Mars Exploration Rover Surface Operations: Driving Spirit atGusev Crater P. Chris Leger, Ashitey Trebi-Ollennu, John R. Wright, Scott A. Maxwell, Robert G. Bonitz,Jeffrey J. Biesiadecki, Frank R. Hartman, Brian K. Cooper, Eric T. Baumgartner, and Mark W. MaimoneJet Propulsion Laboratory, California Institute of TechnologyPasadena, CA, [email protected] - Spirit is one of two rovers that landed on Marsin January 2004 as part of NASA's Mars ExplorationRover mission. As of July 2005, Spirit has traveled over4.5 kilometers across the Martian surface whileinvestigating rocks and soils, digging trenches to examinesubsurface materials, and climbing hills to reachoutcrops of bedrock. Originally designed to last 90 sols(Martian days), Spirit has survived over 500 sols ofoperation and continues to explore. During the mission,we achieved increases in efficiency, accuracy, andtraverse capability through increasingly complexcommand sequences, growing experience, and updates tothe on-board and ground-based software. Safe andprecise mobility on slopes and in the presence of obstacleshas been a primary factor in development of new softwareand techniques.Keywords: Planetary robotics, mobility, MER, Mars,rovers.1. IntroductionNASA's Mars Exploration Rover (MER) missiondeveloped and operates two robotic vehicles tasked withsearching for evidence of past water activity on Mars [1].The 175kg, 1.6m-long rovers, Spirit and Opportunity, eachhave a six-wheeled rocker-bogie suspension, a five degree-of-freedom arm called the Instrument Deployment Device(IDD) [2], four sets of stereoscopic cameras, threespectrometers, a microscopic imager, and a Rock AbrasionTool (RAT) for cleaning and grinding rock surfaces.Spirit landed at Gusev Crater on January 3, 2004.Opportunity, Spirit's sister rover, landed at MeridianiPlanum on January 24, 2004, and is described in a separatepaper which also gives more background on the rovers'mobility software [3]. Spirit's first images showed a rockyplain similar to the Pathfinder [4] and Viking landing sites.Images taken during descent by the Descent Image MotionEstimation System [5] showed several craters whichenabled a rapid localization of Spirit within an orbital imagefrom Mars Global Surveyor. The first twelve sols (Martiandays) after landing constituted the Impact-to-Egress phase,which saw the deployment of the rover's solar arrays, sensormast, and mobility mechanisms, followed by egress fromthe lander [6]. Since it can take up to 26 minutes for a signal fromEarth to reach Mars (and vice-versa), direct teleoperation ofthe rovers is impractical. Power and line-of-sightconstraints also prohibit continuous communication withthe rovers. Instead, the rovers are controlled by commandsequences sent every Martian morning. The sequences areexecuted over the course of the sol, with one or twocommunication passes for downlink (usually via the MarsOdyssey orbiter) during which images and other data arereturned to Earth. The time available each sol for roveroperations is thus affected by how much solar energy isavailable (a function of season, atmospheric conditions,dust on the solar array, and local terrain slope) and whenthe orbiter is within line-of-sight. The first 90 sols ofoperations used a 16-hour planning cycle with separateteams for each rover, but as the teams shifted onto aschedule based on a normal Earth work week rather than anovernight Mars workday, the planning cycle decreased tobetween 5 and 10 hours depending on the phasing of Earthand Mars days. (A Martian day is roughly 40 minuteslonger than an Earth day.)Spirit's mission can be divided into several campaignscharacterized by differing science interests, destinations,terrains, and operational techniques. The first majorobjective was to reach the rim of nearby Bonneville Crater(Figure 1) in the hope of finding exposed outcrops ofbedrock, but none were found. Spirit then began a 3km trekacross the rocky plains toward the Columbia Hills. Uponreaching the hills, Spirit investigated the West Spur ofHusband Hill, the highest peak in the range, for severalmonths while dealing with the approaching Martian winter.Since Spirit is in the southern hemisphere of Mars, moresolar power is available if the rover and its horizontally-mounted solar panels are tilted to the north; this dictated therover's path along the north flanks of Husband Hill as itlater traversed toward Cumberland Ridge, which overlooksthe Tennessee Valley and joins the north face of HusbandHill. Fortuitously, high winds cleaned most of the dust ofSpirit's solar panels on Sol 418. This eliminated the need tostay on north-facing slopes and opened up an easier route toHusband Hill's summit via the mountain's west face.2. Rover Mobility and SoftwareSpirit's six wheel drive, four-wheel steering, androcker-bogie suspension provide for excellent stability,maneuverability, and obstacle negotiation. The rover'sground clearance is 29cm, though operationally we treatrocks or other terrain features taller than 20cm as obstacles.Authorized licensed use limited to: University of Southern California. Downloaded on November 13, 2008 at 13:43 from IEEE Xplore. Restrictions apply.The rover's static tip-over angle is 45 degrees but slippage,rather than drive torque or stability, is usually the limitingfactor when driving on slopes. Sandy slopes of as little as10 degrees can completely block further progress, and onsteep slopes with firmer footing, special techniques arerequired for safe and accurate driving. Spirit's onboard sensors and software provide manymobility and safeguarding capabilities. The inertialmeasurement unit (IMU) produces roll, pitch and yawmeasurements at 8Hz, and stereo camera pairs provideaccurate position knowledge and terrain assessment.Reactive safety checks can halt vehicle motion if thesuspension articulates beyond a preset limit, or if the pitch,roll, or overall tilt exceed the commanded range. Therover's command sequencing language also allows safetysequences to run in parallel with a drive and halt motion onother conditions, such as when the rover enters a manually-defined keep-out zone. When on level terrain, the combination of IMU andwheel odometry data leads to drive accuracy within a fewpercent. In terrain where slip is substantial and highaccuracy is required--either to avoid obstacles or reach adesired location for science observations--visual odometrymust be used, since the rover has no other way of detectingslip as the