Is This Tiny Asteroid a Piece of the Moon?

The moon is the most vis­i­ble object in the night sky, but it’s not the only one that orbits the Earth. There are also some small satel­lites called “qua­si-satel­lites” that fol­low a sim­i­lar path around our planet.

One of these qua­si-satel­lites is a near-Earth aster­oid named Kamo’oale­wa. It was dis­cov­ered in 2016 by a tele­scope in Hawaii. It has a shiny sur­face that reflects light like the moon does. But could it actu­al­ly be part of the moon?

A new study sug­gests that Kamo’oale­wa might have orig­i­nat­ed from the moon as a result of a col­li­sion with anoth­er object. The study, pub­lished in the jour­nal Icarus, ana­lyzed the orbit of Kamo’oale­wa and com­pared it with oth­er pos­si­ble sources of lunar ejecta.

Kamo’oale­wa is not a typ­i­cal satel­lite. It is out­side the Earth­’s Hill Sphere, which is the region where the Earth­’s grav­i­ty dom­i­nates over oth­er forces. The moon is inside the Hill Sphere, and its orbit is rel­a­tive­ly sta­ble. But Kamo’oale­wa is out­side the sphere, and its orbit changes over time due to the influ­ence of oth­er planets.

Kamo’oale­wa belongs to a group of aster­oids called Apol­lo aster­oids, which cross the Earth­’s orbit. It is the small­est, clos­est, and most sta­ble mem­ber of this group. It also belongs to a cat­e­go­ry of co-orbitals, which are objects that share an orbit with a larg­er body. There are three types of co-orbitals: Tro­jans, horse­shoe, and ret­ro­grade/qua­si-satel­lites.

Kamo’oale­wa is a qua­si-satel­lite, which means it orbits around the sun in the oppo­site direc­tion as the Earth, but stays close to the Earth as if it were a satel­lite. It is one of only five known qua­si-satel­lites of the Earth.

The researchers used com­put­er sim­u­la­tions to test dif­fer­ent sce­nar­ios for how Kamo’oale­wa could have become a qua­si-satel­lite. They found that the most like­ly sce­nario was that Kamo’oale­wa was eject­ed from the moon by an impact with anoth­er object about 100 mil­lion years ago. The impact gave Kamo’oale­wa enough speed and direc­tion to escape the moon’s grav­i­ty and enter an orbit around the sun.

The researchers also found that Kamo’oale­wa could remain a qua­si-satel­lite for anoth­er 300 years before becom­ing a horse­shoe co-orbital or leav­ing the Earth­’s vicin­i­ty altogether.

The study pro­vides new insights into the ori­gin and evo­lu­tion of qua­si-satel­lites and oth­er co-orbitals. It also shows that Kamo’oale­wa might be a piece of lunar his­to­ry that we can observe from Earth.