page_title: Jupiter trojan
section_title: Numbers and mass
hierarchical_section_title: Jupiter trojan / Numbers and mass
caption_attribution_description: A (not drawn to scale!) contour plot of the effective potential of a two-body system. (the Sun and Earth here), showing the 5 Lagrange points. An object orbiting in free-fall could trace out a contour (such as the Moon, shown).
context_page_description: The Jupiter trojans, commonly called Trojan asteroids or simply Trojans, are a large group of asteroids that share the planet Jupiter's orbit around the Sun. Relative to Jupiter, each Trojan librates around one of Jupiter's stable Lagrange points: either L₄, lying 60° ahead of the planet in its orbit, or L₅, 60° behind. Jupiter trojans are distributed in two elongated, curved regions around these Lagrangian points with an average semi-major axis of about 5.2 AU.
  The first Jupiter trojan discovered, 588 Achilles, was spotted in 1906 by German astronomer Max Wolf. A total of 7,040 Jupiter trojans have been found as of October 2018. By convention, they are each named from Greek mythology after a figure of the Trojan War, hence the name "Trojan". The total number of Jupiter trojans larger than 1 km in diameter is believed to be about 1 million, approximately equal to the number of asteroids larger than 1 km in the asteroid belt. Like main-belt asteroids, Jupiter trojans form families.
  As of 2004, many Jupiter trojans showed to observational instruments as dark bodies with reddish, featureless spectra.
context_section_description: Estimates of the total number of Jupiter trojans are based on deep surveys of limited areas of the sky. The L₄ swarm is believed to hold between 160,000–240,000 asteroids with diameters larger than 2 km and about 600,000 with diameters larger than 1 km. If the L₅ swarm contains a comparable number of objects, there are more than 1 million Jupiter trojans 1 km in size or larger. For the objects brighter than absolute magnitude 9.0 the population is probably complete. These numbers are similar to that of comparable asteroids in the asteroid belt. The total mass of the Jupiter trojans is estimated at 0.0001 of the mass of Earth or one-fifth of the mass of the asteroid belt.
  Two more recent studies indicate that the above numbers may overestimate the number of Jupiter trojans by several-fold. This overestimate is caused by (1) the assumption that all Jupiter trojans have a low albedo of about 0.04, whereas small bodies may have an average albedo as high as 0.12; (2) an incorrect assumption about the distribution of Jupiter trojans in the sky. According to the new estimates, the total number of Jupiter trojans with a diameter larger than 2 km is 6,300 ± 1,000 and 3,400 ± 500 in the L4 and L5 swarms, respectively. These numbers would be reduced by a factor of 2 if small Jupiter trojans are more reflective than large ones.
  The number of Jupiter trojans observed in the L₄ swarm is slightly larger than that observed in L₅. Because the brightest Jupiter trojans show little variation in numbers between the two populations, this disparity is probably due to observational bias. Some models indicate that the L₄ swarm may be slightly more stable than the L₅ swarm.
  The largest Jupiter trojan is 624 Hektor, which has an mean diameter of 203 ± 3.6 km. There are few large Jupiter trojans in comparison to the overall population. With decreasing size, the number of Jupiter trojans grows very quickly down to 84 km, much more so than in the asteroid belt. A diameter of 84 km corresponds to an absolute magnitude of 9.5, assuming an albedo of 0.04. Within the 4.4–40 km range the Jupiter trojans' size distribution resembles that of the main-belt asteroids. Nothing is known about the masses of the smaller Jupiter trojans. The size distribution suggests that the smaller Trojans may be the products of collisions by larger Jupiter trojans.
caption: A gravitational potential contour plot showing Earth's Lagrangian points; L4 and L5 are above and below the planet, respectively. Jupiter's Lagrangian points are similarly situated in its much larger orbit.
exif: {}