The Mars Exploration Rover mission used the same system of entry descent and landing (EDL), albeight with some modifications as the Mars Pathfinder. Likewise, many design aspects of the Sojourner rover (like the rocker-bogie mobility architecture and the navigation algorithms) were also successfully used on Spirit and her sister Opportunity.
NASA clearly understands the impact of a high profile landing time as she had both her rovers land at prime time (9 PM ET) on Saturday evening. Spirit landed on January 4, 2004, 9:35 PM ET in Gusev Crater. Gusev Crater is a hightly eroded crater 150 kilometers (95 miles) in diameter. Looking at pictures from Mars satellites one seems to see a lake fed by an inflow from a large valley, Ma’adim Vallis, depositing its water into the crater from highlands to the south. At present it is a flat volcanic flood-plain pocked with small craters and strewn with loose rocks.
From the landing site a group of hills was visible, about 2.5 kilometer away. In honour of the perished crew of the Space Shuttle Columbia, the 7 hills received their names. By June 2004, on sol 159, and well into its first extended mission, Spirit had driven to a range named the Columbia Hills, about 1.6 miles (2.6 kilometers) from the landing site, in a quest to find exposed bedrock.
The mission of Spirit was intended to last 90 sols (Mars-days) with a possibility (hoped for) of another 90 sols of surface operations. Spirit did much more than that as it lasted over 6 years (for 2210 sols) until on March 22, 2010 it got stuck in soft soil. Spirit had been examining the surface of Mars and travelling distances in Gusev that had been beyond reach, when she touched down.
As Spirit operated for nearly four Martian years, it experienced several Solar conjunctions while at Mars. A solar conjunction means that the Sun and Mars are standing in a direct line as seen from Earth. The Sun and its corona will then block the communications between the robot on Mars and Earth and to preserve the space craft the machine is powered down and put in safe mode until the solar conjunction has passed.
The first of such a conjunction Spirit experienced between sol 239 and sol 262 (2004.
The second solar conjunction was in October/November 2006. Solar conjunction began on sol 991 (Oct. 16, 2006) and end on sol 1015 (Nov. 10, 2006). During this period, both NASA rovers, Spirit and Opportunity, did not receive any new command loads, but they did send daily downlinks to Earth, averaging 15 megabits of data per transmission. The data was relayed to Earth via NASA’s Mars Odyssey spacecraft in orbit above Mars.
The third solar conjunction started on November 29, 2008 (sol 1746), and communication with the rovers was not possible until December 13, 2008 (sol 1760)
The big science question for the Mars Exploration Rovers was how past water activity on Mars has influenced the red planet’s environment over time. As on Earth, rocks and minerals on Mars contain clues to the past. While there is no liquid water on the surface of Mars today, the record of past water activity on Mars can be found in the rocks, minerals, and geologic landforms, particularly in those that can only form in the presence of water. That’s why the rovers were specially equipped with tools to study a diverse collection of rocks and soils that may hold clues to past water activity on Mars.
Scientific investigations by Spirit and Opportunity tell a story of a sulfate-rich planet that was once wetter and prone to violent explosions from meteorite impacts and volcanoes. At various times, winds blew sand across the surface and water flowed on the surface and soaked the ground. Some of the conditions necessary to sustain life as we know it were present. Whether life ever existed on Mars remains an open question.
Spirit found basaltic rocks only slightly altered by exposure to moisture. The Spirit rover used a suite of scientific instruments to determine that this rock target, called “Clovis,” contained goethite, a mineral that only forms in the presence of water. Spirit also found elevated levels of sulfur, bromine, and chlorine deposited by evaporating water.
Measuring infrared radiation (heat) associated with specific minerals, the rovers identified and mapped minerals in rocks and soils. Key among these was hematite, an iron-bearing mineral often associated with water originally mapped by the Odyssey spacecraft.
Nodular nuggets at the end of short, stalklike features were one of the more unusual features of a softball-size rock nicknamed “Pot of Gold” by rover scientists. Spirit’s scientific instruments determined that the rock contained hematite, a mineral often formed in the presence of water.
Mission members monitoring the Spirit rover on Mars reported on March 12, 2005, that a lucky encounter with a dust devil had cleaned the solar panels of that robot. Power levels dramatically increased and daily science work was anticipated to be expanded.
Exploring in the hills, Spirit discovered a profusion of rocks and soils bearing evidence of extensive exposure to water, including the iron-oxide-hydroxide mineral goethite and hydrated sulfate salts. It found an outcrop rich in carbonate, evidence for wet conditions that were not acidic.
Textures and compositions of materials at a low plateau between hills indicated an early era on Mars when water and hot rocks interacted in explosive volcanism. By driving with one immobile wheel whose motor had worn out after three years on Mars, Spirit serendipitously plowed up a hidden deposit of nearly pure silica. This discovery indicates that the site once had hot springs or steam vents, which are environments that, on Earth, teem with microbial life.
In June 2009, Spirit became embedded in a patch of fine-grained material and was unable to extract itself after a second wheel stopped working. Prevented from parking itself in a position favorable for its solar array to generate energy, Spirit was apparently unable to survive the long southern winter, as no further communications were received.
The Panoramic Camera (Pancam) is a high-resolution color stereo pair of CCD cameras used to image the surface and sky of Mars.
The cameras are located on a “camera bar” that sits on top of the mast of the rover. With these cameras a full 360° panoramic view can be imaged.
The Pancams are actually very small and light weight (270 grams or about 9 ounces) and would fit in the palm of your hand.
Each “eye” of the Pancam carries a filter wheel that gives Pancam its multispectral imaging capabilities. Images taken at various wavelengths can help scientists learn more about the minerals found in martian rocks and soils. Blue and infrared solar filters allow the camera to image the sun.
The Microscopic Imager is a combination of a microscope and a CCD camera that provides information on the small-scale features of martian rocks and soils. It sits on the end of the arm of the rover. Microscopic imaging is used to analyze the size and shape of grains in sedimentary rocks, The Microscopic Imager is Its field of view is 1024 x 1024 pixels in size and it has a single, broad-band filter so imaging is in black and white.
The Hazcams are mounted on the lower portion of the front and rear of the rover where these black-and-white cameras use visible light to capture three-dimensional (3-D) imagery. 3D images are used to avoid having the rover drive into a ditch or crash into rocks. The rover is equipped with software that allows the rover to navigate Mars on its own, making choices without the interference of commands issues from Earth
The Hazcams have a wide field of view to about 120 degrees. The rover uses pairs of Hazcam images to map out the shape of the terrain as far as 3 meters (10 feet) in front of it, in a “wedge” shape that is over 4 meters wide at the farthest distance. It needs to see far to either side because unlike human eyes, the Hazcam cameras cannot move independently; they¹re mounted directly to the rover body.
The two Engineering Navcams are mounted on the mast or the rover. One could say on its ‘neck’. These cameras are also black-and-white cameras using visible light to gather panoramic, three-dimensional (3D) imagery. The Navcam is a stereo pair of cameras, each with a 45-degree field of view. They help the scientist and engineers to plan the route of the rover. And ofcourse they work in tandem with the Hazcams to avoid driving into something.
The Mars Exploration Rovers are also carrying several science instruments, three spectrometers, a grinder (RAT) and a magnet.
The Miniature Thermal Emission Spectrometer (Mini-TES) is an infrared spectrometer , located on the body of the rover at the bottom of the ‘neck’ using a periscope positioned just below the PanCam. The Mini-Tes determines the mineralogy of rocks and soils from a distance by detecting their patterns of thermal radiation. All warm objects emit heat, but different objects emit heat differently. This variation in thermal radiation can help scientists identify the minerals on Mars. Mini-TES searches for minerals that were formed by the action of water, such as carbonates and clays. Mini-TES also looks at the atmosphere of Mars and gathers data on temperature, water vapor, and the abundance of dust.
The second spectrometer is the Mössbauer Spectrometer (MB) is an instrument, mounted on the turret at the end of the rover arm, specially designed to study iron-bearing minerals. Many of the minerals that formed rocks on Mars contain iron, and the soil is iron-rich. The MB determines the composition and abundance of these minerals to a high level of accuracy. This ability can also help us understand the magnetic properties of surface materials. One Mössbauer measurement takes about 12 hours.
The last spectrometer is the Alpha Particle X-Ray Spectrometer (APXS) determines the elemental chemistry of rocks and soils using alpha particles and X-rays. Alpha particles are emitted during radioactive decay and X-rays are a type of electromagnetic radiation, like light and microwaves. The APXS carries a small alpha particle source. The alphas are emitted and bounce back from a science target into a detector in the APXS, along with some X-rays that are excited from the target in the process.
The energy distribution of the alphas and X-rays measured by the detectors is analyzed to determine elemental composition. The elemental composition of a rock describes the amounts of different chemical elements that have come together to form all of the minerals within the rock. Knowing the elemental composition of martian rocks provides scientists with information about the formation of the planet’s crust, as well as any weathering that has taken place. Most APXS measurements are taken at night and require at least 10 hours of accumulation time, although just X-ray alone will only require a few hours.
The Rock Abrasion Tool (RAT), located on the arm of the rover, is a powerful grinder, able to create a hole 45 millimeters (about 2 inches) in diameter and 5 millimeters (0.2 inches) deep into a rock on the Martian surface. It uses three electric motors to drive rotating grinding teeth into the surface of a rock. Two grinding wheels rotate at high speeds. These wheels also rotate around each other at a much slower speed so that the two grinding wheels sweep the entire cutting area. The RAT is able to grind through hard volcanic rock in about two hours. We are using such a tool to see under the outer layer of the rock. This because the interior of a rock may be very different from its exterior and may reveal how the rock was formed and uder what kind of conditions. By having the grinders rotate above the rock it is possible to use the RAT as a duster. That trusted instrument that every geologists uses while out in the field. Alternatively a geologist might blow on the rock. The RAT performs these functions for us.
Once a fresh surface is exposed, scientists can examine the abraded area in detail using the rover’s other science instruments.
Each rover has three sets of magnetic targets that will collect airborne dust for analysis by the science instruments. One set of magnets is carried by the Rock Abrasion Tool (RAT). As the RAT grinds into martian rocks, scientists have the opportunity to study the properties of dust from these outer rock surfaces.
Mars is a dusty place and some of that dust is highly magnetic. Magnetic minerals carried in dust grains may be freeze-dried remnants of the planet´s watery past. A periodic examination of these particles and their patterns of accumulation on magnets of varying strength can reveal clues about their mineralogy and the planet´s geologic history.
A second set of two magnets is mounted on the front of the rover at an angle so that non-magnetic particles will tend to fall off. These magnets can be studied by the Mössbauer and APXS instruments. A third magnet is mounted on the top of the rover deck in view of the Pancam. This magnet is strong enough to deflect the paths of wind-carried, magnetic dust.
The instruments use calibration targets, including a sundial, to determine accurate colors, brightnesses, and other information collected by the instruments.
The rover arm (also called the instrument deployment device, or IDD) holds and manoeuvres the instruments that help scientists get up-close and personal with Martian rocks and soil.
Much like a human arm, the robotic arm has flexibility through three joints: the rover’s shoulder, elbow, and wrist. The arm enables a tool belt of scientists´ instruments to extend, bend, and angle precisely against a rock to work as a human geologist would: grinding away layers, taking microscopic images, and analyzing the elemental composition of the rocks and soil.
At the end of the arm is a turret, shaped like a cross. This turret, a hand-like structure, holds various tools that can spin through a 350-degree turning range. The tools and instruments on the robotic arm are the Microscopic Imager, the Mössbauer Spectrometer, the Alpha Particle X-Ray Spectrometer and the Rock Abrasion Tool. The forearm also holds a small brush so that the Rock Abrasion Tool can spin against it to “brush its teeth” and rid the grinding tool of any leftover pieces of rock before its next bite.
Thirty percent of the mass of the titanium robotic arm comes from the four instruments it holds at the end of the arm. This weight makes manoeuvring the lightweight arm a bit of a challenge — like controlling a bowling ball at the end of a fishing rod. The arm must be as lightweight as possible for the overall health of the mission, and holes are even cut out in places where there is no need for solid titanium.
When the rover starts driving again the arm and the instruments on the ‘hand’ are folded and stored against the body of the rover. The elbow of the robotic arm hooks itself back onto a pin, and the turret has a T-bar that slides back into a slotted ramp. The fit is ver tight and protects the instruments well. Folded like this the arm and turret can withstand shocks of 6 G´s while rover bumps over along the rocky terrain.
“Six G´s” is roughly equivalent to dropping a box onto a hard floor from a height of 20 centimeters (almost 8 inches). During launch and landing, the arm is restrained by a retractable pin restraint, and can withstand even higher loads of 42 G´s.
‘The Spirit of Exploration’, by Stuart Atkinson
(for the Mars rover, “Spirit”)
I am tired. So tired.
Scratching, biting dried-blood dust
Coats and smothers me,
Eating at me, into me,
Planting itches I can never scratch.
I am lame. Where once
I used to dash across this ruddy, rocky land
I can now only crawl; limping
Like a dusty crone
From weathered stone to weathered stone.
Once I scaled a mountain:
High above this boulder-cluttered land stood I,
A martian Queen, triumphant!
But now the hills laugh cruelly
As I drag my useless wheel. Exhausted.
Half a thousand frozen sols
Ago I knew no fear!
Laughing, I scorned the shrunken Sun,
Mocking its meagre, half-hearted heat;
Now I long for its waning warmth.
As dervish dust devils dance giddily past,
Mocking me, scorning my crawling quest
For that same Sun’s precious touch
My blood is ice, I feel it crack
As I haul myself onwards… onwards…
But if I die here, They will find me
One day, after travelling from the Evening Star.
Warm arms will surround me, wrap around me,
Lift me out of my rusted, dusty grave
And brush me clean once more.
One day I’ll stand behind walls of glass,
Warm again, clean again;
Honoured and worshipped by wide-eyed
Martian children not yet born on the day I died.
© Stuart Atkinson 2006