Minor-planet moon

Lore & Background

Prior to the era of the Hubble Space Telescope and space probes, attempts to detect satellites around asteroids were limited to optical observations from Earth. In 1978, stellar occultation observations were claimed as evidence of a satellite for 532 Herculina, but later Hubble imaging did not reveal a satellite, and the current consensus is that Herculina does not have a significant satellite. In 1993, the first asteroid moon was confirmed when the Galileo probe discovered the small Dactyl orbiting 243 Ida in the asteroid belt. The second was discovered around 45 Eugenia in 1998. In 2001, 617 Patroclus and its same-sized companion Menoetius became the first known binary asteroids in the Jupiter trojans. The first trans-Neptunian binary after Pluto–Charon, 1998 WW31, was optically resolved in 2002. Several moons are quite large compared to their primaries: 90 Antiope, Mors–Somnus and Sila–Nunam (95%), Patroclus–Menoetius, Altjira and Lempo–Hiisi (90%, with Lempo–Paha at 50%). The largest known minor-planet moon in absolute size is Pluto's moon Charon, which has about half the diameter of Pluto. There are also several known ring systems around distant objects, such as those around Chariklo and Chiron. The origin of minor-planet moons is not known with certainty; one model suggests they form from debris knocked off the primary by an impact, while other pairings may be formed by gravitational capture.

Reader's Guide

The study of minor-planet moons is significant because their orbits provide estimates of the mass and density of the primary, offering insights into physical properties otherwise inaccessible. Among near-Earth asteroids, satellites tend to orbit at distances of 3–7 primary radii and are two to several times smaller than the primary; tidal stresses from close planetary passes are thought to be responsible for many of these. Among main-belt asteroids, satellites are usually much smaller than the primary (except 90 Antiope) and orbit around 10 primary radii away; many are members of asteroid families, likely fragments from a parent body disrupted by collision. Among trans-Neptunian objects, the two components are often of comparable size with much larger orbital separations (100 to 1000 primary radii), and a significant proportion are expected to be primordial. Pluto's largest moon Charon is large enough to orbit a point outside Pluto's surface, forming a binary system informally called a double dwarf planet. The highest known multiplicities are Pluto (a sextuple system) and 130 Elektra (a quadruple system). As of 2025, the total number of known multiple systems among minor planets is 18.

Did You Know?

• The first asteroid moon confirmed was Dactyl, discovered by the Galileo probe orbiting 243 Ida in 1993.
• 130 Elektra is the only known quadruple minor planet, discovered in 2021.
• Pluto's moon Charon has about half the diameter of Pluto, making it the largest known minor-planet moon in absolute size.
• An estimated 11% of trans-Neptunian objects are thought to be binary or multiple objects.

The Architecture of Discovery and Numbering

Every small body orbiting the Sun—whether an asteroid, a distant trans-Neptunian object, or a dwarf planet—begins its catalog life with a provisional designation, a temporary label such as 1989 AC that marks the moment of first observation. Only after the object has been tracked across at least four oppositions, giving astronomers enough data to pin down its orbit with confidence, does the Minor Planet Center assign a sequential number in parentheses, transforming the entry into a permanent record. This safeguard exists because early cataloging was less rigorous: some bodies were numbered and then vanished from view entirely. The asteroid 719 Albert, lost for nearly eight and a half decades, was finally recovered in 2000, and its return eliminated the last numbered lost object from the registry. An even more dramatic case is 69230 Hermes, first spotted in 1937 but not rediscovered until 2003, meaning its official number arrived decades after its initial sighting. These episodes remind us that the sequence of numbers only loosely tracks the true timeline of discovery.

The Naming Gap and the Limits of Identity

Of the nearly 896,000 numbered minor planets recorded as of May 2026, only about three percent carry a name at all. The vast majority remain known solely by their numerical and provisional designations. A discoverer is granted a window of roughly ten years to propose a name—typically drawn from mythology, geography, or notable figures—but with discovery rates surging due to automated surveys like LINEAR, that window is increasingly irrelevant. The lowest-numbered unnamed body in the catalog, (4596) 1981 QB, and the highest-numbered named one, 875150 Burkegaffneyobs, bookend a reality in which most small Solar System objects will never receive a human-readable label. The first entry, 1 Ceres, discovered by Giuseppe Piazzi in 1801, and the best-known, 134340 Pluto, represent the rare exceptions. For the rest, identity is purely numerical, a reminder that the Solar System's small-body population far outstrips our capacity to individualize it.

Scale, Scope, and the Horizon Ahead

The census of small Solar System bodies is staggering and still accelerating. As of May 2026, 895,910 minor planets hold permanent numbers, accounting for more than half of the 1,547,929 observed objects, the remainder being unnumbered minor planets and comets. Ninety-seven and a third percent of all numbered entries are asteroids residing in the main belt between Mars and Jupiter. Every main-belt asteroid larger than ten kilometers in diameter has already been found, yet the population of smaller bodies dwarfs what we have cataloged: estimates suggest up to ten trillion asteroids of one meter or larger exist out to Jupiter's orbit, and more than a trillion additional minor planets populate the Kuiper belt beyond Neptune. The Vera C. Rubin Observatory, once fully operational, is projected to add roughly five million new minor-planet discoveries over the next decade—an almost tenfold jump over the current total. The catalog that began with a single object in 1801 is on the verge of becoming a record of billions.

The Discoverer Community and Institutional Credit

Behind every catalog entry stands a discoverer—or, in many cases, a survey program, an observatory, or a team of researchers. The Minor Planet Center credits more than a thousand individual astronomers, professional and amateur alike, though many have contributed only a single co-discovery. Approximately three hundred institutional programs and survey sites also carry credit, and a handful of U.S.-based efforts—Spacewatch, LINEAR, MLS, NEAT, and CSS—dominate the statistics. A telling example comes from Palomar Observatory, where the MPC credits the principal investigators Cornelis van Houten, Ingrid van Houten-Groeneveld, and Tom Gehrels for a batch of objects found in 1977, including the Jupiter trojan 189004 Capys. The distinction between a human name and an institutional one underscores how discovery has shifted from solitary telescope work to large-scale, automated sky surveys. As the total climbs by tens of thousands each year, the community of finders grows, yet the credit structure remains a patchwork of individuals, programs, and sites.