Here’s what we know about Earth’s new minimoon



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For only the second time in history, astronomers have discovered a new naturally occurring mini-moon orbiting Earth. The minimoon, known as 2020CD3 (CD3 for short), was first discovered by Kacper Werizchos and Teddy Pruyne using data from Catalina Sky Survey. Once CD3’s orbit was determined to be geocentric, Dr Grigori Fedorets, postdoctoral researcher at Queens University in Belfast, assembled a team of 23 astronomers around the world to make careful observations of the object to to determine his identity. Based on the team’s findings, an article was published on November 24, 2020 in the Astronomical journal, characterizing the minimoon.

One of the main reasons for the painstaking nature of the identification process is the fact that we have been duped before! Rocket boosters on the top floor of lunar missions have been mistaken for asteroids in the past. The Apollo 12 and Chinese Chang’e 2 missions left upper floors in space that briefly passed for minimoons. Another hilarious case of mistaking a man-made object for an asteroid is the legendary ESA Rosetta spacecraft observed by the Catalina Sky Survey in 2007 during an overflight of Earth briefly given the 2007 provisional asteroid designation VN.84. There is something intensely entertaining about a spaceship that was supposed to visit a comet mistaken for an asteroid.

Rosetta and her Philae lander on comet 67P / Churyumov-Gerasimenko. During a flyby of the Earth before reaching the comet, the spaceship was wrongly identified as an asteroid! Credit: ESA

What does CD3 look like and how can we be sure it is not human-made space debris? Like all objects in the solar system, the minimoon is subjected to slight pressure from the solar wind and solar radiation. This effect is directly proportional to the area of ​​the object in question. By measuring the size of an object and observing the deviation of its orbit from that predicted only by the influence of gravity, we can gain useful insight into our target.

Asked about this, the main author of the Astronomical journal paper, Grigori Fedorets, commented: “We compare the surface to the mass. For rocket thrusters, which are hollow, the surface / mass ratio is much higher ”. Another way to look at it is that the sun has Less influences the orbit of CD3 than one would expect for anything made by humans, which leads us to determine that the object is solid. The object is now believed to be made of some kind of silicate. Essentially, as you might expect, it’s a space rock.

Steps-! VB of Apollo 17, identical to the Apollo 12 stage which was mistaken for an asteroid. The hollow nature of these floors means that they are more easily pushed by the pressure of solar radiation. Credit: NASA

How big is our new neighbor? Is this something to fear? What would happen if the minimoon hit the Earth? When space rocks are found near Earth, questions like this inevitably seem to arise. In approaching such frightening questions, Fedorets puts it this way; “This particular minimoon has a diameter of one or two meters. If it hit Earth, it would burn in the atmosphere. Fedorets continues: “… in general; these minimoons are quite small.

Another reason to reassure you is the fact that CD3 has already left the Earth-Moon system! It is by calculating the path of the object back in time that we know that it was a mini-moon at the start. “It was discovered coming out of the Earth-Moon system. Before leaving, he had been captured for 2.7 years. Fedorets explained that this capture time is actually quite long: “According to our simulations, an average minimoon would only be captured for about nine months … that’s a longer capture time than expected.”

Animation of the CD3 2020 route. Credit: Phoenix7777 Data source: HORIZONS System, JPL, NASA
Three long exposures of different colors are stacked to give a true color composite image of 2020CD3. Streaks are stationary stars that are spread out due to the tracking of the movement of the minimoon. Credit: The international Gemini Observatory / NSF’s National Optical-Infrared Astronomy Research Laboratory / AURA / G. Fedorets

Not only did CD3 stay on longer than expected, it also spun slower than most simulated minimoons, with a spin period of once every three minutes. Compared to a huge object like a planet, this may seem fast, but given the small size of this object, it’s a very quiet rotation.

CD3 is only the second natural minimoon discovered, the first being the 2006 RH120, discovered fourteen years earlier. Should we expect this pace to continue? Not if the astronomical community has its way.

Vera Rubin’s next observatory is expected to blow the doors of the minimoon discovery rate. According to Fedorets, the observatory will find many more minimoons. “We would expect to find one every two or three months at best.” With the first light expected in 2021, we could very well be on the verge of a new era in minimoon astronomy (along with the myriad of other discoveries associated with observatories of this magnitude).

The Vera Rubin Observatory under construction in 2017 (known as the Large Synoptic Survey Telescope or LSST at the time) Credit: LSST

Objects like CD3 are particularly fascinating to astronomers. Unlike minerals on the Earth’s surface, which undergo various weathering and geological processes, the asteroid material is pristine. Fedorets remarks: “They [objects like CD3] contain the oldest materials in the solar system, and studying them in detail tells us how the solar system was born and formed. He continues: “It is a rare treat for astronomers to find such an object… it would be exciting to one day visit one of them, touch it with instruments and learn more about the solar system… and we don’t really do a lot of things on these meter to ten meter sized objects. They are not studied much because they are quite difficult to detect.

Extraordinary discoveries in dark matter, gravitational waves, black holes, supernovae, and exoplanets (not to mention both manned and unmanned spaceships) are making headlines. These great stories often prompt many of us to point out that we are living in the golden age of astronomy. We should not neglect the small discoveries close to home like CD3. This sofa-sized chunk of space rock might be a small thing, but the current and future study of this and other similar objects is a big deal.

MORE:
Catalina Sky Inquiry
Rubin Observatory

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