Minor-planet moon

• 1. Introduction

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  A minor-planet moon is a natural satellite in orbit around a minor planet, and as of the latest count, astronomers have identified 625 minor planets known or suspected to carry them. That number would have seemed impossible to earlier generations of sky-watchers, who had no tools capable of detecting such tiny, distant companions. What drives scientists to search for these objects? What do they reveal that nothing else can? And why does the question of how they formed remain genuinely open decades into the search?

• 2. A Discovery Delayed

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  In 1978, stellar occultation observations were put forward as evidence that the asteroid 532 Herculina had a satellite. The claim seemed promising, but when the Hubble Space Telescope later trained its sharper eye on Herculina, no satellite appeared. The current consensus is that Herculina has no significant companion.

  Also in 1978, astronomer Thomas Hamilton published a letter in Sky and Telescope pointing to what looked like simultaneously formed impact craters on Earth, including the Clearwater Lakes in Quebec, as indirect evidence that paired, gravitationally bound objects were real. His suggestion was that such pairs had struck Earth together.

  The same year brought a discovery that would later anchor the whole field: Pluto's largest moon, Charon, was found in 1978, though Pluto itself was still classified as a major planet at the time.

  The long wait for a confirmed asteroid moon ended in 1993, when the Galileo probe flew past 243 Ida in the asteroid belt and spotted a small orbiting body named Dactyl. A second confirmed moon came just five years later, found around 45 Eugenia. By 2001, the Jupiter trojan 617 Patroclus was revealed to have a companion of nearly identical size, Menoetius, making it the first known binary system in that population. The first trans-Neptunian binary after Pluto and Charon was optically resolved in 2002.

• 3. Why They Matter

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  Discoveries of minor-planet moons are scientifically valuable in a way that single-body observations rarely match: when a moon's orbit is measured, the math yields an estimate of the primary's mass and density. Those figures give direct access to the physical properties of the minor planet, properties that are generally not otherwise accessible.

  Density, in particular, tells researchers what a body is made of and whether it is solid rock or a loosely bound pile of rubble. Without a moon to anchor the calculation, such properties must be inferred indirectly, with far greater uncertainty.

  The size relationships between some moons and their primaries also raise questions. Several moons are large compared to their host bodies. 90 Antiope's companion is nearly the same size as the primary itself, at roughly 95% of its diameter. The same ratio applies to Mors-Somnus and Sila-Nunam. Patroclus and Menoetius share a similar mass, as do Altjira and the Lempo-Hiisi pair. Lempo's third body, Paha, sits at around 50%. The largest minor-planet moon in absolute terms is Charon, whose diameter is roughly half that of Pluto.

• 4. What They Are Called

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  Naming these objects has required some flexibility. When a minor planet has one moon, the pair is sometimes called a binary minor planet. When it has two, the term triple is used. If the two components are very close in size, the Minor Planet Center uses the phrase binary companions rather than labeling the smaller body a satellite.

  A good example of a true binary, where both objects share a barycenter that lies between them, is 90 Antiope, identified in that configuration in August 2000. Very small satellites carry the informal label moonlets.

  Pluto and Charon represent an especially striking case. Charon is large enough that the two bodies orbit a common barycenter located outside Pluto's surface. Pluto's own orbit is entirely enclosed by Charon's, making the system what some scientists informally describe as a double dwarf planet. Pluto's four smaller moons, Nix, Hydra, Kerberos, and Styx, orbit the Pluto-Charon system as a whole rather than Pluto alone.

• 5. Where They Form and Why

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  The origin of minor-planet moons is not currently known with certainty. One leading model holds that a moon forms from debris flung off the primary body during an impact. Another proposes that a small object can be captured by the gravity of a larger one passing nearby.

  Impact formation is physically constrained by angular momentum: how massive the components are and how far apart they sit limits what kinds of collisions could have produced them. Close binaries, such as Pluto and Charon, fit this model well. Distant binaries whose components are of comparable size present a harder problem. For an impact to produce such a pair, the event would have had to shed considerable mass, making the scenario less likely.

  Among near-Earth asteroids, a different mechanism may dominate. Since these bodies cross the orbits of the inner planets, tidal stresses during close planetary flybys are thought to be responsible for pulling some of them apart into pairs, with collisions playing a secondary role. In the main belt, many binary systems belong to asteroid families, groupings of bodies believed to share a common ancestor that was shattered in an ancient collision. Among trans-Neptunian objects, a significant fraction of binaries are thought to be primordial, their pairing dating back to the earliest period of solar system formation.

• 6. Populations Across the Solar System

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  Binary systems are not evenly distributed across the solar system, and the differences between populations are substantial. Among asteroids overall, an estimated 2% are thought to carry satellites. Among trans-Neptunian objects, the figure rises to roughly 11%, and the majority of large trans-Neptunian objects have at least one moon, including all four dwarf planets recognized by the International Astronomical Union.

  Near-Earth asteroid moons tend to orbit at distances of about 3-7 primary radii and are typically two to several times smaller than the primary. Main-belt moons are usually much smaller still, orbiting around 10 primary radii out, with 90 Antiope standing as a notable exception. Trans-Neptunian binaries show a very different pattern: the two components are often of comparable size, and their orbital separations are much larger, running from roughly 100 to 1,000 primary radii.

  In the centaur population, only two binaries have been found so far among objects with semi-major axes smaller than Neptune's. Both are ring systems rather than discrete moons: 2060 Chiron and 10199 Chariklo, discovered across the span from 1993 to 2013.

  The distances between binary components span an enormous range. Among asteroids, the closest known pairings, such as 243 Ida and 3749 Balam, lie just a few hundred kilometers apart. The widest known asteroid binary separation belongs to 379 Huenna, at more than 3,000 km. Trans-Neptunian separations run even larger, from 3,000 to 50,000 km.

• 7. Multiple Systems

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  In 2005, the asteroid 87 Sylvia was found to have two satellites, Romulus and Remus, making it the first confirmed triple system. The same year, a second moon was found around 45 Eugenia, and a dwarf planet beyond Neptune was confirmed as only the second trans-Neptunian object after Pluto known to have more than one moon. 216 Kleopatra and 93 Minerva joined the list as triple asteroids in 2008 and 2009 respectively.

  In 2021, 130 Elektra was discovered to have three moons, making it the only known quadruple asteroid. The current count of known multiple systems among minor planets stands at 18, including the Pluto and Haumea systems.

  The highest known multiplicities belong to two objects at opposite ends of the size spectrum. Pluto has five named moons, Charon, Nix, Hydra, Kerberos, and Styx, making it the center of a sextuple system when counting Pluto itself. Haumea has two moons, Hiaka with a radius estimated at around 155 km, and Namaka at around 85 km.

  The triple system around 47171 Lempo offers one of the more unusual arrangements in this catalog. Lempo and its moon Hiisi, which are of roughly equal mass, form a close pair separated by about 867 km. A second moon, Paha, orbits the Lempo-Hiisi binary from a distance of about 7,411 km, circling the inner pair rather than the primary alone.