Earth trojan

Lore & Background

The first Earth trojan discovered was (706765) 2010 TK7, a 300-metre diameter asteroid found using the Wide-field Infrared Survey Explorer (WISE) satellite on 1 October 2010. The second, (614689) 2020 XL5, was discovered by the Pan-STARRS survey on 12 December 2020 and later recognised as an Earth trojan in January 2021; it is 1.2 km in diameter. No known objects are currently thought to be L5 trojans of Earth.

Searches for additional Earth trojans have been conducted. An Earth-based search for L5 objects in 1994 covered 0.35 square degrees of sky under poor conditions and failed to detect any objects, with a limiting sensitivity of magnitude ~22.8, corresponding to C-type asteroids ~350 m in diameter or S-type asteroids ~175 m in diameter. In February 2017, the OSIRIS-REx spacecraft searched the L4 region on its way to asteroid Bennu, finding no additional Earth trojans. In April 2017, the Hayabusa2 spacecraft searched the L5 region while proceeding to asteroid Ryugu, but did not find any asteroids there.

The giant-impact hypothesis proposes a hypothetical planet-sized Earth trojan the size of Mars, named Theia, as the origin of the Moon. The hypothesis states that the Moon formed after Earth and Theia collided, showering material into space that eventually accreted around Earth. Supporters theorise that Earth's large core relative to its volume results from this combination.

Reader's Guide

The survival to the present day of an ancient Earth trojan population is considered reasonably assured provided Earth's orbit was not strongly perturbed since its formation. Modern theoretical models of planet formation find strongly chaotic orbital evolution during the final stages of assembly of the terrestrial planets and the Earth–Moon system. However, during and after this chaotic assembly, a residual planetesimal population of a few percent of Earth's mass likely helped damp orbital eccentricities and inclinations to their observed low values, and provided a 'late veneer' of accreting planetesimals to account for abundance patterns of highly siderophile elements in Earth's mantle. Such a residual population would naturally lead to a small fraction trapped in Earth's Trojan zones as Earth's orbit circularized. In addition to potentially hosting an ancient, long-term stable population of asteroids, Earth's Trojan regions also provide transient traps for near-Earth objects originating from more distal reservoirs like the main asteroid belt. Several other small objects, such as 3753 Cruithne and 469219 Kamoʻoalewa, have been found on orbital paths associated with Earth but are not Earth trojans because they do not librate around a definite Sun–Earth Lagrangian point.

Did You Know?

• Only two Earth trojans have been discovered, both at the L4 Lagrange point.
• The first discovered Earth trojan, 2010 TK7, is about 300 metres in diameter.
• The second Earth trojan, 2020 XL5, is 1.2 km in diameter and was recognised in January 2021.
• No L5 Earth trojans are currently known.

The Hierarchy of Mass and Size

The asteroid population is dominated at its upper end by a single object. Ceres carries roughly one-third of the entire belt's estimated mass, making it more massive than the next fifteen largest bodies combined. The total mass of the belt is generally taken to be about 2.39×10²¹ kilograms, a figure that anchors all proportional comparisons. Beyond the top four entries, however, the ranking becomes genuinely uncertain because astrometric estimates overlap considerably. Masses are typically inferred from the gravitational tugs a body exerts on neighboring asteroids, though a few exceptions exist. The spacecraft Dawn provided direct mass measurements for Ceres and Vesta, while the presence of natural satellites allows precise calculations for bodies such as Sylvia, Camilla, and Elektra. In terms of sheer size, infrared surveys from the IRAS mission identified approximately 140 main-belt asteroids exceeding 120 kilometers in diameter, a threshold that roughly marks the boundary between primordial survivors and collisional fragments. The inner belt, defined as the region inside the 3:1 Kirkwood gap at 2.50 astronomical units, hosts very few of these giants—only Vesta, Fortuna, Hebe, Iris, and Metis qualify.

Visibility and the Albedo Gradient

From the surface of Earth, the asteroid belt is almost entirely invisible to the unaided eye. Vesta stands alone as the only asteroid that regularly achieves naked-eye brightness. Under exceptionally dark skies and at rare perihelic oppositions, a sharp observer might glimpse Ceres, Pallas, or Iris, but these are fleeting opportunities. A useful benchmark for brightness is the +8.3 apparent magnitude reached by Saturn's moon Titan at its peak, a body discovered 145 years before the first asteroid was found. Several asteroids can match or exceed that threshold, yet none from the outer belt ever do. Hygiea and Interamnia, for instance, seldom surpass magnitude 10. The reason lies in the distribution of surface compositions: the brightest, highest-albedo asteroids cluster closer to Mars's orbit, while the darker C-type and D-type bodies dominate the outer regions. Objects with very high orbital eccentricity only flash bright when their perihelion aligns closely with a heliocentric conjunction with Earth, or when they make an exceptionally close pass by our planet. Apophis, for example, typically sits at a dim magnitude of 20 to 22 but will briefly achieve its peak brightness on April 13, 2029.

The Spin Spectrum: From Languid to Breakneck

The rotation periods of minor planets span an astonishing range. The vast majority spin somewhere between two and twenty hours, a pace set by the balance between gravitational cohesion and centrifugal stress. At the slow end, a handful of bodies take at least a thousand hours—over forty-one days—to complete a single turn, making them the most languid spinners in the catalog. At the opposite extreme, the fastest rotators complete a revolution in under one hundred seconds. Strikingly, every one of these ultra-rapid spinners is an unnumbered near-Earth object with a diameter below one hundred meters, too small to have received a permanent designation. Among the numbered asteroids with a confidently determined period, the record holders include 2014 EK24, a sixty-meter stony near-Earth object spinning in 352 seconds, and two main-belt residents: 2005 UW163 at 0.86 kilometers with a 1.29-hour period, and 2000 GD65 at 2.25 kilometers with a 1.95-hour period. Bodies whose periods carry a quality rating below two are flagged as uncertain, reminding us that even a seemingly simple measurement like spin rate can be elusive for small, irregularly shaped rocks.

The Rarity of Retrograde Orbits

Retrograde motion—orbiting in the opposite direction to the Sun's rotation—remains the rarest orbital configuration among minor planets. With orbital inclinations at or beyond 90 degrees (the theoretical maximum being 180 degrees), these objects trace paths that seem to defy the dominant angular momentum of the Solar System. As of March 2018, only 99 such bodies had been identified among the roughly 800,000 known minor planets, a fraction of just 0.01 percent. By contrast, more than 2,000 comets follow retrograde trajectories, making asteroids in this category the most exclusive group in the entire minor-planet census. The high-inclination asteroids that do exist tend to fall into two families: Mars-crossers that may be in the process of being ejected from the inner Solar System, and damocloids, a class of objects with highly inclined and eccentric orbits. A subset of these is thought to be temporarily captured in retrograde resonances with the gas giants, lending them a transient, unstable character that distinguishes them from the vast population of prograde, low-inclination asteroids that fill the rest of the catalog.