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That’s no moon: Earth’s temporary mini-satellites may help protect against disaster

[The unstable orbit of Earth’s temporary satellite]

On February 25, Kacper Wierzchos of the Catalina Sky Survey at the University of Arizona tweeted that since February 15 his team had been observing a small object, now catalogued as 2020 CD3, to ascertain whether it was a natural satellite of the Earth. There is a lot of “space junk” orbiting the Earth by now, but almost all in low orbits which will spiral down, not up, with time; only rocket stages or other shed parts from interplanetary launches are going to be in high orbits.

The period of 2020 CD3 would be 47 days, corresponding to an average distance from the Earth about 4/3 that of the Moon, if it were a stable orbit. But the orbit is highly elliptical, sometimes dipping considerably closer than the Moon, and the perturbing effects of the Moon mean that its path does not exactly repeat from one time to the next.

The best reconstruction of 2020 CD3’s trajectory indicates that it was captured by the Earth about three years ago and probably will not stay in Earth orbit more than a few months longer. Catalina Sky Survey saw a similar object 14 years ago, 2006 RH120, but it drifted away from Earth in 2007. This sparked curiosity about how often such things might happen: a supercomputer simulation in 2012 by Mikael Gravnik of the University of Helsinki and others, studying a hypothetical population of millions of small asteroids in orbits closer to the Sun than Mars,
matching what we believe to be the real population, found that at any given time about 1,000 should be in temporary orbits around the Earth. These TCOs (transiently captured objects) should range in size from one meter in diameter down to a few centimeters, but with an average orbital distance four to five times the distance to the Moon. The average TCO will only orbit three times over the course of about nine months, before perturbations drive them away.

So 2020 CD3 is in a tighter and somewhat more lasting orbit than usual, and it is also somewhat larger than what should usually be the maximum mini-moon, about 2 meters across by3.5 meters long. It is about the size of a car rather than the beach ball size that is usually the largest. It was not noticed for three years because it was only a 20th magnitude object (1st magnitude means one of the brightest stars and 6th magnitude at the limit of naked-eye vision; every 5 magnitudes is a step-down in brightness by a factor of 100), and the thousand tinier TCOs we suspect are out there would be even harder to see. But Gravnik suggests that it would be worthwhile to capture and retrieve one: at present what we know about asteroidal material principally comes from meteorites, which have been heat-blasted and contaminated with terrestrial material as they fall through the atmosphere. Efforts to retrieve pristine asteroidal material have been fraught.


JAXA (the Japanese Aerospace Exploration Agency) launched the Hayabusa probe to asteroid 25143 Itokawa in 2003, but the November 2005 landing did not go well. The pellet gun which was supposed to loosen samples for collection did not fire, and after the craft took off for return to Earth its guidance system failed.

The craft had previously suffered damage to solar panels during a large solar flare while it was outbound, and fuel leakage during the initial landing attempts, and was thought to be cursed. But eventually it was nudged into a trajectory that let it return its sample capsule, into which a little dust (kicked up during the landing) had blown, in 2010. Japan recovered 1500 grains, each about .01 millimeters in diameter. The Hayabusa-2 probe, launched at the end of 2014 to asteroid 162173 Ryugu, contained more sophisticated equipment incorporating lessons from the first try, and from February through June of last year successfully fired a gun to dredge up subsurface material as well as collecting surface samples.

The sample capsule is expected to return later this year. Ryugu is a C-type (“carbonaceous”) asteroid, interesting for studying the question of how common organic materials are in space. But it is not typical of the S-type (“stony”) or M-type (“metallic”) which fall to Earth as meteorites more often. NASA’s Osiris-Rex probe, launched in 2016, has been orbiting C-type asteroid 101955 Bennu since December 2018. After two years of mapping and remotely analyzing Bennu it will land and gather about 60 grams of sample in a brief “TAG” (touch and go) maneuver sometime this year, perhaps July, for return to Earth by 2023, if all goes well.

Plans have had to be rethought in view of some surprising features of Bennu, which turns out to have jagged terrain and unexpected “ejection events” in which plumes of particles are thrown up from the surface. Understanding Bennu is considered important because it is one of the asteroids whose orbit crosses Earth: it will have a series of close encounters most years around September 24 from 2175 to 2199, but as far as we can calculate there is only a 1/2500 chance it will hit us.

Bennu is 490 meters in diameter and an impact would be devastating, though nothing like the Chicxulub (dinosaur killer) asteroid which is estimated to have been 10-80 kilometers. The dynamic studies however indicate that a more typical fate for a near-Earth asteroid is to have its orbit gradually assimilated to Earth’s by successive close encounters. Some will become TCOs drifting in and out of wide orbits like our current friend 2020 CD3. Another stage is illustrated by 2016 HO3, whose distance from the Earth varies from 38 to about 100 times the distance to the Moon: really it is orbiting the Sun, with a period of one year the same as the Earth, wobbling above and below the orbital plane of the Earth, not really circling the Earth.

This loose association is more stable than a closer orbit: it is believed that it started almost a century ago and will continue centuries more. This “quasi-satellite” also has the asteroid designation 469219 Kamo’oalewa. From 1996 to 2006, 2003 YN107 was in a quasi-satellite relationship but now has an orbital period a little under 364 days, and a few other bodies also are believed to slip into and out of quasi-satellite relationships with Earth.

The effect of gradually “domesticating” near-Earth asteroids like this is that their relative velocity with respect to the Earth is much less, and they are less likely to slam into the planet dramatically. Rather they may shed pieces which enter the atmosphere in gentle near-horizontal trajectories, producing fireballs with long visible paths. The Great Meteor Procession of February 9, 1913 which stretched 7,000 miles from northwest Canada across the United States into the Atlantic, was probably the death of one of these mini-moons.

Robert Eckert is a longtime member of the Underground Bunker community and author of the historical novel The Year of Five Emperors.

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