The radar data also enabled the timing sequence on
airbag inflation and backshell RAD rocket firing.
Airbags
Artist's concept of inflated airbags
Airbags
used in the Mars Exploration Rover mission are the same type that Mars
Pathfinder used in 1997. They had to be strong enough to cushion the
spacecraft if it landed on rocks or rough terrain and allow it to bounce
across Mars' surface at highway speeds (about 100 km/h) after landing.
The airbags had to be inflated seconds before touchdown and deflated
once safely on the ground.
The airbags were made of Vectran,
like those on Pathfinder. Vectran has almost twice the strength of
other synthetic materials, such as Kevlar, and performs better in cold
temperatures. Six 100 denier
(10 mg/m) layers of Vectran protected one or two inner bladders of
Vectran in 200 denier (20 mg/m). Using 100 denier (10 mg/m) leaves more
fabric in the outer layers where it is needed, because there are more
threads in the weave.
Each rover used four airbags with six lobes each, all of which
were connected. Connection was important, since it helped abate some of
the landing forces by keeping the bag system flexible and responsive to
ground pressure. The airbags were not attached directly to the rover,
but were held to it by ropes crisscrossing the bag structure. The ropes
gave the bags shape, making inflation easier. While in flight, the bags
were stowed along with three gas generators that are used for inflation.
Lander
MER lander petals opening (Courtesy NASA/JPL-Caltech)
The spacecraft lander is a protective shell that houses the rover,
and together with the airbags, protects it from the forces of impact.
The lander is a tetrahedron shape, whose sides open like petals. It is strong and light, and made of beams and sheets. The beams consist of layers of graphite
fiber woven into a fabric that is lighter than aluminium and more rigid
than steel. Titanium fittings are glued and fitted onto the beams to
allow it to be bolted together. The rover was held inside the lander by bolts and special nuts that were released after landing with small explosives.
Uprighting
After
the lander stopped bouncing and rolling on the ground, it came to rest
on the base of the tetrahedron or one of its sides. The sides then
opened to make the base horizontal and the rover upright. The sides are
connected to the base by hinges, each of which has a motor strong enough
to lift the lander. The rover plus lander has a mass of about 533 kilograms (1,175 pounds).
The rover alone has a mass of about 185 kg (408 lb). The gravity on
Mars is about 38% of Earth's, so the motor does not need to be as
powerful as it would on Earth.
The rover contains accelerometers
to detect which way is down (toward the surface of Mars) by measuring
the pull of gravity. The rover computer then commanded the correct
lander petal to open to place the rover upright. Once the base petal was
down and the rover was upright, the other two petals were opened.
The petals initially opened to an equally flat position, so all
sides of the lander were 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 would be held in place like a bridge above the ground.
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 could then send
commands to the rover to adjust the petals and create a safe path for
the rover to drive off the lander and onto the Martian surface without
dropping off a steep rock.
Moving the payload onto Mars
Spirit's lander on Mars
The moving of the rover off the lander is called the egress phase of
the mission. The rover must avoid having its wheels caught in the airbag
material or falling off a sharp incline. To help this, a retraction
system on the petals slowly drags the airbags toward the lander before
the petals open. Small ramps on the petals fan out to fill spaces
between the petals. They cover uneven terrain, rock obstacles, and
airbag material, and form a circular area from which the rover can drive
off in more directions. They also lower the step that the rover must
climb down. They are nicknamed "batwings", and are made of Vectran
cloth.
About three hours were allotted to retract the airbags and deploy the lander petals.
Rover design
Mars Exploration Rover (rear) vs. Sojourner rover (Courtesy NASA/JPL-Caltech)
Interactive 3D model of the MER
The rovers are six-wheeled, solar-powered robots that stand 1.5 m
(4.9 ft) high, 2.3 m (7.5 ft) wide and 1.6 m (5.2 ft) long. They weigh
180 kg (400 lb), 35 kg (77 lb) of which is the wheel and suspension
system.
Drive system
Drive wheel from the Mars Exploration Rovers, with integral suspension flexures.
Each rover has six wheels mounted on a rocker-bogie
suspension system that ensures wheels remain on the ground while
driving over rough terrain. The design reduces the range of motion of
the rover body by half, and allows the rover to go over obstacles or
through holes that are more than a wheel diameter (250 millimeters
(9.8 in)) in size. The rover wheels are designed with integral compliant
flexures which provide shock absorption during movement. Additionally, the wheels have cleats which provide grip for climbing in soft sand and scrambling over rocks.
Each wheel has its own drive motor. The two front and two rear
wheels each have individual steering motors. This allows the vehicle to
turn in place, a full revolution, and to swerve and curve, making
arching turns. The motors for the rovers have been designed by the Swiss
company Maxon Motor.
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.
Each rover can spin one of its front wheels in place to grind
deep into the terrain. It is to remain motionless while the digging
wheel is spinning. The rovers have a top speed on flat hard ground of
50 mm/s (2 in/s). The average speed is 10 mm/s, because its hazard
avoidance software causes it to stop every 10 seconds for 20 seconds to
observe and understand the terrain into which it has driven.
Power and electronic systems
Circular projection showing MER-A Spirit's solar panels covered in dust in October 2007 on Mars. Unexpected cleaning events have periodically increased power.
When fully illuminated, the rover triplejunction solar arrays generate about 140 watts for up to four hours per Martian day (sol). The rover needs about 100 watts to drive. Its power system includes two rechargeable lithium ion
batteries weighing 7.15 kg (15.8 lb) each, that provide energy when the
sun is not shining, especially at night. Over time, the batteries will
degrade and will not be able to recharge to full capacity.
For comparison, the Mars Science Laboratory's power system is composed of a Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) produced by Boeing.
The MMRTG is designed to provide 125W of electrical power at the start
of the mission, falling to 100W after 14 years of service.
It is used to power the MSL's many systems and instruments. Solar
panels were also considered for the MSL, but RTGs provide constant
power, regardless of the time of day, and thus the versatility to work
in dark environments and high latitudes where solar energy is not
readily available. The MSL generates 2.5 kilowatt hours per day, compared to the Mars Exploration Rovers, which can generate about 0.6 kilowatt hours per day.
It was thought that by the end of the 90-sol mission, the
capability of the solar arrays to generate power would likely be reduced
to about 50 watts. This was due to anticipated dust coverage on the
solar arrays, and the change in season. Over three Earth years later,
however, the rovers' power supplies hovered between 300 watt-hours
and 900 watt-hours per day, depending on dust coverage. Cleaning events
(dust removal by wind) have occurred more often than NASA expected,
keeping the arrays relatively free of dust and extending the life of the
mission. During a 2007 global dust storm on Mars, both rovers
experienced some of the lowest power of the mission; Opportunity dipped to 128 watt-hours. In November 2008, Spirit had overtaken this low-energy record with a production of 89 watt-hours, due to dust storms in the region of Gusev crater.
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. Each rover also has 256 MB of flash memory.
To survive during the various mission phases, the rover's vital
instruments must stay within a temperature of −40 °C to +40 °C (−40 °F
to 104 °F). At night, the rovers are heated by eight radioisotope heater units (RHU), 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
Pancam Mast Assembly (PMA)
Rock Abrasion Tool (RAT)
Alpha particle X-ray spectrometer (APXS) (Courtesy NASA/JPL-Caltech)
The rover has an X band low-gain and an X band high-gain antenna for communications to and from the Earth, as well as an ultra high frequency monopole antenna for relay communications. 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 to Earth at a higher rate. The rovers
use the UHF monopole and its CE505 radio to communicate with spacecraft
orbiting Mars, the Mars Odyssey and (before its failure) the Mars Global Surveyor (already more than 7.6 terabits of data were transferred using its Mars Relay antenna and Mars Orbiter Camera's memory buffer of 12 MB). Since MRO
went into orbit around Mars, the landers have also used it as a relay
asset. Most of the lander data is relayed to Earth through Odyssey and
MRO. The orbiters can receive rover signals at a much higher data rate
than the Deep Space Network can, due to the much shorter distances from
rover to orbiter. The orbiters then quickly relay the rover data to the
Earth using their large and high-powered antennas.
Each rover has nine cameras, which produce 1024-pixel by 1024-pixel images at 12 bits per pixel,
but most navigation camera images and image thumbnails are truncated to
8 bits per pixel to conserve memory and transmission time. All images
are then compressed using ICER
before being stored and sent to Earth. Navigation, thumbnail, and many
other image types are compressed to approximately 0.8 to 1.1 bits/pixel.
Lower bit rates (less than 0.5-bit/pixel) are used for certain
wavelengths of multi-color panoramic images.
ICER is based on wavelets,
and was designed specifically for deep-space applications. It produces
progressive compression, both lossless and lossy, and incorporates an
error-containment scheme to limit the effects of data loss on the
deep-space channel. It outperforms the lossy JPEG image compressor and
the lossless Rice compressor used by the Mars Pathfinder mission.
Scientific instrumentation
The rover has various instruments. Three are mounted on the Pancam Mast Assembly (PMA):
- Panoramic Cameras (Pancam), two cameras with color filter wheels for determining the texture, color, mineralogy, and structure of the local terrain.
- Navigation Cameras (Navcam), two cameras that have larger fields of view but lower resolution and are monochromatic, for navigation and driving.
- A periscope assembly for the Miniature Thermal Emission Spectrometer (Mini-TES), which identifies promising rocks and soils for closer examination, and determines the processes that formed them. The Mini-TES was built by Arizona State University. The periscope assembly features two beryllium fold mirrors, a shroud that closes to minimize dust contamination in the assembly, and stray-light rejection baffles that are strategically placed within the graphite epoxy tubes.
The cameras are mounted 1.5 meters high on the Pancam Mast Assembly.
The PMA is deployed via the Mast Deployment Drive (MDD). The Azimuth
Drive, mounted directly above the MDD, turns the assembly horizontally a
whole revolution with signals transmitted through a rolling tape
configuration. The camera drive points the cameras in elevation, almost
straight up or down. A third motor points the Mini-TES fold mirrors and
protective shroud, up to 30° above the horizon and 50° below. The PMA's
conceptual design was done by Jason Suchman at JPL, the Cognizant
Engineer who later served as Contract Technical Manager (CTM) once the
assembly was built by Ball Aerospace & Technologies Corp., Boulder, Colorado.
Raul Romero served as CTM once subsystem-level testing began. Satish
Krishnan did the conceptual design of the High-Gain Antenna Gimbal
(HGAG), whose detailed design, assembly, and test was also performed by
Ball Aerospace at which point Satish acted as the CTM.
Four monochromatic hazard cameras (Hazcams) are mounted on the rover's body, two in front and two behind.
The instrument deployment device (IDD), also called the rover arm, 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. Universities involved in developing the APXS include the University of Guelph, University of California, and Cornell University
- Magnets, for collecting magnetic dust particles, developed by Jens Martin Knudsen's group at the Niels Bohr Institute, Copenhagen. The particles are analyzed by the Mössbauer Spectrometer and X-ray Spectrometer to help determine the ratio of magnetic particles to non-magnetic particles and the composition of magnetic minerals in airborne dust and rocks that have been ground by the Rock Abrasion Tool. There are also magnets on the front of the rover, which are studied extensively by the Mössbauer spectrometer.
- Microscopic Imager (MI) for obtaining close-up, high-resolution images of rocks and soils. Development was led by Ken Herkenhoff's team at the USGS Astrogeology Research Program.
- Rock Abrasion Tool (RAT), developed by Honeybee Robotics, for removing dusty and weathered rock surfaces and exposing fresh material for examination by instruments on board.
The robotic arm is able to place instruments directly up against rock and soil targets of interest.
Naming of Spirit and Opportunity
Sofi Collis with a model of Mars Exploration Rover
The Spirit and Opportunity rovers were named through a student essay competition. The winning entry was by Sofi Collis, a third-grade Russian-American student from Arizona.
I used to live in an orphanage. It was dark and cold and lonely. At night, I looked up at the sparkly sky and felt better. I dreamed I could fly there. In America, I can make all my dreams come true. Thank you for the 'Spirit' and the 'Opportunity.'
— Sofi Collis, age 9
Prior to this, during the development and building of the rovers, they were known as MER-1 (Opportunity) and MER-2 (Spirit). Internally, NASA also uses the mission designations MER-A (Spirit) and MER-B (Opportunity) based on the order of landing on Mars (Spirit first then Opportunity).
Test rovers
Rover team members simulate Spirit in a Martian sandtrap.
The Jet Propulsion Laboratory maintains a pair of rovers, the Surface System Test-Beds
(SSTB) at its location in Pasadena for testing and modeling of
situations on Mars. One test rover, SSTB1, weighing approximately 180
kilograms (400 lb), is fully instrumented and nearly identical to Spirit and Opportunity. Another test version, SSTB-Lite,
is identical in size and drive characteristics but does not include all
instruments. It weighs in at 80 kilograms (180 lb), much closer to the
weight of Spirit and Opportunity in the reduced gravity of Mars. These rovers were used in 2009 for a simulation of the incident in which Spirit became trapped in soft soil.
SAP software for image viewing
The NASA team uses a software application called SAP to view images collected from the rover, and to plan its daily activities. There is a version available to the public called Maestro.
Planetary science findings
Spirit Landing Site, Gusev Crater
Plains
Although
the Gusev crater appears from orbital images to be a dry lakebed, the
observations from the surface show the interior plains mostly filled
with debris. The rocks on the plains of Gusev are a type of basalt. They contain the minerals olivine, pyroxene, plagioclase,
and magnetite, and they look like volcanic basalt as they are
fine-grained with irregular holes (geologists would say they have
vesicles and vugs).
Much of the soil on the plains came from the breakdown of the local rocks. Fairly high levels of nickel were found in some soils; probably from meteorites.
Analysis shows that the rocks have been slightly altered by tiny amounts
of water. Outside coatings and cracks inside the rocks suggest water
deposited minerals, maybe bromine
compounds. All the rocks contain a fine coating of dust and one or more
harder rinds of material. One type can be brushed off, while another
needed to be ground off by the Rock Abrasion Tool (RAT).
There are a variety of rocks in the Columbia Hills, some of which have been altered by water, but not by very much water.
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| Coordinates: | 14.6°S 175.5°E |
|---|---|
These rocks can be classified in different ways. The amounts and
types of minerals make the rocks primitive basalts—also called picritic
basalts. The rocks are similar to ancient terrestrial rocks called
basaltic komatiites. Rocks of the plains also resemble the basaltic shergottites,
meteorites which came from Mars. One classification system compares the
amount of alkali elements to the amount of silica on a graph; in this
system, Gusev plains rocks lie near the junction of basalt, picrobasalt, and tephite. The Irvine-Barager classification calls them basalts.
Plain’s rocks have been very slightly altered, probably by thin films of
water because they are softer and contain veins of light colored
material that may be bromine compounds, as well as coatings or rinds. It
is thought that small amounts of water may have gotten into cracks
inducing mineralization processes).
Coatings on the rocks may have occurred when rocks were buried and interacted with thin films of water and dust.
One sign that they were altered was that it was easier to grind these rocks compared to the same types of rocks found on Earth.
The first rock that Spirit studied was Adirondack. It turned out to be typical of the other rocks on the plains.
First color picture from Gusev crater. Rocks were found to be basalt. Everything was covered with a fine dust that Spirit determined was magnetic because of the mineral magnetite.
Cross-sectional drawing of a typical rock from the plains of Gusev crater. Most rocks contain a coating of dust and one or more harder coatings. Veins of water-deposited veins are visible, along with crystals of olivine. Veins may contain bromine salts.
Dust
The dust in Gusev Crater is the same as dust all around the planet. All the dust was found to be magnetic. Moreover, Spirit found the magnetism was caused by the mineral magnetite, especially magnetite that contained the element titanium. One magnet was able to completely divert all dust hence all Martian dust is thought to be magnetic. The spectra of the dust was similar to spectra of bright, low thermal inertia regions like Tharsis
and Arabia that have been detected by orbiting satellites. A thin layer
of dust, maybe less than one millimeter thick covers all surfaces.
Something in it contains a small amount of chemically bound water.
Columbia Hills
As the rover climbed above the plains onto the Columbia Hills, the mineralogy that was seen changed.
Scientists found a variety of rock types in the Columbia Hills, and
they placed them into six different categories. The six are: Clovis,
Wishbone, Peace, Watchtower, Backstay, and Independence. They are named
after a prominent rock in each group. Their chemical compositions, as
measured by APXS, are significantly different from each other. Most importantly, all of the rocks in Columbia Hills show various degrees of alteration due to aqueous fluids.
They are enriched in the elements phosphorus, sulfur, chlorine, and
bromine—all of which can be carried around in water solutions. The
Columbia Hills’ rocks contain basaltic glass, along with varying amounts
of olivine and sulfates.
The olivine abundance varies inversely with the amount of sulfates. This
is exactly what is expected because water destroys olivine but helps to
produce sulfates.
The Clovis group is especially interesting because the Mössbauer spectrometer (MB) detected goethite in it.
Goethite forms only in the presence of water, so its discovery is the
first direct evidence of past water in the Columbia Hills's rocks. In
addition, the MB spectra of rocks and outcrops displayed a strong
decline in olivine presence,
although the rocks probably once contained much olivine.
Olivine is a marker for the lack of water because it easily decomposes
in the presence of water. Sulfate was found, and it needs water to
form.
Wishstone contained a great deal of plagioclase, some olivine, and anhydrate (a sulfate). Peace rocks showed sulfur
and strong evidence for bound water, so hydrated sulfates are
suspected. Watchtower class rocks lack olivine consequently they may
have been altered by water. The Independence class showed some signs of
clay (perhaps montmorillonite a member of the smectite group). Clays
require fairly long term exposure to water to form.
One type of soil, called Paso Robles, from the Columbia Hills, may be an
evaporate deposit because it contains large amounts of sulfur, phosphorus, calcium, and iron.
Also, MB found that much of the iron in Paso Robles soil was of the oxidized, Fe3+ form.
Towards the middle of the six-year mission (a mission that was supposed to last only 90 days), large amounts of pure silica
were found in the soil. The silica could have come from the interaction
of soil with acid vapors produced by volcanic activity in the presence
of water or from water in a hot spring environment.
After Spirit stopped working scientists studied old data from the Miniature Thermal Emission Spectrometer, or Mini-TES and confirmed the presence of large amounts of carbonate-rich
rocks, which means that regions of the planet may have once harbored
water. The carbonates were discovered in an outcrop of rocks called
"Comanche."
In summary, Spirit found evidence of slight weathering on
the plains of Gusev, but no evidence that a lake was there. However, in
the Columbia Hills there was clear evidence for a moderate amount of
aqueous weathering. The evidence included sulfates and the minerals
goethite and carbonates which only form in the presence of water. It is
believed that Gusev crater may have held a lake long ago, but it has
since been covered by igneous materials. All the dust contains a
magnetic component which was identified as magnetite with some titanium.
Furthermore, the thin coating of dust that covers everything on Mars is
the same in all parts of Mars.
Opportunity Landing Site, Meridiani Planum
Self-portrait of Opportunity near Endeavor Crater on the surface of Mars (January 6, 2014).
Cape Tribulation southern end, as seen in 2017 by Opportunity rover
The Opportunity rover landed in a small crater, dubbed
"Eagle", on the flat plains of Meridiani. The plains of the landing site
were characterized by the presence of a large number of small spherules, spherical concretions
that were tagged "blueberries" by the science team, which were found
both loose on the surface, and also embedded in the rock. These proved
to have a high concentration of the mineral hematite,
and showed the signature of being formed in an aqueous environment. The
layered bedrock revealed in the crater walls showed signs of being
sedimentary in nature, and compositional and microscopic-imagery
analysis showed this to be primarily with composition of Jarosite, a ferrous sulfate mineral that is characteristically an evaporite that is the residue from the evaporation of a salty pond or sea.
The mission has provided substantial evidence of past water
activity on Mars. In addition to investigating the "water hypothesis", Opportunity
has also obtained astronomical observations and atmospheric data.
The extended mission took the rover across the plains to a series of
larger craters in the south, with the arrival at the edge of a 25-km
diameter crater, Endeavor Crater, eight years after landing. The
orbital spectroscopy of this crater rim show the signs of phyllosilicate rocks, indicative of older sedimentary deposits.
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