Scientists characterize second known minimoon

Astronomers using data collected with the Lowell Discovery Telescope (LDT) have helped characterize only Earth’s second known minimoon, a newly discovered asteroid with the designation 2020 CD3, or CD3 for short. LDT’s observations have helped clarify both the rotational speed and orbit of this tiny body, the latter having helped prove that CD3 is a natural body and not a relic of man-made space debris.

Minimoons are small asteroids temporarily captured while orbiting the Earth. In about a year, they are sent back to interplanetary space. The first known minimoon, 2006 RH120, was detected 14 years ago.

CD3 was discovered on February 15, 2020 by Kacper Wierzchos and Teddy Pruyne via the Catalina Sky Survey, operating out of the University of Arizona’s Lunar and Planetary Laboratory. Due to the scarcity of minimoons, a worldwide effort led by postdoctoral researcher Grigori Fedorets of Queen’s University of Belfast was quickly launched to study this object. Twenty-three researchers from 14 universities in seven countries participated, using several telescopes, including the LDT. The team observed until mid-May 2020 and published their results today in The astronomical journal.

Lowell Observatory astronomer Nick Moskovitz and former Lowell postdoctoral researcher / current Arecibo Observatory scientist Maxime Devogele participated in the effort, assisted in the LDT observation by Quanzhi Ye of the University of Maryland . By measuring the changing brightness of CD3 over time (i.e. its light curve) with the Large Monolithic Imager (LMI) on the LDT, they established its rotational speed at about three minutes. Fedorets said: “The spin rate was probably the biggest unanswered question in this research. The Lowell team has shown that it spins slower than expected for objects in this size range.”

Moskovitz and his colleagues Lowell also used the LMI / LDT combination to accurately measure the position of CD3 in order to refine its orbit. This information, combined with the physical characteristics of CD3 – such as an inferred silicate composition – indicates that it is certainly a natural object. This sets it apart from another recently discovered object, 2020 SO, which scientists believe could be the top stage of NASA’s Surveyor 2 spacecraft.

The study estimates that CD3 is about 1 to 1.5 meters in diameter – about the size of a small car – and that it approached about 13,000 kilometers (8,100 miles) from Earth to the closest approach. Observing such small objects is difficult and requires a telescope large enough to see them. In addition, their transient nature means that the window of time for observing them can quickly close. Enter the 4.3-meter LDT, the flagship telescope of the Lowell Observatory. Its large size and immediate availability make it optimized for such studies. Moskovitz said: “This object was not bright enough to be studied for very long. The fact that we have this telescope in our backyard and are able to react quickly has really made a difference.”

The global response to CD3 could very well serve as a model for future minimoon studies, which scientists expect to occur soon. According to Fedorets, “the minimoons should be discovered in large numbers over the next decade, with the opening of the Vera C. Rubin observatory scheduled for 2023”. This facility is currently under construction in Chile and has an 8.4-meter telescope that will allow astronomers to detect many more small bodies such as minimoons.

Scientists want to know more about these organisms for several reasons. Because minimoons are close to Earth, they are potentially accessible targets for robotic or human exploration. Such efforts will be scientifically useful in understanding the origin of these objects and their relationship to other populations of asteroids and comets in the solar system. These objects may also one day be commercially important as targets for the mining of space resources.