Does the Earth Have “Mini-Moons”?

For centuries, the moon—as we know it—was Earth’s only known natural satellite.  But about a dozen years ago astronomers first identified a new body that eventually became Earth’s second known natural satellite, other than the moon.  They call this type of entity a temporarily-captured orbiter (TCO) and astronomers named it 2006 RH120.  It measures perhaps no more than 3 meters across and orbited the planet for only 13 months before it finally loosed itself from Earth’s gravitational pull, only to be quickly recaptured into solar orbit.  Obviously, this TCO is very small; it is so small, in fact, that NASA had initially thought it was the third stage Saturn S-IVb booster left behind from the Apollo 12 mission.  Closer inspection proved that this was actually an asteroid.

Lead study author Dr. Robert Jedicke, of the University of Hawaii Institute of Astronomy, explains, “These asteroids are delivered towards Earth from the main asteroid belt between Mars and Jupiter via gravitational interactions with the Sun and planets in our solar system.” He goes on to say, “The challenge lies in finding these small objects, despite their close proximity.”

While astronomers have not been able to locate another TCO sighting since the discovery of 2006 RH120, they still propose it is likely there is a “steady state population” of 1-2m diameter objects probably captured and in orbit at any given time, and the number of captured meteoroids probably continues to increase exponentially as they get smaller in size.

Study co-author Dr. Mikael Granvik comments, “We don’t know whether small asteroids are monolithic blocks of rock, fragile sand piles, or something in between,” The planetary scientist for the Luleå University of Technology in Sweden and the University of Helsinki in Finland also adds, “Mini-moons are perfect targets for bringing back significant chunks of asteroid material, shielded by a spacecraft, which could then be studied in detail back on Earth. Mini-moons that spend significant time in orbit around Earth allow us to study the density of these bodies and the forces that act within them, and therefore solve this mystery.”