For the second time in history, astronomers have discovered a new minimoon of natural origin 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. After determining that CD3's orbit is geocentric, Dr. Grigori Fedorets, a postdoctoral fellow at Queens University in Belfast, put together a team of 23 astronomers worldwide to carefully observe the object and determine its identity. Based on the team's results, an article was published in the Astronomical Journal on November 24, 2020, characterizing the minimoon.
One of the main reasons for the care in the identification process is the fact that we have been fooled before! Spent high-tier rocket boosters from lunar missions have been mistaken for asteroids in the past. Both Apollo 12 and the Chinese Chang & # 39; e 2 missions exited the upper tiers in space, which was briefly considered a minimoon. Another fun case of mistaking a man-made object for an asteroid is the legendary ESA Rosetta spacecraft, observed by the Catalina Sky Survey during a flyby on Earth in 2007 and briefly given the 2007 tentative asteroid designation VN84. There's something very entertaining about a spaceship that had to visit a comet that was mistaken for an asteroid.
Rosetta and its lander Philae near comet 67P / Churyumov-Gerasimenko. On a flyby of Earth before reaching the comet, the spaceship was incorrectly identified as an asteroid! Image credit: ESA
How is CD3 and how can we be sure that it is not human-related space debris? Like all objects in the solar system, the minimoon experiences a slight pressure from solar wind and solar radiation. This effect is directly proportional to the surface of the object in question. By measuring the size of an object and observing the deviation of its orbit from the size predicted solely by the influence of gravity, we can gain useful insight into our goal.
When asked, the Astronomical Journal's lead author, Grigori Fedorets, commented: “We compare the surface to the mass. For rocket boosters that are hollow, the surface area to mass ratio is much higher. “Another view is that the sun has less impact on CD3's orbit than we would expect humans to, which leads us to determine that the object is solid. It is now believed that the object is made of some type of silicate. As you would expect, it is essentially a space rock.
S-! VB stage of Apollo 17, identical to the Apollo 12 stage that was mistaken for an asteroid. The hollow nature of these steps means that they can be more easily pushed down by solar radiation pressure. Photo credit: NASA
How big is our new neighbor? Is that something to worry about? What would happen if the minimoon collided with the earth? When space rocks are found near Earth, questions like these inevitably seem to arise. Fedorets addresses such anxious questions like this. This particular minimoon is a meter or two in diameter. If it hit the earth, it would burn up in the atmosphere. "Fedorets continues:" … in general; These minimoons are pretty small. "
Another reason to calm down is the fact that CD3 has already left the Earth-Moon system! By calculating the path of the object backwards in time, we know that it was initially a minimoon. “It was discovered on the way out of the Earth-Moon system. It had been captured for 2.7 years before it went. "Fedorets explained that this acquisition time is actually quite long:" According to our simulations, an average minimoon would only be acquired for about nine months. It is a longer acquisition time than expected. "
Animation of the path from 2020 CD3. Photo credit: Phoenix7777 Data source: HORIZONS System, JPL, NASAThree long exposures in different colors are stacked to form a composite, true color image of 2020CD3. The streaks are stationary stars that are smeared due to tracking the movement of the minimon. Credit: The Gemini International Observatory / NSF's National Research Laboratory for Infrared Optical Astronomy / AURA / G. Fedorets
Not only did CD3 stay longer than expected, it also spun slower than most simulated mini-moons, rotating once every three minutes. This may seem quick compared to a huge object like a planet, but given the object's small size, it's an extremely leisurely rotation.
CD3 is only the second natural minimoon to be discovered. The first is 2006 RH120, which was found fourteen years earlier. Should we expect this pace to continue? Not when the astronomical community has its way.
The upcoming Vera Rubin Observatory should blow the doors of the minimoon discovery rate. According to Fedorets, the observatory will find a lot more minimoons. "We'd expect to find one every two or three months at best." With the first light expected in 2021, we may be on the verge of a new era in minimoon astronomy (along with the myriad of other discoveries associated with observatories of this magnitude).
Vera Rubin Observatory under construction in 2017 (known at the time as the Large Synoptic Survey Telescope or LSST) Credit: LSST
Objects like CD3 are particularly fascinating for astronomers. In contrast to the minerals on the earth's surface, which are subject to various weathering and geological processes, asteroid material is flawless. Fedorets notes, "They (objects like CD3) contain the oldest material in the solar system, and when we examine them in detail, we can learn 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 visit one of these one day, touch it with some instruments and learn more about the solar system. And we don't really know that much about these objects up to ten meters in size. They aren't studied much because they are pretty hard to spot. "
Extraordinary discoveries in dark matter, gravitational waves, black holes, supernovae and exoplanets (not to mention manned and unmanned spacecraft) are making big headlines. These tremendous stories often inspire many of us to remark that we are living in a golden era of astronomy. We shouldn't overlook small discoveries like CD3. That couch-sized lump of space rock might seem like a trifle, but current and future study of this and similar objects is a big deal.
Catalina Sky Survey