Has a Stray Piece of the Moon Become a Quasi-satellite of Earth?

Chipping the Moon
An creative illustration exhibits Kamo’oalewa as impression ejecta from the lunar floor — one of the hypotheses proposed to elucidate the origin of this asteroid.
Juan A. Sanchez / PSI

The Moon has orbited Earth for billions of years. But on this century, astronomers have found 5 small asteroids which can be truly “quasi-satellites” that journey in in part with our planet round the Sun. Now the first detailed observations of a quasi-satellite recommend that it (and maybe others) could be a chip off the Moon.

Earth’s gravity impacts the orbits of quasi-satellites, and so they circle the Sun in a year like our planet. But they neither orbit Earth nor come inside the Hill sphere, the place terrestrial gravity dominates.

Discovered in 2016, the closest and smallest quasi-satellite, 469219 Kamo‘oalewa (2016 HO3), is the first to come within the observing range of large telescopes. Now five years of observations reported in Nature Communications Earth & Environment show that the 50-meter object has a color surprisingly like that of samples Apollo astronauts collected from the lunar highlands half a century ago. In comparison, very few asteroids have been found to come from the Earth-Moon system, says coauthor Juan Sanchez (Planetary Science Institute).

Not Exactly Satellites

What makes a quasi-satellite an odd duck in the world of asteroids is the gravitational resonance that locks its orbital period with that of Earth or some other planet. The orbit of the much lighter quasi-satellite is more elongated, or eccentric, than that of the planet, so the quasi-satellite spends most of its time inside and outside of the planet’s orbit.

Orbit of a quasi-satellite (diagram)
This diagram exhibits a quasi-satellite orbit relative to Earth.
Wikimedia Commons

The quasi-satellite alternately falls behind the planet when it’s on the exterior and strikes forward when it’s on the inside. That movement takes the quasi-satellite round the planet however exterior of the Hill sphere, the area the place a planet’s gravity holds “real” satellites in orbit.

The dynamics of a quasi-satellite orbit are much like these of a horseshoe orbit, wherein an object traces a “U” form from the planet’s perspective, going from close to the L4 Lagrange level to the L3 and L5 factors, then again to the L4 level.

Horseshoe orbit (animation)
This animation exhibits a totally different near-Earth asteroid, (419624) 2010 SO16, in a horseshoe orbit relative to Earth. The animation covers the years 1600 to 2500.
Wikimedia Commons; Data: HORIZONS System / JPL / NASA

In truth, objects can simply shift between quasi-satellite and horseshoe orbits: Kamo‘oalewa probably switched from a horseshoe orbit to a quasi-satellite orbit about a hundred years ago and will switch back in another three centuries or so, according to a paper published in Monthly Notices of the Royal Astronomical Society by Carlos and R. de la Fuente Marcos (Complutense University of Madrid) shortly after the asteroid’s discovery.

These shifts can complicate counting what number of objects are in every state. Earth presently has 5 quasi-satellites and about a dozen in horseshoe orbits. Single quasi-satellites have been noticed for Venus and Neptune, too. Models present that quasi-satellites might stay round the outermost planets, Uranus and Neptune, for the life of the Sun. Jupiter can maintain quasi-satellites for almost 10 million years, and Saturn for about 100,000 years.  



Hints of a Lunar Origin

Kamo‘oalewa came within about 0.1 astronomical unit of Earth last April, which allowed both the Large Binocular Telescope and the Lowell Discovery Telescope to collect spectral data at visible and infrared wavelengths. That data yielded crucial information on the object’s shade.

Kamoʻoalewa displays extra mild at longer wavelengths of 1.5 to 2.5 microns, Sanchez explains, which makes it extra like the samples collected throughout the Apollo missions and fewer like different near-Earth asteroids and meteorite samples. He provides that the distinction comes from floor composition and the extent of space weathering, wherein micrometeorites and solar wind particles change floor supplies.

The similarity with lunar samples means that Kamoʻoalewa could be a chunk of the Moon, maybe blasted into space by an asteroid impression. But shade isn’t the solely proof for that situation. The object’s eccentricity and orbital inclination are additionally fairly small for what’s typical of captured objects. When close to the Earth-Moon system, Kamoʻoalewa strikes at solely 10–25% the speed of typical near-Earth asteroids. The gradual speed additionally helps a Moon origin.

Kamo‘oalewa’s shut approaches to Earth could provide distinctive prospects in the future. The quasi-satellite comes shut sufficient that the Chinese National Space Administration has designated it the goal for a complex mission to gather and return samples from an asteroid. Plans name for launch in 2024 and arrival in 2025.


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