Вадим Дудченко
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Using the Large Binocular Telescope and the Lowell Discovery Telescope, astronomers have conducted a comprehensive physical characterization of the near-Earth asteroid (469219) Kamo’oalewa and assessed its affinity with other groups of near-Earth objects (NEOs). They’ve found that Kamo’oalewa’s spectrum matches lunar rocks from NASA’s Apollo missions, suggesting it originated from the Moon.

An artist’s impression of Kamo’oalewa near the Earth-Moon system. Image credit: Addy Graham / University of Arizona.

Earth’s quasi-satellites are a class of near-Earth bodies that orbit the Sun but remain close to our home planet, because they are faint and difficult to observe.

Kamo’oalewa is the most stable of the five known quasi-satellites of the Earth, with a dynamical lifetime of a few hundred years.

Also designated as 2016 HO3, the asteroid was discovered in 2016 by astronomers using the PanSTARRS telescope.

Kamo’oalewa is between 46 and 58 m (150-190 feet) in diameter, and gets as close as about 14.5 million km (9 million miles) from Earth.

As a quasi-satellite, its orbit is very Earth-like, with semi-major axis within 0.001 AU of Earth’s, a low eccentricity of just 0.1, and a modest inclination of about 8 degrees to the ecliptic.

As it orbits the Sun with a ~1 year orbital period, it takes a quasi-satellite path relative to Earth, that is, it makes retrograde loops around Earth with a ~1 year period.

“Its orbit is also not typical of near-Earth asteroids,” said Professor Renu Malhotra, a planetary scientist in the Lunar and Planetary Laboratory at the University of Arizona, Tucson.

“It is very unlikely that a garden-variety near-Earth asteroid would spontaneously move into a quasi-satellite orbit like Kamo’oalewa’s.”

“It will not remain in this particular orbit for very long, only about 300 years in the future, and we estimate that it arrived in this orbit about 500 years ago.”

The astronomers obtained and analyzed broadband photometric and visible spectra data from the Multi-Object Double Spectrograph (MODS) instrument on the Large Binocular Telescope and the Large Monolithic Imager on the Lowell Discovery Telescope.

“I looked through every near-Earth asteroid spectrum we had access to, and nothing matched,” said Benjamin Sharkey, a graduate student in the Lunar and Planetary Laboratory at the University of Arizona, Tucson.

The researchers found that Kamo’oalewa rotates with a period of 28.3 minutes; its spectrum is indicative of a silicate-based composition, but with reddening beyond what is typically seen amongst asteroids in the inner Solar System.

“The natural question that arises is: what is Kamo’oalewa’s origin? The answers are speculative,” they said.

“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 simulations.”

“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 points. 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 Kamo’oalewa originates in the Earth-Moon system, perhaps as impact ejecta from the lunar surface or as a fragment of a parent NEO’s tidal or rotational break up during a close encounter with Earth-Moon.”

“The reflectance spectrum of Kamo’oalewa lends support to the lunar ejecta hypothesis,” they said.

“An origin near or within the Earth-Moon system is further supported by the low value of the relative velocity — 2-5 km/s — of Kamo’oalewa during its close approaches to Earth-Moon, whereas NEOs have larger relative velocities at close approaches, with an average of 20 km/s.”

The results were published in the journal Communications Earth and Environment.

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B.N.L. Sharkey et al. 2021. Lunar-like silicate material forms the Earth quasi-satellite (469219) 2016 HO3 Kamo’oalewa. Commun Earth Environ 2, 231; doi: 10.1038/s43247-021-00303-7

 



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