Photometric Observations of Tiny Near-Earth Asteroids during the Close Approaches

Introduction
Studying near-Earth asteroids (NEAs) is fundamental to understanding material transportation in our solar system and mitigating the hazard of asteroid impacts. Most NEAs have their origins in the main belt between the orbits of Mars and Jupiter. Asteroidal fragments are thought to be generated from collisional events in the main belt and gradually drifted by the Yarkovsky effect, which is a thermal force caused by radiation from the Sun. During the orbital evolution, the rotation states of the object are changed by the YORP effect, which arises from the asymmetricity of scattered sunlight and thermal radiation from the surface. The orbital elements and rotation states are thus valuable tracers of their dynamical evolution, whereas recently mysterious results have been reported in various contexts. Therefore, the formation mechanism, dynamical evolution, and surface properties as well as interior structures of NEAs are not well understood. One point that should be emphasized is the difference in environments between large and tiny (diameter less than 100 m) bodies. If the environment of tiny asteroids such as surface properties and internal structures are different from those of large asteroids, we could not use empirical relationships obtained in previous studies. Tiny asteroids in the main belt are too faint to characterize even using 8 m class telescopes. Only NEAs during the close approaches and a few spacecraft mission targets give us knowledge about tiny asteroids. However, it is not easy to characterize tiny NEAs; the number of targets is very limited for spacecraft missions, whereas ground-based observations are restricted by limited observational windows, fast rotation, and large apparent motion on the sky during the close approaches. We present the results of three observational studies of tiny asteroids, all of them stand alone [1][2][3]. We have overcome the difficulties in observations of tiny asteroids by quick response or well-planned campaign observations using high-speed CMOS cameras. 

Observations & Results
First [1], we have obtained optical lightcurves of 108 tiny NEAs, and successfully derived the rotation periods of 52 tiny NEAs. We statistically confirmed that there is a certain number of tiny fast rotators in the NEA population, which have been missed with any previous surveys. Moreover, we discovered the tentative critical rotation period of 10 s for tiny asteroids. The critical rotation period of 10 s could be explained by a nongravitational effect considering the tangential YORP effect.

Next [2], we have conducted optical multicolor photometry of the tiny NEA 2015 RN₃₅ across a wide range of phase angles (2–30 deg). We showed that the slope of a visible spectrum of 2015 RN₃₅ is as red as asteroid (269) Justitia, one of the very red objects in the main belt. In conjunction with the shallow slope of the phase curve, we suppose that 2015 RN₃₅ is a high-albedo A-type asteroid. The other interpretation is that the shallow slope comes from the lack of fine grains on its surface due to the weak gravity and strong centrifugal force.

Finally [3], we have conducted optical multicolor photometry and polarimetry of the NEA pair candidate 2010 XC₁₅. The color indices of 2010 XC₁₅ are derived as g-r=0.435±0.008, r-i=0.158±0.017, and r-z=0.186±0.009 in the Pan-STARRS system. The linear polarization degrees of 2010 XC₁₅ are a few percent across a wide range of phase angles (58–114 deg). We found that 2010 XC₁₅ is an E-type NEA on the basis of its photometric and polarimetric properties. Taking the similarity of not only physical properties but also dynamical integrals and the rarity of E-type NEAs into account, we suppose that 2010 XC₁₅ and 1998 WT₂₄ are of common origin. These two NEAs are the sixth NEA pair and the first E-type NEA pair ever confirmed, possibly formed by rotation fission. 

Discussion
Combining the parts into a whole, we revealed the nature of tiny NEAs. As for surface properties of tiny NEAs, we found that the tiny fast-rotating NEA 2015 RN₃₅ may lack the regolith on its surface. This is explained with the lack of fine grains on its surface due to the weak gravity and/or strong centrifugal force caused by the fast rotation. We showed that asteroid pairs share similar surface properties even in the small size range (100–400 m in diameter). The slightly bluer spectrum of 2010 XC₁₅ is indicative of a lack of fine grains on the surface. Thus, we found direct evidence of the size dependence of the surface properties of NEAs; small NEAs may lack fine grains on the surface. Our conclusion, tiny NEAs lack fine regolith on the surface, is consistent with a recent study about histories of surface regolith considering both removal and production of them. As for interior structures, we succeeded in explaining the observed diameter and rotation period relation assuming the tiny NEAs are monolithic asteroids. This may imply that our assumption is correct; tiny asteroids are actually monolithic asteroids. In conjunction with the previous studies that imply some tiny asteroids are covered with fine grains or porous rocks, we suppose that the tiny asteroids might have porous structures. Throughout the dissertation, we conjecture that tiny asteroids are free from fine grains on the surface regardless of spin states, and possibly have porous interior structures. Our understanding of the formation mechanism of NEAs gives a strong constraint on the size frequency distribution of asteroids. Moreover, knowledge about the surface properties of tiny NEAs helps the understanding of the current mysterious results of tiny NEAs. The nature of tiny asteroids will be clearer in the next decade with the advent of such as the Rubin Observatory Legacy Survey of Space and Time, the Near Earth Object Surveyor, and the University of Tokyo Atacama Observatory as well as future spacecraft missions.

Acknowledgments
I would like to thank all those who were involved in this dissertation. This work was supported by JSPS KAKENHI Grant Numbers JP22K21344 and JP23KJ0640.

References

[1] Beniyama et al., 2022, PASJ, 74, 877–903.
[2] Beniyama et al., 2023a, AJ, 166, 229–241.
[3] Beniyama et al., 2023b, ApJ, 955, 143–158.