Engineering:Sojourner (rover)

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The first NASA Mars rover on Mars Pathfinder
Sojourner
Sojourner on Mars PIA01122.jpg
Sojourner rover pictured by Pathfinder lander
Mission typeMars rover
OperatorNASA
WebsiteOfficial website
Mission durationPlanned: 7 sols (7 days)
Mission end: 83 sols (85 days)
From arrival on Mars
Spacecraft properties
Dry mass11.5 kilograms (25 lb) (Rover only)
Start of mission
Launch dateDecember 4, 1996, 06:58:07 UTC
RocketDelta II 7925 D240
Launch siteCape Canaveral Air Force Station LC-17B
ContractorMcDonnell Douglas
Deployed fromMars Pathfinder
Deployment dateJuly 5, 1997 (1997-07-05)
End of mission
Last contactSeptember 27, 1997 (1997-09-28)
Mars Pathfinder Insignia.png
Mars Pathfinder mission patch
NASA Mars rovers
Spirit →
 

Sojourner is a robotic Mars rover that landed on July 4, 1997,[1] in the Ares Vallis region. Sojourner was operational on Mars for 92 sols (95 days). The rover was the first wheeled vehicle to rove another planet, and was part of the Mars Pathfinder mission.[2] It had front and rear cameras and hardware to conduct several scientific experiments. Designed for a mission lasting 7 sols, with possible extension to 30 sols,[3] it was ultimately active for 83 sols (85 Earth days). The rover communicated with Earth through the Pathfinder base station, which had its last successful communication session with Earth at 3:23 a.m. PDT on September 27, 1997.[1][4] The last signal received from the rover was on the morning of October 7, 1997.[5] The Sojourner mission formally ended four months later on March 10, 1998, after exhausting all further options. Sojourner traveled a distance of just over 100 meters (330 ft) by the time communication was lost.[6] Its final confirmed command was to stay stationary until October 5, 1997, (sol 91) and then drive around the lander,[7] though there is no indication that it was able to do so.

Mission

Main page: Astronomy:Mars Pathfinder
Sojourner at the JPL

Sojourner was an experimental vehicle, whose main mission was to test some technical solutions developed by the engineers of the NASA research laboratories in the Martian environment.[8] On the one hand, it was necessary to verify whether the design strategy followed had allowed the construction of a vehicle suitable for the environment it would encounter, despite the limited knowledge of it; on the other hand, the careful analysis of the performances that would have taken place on Mars would have made it possible to develop solutions to any critical issues identified or to introduce improvements in general in view of subsequent planetary exploration missions.

Particularly innovative aspects were represented by the semi-automatic navigation system and the locomotion system. Furthermore, the effects that the dust present on Mars would have on the photovoltaic panels that powered the lander and the rover were not known with certainty.

These objectives required careful selection of the landing site to balance the technical requests with the scientific ones.[9] A large plain was needed for the probe to land and a rocky terrain to verify the rover's systems. The choice fell on Ares Vallis, in Chryse Planitia, characterized by alluvial-looking rock formations. Scholars believed that the analysis of the rocks, present in what appeared to be the outlet of a huge drainage channel, could have confirmed the past presence of liquid water on the surface of Mars, as well as providing details on the surrounding areas, from which those rocks had been eroded.[9][10]

Technical characteristics

Sojourner in the production phase.

Developed by NASA's Jet Propulsion Laboratory (JPL), Sojourner was a 6-wheeled vehicle 65 cm long, 48 cm wide and 30 cm high. In the cruise phase it occupied a space of 18 cm in height. It weighed a total of 11.5 kg.[11][12] Another 6 kg of equipment was required to maintain communications between the rover and the lander; these equipment was mounted on the lander.[12]

It could reach a distance of 500 m from the lander and a maximum speed of 1 cm/s.[11]

Sojourner has solar panels and a non-rechargeable battery, which allowed limited nocturnal operations. Once the batteries were depleted, it could only operate during the day.[3] The batteries are lithium-thionyl chloride (LiSOCl2) and could provide 150 watt-hours.[13] The batteries also allowed the health of the rover to be checked while enclosed in the cruise stage while en route to Mars.[14]

0.22 square meters of solar cells could produce a maximum of about 15 watts on Mars, depending on conditions.[13] The cells were GaAs/Ge (Gallium Arsenide/Germanium) and capable of about 18 percent efficiency. They could survive down to about −140° Celsius (−220 °F).[14]

Locomotion system

Side view

The wheels, made of aluminum, were 13 cm in diameter and 7.9 cm in width. They were equipped with serrated stainless steel tracks capable of generating a pressure of only 1.65 kilopascals in optimal conditions on soft ground.[15] However, no such need arose during the operational phase.[15] All wheels were driven, each by an independent motor.[8] The first and third wheels were steering. A six-wheel steer configuration was considered, but rated too heavy in weight.[15] As rover rotated on itself, it drew a circle 74 cm in diameter.[8]

The wheels were connected to the frame through specially developed suspension to ensure that all six were in contact with the ground even on rough terrain.[15][16] Referred to as "Rocker-Bogie", they were developed by JPL's Don Bickler for the experimental "Rocky" vehicles, of which the Sojourner is the eighth version.[17] They consisted of two elements: the first, called "Bogie", connected the front wheel with the central one; the second, "Rocker", connected the rear wheel with the other two. The system did not include the use of springs or other elastic elements, which could have increased the amount of pressure exerted by the individual wheels.[15] This system allowed the overcoming of obstacles up to 8 cm high,[11] but would theoretically have allowed to overcome them equal to about 30% of the rover's length (20 cm).[15] The suspension system was also given the ability to collapse on itself, so that, in the cruising configuration, the rover would occupy a height of only 18 cm.[18]

The locomotion system turned out to be suitable for the environment encountered on Mars - very stable and able to allow forward and backward movements with similar ease[11] - and was adopted with appropriate precautions in the subsequent missions of the Spirit and Opportunity rovers.[16]

Sojourner in the cruise configuration.

In the ten-year development phase that led to the realization of Sojourner, various alternative solutions were examined, which could take advantage of the long experience gained at the Jet Propulsion Laboratory in the development of vehicles for the Moon and Mars.[17] The use of four or more legs was excluded for three reasons: a low number of them would have limited the movements and consequently the freedom of action of the rover, however, increasing the number would have led to a significant increase in complexity; proceeding in this configuration would also have required knowledge of the space in front (the ground corresponding to the next step) and this would have entailed further difficulties.[16] The choice of a vehicle on wheels instead solved most of the stability problems, led to a reduction in weight and improved efficiency and control compared to the previous solution.[16] The simplest configuration consists of a four-wheel system, which however encounters difficulties in overcoming obstacles. The solutions with six or eight wheels are better, with the rear ones able to push and thus allow the obstacle to be overcome. Finally, between the two, the six-wheeled one was preferred because it is lighter and simpler.[16]

Instrumentation

Schematic representation of the lander
Schematic representation of the rover

Sojourner's central processing unit (CPU) is an 80C85 with a 2 MHz clock, addressing 64 Kbytes of memory. It has four memory stores; the previously mentioned 64 Kbytes of RAM (made by IBM) for the main processor, 16 Kbytes of radiation-hardened PROM (made by Harris), 176 Kbytes of non-volatile storage (made by Seeq Technology), and 512 Kbytes of temporary data storage (made by Micron). The electronics were housed inside the Warm Electronics Box inside the rover.[3]

Sojourner communicated with its base station using a 9,600 baud radio modem, although error-checking protocols limited communications to a functional data rate of 2,400 baud with a theoretical range of about half a kilometer. Under normal operation, it would periodically send a "heartbeat" message to the lander. If no response was given, the rover could autonomously travel back to the location at which the last heartbeat was received. If desired, this same strategy could be used to deliberately extend the rover's operational range beyond that of its radio transceiver, although the rover rarely traveled further than 10 meters from Pathfinder during its mission.[3]

Power board (bottom side) and CPU board (top side)

The UHF radio modems worked similar to walkie-talkies, but sent data, not voice. It could send or receive, but not both at same time, which is known as half-duplex. The data was communicated in bursts of 2 kilobytes.[19]

Sojourner operation was supported by "Rover Control Software", which ran on a Silicon Graphics Onyx2 computer back on Earth, and allowed command sequences to be generated using a graphical interface.[20] The rover driver would wear 3D goggles supplied with imagery from the base station and move a virtual model with the spaceball controller, a specialized joystick. The control software allowed the rover and surrounding terrain to be viewed from any angle or position, supporting the study of terrain features, placing waypoints, or doing virtual flyovers.[20]

The rover had three cameras: two monochrome cameras in front, and a color camera in the rear. Each front camera had an array 484 pixels high by 768 wide. The optics consisted of a window, lens, and field flattener. The window was made of sapphire, while the lens objective and flattener were made of zinc selenide.[21] The rover was imaged on Mars by the base station's IMP camera system, which also helped determine where the rover should go.[22]

There were two experiments on board: the Wheel Abrasion Experiment (WAE) and the Material Adherence Experiment (MAE).

Alpha Particle X-Ray Spectrometer

Main page: Astronomy:Alpha particle X-ray spectrometer
MER APXS PIA05113.jpg Back of Sojourner and its Alpha Proton X-Ray Spectrometer.png
MSL - Alpha Particle X-ray Spectrometer (APXS).jpg
Alpha particle X-ray spectrometer (top left), APXS at the back of the Mars Pathfinder Sojourner rover (right), MSL Curiosity's alpha particle X-ray spectrometer, with a ruler (bottom left).

The Alpha Particle X-Ray Spectrometer was a spectrometer capable of determining the chemical composition of the rocks and dust of the Martian soil by analyzing the return radiation in its alpha, proton and X-ray components, resulting from the exposure of the sample to a radioactive source contained in the instrument.[23][24] The instrument had Curium-244[25] which emits alpha particles with an energy of 5.902 MeV. When the incident radiation impacted the surface of the analyzed sample, it was partly reflected and partly interacted with matter. The interaction of the alpha particles with the atomic nuclei led to the production of protons, while that with the electrons of the innermost orbitals determined the emission of X-rays. The instrument was designed to detect the energy of the three components of the return radiation. This would made possible to identify the atoms present (and their quantities) in a few tens of micrometers below the surface of the analyzed sample.[26] The detection process was rather slow and each measurement can take up to ten hours.[27]

The instrument was designed for the Russian Mars-96 mission,[25] that failed at launch. The alpha particle and proton detectors were provided by the Chemistry Department of the Max Planck Institute, while the University of Chicago had developed the X-ray detector.[24]

During each measurement the front surface of the instrument had to be in contact with the sample.[24] For this to be possible, the APXS was mounted on a robotic arm, the so-called Alpha-Proton-X-ray Spectrometer Deployment Mechanism, referred to by the acronym ADM. The ADM was an anthropomorphic actuator, equipped with a wrist capable of rotations of ±25°.[27]

The dual mobility of the rover and the ADM increased the potential of the instrument[27] - the first of its kind to reach Mars.[25]

Wheel Abrasion Experiment

The wheel affected by the Wheel Abrasion Experiment.

The Wheel Abrasion Experiment (WAE) was designed to measure the abrasive action of Martian soil on thin layers of aluminum, nickel and platinum and thus deduce information on the grain size of the soil at the landing site. For this purpose, 15 layers were mounted on one of the two central wheels, five of each metal, with a thickness between 200 and 1000 ångström, and electrically isolated from the rest of the rover. By directing the wheel appropriately, the incident sunlight was reflected towards an optical photovoltaic sensor, positioned near it. The analysis of the collected signal made possible to determine the desired information.[28] In order for the abrasive action to be significant on the mission schedule, the rover was scheduled to stop at frequent intervals and, with the other five wheels braked, force the wheel of the WAE to rotate to increase wear.[29] Following the conduct of the experiment on Mars, attempts were made to reproduce the effects observed in the laboratory.[29]

The interpretation of the results proposed by Ferguson et al. suggests that the soil at the landing site was made up of fine-grained dust of limited hardness, with a grain size of less than 40 µm.[29]

The instrument was developed, built and directed by the Lewis' Photovoltaics and Space Environments Branch of the Glenn Research Center.[29]

Material Adherence Experiment

The Material Adherence Experiment (MAE) was designed by engineers at the Glenn Research Center to measure the daily accumulation of dust on the back of the rover and the consequent reduction in the energy conversion capacity of photovoltaic panels.[30][31] It consisted of two sensors.[30]

The first was composed of a photovoltaic cell, covered by a transparent glass that could be removed on command. Near local midday various measurements of the cell's energy yield were made, both with the glass in place and rotated. From the comparison it was possible to deduce how much the dust coverage had decreased the cell yield.[30] The latter was also compared with that of a second photovoltaic cell, exposed to the Martian environment.[30]

The second sensor used was Quartz crystal microbalance (QCM) to measure the weight per surface unit of the dust deposited on the sensor itself.[30]

During the mission, a daily rate equal to 0.28% of percentage reduction in the energy efficiency of the photovoltaic cells was recorded, independent of the behavior of the rover - stationary or in motion.[31] This suggests that the settling dust was suspended from the atmosphere and was not, instead, raised by the rover's movements.[28]

Control system

Sojourner overcomes a height difference.

Since it was established that the transmissions relating to driving the Sojourner occur once every sol, it was necessary to equip the rover with a computerized control system to guide its movements independently.[32]

A series of commands had been programmed, providing an appropriate strategy for overcoming any obstacles. One of the primary commands was the "Go to Waypoint". A local reference system was envisaged (of which the lander was the origin), whose coordinate directions were fixed at the moment of landing taking the direction of the north as a reference.[32] During the communication session, the rover received a command string from Earth containing the coordinates of the arrival point, which it would have to reach autonomously.

The algorithm implemented on the on-board computer attempted, as a first option, to reach the obstacle in a straight line from the starting position. A system of photographic objectives and laser emitters made it possible to identify obstacles in the way along this path. The on-board computer was programmed to search for the signal produced by the lasers in the images shot through the cameras: in the case of a flat surface and no obstacles, the position of this signal was unchanged with respect to the reference one stored in the computer; any deviation from this position also made possible to identify the type of obstacle in the way.[32] The photographic scan was performed after each advance equal to the diameter of the wheels (13 cm) and before each turn.[8]

One of the obstacle detection images taken by Sojourner. The laser trace is clearly visible.

In case of confirmed presence of the obstacle (it was foreseen the possibility that three false positives out of twenty detections carried out before proceeding), the computer commanded the execution of a first strategy to avoid it. The rover, still on itself, rotated until the obstacle was no longer in sight. Then, after having advanced for half of its length, it recalculated a new straight path that would lead it to the point of arrival. At the end of the procedure, the computer had no memory of the existence of the obstacle.[32] As already mentioned, the steering angle of the wheels was controlled through potentiometers.[8]

In particularly uneven terrain, the procedure described above would have been prevented by the presence of a large number of obstacles. There was therefore a second one, indicated as "thread the needle", consisting in proceeding between two obstacles along the bisector between them, as long as they were sufficiently spaced to allow the rover to pass. If, before reaching a predetermined distance, the rover had encountered a clearing, it would have had to resume the primary procedure: rotate on itself to calculate a new straight trajectory to reach the goal; conversely, the rover would have had to go back and try a different trajectory.[32] As a last resort, there were contact sensors distributed on the front and rear surfaces of the rover.

To facilitate the rover's direction, an appropriate rotation on the spot could also be commanded from the Earth. The command in this case was "Turn" and was performed thanks to a gyroscope.[8]

Three accelerometers that measured the components of the acceleration of gravity along three perpendicular directions, made possible to measure the slope of the surface. The rover was programmed to deviate from routes that would have required a slope greater than 30°,[32] though it was designed not to tip over even when tilted 45°.[8]

The length of the distance traveled was finally determined by the number of revolutions of the wheels.[32]

During the operations phase on Mars, the sequences of the most complex commands to be sent to Sojourner were previously verified on a sister rover, the Marie Curie, in the laboratories of the JPL.[33]

Name

Sojourner Truth.

The name Sojourner was chosen through a competition held in March 1994 by the Planetary Society in collaboration with the Jet Propulsion Laboratory (JPL), which lasted one year and extended to students under the age of 18 from any country, who were invited to indicate a "heroine to whom to dedicate the rover" and to produce an elaborate in which they should have highlighted the merits and how they could have adapted to the Martian environment.[34] The initiative was publicized in the United States through the magazine Science and Children of the National Science Teachers Association, on which appeared in the January 1995.

3,500 papers were received from Canada, India, Israel, Japan, Mexico, Poland, Russia and the United States, of which 1,700 by students aged between 5 and 18.[34] The selection of the winners was made on the basis of various factors: the quality and creativity of the work, taking into consideration the age of each competitor; the appropriateness of the name for a Martian rover; the competitor's knowledge of the heroine; and the probe mission.

The winning paper was written by 12-year-old Valerie Ambroise of Bridgeport, Connecticut, who suggested dedicating the rover to Sojourner Truth,[35] African-American abolitionist and women's rights advocate, who chose as her mission to "cross long and broadly the country "asking that everyone be recognized the right to be free and women be guaranteed equality.

The second-place prize went to Deepti Rohatgi, 18, of Rockville, Maryland, who proposed Marie Curie, a Nobel Prize-winning Franco-Polish chemist. Third place went to Adam Sheedy, 16, of Round Rock, TX, who chose Judith Resnik, a United States astronaut and Space Shuttle crew-member, who died in the 1986 Challenger disaster.[36] The rover was also known as Microrover Flight Experiment abbreviated MFEX.[22]

Operations

Route of the rover projected on an image taken by the lander.

Sojourner reached Mars on July 4, 1997, after 7 months of cruising aboard the Mars Pathfinder probe. It operated in Ares Vallis, in a region called Chryse Planitia,[37] from July 5[38] to September 27, 1997, when the lander cut off communications with Earth.[37] In the 83 sols of activity (equal to twelve times the expected duration for the rover), Sojourner traveled a total of 104 m, always remaining within 12 m of the lander;[25] collected 550 images;[37] performed 16 analyzes through the APXS, 9 of rocks and the remainder of the soil,[25] performed eleven Wheel Abrasion Experiments and fourteen experiments on soil mechanics in cooperation with the lander.[8][39]

Rock analysis

The rover on the surface of Mars photographed by the lander.

The rocks at the landing site were given names of cartoon characters. The first analysis was carried out during the third sol on the one called "Barnacle Bill". The composition was determined by the APXS spectrometer, which took 10 hours for a complete scan. The tenth sol was analyzed through the spectrometer the rock "Yogi".[38][40] It has been suggested that the conformation of the land close to the rock, even visually at a lower level than the surrounding surface, was derived from the evaporation of water carried there by a flood.[41]

Both rocks turned out to be andesites.[42] This caused some surprise among scholars, because they are rocks that are formed by geological processes that require an interaction between materials of the crust and the mantle. However, lacking information on the surrounding highlands, it was not possible to grasp all the implications of the discovery.[42]

The rover was then directed to the next target and the fourteenth sun analyzed the rock referred to as "Scooby-Doo", also collecting images of "Casper".[38] Both were deemed to be consolidated deposits.[28]

Another rock, called "Moe", showed signs attributable to wind erosion. Most of the rocks analyzed showed a high silicon content. In a region nicknamed Rock Garden, the rover encountered crescent moon shaped dunes, similar to the dunes on earth.

The landing site turned out to be rich in varied rocks. Some of them of clear volcanic origin, such as "Yogi"; others, on the other hand, were conglomerates, for whose origin various alternatives have been proposed. A first hypothesis foresees their formation in the presence of water, in the distant past of the planet.[28] In support of this, there would be the detection of high silicon contents, also explainable as a consequence of sedimentation processes and the discovery of rounded-looking rocks, of various sizes, addition to the fact that the valley has shapes compatible with a river channel environment.[10] On the other hand, the smaller rounded stones may also have been generated during a surface impact.[28]

Sojourner in popular culture

Sojourner performs spectrometer measurements on the "Yogi" rock.
  • In the 2000 film Red Planet, the crew of the first manned mission to Mars survives the crash-landing of their entry vehicle, but their communications equipment is destroyed so they cannot contact their recovery vehicle in orbit. To reestablish contact before being presumed dead and left behind by the pilot of their recovery vehicle, the crew goes to the site of the Pathfinder rover, from which they salvage parts to make a basic radio.[43]
  • In the 2005 season 4 Star Trek: Enterprise episode "Terra Prime", Sojourner is briefly seen on the surface of Mars as a monument. Sojourner appeared in the opening titles of Enterprise lying dormant and covered in dust. A further shot showed a plaque which denoted the landing spot of the rover on board the Carl Sagan Memorial Station.[44]
  • In the 2011 novel The Martian by Andy Weir, and the 2015 film based on it, the protagonist Mark Watney, stranded on Mars, recovers the Pathfinder lander, and is able to use it to contact Earth.[45] In the movie, he is later seen in his Mars outpost, the Ares III Hab, with the Sojourner roving around.
  • Sojourner was included in the Robot Hall of Fame by Carnegie Mellon University.[46]
  • The rover appears in the television series The Big Bang Theory, where it is piloted by Howard Wollowitz, who causes Sojourner to end up in a crater.

The Rover Team

The development of the rover and its instruments as well as its guidance during operations on Mars were entrusted to a group of engineers from NASA collectively referred to as "The Rover Team". They were: JR Matijevic, J. Crisp, DB Bickler, RS Banes, BK Cooper, HJ Eisen, J. Gensler, A. Haldemann, F. Hartman, KA Jewett, LH Matthies, SL Laubach, AH Mishkin, JC Morrison, TT Nguyen, FA Comoglio, AR Sirota, HW Stone, S. Stride, LF Sword, JA Tarsala, AD Thompson, MT Wallace, R. Welch, E. Wellman and BH Wilcox of the Jet Propulsion Laboratory; and D. Ferguson, P. Jenkins, J. Kolecki, GA Landis, D. Wilt from NASA Lewis Research Center.

Gallery

Mars Pathfinder panorama with rock names and the Sojourner rover.

Comparison to later Mars rovers

Two spacecraft engineers stand with a group of vehicles providing a comparison of three generations of Mars rovers developed at NASA's Jet Propulsion Laboratory. The setting is JPL's Mars Yard testing area. Front and center is the flight spare for the first Mars rover, Sojourner, which landed on Mars in 1997 as part of the Mars Pathfinder Project. On the left is a Mars Exploration Rover Project test rover that is a working sibling to Spirit and Opportunity, which landed on Mars in 2004. On the right is a Mars Science Laboratory test rover the size of that project's Mars rover, Curiosity, which landed on Mars in 2012. Sojourner and its flight spare, named Marie Curie, are 2 feet (65 centimeters) long. The Mars Exploration Rover Project's rover, including the "Surface System Test Bed", are 5.2 feet (1.6 meters) long. The Curiosity rover and "Vehicle System Test Bed" rover, on the right, are 10 feet (3 meters) long.
Comparison of wheels of Sojourner, Spirit, Opportunity, and Curiosity rovers.

Sojourner's location in context

See also

References

This article was partly translated from the Italian Wikipedia article. For original, see :it:Sojourner.

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Further reading

  • A. Mishkin (2004). Berkeley Books. ed. Sojourner: An Insider's View of the Mars Pathfinder Mission. ISBN 978-0-425-19839-1. 
  • The Rover Team (1997). "The Pathfinder Microrover". J. Geophys. Res. 102 (E2): 3989–4001. doi:10.1029/96JE01922. 

External links