Cruithne: Earth's quasi-satellite that never became a second moon

Cruithne and asteroids playing cat and mouse with Earth

There is a rock about 5 km in diameter that shares an orbit with Earth, albeit on a different plane, and approaches us every 385 days, seeming to stay but then retreating back the way it came. It is called 3753 Cruithne and is not alone: astronomers classify them as co-orbital objects, a category that also includes 2004 GU 9 and 2003 YN 107. The question hovering over any conversation about them is simple: why doesn't it end up falling towards us and becoming a second moon?

The short answer is that gravity does not work like a vacuum cleaner. The long answer involves orbital resonances, inclined planes, and a choreography that celestial mechanics solves with a precision many find suspicious.

What does it miccionan for an asteroid to "retreat" in space?

In space there is no up or down, nor an intrinsic sense of movement. When it is said that Cruithne "retreats," an apparent trajectory is being described, not a whim of the object. Retrograde motion, in its astronomical sense, is real orbital or rotational motion in the opposite direction to the rotation of the central body; in its astrological sense, it is an optical illusion seen from Earth to which effects on character or destiny are attributed. It is advisable not to mix both things, although they share vocabulary.

What Cruithne does is trinc an elliptical orbit that, seen from our planet, draws a kind of horseshoe. It approaches, moves away, approaches again from the opposite side and repeats the cycle. It neither escapes nor gets trapped. It is a dynamic equilibrium, not indecision.

The Moon, the Sun and the coincidence that does not cease

The thread quickly drifts toward another recurring obsession: the Moon has the exact size and distance to cover the Sun during a total eclipse. What are the odds? The uncomfortable answer is that the Moon was closer in the past and its apparent size was larger; billions of years from now it will be further away and will not cover the solar disk. The coincidence is not eternal, it is a temporal window. That Homo sapiens developed self-awareness just within that window is, depending on how you look at it, an astonishing coincidence or simply a consequence of being here to tell it.

To this is added tidal locking: the Moon takes as long to rotate on its axis as it does to orbit the Earth, which is why we always see the same face. It is not an exclusive phenomenon. Io, Europa, Ganymede and Callisto do so relative to Jupiter; Titan relative to Saturn; Triton relative to Neptune. Tidal coupling is the norm, not the exception.

The Moon recedes and days lengthen

Lunar attraction slows down Earth's rotation. Days are getting longer, an effect measurable in photosynthetic fossils with daily periodic growth. The Sun-Earth-Moon system is a three-body problem that adjusts slowly, and that slowness is what has allowed persistent life to find a stable framework. Some argue that all this constitutes a "rare Earth," a planet with improbable astronomical advantages. Others respond that the anthropic principle explains why we observe a universe compatible with our existence: if it were not, we would not be here to ask.

Interstellar comets and the Moon's noise

The conversation jumps to interstellar objects such as comet 3I/ATLAS, discovered on July 1, 2025 from Chile, the third confirmed after 1I/ʻOumuamua and 2I/Borisov, with an eccentricity of 6.14 and a hyperbolic excess velocity of 58 km/s relative to the Sun. It poses no threat: it will pass no less than 270 million kilometers away. Comet C/2025 R2 SWAN will approach to 39 million kilometers on October 20, 2025, about a hundred times further away than the Moon.

And in the background, a curiosity that sounds like legend: seismographs left on the Moon recorded resonances lasting up to three hours after the impact of the Apollo 13 Saturn rocket. The Moon, apparently, rings like a gong.

Why doesn't Cruithne end up falling towards Earth?

Because its orbit and ours are in resonance. The asteroid does not "decide" to return: combined gravitational perturbations from the Sun and Earth keep it on a repeating stable trajectory. There is no regret, there is mechanics. The open question is another: if so many orbital, thermal and chemical coincidences align to allow us to exist, are we facing a tuned universe or the inevitable bias of one who can only observe from within?