Does Earth have a second moon? Some suggest so. The asteroid known as Kamoʻoalewa is the planet's most stable quasi-satellite in terms of its orbit, but while it likely might not qualify as an actual moon, it may in fact be a piece of ours, a new study has suggested.
Discovered in 2016 by scientists at the University of Hawaii, the asteroid, also designated 469219 (2016 HO3), takes its name from the Hawaiian language and is a combination of words meaning "the," "fragment," "of" and "to oscillate."
The asteroid itself is only around 46-58 meters in diameter, comparable in size to the Leaning Tower of Pisa in Italy or the Cinderella Castle at Disney World. As such, while its proximity to the planet has caused it to be labeled a Near-Earth Object (NEO), it is not considered a Potentially Hazardous Asteroid (PHA), which are at least 140 meters in size.
But what is especially interesting about it is its orbit and relationship to Earth.
Kamoʻoalewa is designated as an Apollo-class asteroid, meaning its orbit around the Sun frequently puts it close to Earth. However, the Earth's orbit has an effect on the asteroid as well. Essentially, it rotates around the Sun closer than Earth, but often crosses to outside Earth's orbit.
Earth's gravity influences it further, as it bobs around the planet's orbital plane in what Paul Chodas, manager of the NASA Center for NEO Studies at the Jet Propulsion Laboratory, described as a game of leapfrog. This is a process that has taken place for nearly a century and is expected to last for centuries more.
For this reason, it is referred to as Earth's "constant companion" and is considered the best example of a quasi-satellite.
"The asteroid's loops around Earth drift a little ahead or behind from year to year, but when they drift too far forward or backward, Earth's gravity is just strong enough to reverse the drift and hold onto the asteroid so that it never wanders farther away than about 100 times the distance of the Moon," Chodas said in 2016.
"The same effect also prevents the asteroid from approaching much closer than about 38 times the distance of the Moon," he said. "In effect, this small asteroid is caught in a little dance with Earth."
The Earth has one true satellite, the Moon, and is said to have five known quasi-satellites. Of them, Kamoʻoalewa is by far the most stable. But it seems to be very different than other asteroids.
THE RECENT study, published in the academic journal Communications, Earth & Environment, began in 2016 and examined the asteroid using the Large Binocular Telescope and the Lowell Discovery Telescope to perform a comprehensive glance, and found that the asteroid is reddened, referring to its particular pattern of reflected light spectrum. That itself may not be too out of the ordinary, but the amount of reddening is far more than what has been seen in other asteroids in the inner solar system.
What could this mean?
There is one other object in the inner solar system that has the best spectral match: the Moon.
Analyzing lunar samples taken from the Apollo 14 mission, the researchers found it to be the closest match. Essentially, this means that Kamoʻoalewa seems to have broken off from the lunar surface at some point.
While this is possible, it is also completely unprecedented.
"I looked through every near-Earth asteroid spectrum we had access to, and nothing matched," study lead author Ben Sharkey, a graduate student at the University of Arizona, said in a statement, regarding how there are no other known NEOs thought to have detached from the Moon.
Unsure at first and struggling for three years, the researchers had to take another observation of the asteroid. However, COVID-19 shut down the telescopes in April 2020, costing them the chance for another glance. This was difficult, as Kamoʻoalewa is extremely faint, four million times fainter than the faintest visible star in the sky. Seeing it without a telescope is impossible.
But in the spring of 2021, they finally were able to observe Kamoʻoalewa, and the pieces started to come together.
The observations supported the idea that it detached from the Moon – and there is further evidence to support this, according to University of Arizona's Renu Malhotra, who led the study's orbit analysis portion, another clue is the orbit.
Kamoʻoalewa's orbit isn't typical of other NEOs, and is very similar to Earth's – just with a slight tilt.
"It is very unlikely that a garden-variety near-Earth asteroid would spontaneously move into a quasi-satellite orbit like Kamoʻoalewa's," she explained. "It will not remain in this particular orbit for very long, only about 300 years in the future, and we estimate that it arrived in this orbit about 500 years ago."
How it detached from the Moon remains a mystery, and Malhotra's lab is currently working to further investigate it.
But they aren't the only ones interested in this asteroid.
Kamoʻoalewa is currently slated to be one of the targets for China's planned ZhengHe mission. Set to be launched in 2024-2025, this robotic probe will head for the NEO as well as the comet 311P/PanSTARRS to collect samples.
When these samples eventually return, the nature of the asteroid will become clearer.