Why should we care if Kamo`oalewa is a lunar ejecta?
Asteroids are a window into the past of the Solar System. They are the leftover bricks from the formation of the rocky planets and carry clues about the big events in the history of our planetary system. Asteroids may have also been important for the appearance of life on Earth, by bringing water and organic substances to our planet.
The current models for estimating the number and distribution of NEAs consider only sources from the main asteroid belt (between Mars and Jupiter) and the Kuiper belt (beyond Neptune). However, we found that these models do not account for objects with orbits similar to Kamo`oalewa's. This brings up the need to consider other origin scenarios, in particular, that it may have originated as a lunar fragment from a meteoroid impact on the moon.
This different origin would make this object also of interest for cosmo-chemical studies, since it would be a sample of ancient lunar material. Kamo`oalewa’s proximity to Earth and its exceptional orbital stability make it a good candidate for a future space mission.
Kamo`oalewa could be the first of a larger population of lunar fragments in the Earth’s orbital space, awaiting discovery with upcoming space survey projects like the Near-Earth Object Surveyor and the Vera Rubin Observatory.
What makes Kamo`alewa special?
Kamo`oalewa is currently classified as an Earth's quasi-satellite (QS). Although a few other quasi-satellites of Earth are known, Kamo`oalewa is the most stable among them. Orbital computations indicate that it will remain in this kind of orbit for centuries. These simulations also show that when departing this orbit, it will move to a related type of co-orbital motion, known as a horseshoe (HS) orbit, only to later return to the quasi-satellite state in a few centuries (see video below). Kamo`oalewa’s HS-QS transitions persist for millions of years, both in the past and in the future. Asteroids co-orbiting with Earth for such long timescales are uncommon.
Kamo`oalewa's orbital shape and tilt are also atypical of other known NEAs. Its reflectance spectrum has been found to be unlike other known NEAs, but more closely resembles lunar silicate rocks. This is a key evidence for the lunar origin of this object.
Our simulations and results
Previous studies have not reported the possibility of lunar ejecta reaching Kamo`oalewa-like co-orbital states with Earth. In order to test the lunar ejecta hypothesis for this object, we simulated the motion of particles launched from the surface of the Moon. We considered four representative launch sites (see figure below) on the Lunar surface, from which particles were launched with a variety of speeds and directions. We included the gravitational forces from the Sun, the Moon, and all the planets. The motion of the particles is tracked for 5,000 years to determine if a co-orbital state similar to Kamo`oalewa's is reached.
Our results show that most of the ejected particles achieve either Aten- or Apollo-like orbits, similar to typical near-Earth asteroids. In the diagram of orbital eccentricity versus semi-major axis, it can be observed how the outcomes face a dynamical barrier for entry into the region with a=1 au, that is, the co-orbital region. This barrier is however slightly porous (likely because of the small forces from the distant planets), and in our simulations a small fraction of particles, denoted as Kamo`oalewa-like (KL) particles, reached this region and showed persistent HS-QS transitions.
We found that 6.6% of launched particles reached some co-orbital state during this time lapse, while 0.8% exhibited at least one HS-QS transitions, and some of these persisted for tens of thousands of years. The launch conditions most favored for such an outcome are those with launch velocities slightly above the lunar escape velocity (~2.4 km/s) and launch locations from the Moon’s trailing side.
These minority outcome events have not been previously reported. The existence of these outcomes lends credence to the hypothesis that Kamo‘oalewa could indeed be lunar ejecta and show that rare orbital pathways from the lunar surface to Earth’s co-orbital space of HS-QS outcomes do exist.