What’s up with Earth’s new mini-moon? Prof. Carolin Frueh offers insight
If you thought the Earth had only one moon, you may be surprised to learn it has company. Astronomers recently discovered what’s believed to be a tiny asteroid, about the size of a compact car, orbiting our planet. You can’t see it with the naked eye; it’s visible only through powerful telescopes. Called the 2020 CD3 mini-moon, the object may have been circling the Earth for three years, but it’s becoming fainter, indicating it may leave Earth’s gravitational pull in a few weeks.
Despite its miniscule size and short stay, the new Earth satellite and its detection are significant, says Carolin Frueh, an assistant professor of aeronautical and astronautical engineering. Dr. Frueh knows a thing or two about detecting and assessing objects in space, so we asked her to share her perspectives on 2020 CD3, why it’s important now, and what it may mean for the future.
Please tell us about your experience related to the discovery of the Earth’s new mini-moon.
I work in my research group with my telescope, the Purdue Optical Ground Station, in tracking, detecting and characterizing objects in orbit around the Earth or in the cislunar region. Most of these objects are satellites and space debris, but they also include deep-space objects, captured in Earth orbit on occasion. One example was the WT1190F object. In a collaboration with colleagues from Italy, observations were taken, and the object was characterized as human-made. Upon evaluation of light curves, spectra and the orbit, two likely candidate objects could be identified: the Athena II Trans-Lunar Injection Stage of the Lunar Prospector mission and the ascent stage of the Apollo 10 lunar module, aka ‘‘Snoopy.”
Why is this newly found object called a “moon” anyway? At only 6x11 feet, is it large enough to be a moon?
Ah, the classification “moon” has nothing to do with the size. Any natural object orbiting a planet — in our case, the Earth — is a moon. It is dubbed the “mini-moon” because it is so much smaller than the moon we know.
It is also expected to not be a permanent Earth moon because orbital perturbations, caused by gravitational forces from the sun, our usual moon and other bodies, as well as solar radiation pressure, will drive the tiny moon out of the capture region of the Earth. Thus, mini-moons sometimes are called temporarily captured objects.
How do we know it is a natural object and not space debris (a human-made object)?
Upon discovery, as only non-resolved images (images with no details on the object) are available because of the large distance to the object, it is not immediately clear if it is a natural object or a human-made one. The strongest indication that it is actually a natural object so far is that its area-to-mass ratio seems to be small.
The definitive assessment is made by determining the orbit and by estimating the orbital perturbation due to solar radiation pressure, which is directly proportional to the area-to-mass ratio. Natural objects tend to take shapes that minimize the area relative to the mass. This principle applies most perfectly to a round object, but it even holds for asteroids, which can have significantly non-spherical shapes such as a dumbbell shape. While a dumbbell shape has a higher area-to-mass ratio than a spherical object, that ratio is still significantly smaller than most human-made satellites, just by the way satellites are built. This can be illustrated, for example, by the classical configuration of a box-wing setup, in which solar panels that carry little mass and, contrary to nature, expose large surfaces, are unfolded.
For instance, when WT1190F was detected in a much similar fashion, we became almost certain it was a human-made object because of its significantly higher area-to-mass ratio compared with natural objects. (WT1190F impacted the Earth and plummeted into the ocean Nov. 13, 2015.)
Is the newly discovered mini-moon the smallest known moon in our solar system today?
Among detected objects that are mini-moons around the Earth, 2020 CD3 is only the second one (2006 RH120 being the first). But it is assumed that at any given time, there are small asteroids temporarily captured by Earth gravity. As those are not detected, they are likely to be smaller. Thinking about the solar system, the planets featuring ring systems, technically speaking, have rings consisting of moon objects. The sizes of those “moons” in the rings are only limited by their material composition.
Why was 2020 CD3 found only recently? Why didn’t anyone see it before, when it is estimated to have been around for about three years?
The object is faint (~magnitude 20), so many standard telescopes could not detect it. Roughly speaking, a good observation location is needed, in combination with a telescope with an aperture of one meter or more when not tracking the object (when the orbit is not known a priori, meaning based on theoretical deduction). A smaller-aperture telescope can suffice when observing in a particular direction for an extended time, in combination with advanced image processing techniques that are stacking the single images into a combined one, increasing the signal of the faint object. Only a small community of researchers regularly scans the sky for asteroids, in particular to detect those that could collide with the Earth, so faint objects are easy to miss.
What can researchers learn from observing the newly detected mini-moon?
As the moon does not appear to be on a collision course with the Earth, there is scientific value in studying its composition and characterizing the object. Observing how it reflects light provides information on the surface materials. From brightness measurements over time, approximated shapes can be determined. Both are extremely tricky in this case, as the object is so small and hence faint for the given distance to the observer on the Earth.
Is the discovery of this mini-moon part of an increasing trend of finding new objects in space, also including the NASA intern’s recent discovery of a new planet?
Yes, object detection capabilities have improved and are continuing to do so. For one, with the arrival of digital imaging sensors using CCD (charge-coupled devices) and CMOS (Complementary Metal Oxide Semiconductor) cameras, a lot more telescope imagery can be collected and easily stored. As with the story of the NASA intern, there are collected data sets available. With the revolution in computing power, a lot of hands-on work on the processing side has been replaced with algorithms. Thanks to advanced computing power, more elaborate algorithms for object detection can be run in reasonable time scales, even allowing archived data to be rerun when new methods become available. With robotic mounts and optimization techniques, telescope time and usage are being maximized, covering more sky area per night.
These advances are continuing to enhance detection capabilities. Researchers worldwide, including me, are working to develop ways to detect fainter objects and also to scan the sky most efficiently, which will necessarily lead to the detection of new objects, natural or human-made.
As it is suspected that at any given time there is a small asteroid being captured in Earth gravity, it is likely that more mini-moons and temporary captured objects will be detected while they orbit the Earth, before they leave the Earth gravity capture again. Overall, with increasing detection capabilities, the knowledge of objects in the asteroid belt in general (objects that are orbiting the sun) will improve.
Carolin Frueh, PhD
Assistant Professor, School of Aeronautics & Astronautics,
College of Engineering, Purdue University
 A. Buzzoni, G. Altavilla, S. Fan, I. Froppiani, C. Frueh, M. Micheli, J. Nomen, N. Sanchez Ortiz, Physical characterization of the deep-space debris WT1190F: a testbed for advanced SSA techniques, Advances in Space Research, Vol. 63, Issue 1, pp. 371–393, doi:10.1016/j.asr.2018.07.025, 2019