One Of Our Near-Earth Asteroid Buddies Could Be A Fragment Of The Moon

Artist's impression of Kamo`oalewa making a return trip past the Moon with the Earth in the distance. Image Credit: Addy Graham/University of Arizona

Quasi-satellites are asteroids that stick closely to a planet, including near-Earth asteroids that take the near part rather seriously. Quasi-satellites' origins are mysterious. Kamo`oalewa, a particularly interesting space rock, never gets more than 100 times the distance to the Moon away from us, but new research reveals it was probably once part of the Moon, presumably having been blasted off by an asteroid collision.

Initially known as 2016 H03, Kamo`oalewa was discovered using the PanSTARRS telescope in Hawaii. It's hard to study because it is only 55-60 meters (150-190 feet) across. Its orbit only makes it visible for part of every April, and then only to very large telescopes.

University of Arizona graduate student Ben Sharkey collected Kamo`oalewa's spectrum and found it was unlike any other asteroid ever seen. However, he reports in Communications Earth and Environment, there was one known object with the same spectrum and it's the only place humans have visited beyond the Earth.

At first glance, Kamo`oalewa's spectrum looked similar to silicate asteroids but when Sharkey and co-authors used longer exposure times they found its spectral slope is distinctively skewed to the red. After ruling out comparisons with particularly iron-rich asteroids the authors found the only match they could find were certain rocks from the surface of the Moon.

The paper acknowledges we don't yet know enough about Kamo`oalewa to be confident of its origins. Indeed co-author Dr Vishnu Reddy pointed out in a statement: “We doubted ourselves to death.” However, meteorites have been found that were knocked off the Moon in impacts, so we know such objects should be out there.

Moreover, both Kamo`oalewa's spectra and its orbit are consistent with it having been knocked off the Moon when a larger object hit. "It's easier to explain with the moon than other ideas," Sharkey said. Three smaller objects were found last year with orbits similar enough to Kamo`oalewa they could all be pieces of a larger asteroid.

Only five quasi-satellites are confirmed, but their discoveries are so recent (the first was found in 2004) many more are probably waiting to be discovered. Most quasi-satellites have quite unstable orbits, having been ordinary near-Earth asteroids previously, and returning to that status after a few years or decades of association with Earth. However, Kamo`oalewa is more stable. It's expected to keep its quasi-satellite status for 300 years, having so far been at it for 100.

Previously Kamo`oalewa, whose name comes from a Hawaiian creation chant and refers to an offspring that travels on its own, had a “horse-shoe shaped path” with an orbital diameter alternately a little shorter and longer than our own. Even during these more distant phases, Kamo`oalewa has a relative velocity 10 times lower than most near-Earth objects, relative to the Earth-Moon system.

In the process of examining its composition, Sharkey and co-authors also found Kamo`oalewa rotates every 28.3 minutes; any future missions there need to be ready for some serious motion sickness.

Kamo`oalewa's orbit while a quasi-satellite (green) and in a horseshow orbit (black). The top graph shows the diameter of its orbit from teh Sun, the middle shows time variation of the mean longitude relative to Earth’s; and the bottom is time variation of Kamoʻoalewa’s geocentric distance. Image Credit: Sharkey et al/Communications Earth and Environment



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