Scientists reveal new features of solar wind interactions with asteroids

In recent years, small objects (asteroids, comets, etc.) have become a hot spot for human deep space exploration. On the one hand, small bodies retain material from the beginning of the formation of the solar system, which can provide clues to the study of the origin of the solar system and life. On the other hand, NEOs pose a threat to the security of the Earth, so it is necessary to examine their spatial distribution and orbital characteristics in detail. China’s Tianwen-2 mission will sample and return the near-Earth small object 2016HO3, conduct orbital research on the main-belt comet 311P, and obtain their orbital parameters, surface material composition, magnetization intensity, internal structure and surrounding space environment characteristics, which provides data for exploring the formation and evolution of small objects and the migration process of surface materials. Due to the lack of observational data, little is currently known about the space environment around small objects, which will pose a safety hazard to the close detection of small objects, especially for sampling operations. For NEO 2016HO3, its surface will be charged by sunlight and solar wind bombardment. Due to the small size, the surface of 2016HO3 may have a strong electric field, which will bring discharge risks to surface sampling. At the same time, 2016HO3 represents a class of atmosphereless objects that interact with the solar wind and form a wake-up structure on the sunny side. Similar trails also exist on the Moon, but 2016HO3 is more than 3 orders of magnitude smaller than the Moon, and its trail may have new features different from the Lunar trail.

Xie Lianghai, associate researcher at the Key Laboratory of Solar Activity and Space Weather, National Space Science Center, Chinese Academy of Sciences, established a three-dimensional PIC model of the interaction between the solar wind and asteroid 2016HO3 for quantitative analysis of the electric field and plasma characteristics around 2016HO3. It is found that the highest surface potential of asteroid 2016HO3 occurs near the sunset point, up to +12V, the lowest potential on the sunny side is about -35V, the corresponding electric field on the sunny side is about +2V/m, and the potential on the sunny side is about -5V/m. The maximum electric field occurs near the morning and evening line and can be greater than 10V/m. In addition, the analysis of different rotation states (Cases1-3) shows that when the 2016HO3 major axis is perpendicular to the solar wind (Case1), the electric potential generated is the largest in size and spatial range, and the electric field near the morning and evening line can reach 20V/m.

The study found that the electric characteristics of different solar wind conditions found that the solar wind velocity causes an increase in the potential on the sunny side, and the solar wind temperature causes a decrease in the potential on the sunny side. In addition, photoionization dominates on the sunny side, and photoelectron sheaths are formed near the near surface, with electron densities up to 107.5m-3. On the dorsal side, solar wind ions are blocked and absorbed by asteroid 2016HO3, forming a low-density cavity with densities below 1m-3. The surrounding solar wind ions will try to fill the density cavity, bringing compressive waves propagating inward and sparse waves propagating outward, and eventually forming a tapered wake structure under the action of solar wind convection. Similar wake structures on the Moon have been studied, and self-similar plasma expansion theory has been established. This theory mainly considers the effect of thermal motion and bipolar diffusion electric field on ion filling, and the magnitude of the obtained cone angle depends on the ratio of ion sound velocity and solar wind velocity. This theory can better explain the observed lunar wake structure and has been widely used to study the wake structure of other non-atmospheric objects. However, the study found that the overall size of the simulated 2016HO3 wake cone angle is larger than the theoretical model value (Figure 2), especially the deionization sound velocity and solar wind velocity, and the cone angle also changes with solar wind density. Based on this, this study proposes that in addition to thermal motion and bipolar diffusion, the negative surface charge on the sunny surface will also accelerate the filling of solar wind ions to the density cavity, and bring faster filling speed and large wake cone angle. On the moon, because the surface plasma sheath is smaller than the wake size, the effect of the surface electric field is not obvious. For 2016HO3, the thickness of the surface plasma sheath is comparable to the transverse scale of the wake, so the role of the surface electric field becomes more important.

Figure 1. Electric potential and electric field distribution around asteroid 2016HO3

Figure 2. For the wake structure under different solar wind conditions, the white numbers in the figure are the simulated cone angle size, and the red numbers are the cone angle size given by the theoretical model

This study quantitatively gives the electric field and plasma density distribution around asteroid 2016HO3, which provides a basis for the control of surface potential difference in the asteroid sampling process of China’s Tianwen-2 mission and the formulation of space environment detection scheme. In addition, the study discovered a new theory of wake formation that improved the understanding of the interaction of the solar wind with small bodies. The above results have important reference significance for exploring the space environment of other non-atmospheric objects. The findings were published in The Astrophysical Journal. (Source: National Space Science Center, Chinese Academy of Sciences)

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