9. Trafficability

2005: IVA-6
2010: 4
Priority: Low

Investigation: Assess landing site-related hazards, including those related both to safe landing and safe operations within the possible area to be accessed by possible elements of a human mission.

Artist's rendering of a rover encountering rocks while traversing the Martian surface. Image credit: NASA/JPL-Caltech
Trafficability refers to the ability of a vehicle to traverse any given terrain. At a landing site especially, it is important to make sure that there are no big rocks or other potential hazards that would adversely affect operations. Landing sites are selected based on finding an acceptable balance between terrain hazards and scientific interest, since sometimes the most potentially interesting sites are also the most dangerous. Hazards are categorized into two groups: those related to landing safely, and those related to movements along the Martian surface.

This image shows the large rut created by the Opportunity rover as it extracted itself from the section of Meridiani dubbed "Purgatory Dune." Image credit: NASA/JPL-Caltech/Cornell (Jim Bell)
The main factors relevant to landing safety are: size and concentration of surface rocks, terrain slopes, and concentration of dust. When traversing a surface, rock fields, terrain slopes, and soft soil (see above picture) are hazards to be considered. Measurements from previous rover expeditions as well as orbital imaging can help reduce risks involved by allowing scientists to identify obstacles, dust, and soft material in potential landing areas. This knowledge would enable effective and efficient expedition planning.

Click on the expanding links below to see the 2005 and 2010 versions of the investigation.

2005 Version of Investigation (old version)

2010 Version of Investigation (current version)

Many investigations have led to better understandings of trafficability on the Martian surface. The movements of the Mars Exploration Rovers (MER) Spirit and Opportunity in particular have contributed greatly to the field of terramechanics – rover wheel torques, ruts, and trenches have been analyzed to determine internal friction angles and provide rolling statistics. HiRISE, TES, and THEMIS data are also used to map surfaces and determine properties and potential hazards that the rovers may encounter.

Currently, there is a lot of research going on in the field of terramechanics (study of interaction of wheels with various surfaces), mostly focused on discrete element modeling and dynamic modeling packages. These programs use known information about the properties of soil to create realistic terrains to simulate rover drive paths as accurately as possible. The results of this new research would allow scientists and engineers to identify risky areas along a rover’s driving route and alter its path to avoid slipping or sinking into the ground.

Although the operation of the MER rovers has significantly improved the general understanding of the issues related to trafficability on the martian surface, an assessement would need to be made on a site-by-site basis given the range of mobile elements associated with a human mission.

The following list of references includes sources that touch on all the above topics.