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

Introduction:  This paper presents the ongoing development of the “Terrain Testing Instrument (TTI)”, being an international payload for the planned Chinese Chang’e 8 lunar landing and roving mission in 2029.

Science Background:  The lunar surface is covered by a layer of granular material referred to as the regolith which is derived through comminution of bedrock and float rocks by impact gardening and deep thermal cycling. Thickness of the regolith is typically several meters but varies between mare and highland regions as well in response to local processes. Ground penetrating radar has been used successfully on the Chinese lunar rovers Yutu and Yutu-2 to inform the stratigraphy and thickness of the regolith at fine scale. Knowledge of the physical properties of the regolith in terms of bearing and shear strength is critical for designing equipment such as ground vehicles (rovers), excavation equipment, and elements of future outposts. This will become ever more important as crewed missions will again be sent to the lunar surface, outposts will be constructed, and mining will be performed to extract local resources.

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 key physical properties of the regolith column of the Moon in various locations. This was done by penetrometry (on the Apollo missions) and vane-cone instruments (PROP-M instrument on the Lunokhod rovers). On the lunar landing and roving missions of the modern era however, no dedicated instruments have yet been flown to measure regolith physical properties. It will be particularly important to understand bearing and shear strength in the South polar region where extensive landing, roving, mining, and construction activities are foreseen over the next several decades. A general assumption is that regolith in the lunar South polar terrain would broadly resemble lunar highland regolith, as argued in the lunar geological community on the grounds of structural geology and geological mapping of the Moon. But direct measurements of physical properties will be indispensable ahead of crewed missions. The same holds true for understanding volatile contents of the regolith, at spatial scales relevant to roving and excavation. 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.

To address this gap in critical knowledge, the authors of the present abstract are developing the so-called Terrain Testing Instrument (TTI) which has been selected as an international payload for the planned Chinese Chang’e 8 lunar landing and roving mission slated to launch in 2029.

Instrument Concept and Design:  The TTI will measure regolith penetration resistance via a cone penetrometer and shear strength via a shear vane, with both techniques combined in a so-called vane-cone instrument. A permittivity sensor 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 and relative permittivity.

A linear translation mechanism drives the vane-cone assembly into the regolith, followed by rotation of the front shear vane to indicate shear resistance as a function of shear angle.

Depth range of the instrument is ~5 cm as it will be carried on the relatively small JINNAH-1 rover contributed by SUPARCO, with limited available reaction force. The TTI will perform multiple measurement sequences at various locations during the first lunar day following landing. TTI overall mass is ~1.5 kg, including its electronics unit which is accommodated inside the rover.

Development Status:  The instrument Structural Model (SM) has already been delivered, to be followed by the Electrical Model (EM) in the summer of 2026.

The TTI is being developed by an international team of entities from Germany, the Netherlands, France, China, and the two Special Administrative Regions Macau and Hong Kong.

References:  [1] Heiken, Vaniman, Carrier (1991). Lunar Sourcebook: regolith penetration resistance and shear strength. [2] L. Ding. L. Richter, and 44 co-authors (2022). Sci. Robot. 7, eabj6660. [3] H. Seifamiri, R. C. Anderson, R. Boudreaulta, R. de Moraes, C. Dickinson, N. Gelino, P. Maghoul (2025). Geotechnical Properties of Lunar Regolith for Excavation and Construction on the Moon.

 

Fig. 1: JINNAH-1 rover with the TTI external assembly mounted on its rear panel. Electrodes of the permittivity sensor (PP) are integrated with the TTI shear vanes. The TTI electronics reside inside the rover body.

 

Fig. 2: Installing the Structural Model of the TTI external assembly on the rover Structural Model, ahead of vibration testing (March 2026).