Jupiter trojan
German astronomer Max Wolf spotted the first Jupiter trojan in February 1906. He named this asteroid 588 Achilles and found it at the L4 Lagrangian point of the Sun-Jupiter system. This discovery came more than a century after Italian mathematician Joseph-Louis Lagrange predicted such points would exist in 1772. Lagrange had studied the restricted three-body problem to show that small bodies could be trapped near these equilibrium locations. E.E. Barnard made an earlier observation in 1904 but misidentified the object as Saturn's moon Phoebe. That object was later identified as 12126 Chersidamas when its orbit was calculated in 1999. By 1938 astronomers had detected only eleven Jupiter trojans total. The count grew to fourteen by 1961 before accelerating rapidly with better instruments. January 2000 saw 257 known objects while May 2003 brought the number to 1,600. Current records list 9,694 trojans at the leading point and 5,628 trailing behind.
Johann Palisa of Vienna suggested naming all asteroids in Jupiter's L4 and L5 points after heroes from the Trojan War. Asteroids ahead of Jupiter carry names of Greek heroes like Achilles while those behind bear names of Trojans like Hector. Two early discoveries broke this rule because they were named before the convention existed. 617 Patroclus sits in the Trojan camp despite being a Greek hero. 624 Hektor resides in the Greek camp even though he fought for Troy. The International Astronomical Union changed these rules in 2018 during their 30th General Assembly in Vienna. They allowed smaller asteroids with diameters under 22 kilometers to be named after Olympic athletes instead. As of June 2025 twenty-four Jupiter trojans have received athlete names. Russian biathlete Anastasiya Kuzmina has an asteroid bearing her name. Belarusian fencer Elena Novikova-Belova also claims one. Polish ski jumpers Adam Małysz and Kamil Stoch each have dedicated asteroids. Canadian Paralympic gold medalist Hilda Binns appears on the list too. Italian wheelchair fencer Bebe Vio has an asteroid named for her achievements.
Deep sky surveys estimate the L4 swarm holds between 160,000 and 240,000 asteroids larger than two kilometers across. About 600,000 objects exceed one kilometer in diameter within that same group. If the trailing swarm contains similar numbers then over one million Jupiter trojans exist at this size threshold. These figures match comparable counts found in the main asteroid belt. Total mass reaches only 0.0001 times Earth's mass or roughly one-fifth of the entire asteroid belt. Two recent studies suggest these estimates might be several times too high due to incorrect assumptions about reflectivity. Small bodies may reflect light more strongly than previously thought with albedos reaching 0.12 instead of 0.04. Newer calculations place the count of large L4 objects around 350,000 while L5 holds approximately 280,000. The largest known trojan measures 203 kilometers wide and carries the name 624 Hektor. Few large objects exist compared to the vast population of smaller ones. Numbers grow quickly as size decreases down to 84 kilometers before flattening out again.
Jupiter trojans orbit between 5.05 and 5.35 astronomical units from the Sun. Their average semi-major axis sits near 5.2 AU with a spread of plus or minus 0.15 AU. Each swarm stretches roughly 26 degrees along Jupiter's path covering about 2.5 AU total distance. Widths reach two Hill radii which equals 0.6 AU for this planet. Many trojans maintain orbital inclinations up to 40 degrees relative to Jupiter's plane. They do not stay fixed next to Jupiter but librate slowly around equilibrium points. Average libration periods span about 150 years. Amplitudes vary from 0.6 degrees to 88 degrees averaging 33 degrees. Simulations show some objects follow complex horseshoe trajectories moving between Lagrangian points. No Jupiter trojan currently follows such an orbit though Neptune has one example. The L4 swarm appears slightly more stable than its trailing counterpart according to some models. Observational bias likely explains why counts differ between the two groups since brightest objects show little variation.
Two primary theories explain how Jupiter trojans formed and evolved over time. One model suggests they accreted alongside Jupiter during its initial growth phase lasting roughly 10,000 years. About half of remaining planetesimals became trapped through this efficient capture mechanism. Problems arise because predicted numbers exceed observed populations by four orders of magnitude. Current inclinations also contradict predictions from simple capture models. Simulations show no Saturnian trojans exist despite expectations if this theory held true. Another approach proposes capture occurred later during giant planet migration described by the Nice model. Instability arose thousands of years after Solar System formation when Jupiter and Saturn crossed their 1:2 mean-motion resonance. Uranus and Neptune scattered outward disrupting the primordial Kuiper belt and throwing millions inward. New objects entered unstable regions and were captured as Jupiter and Saturn separated. This process allows wide inclination ranges resulting from multiple planetary encounters. Revised versions suggest ice giants like Uranus or Neptune could be scattered inward onto Jupiter-crossing orbits before being ejected
again. Such events cause existing trojans to escape while new ones with similar semi-major axes get captured.
NASA selected Lucy as one of two Discovery Program missions on the 4th of January 2017. The spacecraft launched the 16th of October 2021 and will reach the L4 Trojan cloud in 2027 following Earth gravity assists. It visits a main-belt asteroid en route before returning near Earth for another assist. The mission plans to explore seven different Jupiter trojans total. First stops include 588 Achilles at the leading point. Later targets involve 617 Patroclus within the trailing swarm. Scientists aim to study binary systems and measure densities directly. Data collection helps clarify whether these objects resemble comets or Kuiper belt bodies more closely. Surface composition analysis seeks answers about water ice presence and organic materials. Rotational characteristics provide clues regarding collisional history compared to other asteroids. The mission timeline extends through multiple flybys designed to maximize scientific return before concluding operations.
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Common questions
Who discovered the first Jupiter trojan and when?
German astronomer Max Wolf spotted the first Jupiter trojan in February 1906. He named this asteroid 588 Achilles and found it at the L4 Lagrangian point of the Sun-Jupiter system.
How many Jupiter trojans are known as of June 2025?
Current records list 9,694 trojans at the leading point and 5,628 trailing behind. As of June 2025 twenty-four Jupiter trojans have received athlete names under new naming rules.
What is the largest known Jupiter trojan and how wide is it?
The largest known trojan measures 203 kilometers wide and carries the name 624 Hektor. Few large objects exist compared to the vast population of smaller ones.
When did NASA launch the Lucy mission to study Jupiter trojans?
NASA selected Lucy as one of two Discovery Program missions on the 4th of January 2017. The spacecraft launched the 16th of October 2021 and will reach the L4 Trojan cloud in 2027 following Earth gravity assists.
Where do Jupiter trojans orbit relative to the Sun and Jupiter?
Jupiter trojans orbit between 5.05 and 5.35 astronomical units from the Sun with an average semi-major axis near 5.2 AU. Each swarm stretches roughly 26 degrees along Jupiter's path covering about 2.5 AU total distance.