Jupiter trojan

• 1. Introduction

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  Jupiter trojans are a vast population of asteroids that share the orbit of the solar system's largest planet, locked into gravitational balance points sixty degrees ahead and sixty degrees behind Jupiter as it circles the Sun. For most of human history, no one knew they existed. Then, in February 1906, astronomer Max Wolf of Heidelberg spotted an unexpected object hovering at one of those invisible balance points, and everything changed. That asteroid was named 588 Achilles, and it opened the door to a hidden world. How do more than fifteen thousand known asteroids come to share a planet's orbit without ever crashing into it? Where did they come from in the first place? And what might they tell us about the violent early history of our solar system?

• 2. Lagrange's Prediction

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  In 1772, Joseph-Louis Lagrange, an Italian-born mathematician working through the mathematics of the three-body problem, showed that a small object sharing an orbit with a much larger planet would not necessarily collide with it. If positioned sixty degrees ahead or sixty degrees behind, the object would instead become trapped, slowly wobbling around that equilibrium point in what physicists call libration. These stable positions are now called Lagrange points, specifically the L4 and L5 points. Lagrange's prediction sat unverified for more than a century before the sky produced its proof.

  E. E. Barnard made the first recorded observation of what turned out to be a trojan in 1904, spotting the object later catalogued as 12126 Chersidamas. Barnard thought he might be looking at the Saturnian moon Phoebe, which lay only two arc-minutes away in the sky at the time. The object's true identity as a trojan was not confirmed until its orbit was calculated in 1999, nearly a century after Barnard's observation.

  Wolf's discovery of 588 Achilles in early 1906 was the first accepted find. Fellow German astronomer August Kopff followed quickly, identifying 624 Hektor and 617 Patroclus in 1906 and 1907. Patroclus was particularly significant because it was the first asteroid found at the trailing L5 point rather than the leading L4 swarm. By 1938, only eleven Jupiter trojans were known. That number crept to fourteen by 1961, then accelerated: 257 had been catalogued by January 2000, and more than 1,600 by May 2003.

• 3. Names and Rules

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  Johann Palisa of Vienna, the first person to calculate accurate orbits for Jupiter trojans, proposed the naming convention that still governs them today. Asteroids in the leading L4 swarm take names from Greek heroes of the Trojan War, while those in the trailing L5 swarm are named after the heroes of Troy. The two groups are sometimes called the Greek camp and the Trojan camp accordingly.

  The system was devised after the first few trojans had already been named, which produced two famous exceptions. Patroclus, a Greek hero, ended up in the Trojan node, earning it the informal label of a Greek spy. Hector, the great Trojan warrior, sits in the Greek node, making him a Trojan spy in enemy territory.

  By 2018, the International Astronomical Union recognized a practical problem: the known trojan population had grown so large that Trojan War figures were running out. At its 30th General Assembly in Vienna, the IAU amended the convention. Trojans smaller than roughly 22 kilometers in mean diameter could now be named after Olympic or Paralympic athletes. As of the 4th of May 2026-38 Jupiter trojans carry the names of athletes, including 57915 Mahuchikh, named for Ukrainian high jumper Yaroslava Mahuchikh, who set a women's high jump world record of 2.10 meters in 2024, and 247341 Shaulladany, named for Shaul Paul Ladany, a racewalker who survived both the Holocaust and the massacre at the 1972 Munich Olympics.

• 4. Population and Size

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  More than fifteen thousand Jupiter trojans have been confirmed, but the actual population is believed to be far larger. Estimates based on deep sky surveys suggest between 160,000 and 240,000 trojans with diameters larger than 2 km in the L4 swarm alone, with about 600,000 exceeding 1 km across both swarms combined. If similar numbers exist in the other swarm, the total population of trojans larger than 1 km probably exceeds one million, roughly matching the count of comparable asteroids in the main asteroid belt.

  Two studies published in 2008 and 2009 suggested those figures may be overestimates by several times, partly because earlier work assumed all Jupiter trojans have a low albedo of about 0.04, when smaller bodies may actually average an albedo as high as 0.12. The revised estimates put the L4 population at 6,300 plus or minus 1,000 and the L5 population at 3,400 plus or minus 500 for bodies larger than 2 km.

  The largest confirmed trojan is 624 Hektor, with a mean diameter of 203 plus or minus 3.6 km. Below 84 km in diameter, the number of trojans rises sharply, much faster than the equivalent size distribution in the asteroid belt. Within the 4.4-40 km range, though, the size distribution resembles that of main-belt asteroids, suggesting that smaller trojans may be collision fragments from larger ones. The total mass of all Jupiter trojans combined is estimated at about 0.0001 of Earth's mass, or one-fifth the mass of the entire asteroid belt.

• 5. Dark Bodies in Motion

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  Jupiter trojans appear to observational instruments as dark, irregularly shaped objects with reddish, featureless spectra. Their geometric albedos typically fall between 3 and 10 percent. The asteroid 4709 Ennomos stands out with the highest albedo of any known Jupiter trojan, at 0.18, which may hint at the presence of water ice on its surface.

  No firm chemical evidence has been collected from any trojan surface. Some objects, including 911 Agamemnon and 617 Patroclus, show very weak absorptions at 1.7 and 2.3 micrometers that might point to organics. The leading hypothesis is that trojan surfaces are coated in tholins, organic polymers produced by the Sun's radiation acting on simpler molecules. A trojan's spectrum can be matched to a mixture of water ice, large quantities of carbon-rich material resembling charcoal, and possibly magnesium-rich silicates.

  In 2006 a team from the Keck Observatory in Hawaii measured the density of the binary trojan 617 Patroclus as less than that of water ice, at 0.8 grams per cubic centimeter. That figure, if representative, would place trojans closer in composition to comets or Kuiper belt objects than to typical rocky asteroids. But 624 Hektor, whose density is determined from its rotational lightcurve at 2.480 grams per cubic centimeter, is far denser than Patroclus, suggesting the two populations may not share the same origin story.

  Rotation data also points to something unusual. A 2008 study from Calvin College examined a sample of ten trojans and found a median spin period of 18.9 hours, significantly slower than the 11.5-hour average for main-belt asteroids of similar size. Lower rotational speed might indicate lower average density, which in turn would be consistent with formation in the outer solar system.

• 6. Origins and the Nice Model

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  Two major theories attempt to explain how Jupiter trojans ended up where they are. The first holds that they formed in roughly the same region of the solar system as Jupiter itself. During Jupiter's final growth phase, the planet accreted enormous amounts of hydrogen and helium over a period of only about 10,000 years, multiplying its mass by a factor of ten. Nearby planetesimals were swept up by the planet's expanding gravity in a process so efficient that roughly half of all remaining planetesimals in the region were captured. The problem is that this mechanism predicts a captured population four orders of magnitude larger than what is actually observed, and it predicts orbital inclinations smaller than those seen in real trojans.

  The second theory connects trojan capture to the Nice model, a description of planetary instability that occurred 500-600 million years after the solar system's formation. In this scenario, Jupiter and Saturn crossed a 1:2 mean-motion resonance, triggering a cascade of gravitational encounters. Uranus and Neptune were scattered outward into the primordial Kuiper belt, flinging millions of objects inward toward the inner solar system. The existing trojan population at that time was destabilized by a secondary resonance, clearing the Lagrange points. Objects scattered inward by Uranus and Neptune could then be recaptured as trojans when Jupiter and Saturn's orbits separated. The wide range of orbital inclinations seen in today's trojan population matches what this chaotic capture process would produce.

  A revised version of the Nice model adds an ice giant to the story. In this variant, one of the outer planets, possibly a fifth giant that no longer exists, was scattered inward onto a Jupiter-crossing orbit before being flung back out. Jupiter's semi-major axis jumped during these encounters, releasing old trojans and capturing new ones with different orbital properties. Following the encounters, the ice giant could have passed through one of the Lagrange points itself, depleting that region relative to the other.

• 7. Lucy and What Comes Next

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  On the 4th of January 2017, NASA announced that a spacecraft called Lucy had been selected as one of its next two Discovery Program missions. Lucy launched on the 16th of October 2021 and is set to visit seven Jupiter trojans, reaching the first of them in 2027 after two gravity assists from Earth and a flyby of a main-belt asteroid. After its tour of the L4 swarm, the spacecraft will return to Earth for another gravity assist and then travel to the L5 swarm, where it will visit 617 Patroclus.

  The long-term fate of the trojans themselves is uncertain. Multiple weak resonances with Jupiter and Saturn cause individual trojans to behave chaotically over billions of years. Simulations suggest that up to 17 percent of Jupiter trojans are on unstable orbits over the age of the solar system. Some escaping trojans may become temporary satellites of Jupiter; others may become Jupiter-family comets as they drift closer to the Sun and surface ice begins to sublimate. Researchers estimate that roughly 200 escaped trojans larger than 1 km in diameter may currently be traveling through the broader solar system, with a small number potentially on Earth-crossing orbits. The binary trojan 617 Patroclus, with its orbital separation of just 650 km between the two components, will be among the first of these ancient objects examined up close when Lucy arrives.