All About Earth’s New Minimoon
For only the second time in history, astronomers have discovered a new, natural-origin, minimoon orbiting the Earth. The minimoon, known as 2020CD3 (CD3 for short), was first discovered by Kacper Werizchos and Teddy Pruyne using data from the Catalina Sky Survey. Once CD3’s orbit was determined to be geo-centric, Queens University Belfast Post-Doctoral Research Fellow Dr. Grigori Fedorets assembled a team of 23 astronomers worldwide to make careful observations of the object to determine its identity. Based on the team’s findings, a paper was published on November 24, 2020, in the Astronomical Journal, characterizing the minimoon.
One of the main reasons for the careful nature of the identification process is the fact that we’ve been fooled before! Spent upper stage rocket boosters from lunar missions have been mistaken for asteroids in the past. Both Apollo 12 and the Chinese Chang’e 2 missions left upper stages in space that briefly passed for minimoons. Another hilarious case of mistaking a human-made object for an asteroid is the legendary ESA Rosetta spacecraft observed by the Catalina Sky Survey in 2007 during a flyby of the Earth was briefly given the provisional asteroid designation of 2007 VN84. There’s something intensely entertaining about a spacecraft that was bound to visit a comet being mistaken for an asteroid.
What is CD3 like, and how can we be sure it isn’t human-related space debris? Like all solar system objects, the minimoon experiences a slight pressure from the solar wind and solar radiation. This effect is directly proportional to the surface area of the object in question. By measuring the size of an object and observing the deviation in its orbit from that predicted solely from the influence of gravity, we can gain useful insight into our target.
When asked about it, the lead author of the Astronomical Journal paper, Grigori Fedorets, commented, “We compare the surface area to the mass. For rocket boosters, which are hollow, the surface to mass ratio is much higher”. Another way to look at it is that the sun has less influence on the orbit of CD3 than we would expect for anything human-made, leading us to determine that the object is solid. It is now thought that the object is made of some kind of silicate. Essentially, as one might expect, it’s a space-rock.
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