Sharkey and 9 co-authors, reported late 2021 that little is known about Earth quasi-satellites, a class of near-Earth small solar system bodies that orbit the sun but remain close to the Earth, because they are faint and difficult to observe. The authors used the Large Binocular Telescope (LBT) and the Lowell Discovery Telescope (LDT) to conduct a comprehensive physical characterization of quasi-satellite (469219) Kamoʻoalewa and assess its affinity with other groups of near-Earth objects.
The authors found that (469219) KamoÊ»oalewa rotates with a period of 28.3 (+1.8/−1.3) minutes and displays a reddened reflectance spectrum from 0.4–2.2 microns. This spectrum is indicative of a silicate-based composition, but with reddening beyond what is typically seen amongst asteroids in the inner solar system. The authors compared the spectrum to those of several material analogs and concluded that the best match is with lunar-like silicates. This interpretation implies extensive space weathering and raises the prospect that Kamo’oalewa could comprise lunar material.
The authors concluded that the natural question that arises is: what is Kamo’oalewa’s origin? The answers are speculative. One possibility is that it was captured in its Earth-like orbit from the general population of NEOs. Its low eccentricity and inclination are, however, rather atypical of such captured co-orbital states found in numerical simulations42.
Another possibility is that Kamo’oalewa originates from an as-yet undiscovered quasi-stable population of Earth’s Trojan asteroids orbiting near Earth’s L4 and L5 Lagrange points43,44. This hypothesis can be tested in future deeper and wider observational surveys of the Earth-Sun Trojan regions, supplemented with theoretical investigation of dynamical pathways between Earth Trojans and quasi-satellites.
A third possibility is that KamoÊ»oalewa originates in the Earth-Moon system, perhaps as impact ejecta from the lunar surface45 or as a fragment of a parent NEO’s tidal or rotational break up during a close encounter with Earth-Moon46. Three NEOs, 2020 PN1, 2020 PP1, and 2020 KZ2, have been identified as having orbital parameters cluster near KamoÊ»oalewa’s closely enough that they may be break-up companions47.
The reflectance spectrum of KamoÊ»oalewa (as reported in the present work) lends support to the lunar ejecta hypothesis. An origin near or within the Earth-Moon system is further supported by the low value of the relative velocity, v ≈ 2–5 km/s, of KamoÊ»oalewa during its close approaches to Earth-Moon, whereas NEOs have larger relative velocities at close approaches, with an average v ≈ 20 km/s48,49.
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