A provisional moon is a newly discovered natural satellite that has been observed orbiting a planet but hasn’t yet received a permanent name. It sits in a kind of astronomical waiting room: confirmed enough to get an official temporary label, but not yet fully verified and named. The term covers everything from tiny rocks spotted circling Jupiter to school-bus-sized asteroids temporarily caught in Earth’s gravity.
How Provisional Designations Work
When astronomers discover a new moon orbiting any planet, the International Astronomical Union (IAU) assigns it a provisional designation rather than an immediate name. This designation follows a specific format: the prefix S/ (for satellite), followed by the year of discovery, a letter indicating the planet, and a number showing the order of discovery that year. The planetary codes are straightforward: J for Jupiter, S for Saturn, U for Uranus, N for Neptune, M for Mars, and E for Earth.
So a designation like S/2003 J 2 tells you this was a satellite discovered in 2003, orbiting Jupiter, and it was the second satellite found that year. The Minor Planet Center, hosted by the Center for Astrophysics at Harvard and Smithsonian and funded by NASA, maintains the official registry tracking these discoveries.
A moon keeps its provisional label until astronomers can confirm its orbit through repeated observations over multiple years. Only after that confirmation does the IAU’s naming committee assign a permanent name, typically drawn from mythology connected to the planet’s existing naming theme. Jupiter’s moons get names from Greek and Roman mythology, Saturn’s from Norse and other traditions, and so on. Some moons sit in provisional status for years or even decades before getting a proper name.
Why So Many Moons Are Still Provisional
The outer planets have staggering moon counts. Jupiter has 95 known moons, and Saturn leads with 274 as of March 2025. Many of these are tiny, irregular objects just a few kilometers across, discovered in batches by modern survey telescopes. Confirming the orbit of a small, faint moon circling a distant planet takes time. You need observations across multiple years to nail down the orbital path precisely enough to be sure the object is genuinely bound to the planet and not just passing through.
This is why large groups of moons at Jupiter and Saturn carried provisional designations for years after their initial discovery. Some were even “lost” after their discovery apparition, only to be recovered later when telescope technology improved or when targeted follow-up observations were made.
Earth’s Temporary Mini-Moons
Earth occasionally captures its own provisional moons, though they work quite differently from the permanent satellites of the outer planets. These objects, called mini-moons, are small asteroids that wander close enough to Earth to get temporarily trapped by our planet’s gravity. They stick around for weeks, months, or sometimes years before drifting back into their regular orbit around the Sun.
Astronomers split mini-moons into two categories based on how long they stay. A “temporarily captured orbiter” completes at least one full loop around Earth before leaving. A “temporarily captured flyby” gets pulled in by Earth’s gravity but escapes before finishing a full orbit. Of the five mini-moons observed so far, only two qualified as true temporarily captured orbiters.
The first known mini-moon, 1991 VG, arrived in late 1991 and departed in early 1992. The record-holder, 2020 CD3, stayed in Earth’s gravitational grip for more than two years, making it the longest-captured mini-moon ever observed. The most recent, 2024 PT5, was captured on September 29, 2024, and escaped just 57 days later on November 25. It measured roughly the size of a school bus (about 10 meters across), far too small to see with the naked eye.
These mini-moons tend to come from a group called the Arjuna asteroids, which orbit the Sun on paths very similar to Earth’s. That similarity in speed and trajectory makes it easier for Earth’s gravity to grab them temporarily.
Mini-Moons vs. Quasi-Moons
Mini-moons are sometimes confused with quasi-moons, but the two are fundamentally different. A mini-moon genuinely orbits Earth, even if only for a short time. It’s inside Earth’s gravitational influence, looping around our planet the way our permanent Moon does, just briefly.
A quasi-moon, by contrast, never actually orbits Earth. It orbits the Sun on a path so similar to Earth’s that, from our perspective, it appears to circle us. It’s an optical illusion of orbital mechanics. All of Earth’s known quasi-moons are temporary companions too, but on much longer timescales: they maintain their quasi-moon relationship for hundreds or thousands of years before gradually shifting into different orbital patterns around the Sun.
The practical difference matters. Mini-moons average less than a year in Earth’s company. Quasi-moons can shadow Earth for millennia but are never gravitationally bound to it.
What Happens After Provisional Status
For moons of the outer planets, the path from provisional to permanent is a matter of patience. Once enough observations confirm a stable, repeating orbit, the discoverer can propose a name to the IAU. The naming committee reviews the proposal, checks it follows the planet’s naming conventions, and either approves or requests changes. The provisional S/ designation then gets retired in favor of the new name.
For Earth’s mini-moons, the situation is different. Because they leave, they never become permanent moons. They keep whatever asteroid designation they were given (like 2024 PT5) and return to being classified as near-Earth objects once they escape our gravity. Their brief time as Earth satellites is noted in the scientific record, but they don’t go through the same naming process as a permanent satellite of Jupiter or Saturn would.