Lunar-like silicate material forms the Earth quasi-satellite (469219) 2016 HO3 Kamoʻoalewa

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By Ben Sharkey, Audrey Thirouin, and Vishnu Reddy

Our paper on the natural Earth quasi-satellite Kamo`oalewa, discovered in 2016 by the PanSTARRS survey in Hawaii, is the result of a longstanding observational campaign. The main difficulty of this work is simply that Kamo`oalewa is extremely faint despite being so close to the Earth – while we can detect it as an object streaking across the sky, assessing its physical characteristics is much more challenging. For asteroids, the first quantities we typically want to measure are (1) how their brightness varies with time as the object rotates (the object’s “lightcurve,” which is related mainly to how the asteroid is shaped and how it spins), and (2) its reflectance spectrum, which measures how efficiently the asteroid’s surface reflects sunlight at different wavelengths of light and can infer its surface composition. When this project began, there was even a question about whether it could be a remnant rocket body!

Our thinking evolved over the five years we collected data, as we began to realize that Kamo`oalewa had exciting and unique properties. The first challenge we faced was its inherent faintness. Even at its closest approach, Kamo`oalewa was 4 million times fainter than the faintest star humans can see in a dark sky. It also meant that we could not characterize its composition with the 3m NASA Infrared Telescope Facility (IRTF), one of the most prolific telescope for investigating asteroid physical properties. We realized that we needed the largest possible aperture we could lay our hands on and that meant the 8.4m Large Binocular Telescope (LBT), which is operated in part by the University of Arizona. A discussion over coffee with the LBT team led to the first proposal in 2017 that enabled us to take a first look at Kamo`oalewa a year after its discovery.

We began by observing this object at visible wavelengths, which are easier to measure than the infrared, and because we wanted first to assess the object’s lightcurve. Our images were obtained over two nights in April 2017 with the LBT and with the 4.3m Lowell Discovery Telescope (LDT). We found that Kamo’oalewa has a day and night cycle of about 28 minutes which is typical for a Near-Earth Object (NEO) in this size range, and its shape is likely elongated. Also, we found that the way Kamo’oalewa reflects visible-wavelength sunlight was like certain classes of silicate-rich (rocky) NEOs. This led us to believe that Kamo`oalewa could be a typical asteroid caught in a unique orbit close to the Earth. But we kept looking.

One of the reasons why small body observers try so hard to make measurements in the infrared is because many different materials look the same when you only measure wavelengths the human eye can see. In the infrared, minerals that make up the surfaces of most small bodies have diagnostic spectral signatures that enable us to constrain their composition. We often liken the process of spectroscopy to measuring the “fingerprints” of minerals. The next year, we found something peculiar – when you observe Kamo`oalewa near wavelengths of 1 micron, Kamo`oalewa looks markedly different from what we expected! This difference was so stark, that for the first few weeks after we processed this data, we assumed we were doing something wrong. But our expectations were the problem, not the data. 

We found that Kamo`oalewa is brighter in the infrared than we thought it would be. And then we started asking ourselves what could cause this behavior. To asteroid spectroscopists, one of the first materials that comes to mind is metal. But another material that behaves this way is lunar silicates. Separating different hypotheses for the composition of Kamo`oalewa is difficult without being able to examine this object closer (say, with a spacecraft, like the recent Osiris Rex and Hayabusa missions to NEOs). However, we had one more test to perform.

 Unfortunately, our observing campaign, like everything else in the world, was disrupted by the COVID-19 pandemic. We had planned this final test at LBT to confirm our findings in April 2020. Operations at the telescope were halted before these could be conducted as pandemic protocols were created and implemented so that engineers, staff, and operators could do their jobs safely. Of course, this hardship is nothing compared to the many difficulties we have all experienced during the pandemic, but we mention this to emphasize that science has been affected as well. Fortunately for us, Kamo`oalewa could wait for another year.

 In March and April of 2021, we were able to collect infrared images of Kamo`oalewa at longer wavelengths than we could reasonably achieve with spectroscopy. In these observations, we were able to detect the asteroid and found that the trend of Kamo`oalewa being brighter than we expected continued to at least 2.2 microns! When you plot different materials’ reflectance properties as we do in Figure 2 of the paper, this shows that there aren’t any clear matches to Kamo`oalewa among the known library of NEOs. But lunar material is a close fit.

 So, this leads us to the conclusions of the paper. We detected that Kamo`oalewa is significantly brighter in the infrared than we would expect for a “normal” asteroid. This is reminiscent of how lunar material looks in the infrared. Kamo`oalewa itself has an orbit that will keep it near to the Earth for hundreds of years. Combining each of these details leads to the exciting possibility that Kamo`oalewa itself could have formed from lunar material. Such a scenario would be incredibly exciting, but maybe we shouldn’t be surprised!

 We have found lunar meteorites on the Earth, so we know that fragments of the moon have been flung off in recent history (geologically speaking). But what if we could visit a such a fragment, without it being subject to alteration by entering the Earth’s atmosphere? Could we learn when Kamo`oalewa was formed? If so, could we figure out where it came from? For now, who can say? But if we keep looking, and keep asking these questions, one day soon we might be able to find out.

 One final point to stress is that telescope data doesn’t get collected by accident. This work took the effort of teams at two different Arizona-based telescopes to organize, plan, and execute observations. Telescopes rely on teams of operators and support staff in order to function and provide cutting-edge data for all of their users, and we are grateful for the support we received from LBT and LDT.

 

 

Ben Sharkey

PhD Candidate, University of Arizona