Lessons for the future on asteroid's surface

In November 2005, the Hayabusa spacecraft performed touchdown rehearsals, imaging navigation tests, and two touchdowns on a ∼300m asteroid, Itokawa, which is by far the smallest asteroid ever studied at high resolution.¹ These operations provided close-up images with resolutions up to 6mm/pixel,² which revealed that Itokawa is covered by fine- and coarse-grained materials, from granules to boulders up to tens of meters.³ These findings are completely different from previous expectations of small asteroids, which were believed to have no regolith (loose material overlying the surface). Itokawa appears to be a so-called rubble-pile asteroid where sorting processes have acted to redistribute materials according to grain size.⁴

The left side of Figure 1 is one of the highest-resolution images of the surface of Itokawa, a close-up of the region indicated on the right. The photograph shows the boundary area between the boulder-rich rough terrain and the pebble-covered smooth terrain, which is called the Muses Sea region. It is obvious from close-up images that the asteroid's surface is covered with unconsolidated gravels and boulder fields, which are typically piled on each other without being buried by fines (very small particles).⁴ The existence of unconsolidated gravels, especially the ponds of well-sorted pebbles in smooth terrains, was surprising because the surface of a small asteroid is believed to keep only a small amount of regolith due to the low escape velocity. In fact, Figure 1 appears to be similar to a terrestrial landslide deposit.

Figure 1. One of the highest-resolution images of the surface of Itokawa (left) shows the boundary between rough terrain and the smoother pebble-covered Muses Sea. The location of the close-up is indicated on a global image of Itokawa (right).

We have already reported well-developed imbrications (overlapping patterns) in several close-up images.⁴ The imbrications indicate the migrations of boulders and gravels (from lower left to upper right in this image), whose directions always match the local gravitational slopes. This indicates that fluidization occurred on a global scale, resulting in the regolith segregations and local concentrations on the surface of Itokawa. The fluidity is believed to be triggered by impact-induced vibrations.⁴

Despite surface gravity verging on about 0.01milligee, Itokawa not only retains particulate material, but shows evidence of gravitational sorting based on grain size and possible downslope regolith migration. This fact has significant implications for future mining of small asteroids. First, a very careful approach may be required to move rubble-pile asteroids in their entirety to other orbits, due to their low strength. Second, if Itokawa is typical, it should be possible to find large boulders on such bodies and transport them in their entirety into low Earth orbit for use in space manufacturing. For example, a 25m-wide block, such as one of the larger boulders identified on Itokawa, may have a mass of around 50,000 metric tons. This is 200 times the mass of the International Space Station, which suggests that major space construction projects could use such a boulder's material. If it were a carbonaceous chondrite type of asteroid instead of an S-type (stony), this mass could contain anywhere from 1500 to 7000 tons of water and 500 to 1500 tons of carbon, and thousands of tons of many potentially useful metals and oxides. Finally, some asteroids may be highly heterogeneous, both chemically and mineralogically. This heterogeneity may provide resources of varied materials as well as an exploration challenge.