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At the end of September, a passing asteroid, dubbed 2024 PT5, was temporarily captured by Earth’s gravity and will orbit the Earth once before leaving once again at the end of November. Such a temporary gravitational capture of an asteroid is called a mini-moon event. They’re rare, but they do happen.
This temporary capture brought to mind a thread I encountered on X/Twitter earlier this year about another surprising, and quite complicated asteroid capture involving another planet. The thread is here:
Briefly, Latif Nasser, cohost of the RadioLab podcast, noticed that a poster depicting the Solar System decorating his 2-year-old son’s bedroom (a detail from the poster is the title image) shows the planet Venus having a moon called Zoozve. As one devoted to space science from an early age, I have known since childhood that Venus had no moons, and if you consult any reference, it will tell you the same thing. Being a reporter, Nasser pursued how the Venus depicted on this poster had acquired this moon, and why its name is so weird. It turns out Zoozve is a typo; the body’s name was actually 2002 VE68, but the poster’s illustrator took the numbers to be letters. Part of the podcast involves how Nasser managed to get the International Astronomical Union to change the official name of this object to Zoozve, and if you want to hear about that, go listen to the podcast (link below). What interests me more here is the dynamics of Zoozve’s movement, and how, while it does orbit Venus, it doesn’t quite fit our normal picture of a moon. Indeed, it is called a quasi-moon, the first astronomical object to be placed in that category.
What I learned about the Solar System as a kid is that all the planets orbit the Sun. Most of the planets have moons orbiting around them in orbits analogous to the orbits of the planets around the Sun. Later, when learning Newtonian physics, it was explained that Newton’s law of gravitation led naturally to two bodies orbiting each other around their center of gravity in an elliptical orbit. (We usually say that the planets revolve around the Sun, but all bodies, including the Sun, revolve around the center of gravity of the Solar System. Because the Sun is so much more massive than the planets, that center of gravity is close to the Sun, but mainly because Jupiter is so massive, the Solar System’s center of gravity is usually just outside the Sun, near its surface.) That well-behaved elliptical orbit is the solution of what we call the two-body problem, where we only consider the motions and forces acting between two isolated objects, for example the Sun and one planet, or the Earth and the Moon, in the absence of any other bodies.
However, the Solar System consists of way more than two isolated bodies. Nobody has definitively proven that the motions of three or more bodies bound by gravity can’t be solved exactly, but neither has anyone obtained a simple, clean solution for such a system. In general, numerical solutions of such systems can display chaotic motion that is not consistent and regular, and predictability becomes highly dependent on how many decimals were included in the starting parameters of a simulation. (This is the physical basis for the Netflix series The Three Body Problem and the book on which it was based.) Now the Solar System has been in its present state for billions of years. While the various planets do exert gravitational force on each other (Jupiter in particular), one can be reasonably sure that any chaotic aspects of planetary motions just destroyed themselves in the early history of the Solar System. At least as far as the planets go, we’re not going to be doomed by chaotic motion leading to the Earth falling into the Sun, for example.
But then there are the asteroids, those small (relative to planets) chunks of rock that also orbit the Sun. Some of these asteroids get captured by a planet’s gravity and become a moon, but most of them just orbit the Sun. Zoozve could be classed as a captured asteroid. Again, when I was a kid, I learned that most of these are in the Asteroid Belt between the orbits of Mars and Jupiter, but in fact, there are other asteroid belts, and bodies in these other belts can pass close to the Earth and the other planets. This became a concern starting the late 1980s, when it became clear that the collision of a large asteroid with the Earth could wipe out much of life on Earth, not to mention human civilization. By that time, the generally accepted scenario of the extinction at the end of the Cretaceous era was that it was caused by just such a collision. In an effort to prevent such a collision in the future, astronomers have spent succeeding years searching for and cataloguing as many asteroids as they could find. If you know one’s coming, and you have enough time, you can prevent the collision. It was during this search that Zoozve (2002 VE68) was found.
So, does Zoozve revolve around Venus? Yes… but. Zoozve’s orbit is different from that of the typical moon around its planet. The size of Zoozve’s orbit around Venus is very large, such that it actually crosses Earth’s orbit on one side and Mercury’s on the other. Further, Zoozve’s orbit around Venus is not an ellipse. The reason for this is that Zoozve is not so much orbiting Venus as it is revolving around the Sun in an orbit that regularly brings it within Venus’ gravitational field. Unlike a proper moon, where one only need to consider the interaction between planet and moon to get the correct orbit, it’s necessary to include the interactions of three bodies, Zoozve, Venus, and the Sun, to explain Zoozve’s dynamics. This is a true three-body system. You can see in the video below that while Zoozve’s orbit around the Sun appears to be a regular ellipse, its orbit around Venus is a much stranger shape. (Note that the perspective changes in the course of the video.)
As we might suspect from the chaotic nature of three-body dynamics, Zoozve’s orbit is not stable in the long term. Researchers estimate that Zoozve entered into its current orbit about 7000 years ago, and it will probably leave again some thousands of years in the future.
It turns out that quasi-moons are not rare in our Solar System. Seven have been discovered around Earth, and most of the asteroids in the Asteroid Belt between Mars and Jupiter are quasi-moons of Jupiter. The orbits of these quasi-moons can be quite strange, even stranger than Zoozve’s around Venus. For more details, you can listen to the RadioLab podcast about Zoozve here: www.wnycstudios.org/…
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