Plotting Kamo’oalewa’s orbit
How the asteroid became Earth’s moon-like buddy
1 Orbiting the Sun
Kamo‘oalewa is also known as 2016 HO3, and just like Earth it orbits the Sun, taking just over a year to do so.
2 Staying close
Kamo‘oalewa also circles Earth, and it has become a stable quasi-satellite. It doesn’t venture too far away as both the asteroid and Earth orbit the Sun.
3 Weaving in and out
The asteroid spends roughly half of the time closer to the Sun than Earth; the objects leapfrog so that the other half of the time it is farther away.
4 gravitational pull
This unusual orbit is due to Earth’s gravity being strong enough to hold onto the asteroid. It doesn’t venture farther than 100 times the distance of Earth to the Moon.
it would be the only known asteroid with a lunar origin. It also opens up a whole new line of inquiry that explores how the piece came to break away and when it happened. Of course, such events have occurred many times in the past. “You can see craters on the Moon with your own eyes,” explains Sharkey. But we think of most asteroids as coming from the asteroid belt.
“We know material from the Moon has been ejected in the past, some of it coming to Earth as meteorites,” Sharkey says. “Hundreds of lunar meteorites have been collected on Earth, so impacts on the Moon are not uncommon. But the question is whether we can find a link in the processes. Is there a possibility that material from the Moon that hasn’t hit Earth is out there, yet to be found? We could tell a lot about how material has mixed or moved around the Solar System with time.”
Sharkey and his team knew early on that Kamo‘oalewa wasn’t a standard near-Earth asteroid. During observations it wasn’t reflecting brightly in particular infrared frequencies, even though it was made of common silicates in the same way as other asteroids. Sharkey sought to match data gathered on Kamo‘oalewa with the light reflecting off other near-Earth asteroids. But Kamo‘oalewa’s dimmer reflection pointed to it being composed of a different material. Researchers just needed to be totally sure there had not been a mistake.
“The spectral signature of some other asteroids can actually look very similar to what Kamo‘oalewa looks like at visible wavelengths,” Sharkey says. “We had this expectation that it would basically reflect infrared light in a similar way to how it did in the visible, but it displayed a redder reflectance spectrum. We tried to run through different scenarios and think about the different ways to change how asteroids reflect light. This doesn’t have to do with just the material it’s made of. It could be to do with different textures of the surface – scuffing can make something appear darker or change its colour, for instance, altering how asteroids are reflecting light.
“But we tried to say if we take a more typical asteroid and apply this effect, can we come up with a satisfying answer and achieve a reflection identical to what we saw with Kamo‘oalewa? And our argument was that we can’t get anything else to work. Then we saw that there’s a source of material that has a very good match to this spectrum which is nearby: space-weathered lunar silicates.” It opened up a winning line of inquiry.
As luck would have it, one of Sharkey’s PhD advisors had studied samples collected from the
Fra Mauro formation in the lunar highlands which had been brought back to Earth by the Apollo 14 mission in 1971. “It was a kind of funny moment because he just sort of mentioned it randomly, and it was like, ‘hey, you should probably think about this comparison too’,” Sharkey laughs. By comparing Kamo‘oalewa’s pattern of reflected light to those lunar samples, a near-perfect match was found. This suggested that Kamo‘oalewa indeed originated from the Moon, with the spectral characteristics being consistent with silicate material showing a high degree of space weathering, such as solar wind particles or micrometeorite bombardment.
Even so, Sharkey and his advisor, University of Arizona associate professor of lunar and planetary sciences Vishnu Reddy, harboured doubts. Although a second set of data was obtained in 2019, they were frustrated that they couldn’t observe Kamo‘oalewa in 2020 because the COVID-19 pandemic caused the Large Binocular Telescope to be shut down. Fortunately, it was back up and running in April this year, allowing another observation, and this finally convinced the team that they were correct.
But Kamo‘oalewa isn’t going to hang around. According to the study’s coauthor, University of Arizona planetary sciences professor Renu Malhorta, who has been studying the orbit, it will wave goodbye in about 300 years time, once it frees itself from the gravitational ropes that are keeping it around Earth. “It will not remain in this particular orbit for very long, only about 300 years in the future,” Malhorta affirms, adding that “it arrived in this orbit about 500 years ago”. But what exact path it will take when it goes on its lonely
“We could tell this object had a unique orbit, so there was an immediate interest in characterising it” Ben Sharkey
“It will not remain in this particular orbit for very long, only about 300 years in the future; it arrived in this orbit about 500 years ago” Renu Malhorta
adventure is not entirely certain. What we do know, however, is that it’s gravitationally bound to the Sun rather than Earth, and that’s why it can’t actually be classed as a moon, even though it’s made up of a bit of one.
“Orbital dynamics are outside my normal expertise,” says Sharkey, “but one of the big challenges is tracing the exact path of an object like this beyond a few hundred years. Your uncertainties just become so large that it’s really difficult to say anything with confidence besides the sort of general ideas about the state of the object. Beyond 300 years or so, it’s a question of the exact path it takes, but it’s not going to shoot off in some new direction. It’s going to be a more gentle progression than that.”
This still gives plenty of time to make more observations, and while there’s no chance anyone is going to be thinking of landing a human on this particular ‘moon’, a sample-return mission to Kamo‘oalewa is being planned by the China National Space Administration, with a launch pencilled in for 2025. It’s also entirely possible that Kamo‘oalewa is not alone, and the orbits of three other near-Earth objects could even be linked. “I feel at this point that anything could be a surprise, or not a surprise, and the honest answer is that we really don’t know,” Sharkey says. “I think that’s part of why this kind of study adds excitement, at least from my perspective. But we didn’t really have a good handle on the question of what Kamo‘oalewa was beforehand and whether other objects could be related in similar ways. The next step is to keep asking that question.”