The Moon dominates our view of the night sky. But it's not the only thing orbiting Earth. A small number of what scientists call quasi-satellites also orbit Earth.
One of them is called Kamo'oalewa, and it's a near-Earth asteroid. It's similar to the Moon in some respects. Could it be a chunk of the Moon?
Kamo'oalewa was discovered in 2016 with the Pan-STARRS at Haleakala Observatory. It's an unusual object because its orbit changes over time. But as it changes, it always stays near Earth.
Its surface is also remarkable. It reflects light the same way the Moon does, thanks to the presence of silicates. That's an intriguing clue to its origins, but it's not the only clue. While Kamo'oalewa isn't the only quasi-satellite, nor the only one in the Apollo group, it's the smallest, closest, and most stable of them.
A new study examines the object's orbit to understand if it could be ejecta from the Moon. The study is Orbital pathways for a Lunar-Ejecta Origin of the Near-Earth Asteroid Kamo`oalewa. The first author is Jose Daniel Castro-Cisneros from the Department of Physics at the University of Arizona.
Sometimes, small bodies in the Solar System don't follow heliocentric orbits. Instead, due to orbital resonances, they can share the orbit of a massive planet. These are called co-orbital objects, and the Jupiter Trojans are a group of such objects.
There are three main types of co-orbitals: Trojan/tadpole (T), horseshoe (HS), and retrograde satellite/quasi-satellite (QS.) The two types that are important in this research are the latter two: HS and QS.
Kamo'oalewa is beyond Earth's Hill Sphere, which is a region of space that dominates the attraction of satellites. The Moon is inside the Hill Sphere, and though its orbit is subject to small perturbations and changes, it's fairly stable. But Kamo'oalewa is outside the sphere, and its orbit is highly elliptical. It's called a quasi-satellite because the Sun exerts more pull on it than Earth does.
Earth has 21 co-orbital objects: two are Trojans, six are in the QS state, and 13 are undergoing HS motion. But Kamo'oalewa is different than the other QS objects.
The other 20 are only temporarily in their co-orbital states, usually for less than a few decades, while Kamo'oalewa persists. It transitions back and forth between HS motion and QS motion and has done this for centuries. It'll keep doing it for centuries.
Why is that? What about its origins compels it to follow this orbit?
"Considering its Earth-like orbit and its physical resemblance to lunar surface materials, we explore the hypothesis that it might have originated as a debris-fragment from a meteoroidal impact with the lunar surface," the paper states.
Since they can't go back in time and watch the Moon during its long history of bombardment, scientists do the next best thing. They use computers to simulate events with a wide variety of variable values and see what they find. In this paper, the researchers modeled particles ejected from the Moon by collisions.
"We carry out numerical simulations of the dynamical evolution of particles launched from different locations on the lunar surface with a range of ejection velocities," they write.
Most of the particles in their simulation leave the vicinity of the Earth and its Moon and transition into orbits around the Sun, which is not surprising. The Sun's dominant mass influences everything in the Solar System.
But some – only a small number – don't enter heliocentric orbits. Instead, they take up orbits similar to Kamo'oalewa's orbit. "As these ejecta escape the Earth-Moon environment and evolve into heliocentric orbits, we find that a small fraction of launch conditions yield outcomes that are compatible with Kamo'oalewa's dynamical behavior," they write.
The ones that do mimic Earth's smallest and most stable quasi-satellite have one thing in common: launch velocity. "The most favored conditions are launch velocities slightly above the escape velocity from the trailing lunar hemisphere," the researchers explain.
Kamo'oalewa has a moderate ecliptic inclination of about 8°. In the simulation, most ejected particles have inclinations even smaller than that, usually between 1° and 3°. But some of them reached higher inclinations similar to Kamo'oalewa's.
The simulations show that Kamo'oalewa needn't have begun its journey with its larger inclination compared to other particles. Its inclination also doesn't stay at 8°. During close approaches to Earth, it experiences jumps in inclination that build up over hundreds of years then dissipate over thousands of years.
"These results demonstrate that Kamo'oalewa's inclination could have arisen from a smaller initial inclination by means of kicks at close approaches during its HS state," the authors explain.
The Moon's surface is covered in impact craters, and the historical record held in those craters constitute a good test for the lunar impact hypothesis for Kamo'oalewa. "The lunar ejecta velocities (in excess of lunar escape speed, 2.4 km/s) needed to obtain the co-orbital outcomes appear to be achievable in meteoroidal impacts on the Moon," the authors write.
Impacts on the lunar surface routinely have impact speeds of 22 kilometers/second (13.7 miles/second) and can be as high as 55 kilometers/second. Other simulation studies show that impacts with those speeds can eject debris traveling as fast as 6 km/second, well above the 2.4 kilometers/second threshold for escape.
Lunar crater studies also show that large impact craters greater than 33 kilometers diameter occur once every 25 million years, and those large craters are likely sources of impact ejecta traveling fast enough to escape the Moon. The authors say that a future still will have to address which specific crater might have been the source for Kamo'oalewa.
"We leave to a separate study to investigate whether a lunar crater of appropriate size and age and geographic location can be consistent with the lunar ejecta hypothesis for the provenance of Kamo'oalewa," they write.
If scientists can prove that Kamo'oalewa is a chunk of the Moon, that opens up some intriguing possibilities. It would be the first one, and it would "… be of great interest for cosmochemical study as a sample of ancient lunar material," the authors write.
There's some talk of missions to Kamo'oalewa, but they may be modest. In 2017, a team of graduate research assistants presented a plan to send a small spacecraft to the asteroid. Their proposal was called The Near-Earth Asteroid Characterization and Observation (NEACO) mission.
In a 2019 conference paper, a group of NASA scientists proposed the New Moon Explorer mission. It would be a small spacecraft mission. Both concepts focused on determining the asteroid's mass, density, composition, regolith characteristics, and other properties.
Kamo'oalewa is small, maybe as little as 40 meters (131 feet) in diameter. But that hasn't stopped China from developing a more ambitious mission of their own. It's called Tianwen-2, and along with the spacecraft itself, there will be a nano-orbiter and a nano-lander.
The nano-lander will take a sample of the asteroid that'll be returned to Earth for analysis. Tianwen-2 is due to launch in 2025, and will also visit the main-belt comet 311P/PANSTARRS.
If one or all of these missions is successful, we may finally know if Kamo'oalewa is indeed a chunk of the Moon.