469219 Kamoʻoalewa

469219 Kamoʻoalewa, provisionally designated , is a very small asteroid, fast rotator and near-Earth object of the Apollo group, approximately 40-100 m in diameter. At present it is a quasi-satellite of Earth, and currently the second-smallest, closest, and most stable known such quasi-satellite (after ). The asteroid was discovered by Pan-STARRS at Haleakala Observatory on 27 April 2016. It was named, a Hawaiian word that refers to an oscillating celestial object. The object's Earth-like orbit and its composition of lunar-like silicates may be a result of it being lunar ejecta.

Discovery and naming
Kamoʻoalewa was first spotted on 27 April 2016, by the Pan-STARRS 1 asteroid survey telescope on Haleakalā, Hawaii, that is operated by the University of Hawaii's Institute for Astronomy and funded by NASA's Planetary Defense Coordination Office. The name Kamoʻoalewa is derived from the Hawaiian words ka 'the', moʻo 'fragment', referring to it being a piece broken off a larger object, a 'of', and lewa 'to oscillate', referring to its motion in the sky as viewed from Earth. The official was published by the Minor Planet Center on 6 April 2019 (M.P.C. 112435).

Orbit and classification
Kamoʻoalewa orbits the Sun at a distance of 0.90–1.10 AU. Although the period as of 2022 is about 366 days, its longer-term average period is closer to 365 days since it is a quasi-satellite of Earth and will continue to be so for hundreds of years. Its orbit has an eccentricity of 0.10 and an inclination of 8° with respect to the ecliptic. It presently has an Earth minimum orbital intersection distance of 0.031 AU that translates into 12 lunar distances. This distance is well outside of Earth's Hill sphere of 1.5 e6km.

Quasi-satellite of Earth
In a frame of reference that rotates once a year, so that the Earth and the Sun remain relatively stationary, Kamoʻoalewa appears to circle elliptically around the Earth. The object is beyond the Hill sphere of Earth and the Sun exerts a much stronger pull on it than Earth does. Although it is too distant to be considered a true natural satellite of Earth, it is the best and most stable example to date of a near-Earth companion, or quasi-satellite.

Paul Chodas, manager of NASA's Center for Near-Earth Object Studies (CNEOS) at the Jet Propulsion Laboratory (JPL) in Pasadena, California, commented on the orbit:

"Since loops around our planet, but never ventures very far away as we both go around the Sun, we refer to it as a quasi-satellite of Earth. One other asteroid –  –  followed a similar orbital pattern for a while over 10 years ago, but it has since departed our vicinity. This new asteroid is much more locked onto us. Our calculations indicate  has been a stable quasi-satellite of Earth for almost a century, and it will continue to follow this pattern as Earth's companion for centuries to come."

In its yearly trek around the Sun, Kamoʻoalewa spends approximately half of the time closer to the Sun than Earth is (that is, the asteroid is inside the Earth's orbit) and passes ahead of our planet, and approximately half of the time farther away (crosses outside Earth's orbit), causing it to fall behind. Also, its orbit is tilted a little, causing it to bob up and then down once each year through Earth's orbital plane. In effect, this small asteroid is caught in a game of leap frog with Earth that will last for hundreds of years.

Chodas explained how the asteroid's orbit also undergoes a slow, back-and-forth twist over multiple decades: "The asteroid's loops around Earth drift a little ahead or behind from year to year, but when they drift too far forward or backward, Earth's gravity is just strong enough to reverse the drift and hold onto the asteroid so that it never wanders farther away than about 100 times the distance of the moon. The same effect also prevents the asteroid from approaching much closer than about 38 times the distance of the moon. In effect, this small asteroid is caught in a little dance with Earth."

Kamoʻoalewa is currently the most stable among the quasi-satellites of Earth that have been discovered and will remain in that orbit for about the next 300 years. But it alternates between stages of being a quasi-satellite and of being in a horseshoe orbit. In the 24th century it will go back to being in a horseshoe orbit. The closest Earth approach was on December 27, 1923 at 12.44 e6km. By late May 2369, the asteroid will be 2.0 AU from Earth. The Earth-like orbit may be a result of it being lunar ejecta. After limiting crater size based on simulations of impacts that could eject a piece this large into a quasi-satellite orbit, and limiting crater age to between 1 and 10 million years based on the instability of the asteroid's orbit, a 2024 study found that the most likely origin is the 22 km wide Giordano Bruno crater.

Most objects in this kind of orbit are eventually perturbed out of being in an Earth-co-orbital state and hit the Earth, Venus, or the Sun or are ejected from the solar system, and Kamoʻoalewa will probably hit the Earth in the next 100 million years.

Physical characteristics
The size of Kamoʻoalewa has not yet been firmly established, but it is approximately 40 –. Based on an assumed standard albedo for stony S-type asteroids of 0.20, its absolute magnitude of 24.3 corresponds to a 41 m diameter.

Photometric observations in April 2017 revealed that Kamoʻoalewa is a fast rotator. Lightcurve analysis gave a rotation period of 0.467 ± and a brightness variation of $0.041 km$ magnitude (U=2).

In 2021, a comprehensive physical characterization of Kamoʻoalewa was conducted using the Large Binocular Telescope and the Lowell Discovery Telescope, which found that the asteroid is composed of lunar-like silicates and may be an impact fragment from the Moon.

Proposed missions
During the 2017 Astrodynamics Specialist Conference held in Stevenson in the U.S. state of Washington, a team composed of graduate research assistants from the University of Colorado Boulder and the São Paulo State University (UNESP) was awarded for presenting a project denominated "Near-Earth Asteroid Characterization and Observation (NEACO) Mission to Asteroid (469219) ", providing the first baselines for the investigation of this celestial object using a spacecraft. Recently, another version of this work was presented adopting different constraints in the dynamics.

The China National Space Administration (CNSA) is planning a robotic mission that would return samples from Kamoʻoalewa. This mission, Tianwen-2, is planned to launch in 2025.