Every April for the next several hundred years, a small asteroid is visible to a select few telescopes on Earth. That asteroid is Kamoʻoalewa, a space rock that a team of astronomers says is likely an ancient fragment of our Moon.
The astronomers recently observed the extremely faint Kamoʻoalewa through the Large Binocular Telescope in Arizona and analyzed the way light reflected off it. They found that the asteroid’s spectra matched lunar rocks from NASA’s Apollo mission, a strong indicator that the rock is actually Moon rock. Their results are published today in Communications Earth & Environment.
“We looked at the spectral behavior of asteroids that the scientific community has studied before, and we couldn’t find a good match in any known library,” said Benjamin Sharkey, lead author of the paper and an astronomer at the University of Arizona, in an email to Gizmodo. “We then asked ourselves if any physical processes not related to material composition could reconcile known asteroids with our data. We couldn’t think of anything that fit. Then, when we looked at lunar materials, we found that they provided a much more direct match to our observations.”
Kamoʻoalewa is a Hawaiian word that alludes to the object’s oscillating visibility in the sky; in April you see it, and then you don’t. The asteroid doesn’t inhabit the solar system’s main asteroid belt between Mars and Jupiter, as most do. Instead, this particular 200-foot object is a quasi-satellite, meaning it’s a near-Earth asteroid that does orbit the Sun but is closer to Earth than other asteroids. (In this case, close means about 9 million miles from our planet; the Moon is about 250,000 miles away at its farthest.)
The rock was originally discovered in 2016 by the PanSTARRS telescope in Hawaii. Sharkey’s team spent the next three years investigating Kamo’oalewa’s origins, with some hiatus due to telescope closures caused by the covid-19 pandemic. They took their final data this spring, some “much needed follow-up observations,” Sharkey said in a University of Arizona news release, adding that the asteroid “is easier to explain with the Moon than other ideas.”
“Asteroids are made of minerals that have unique spectral signatures when you look in the infrared light,” said co-author Vishnu Reddy, an astronomer at the University of Arizona, in an email to Gizmodo. “When we first looked at Kamoʻoalewa, we detected a mineral called pyroxene which is relatively common. What was interesting was this mineral pyroxene was altered in a way that is very similar to what we seen on the Moon. That sent me thinking that we should look at the Apollo samples.”
The team compared the asteroid’s reflected light with the way other near-Earth asteroids reflect light, and the Apollo rocks were the nearest match. Besides the light reflectance data, the team said there were other characteristics of Kamoʻoalewa that set it apart, like its location in space.
“It is very unlikely that a garden-variety near-Earth asteroid would spontaneously move into a quasi-satellite orbit like Kamoʻoalewa’s,” said co-author Renu Malhotra, a planetary scientist at the University of Arizona, in the release. Malhotra’s lab is separately working to determine the asteroid’s origins (that is, when and how exactly it broke away from the Moon).
Malhotra added that Kamoʻoalewa’s current orbit will last. Her team estimates the entered its current orbit around 500 years ago and has about 300 years left there. So if we’re going to confirm that the rock is a slice of the Moon, we’ve got three centuries to do it.
Unlike this nearby rock, most asteroids are thought to have formed in the very early years of the solar system, when the planets were coalescing. A NASA mission called Lucy launched last month on a journey to study a group of them called the Trojans, in an effort to learn more about what conditions were like billions of years ago.