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Mars Rovers: Sojourner


Sojourner was the first exploration rover sent to Mars. It was part of the Mars Pathfinder mission which consisted in sending to Mars a lander and rover. Sojouner traveled inside the lander, later named Carl Sagan Memorial Station, for almost seven moths arriving to the red planet's Aris Vallis region on July 4th, 1997.  The main goal of the Pathfinder mission was to demonstrate that it was possible to send a lander and a rover to another planet at a lower cost than previous missions. One day after the landing, on July 5th, the small rover was deployed, and made its first run on Mars soil.




Sojourner was 65cm (2.13ft) long, 48cm (1.57ft) wide, and 30 cm (1ft) tall, and it weighed about 11.5kg (25.35lbs). It was provided with a six-wheeled structure to move around. The wheels were 13cm (5.13in) of diameter, and they where made of aluminum, stainless steal threads and cleats to provide traction. Also, the rover had a Rocket-Bogie suspension system, which doesn't use axels or strings, providing independent movement to each wheel. Rocket-Bogie suspension provided great stability to the rover and allowed the rover's wheels to adapt to the geometry of the terrain and cross obstacles with a height of twice the wheels' diameter, indeed could tip as much as 45° when climbing a rock without tipping over. Three motion sensors, placed along the rover's frame, were used to detect excessive tilt. In addition, front and real wheels were independently steerable allowing the rover to turn.
Rocker Bogie System

The rover was powered by a 0.2 cm2 solar panel and 9 D size batteries. The solar panel provided 16 W of energy. The batteries were used when there was either too little or no sunlight. Although, most of the time, they were used during the night while the rover was performing some experiments. The batteries were cased inside three tubes (3 batteries on each tube) and strapped together around the rover suspension.

Power Supply System

Sojourner was controlled from the Earth, and all the communications were done through the Deep Space Network (DSN). All the command sequences where generated on a Silicon Graphics Onyx2, and special software was used to simulate the rover movements on Mars. This simulators helped operators to calculate the coordinates that were used to tell the rover where to go avoiding any kind danger that could damage the rover such as running into a rock or falling from a cliff. Moreover, rover telemetry and images took by the lander and rover were used to get a more realistic view of the surroundings.
When a set of commands was completed, they were sent to Mars using the DNS. In Mars, the lander received the information and transferred it to the rover. All the communication between the rover and DNS was done through the lander as intermediary. The communication between the rover and the lander was half-duplex at a frequency of 459.7 MHz (UHF) and bandwidth of 25 KHz, and it was controller by the rover. During the day, the rover requested for any command sequence sent from Earth, and when no commands were available, the rover transmitted any data collected (such as images or telemetry). The lander saved this data and transferred it when the communication to the DNS was available.

However, due to the distance between Earth and Mars, all the information took about 8 – 12 minutes to arrive to the lander, and real-time driving couldn't be possible. Although, drivers set the most secure path for the rover, the two front cameras along with a laser system were used for proximity scanning and hazard detection.


A motor encoder and potentiometer were added to provide motion control and according to the readings, motors were periodically turn on or off. When motors were off, the computer performed two tasks updating its measurements of the distance traveled to provide an stimate of progress to the goal location, and reading and processing the camera and laser array data values to detect obstacles. The vehicle is stereed autonomously to avoid obstacles and continue to achieve the commanded goal location. 
All control and command processing was done by the rover CPU which consisted in a 10Khz Intel 80C85 with 512kB of RAM and 176kB of flash. To kept al the rover's electronics safe from Mars' extreme temperatures, which can reach up to -110°C (-230°F) during the night, it was place inside a box coated with high and low emissivity paints on the exterior and silica aerogel (solid smoke) on its interior. This box was called WEB that stands for Warm Electronic Box. The rover also had a heating system to keep the internal temperature between the ranges of -40 to +40°C (-40 to +104°F).
The rover was equipped with various instruments for experimentation. An Alpha-Proton X-ray Spectrometer (APXS) was used for rock and soil analysis. The Alpha-Proton spectrometer was provided by the Max Plank Institute and the X-ray spectrometer by the University of Chicago, and it was designed to measure the elemental chemistry of the Martian surface material to gain more detailed understanding of the geological and geochemical processes within Mars. The APXS consisted in a sensor mounted at the end of deployment mechanism (robotic arm) located in the back of the rover.  The deployment mechanism placed the APXS in contact with a rock or soil surface during the analysis that takes 10 hours to be completed. The result of the experiments was collected and processed inside the electronic box (WEB).


To support the experiments, three cameras, manufactured by Eastman Kodak, were added to the rover. All the cameras were CCD and had 4mm wide-angle lens. Two cameras were located on the front of the rover and the third one on the back. The front cameras were black and white, and where mounted horizontally spaced to provide stereo coverage. The back camera was color and used to take pictures of the APXS target area and the rover track on the ground to estimate soil properties.

Images took with the two front cameras. Right - left camera, Middle - right camera, Left - Anagliph image obtained combining the two images

Image took with the back camera

Rover Material Adherence Experiment (MAE) was performed to measure the rate of dust deposition and create models of atmospheric transport dust. A solar censor and quartz crystal monitor (QCM), mounted on the front left corner of the rover, were used to perform this experiment. The solar censor consisted in a solar cell cover by a silica plate which was used to collect the dust that settle on its surface. A comparison of the output of the cell uncovered with the output when the plated was covering it gave the degradation in performance due dust. In the other hand, QCM consisted in two quartz crystals with matching characteristics, but one was isolated and the other one, covered with a thin layer of adhesive, was exposed to the environment. Similar to the Solar Censor, the comparison of the measurements between both crystals provide amount of dust colleted during time.



The wheel abrasion experiment (WAE) study the abrasive wear of the soil. Films of aluminum, nickel, and platinum over black anodized aluminum strips attached to the center of the thread of the middle left rover wheel. Film reflectivity was measured over time by a photovoltaic sensor. Changes in reflectivity indicate the amount of abrasion due to rolling wear.


Barnacle Bill was the first rock visited and analyzed by the rover on Sol 3 (a sol is a day on Mars equivalent to 24 hours and 39 minutes).


Left - Anagliph of the pictures took with the lander's camare. Middle - Sojourner performing APXS analisys. Right - Picture of APSX test took with the back camera

Although, the mission was programmed to last a week to a month, in the end it last 83 (almost three months). Mars Pathfinder mission accomplished its main goal of demonstrating that it was possible to send spacecraft with a lot of instruments at a lower cost, and Soujourner became the forerunner of the next rover missions.  During its time on Mars, Sojourner performed chemical analysis at sixteen different locations near the lander and took 550 photographs. The sojourner works from July 5, 1997 to September 27, 1997, when the last contact was done. The mission was officially ended on March 1998 due to problems with communication. In 2003, Sojourner was inducted into the Robot Hall of Fame.

Map of the places visited by the rover
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