The Terrain Testing Instrument (TTI) as a Selected Payload for the Chang'e 8 Lunar Landing Mission

This contribution describes the so-called Terrain Testing Instrument (TTI) which has recently been selected as an international payload for the planned Chinese Chang’e 8 lunar landing and roving mission targeting a high southern latitude landing site. The TTI will measure lunar regolith penetration resistance and shear strength, in a so-called vane-cone instrument that combines a cone penetrometer and a shear vane. A permittivity sensor using a novel patch electrode arrangement is integrated with the vane-cone and will allow to infer bulk density and ice content of the regolith, derived from measurement of the dielectric properties such as relative permittivity.

In the initial phase of lunar surface exploration by the United States and the Soviet Union, dedicated instruments were designed and used to measure in situ some key physical properties of the regolith column in various locations. On the lunar landing and roving missions of the modern era however, no instruments have yet been flown for such purposes. It will however be particularly important to understand regolith stress-strain behavior in the South polar region as well as local volatiles contents, as extensive landing, roving, mining, and construction activities are foreseen there over the next several decades. A general assumption is that regolith in the South polar area would broadly resemble lunar highland regolith. But direct measurements will be indispensable ahead of crewed missions.

Volatiles constitute an important resource while at the same time sublimation of ices from an icy regolith in response to loading and thermal dissipation from human-emplaced structures can lead to subsistence of the ground, thus constituting a hazard. The TTI will address these critical gaps in knowledge.

The TTI is a slender penetrometer with a frontal shear vane for quasi-static regolith intrusion. A linear translation mechanism will drive it into the regolith while resistance vs. depth is recorded, followed by rotation of the shear vane to indicate shear resistance as a function of shear angle. Depth range of the TTI instrument is ~10…20 cm. It will be carried on a mobile rover and perform multiple measurement runs during the Chang’e 8 mission at various locations, thanks to the mobility of the carrying platform. The TTI overall mass is ~1.5 kg. The instrument is being developed by an international team of entities from Germany, the Netherlands, and China.