Jupiter trojan
The Jupiter trojans, commonly called trojan asteroids or simply trojans, are a large group of
The first Jupiter trojan discovered, 588 Achilles, was spotted in 1906 by German astronomer Max Wolf.[2] More than 9,800 Jupiter trojans have been found as of May 2021[update].[3] 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,[1] approximately equal to the number of asteroids larger than 1 km in the asteroid belt.[4] Like main-belt asteroids, Jupiter trojans form families.[5]
As of 2004[update], many Jupiter trojans showed to observational instruments as dark bodies with reddish, featureless spectra. No firm evidence of the presence of water, or any other specific compound on their surface has been obtained, but it is thought that they are coated in tholins, organic polymers formed by the Sun's radiation.[6] The Jupiter trojans' densities (as measured by studying binaries or rotational lightcurves) vary from 0.8 to 2.5 g·cm−3.[5] Jupiter trojans are thought to have been captured into their orbits during the early stages of the Solar System's formation or slightly later, during the migration of giant planets.[5]
The term "Trojan Asteroid" specifically refers to the asteroids co-orbital with Jupiter, but the general term "
Observational history
In 1772, Italian-born mathematician
The first accepted discovery of a trojan occurred in February 1906, when astronomer
Nomenclature
The custom of naming all asteroids in Jupiter's L4 and L5 points after famous heroes of the Trojan War was suggested by Johann Palisa of Vienna, who was the first to accurately calculate their orbits.[2]
Asteroids in the leading (L4) orbit are named after Greek heroes (the "Greek node or camp" or "Achilles group"), and those at the trailing (L5) orbit are named after the heroes of Troy (the "Trojan node or camp").[2] The asteroids 617 Patroclus and 624 Hektor were named before the Greece/Troy rule was devised, resulting in a "Greek spy", Patroclus, in the Trojan node and a "Trojan spy", Hector, in the Greek node.[14][17]
Numbers and mass
Estimates of the total number of Jupiter trojans are based on deep surveys of limited areas of the sky.[1] The L4 swarm is believed to hold between 160,000 and 240,000 asteroids with diameters larger than 2 km and about 600,000 with diameters larger than 1 km.[1][11] 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.[15] These numbers are similar to that of comparable asteroids in the asteroid belt.[1] 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.[11]
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;[18] (2) an incorrect assumption about the distribution of Jupiter trojans in the sky.[19] 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.[19] These numbers would be reduced by a factor of 2 if small Jupiter trojans are more reflective than large ones.[18]
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.[5] Some models indicate that the L4 swarm may be slightly more stable than the L5 swarm.[10]
The largest Jupiter trojan is 624 Hektor, which has a mean diameter of 203 ± 3.6 km.[15] 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.[10] The size distribution suggests that the smaller Trojans may be the products of collisions by larger Jupiter trojans.[5]
Trojan | Diameter (km) |
---|---|
624 Hektor | 225 |
617 Patroclus | 140 |
911 Agamemnon | 131 |
588 Achilles | 130 |
3451 Mentor | 126 |
3317 Paris | 119 |
1867 Deiphobus | 118 |
1172 Äneas | 118 |
1437 Diomedes | 118 |
1143 Odysseus | 115 |
Source: JPL Small-Body Database, NEOWISE data
|
Orbits
Jupiter trojans have orbits with radii between 5.05 and 5.35 AU (the mean semi-major axis is 5.2 ± 0.15 AU), and are distributed throughout elongated, curved regions around the two Lagrangian points;
Jupiter trojans do not maintain a fixed separation from Jupiter. They slowly librate around their respective equilibrium points, periodically moving closer to Jupiter or farther from it.
Dynamical families and binaries
Discerning dynamical families within the Jupiter trojan population is more difficult than it is in the asteroid belt, because the Jupiter trojans are locked within a far narrower range of possible positions. This means that clusters tend to overlap and merge with the overall swarm. By 2003 roughly a dozen dynamical families were identified. Jupiter-trojan families are much smaller in size than families in the asteroid belt; the largest identified family, the Menelaus group, consists of only eight members.[5]
In 2001,
Physical properties
Jupiter trojans are dark bodies of irregular shape. Their geometric albedos generally vary between 3 and 10%.[15] The average value is 0.056 ± 0.003 for the objects larger than 57 km,[5] and 0.121 ± 0.003 (R-band) for those smaller than 25 km.[18] The asteroid 4709 Ennomos has the highest albedo (0.18) of all known Jupiter trojans.[15] Little is known about the masses, chemical composition, rotation or other physical properties of the Jupiter trojans.[5]
Rotation
The rotational properties of Jupiter trojans are not well known. Analysis of the rotational
In 2008 a team from
Composition
A team from the
Origin and evolution
Two main theories have emerged to explain the formation and evolution of the Jupiter trojans. The first suggests that the Jupiter trojans formed in the same part of the Solar System as Jupiter and entered their orbits while it was forming.[10] The last stage of Jupiter's formation involved runaway growth of its mass through the accretion of large amounts of hydrogen and helium from the protoplanetary disk; during this growth, which lasted for only about 10,000 years, the mass of Jupiter increased by a factor of ten. The planetesimals that had approximately the same orbits as Jupiter were caught by the increased gravity of the planet.[10] The capture mechanism was very efficient—about 50% of all remaining planetesimals were trapped. This hypothesis has two major problems: the number of trapped bodies exceeds the observed population of Jupiter trojans by four orders of magnitude, and the present Jupiter trojan asteroids have larger orbital inclinations than are predicted by the capture model.[10] Simulations of this scenario show that such a mode of formation also would inhibit the creation of similar trojans for Saturn, and this has been borne out by observation: to date no trojans have been found near Saturn.[28] In a variation of this theory Jupiter captures trojans during its initial growth then migrates as it continues to grow. During Jupiter's migration the orbits of objects in horseshoe orbits are distorted causing the L4 side of these orbits to be over occupied. As a result, an excess of trojans is trapped on the L4 side when the horseshoe orbits shift to tadpole orbits as Jupiter grows. This model also leaves the Jupiter trojan population 3–4 orders of magnitude too large.[29]
The second theory proposes that the Jupiter trojans were captured during the migration of the giant planets described in the Nice model. In the Nice model the orbits of the giant planets became unstable 500–600 million years after the Solar System's formation when Jupiter and Saturn crossed their 1:2 mean-motion resonance. Encounters between planets resulted in Uranus and Neptune being scattered outward into the primordial Kuiper belt, disrupting it and throwing millions of objects inward.[30] When Jupiter and Saturn were near their 1:2 resonance the orbits of pre-existing Jupiter trojans became unstable during a secondary resonance with Jupiter and Saturn. This occurred when the period of the trojans' libration about their Lagrangian point had a 3:1 ratio to the period at which the position where Jupiter passes Saturn circulated relative to its perihelion. This process was also reversible allowing a fraction of the numerous objects scattered inward by Uranus and Neptune to enter this region and be captured as Jupiter's and Saturn's orbits separated. These new trojans had a wide range of inclinations, the result of multiple encounters with the giant planets before being captured.[31] This process can also occur later when Jupiter and Saturn cross weaker resonances.[32]
In a revised version of the Nice model Jupiter trojans are captured when Jupiter encounters an ice giant during the instability. In this version of the Nice model one of the ice giants (Uranus, Neptune, or a lost fifth planet) is scattered inward onto a Jupiter-crossing orbit and is scattered outward by Jupiter causing the orbits of Jupiter and Saturn to quickly separate. When Jupiter's semi-major axis jumps during these encounters existing Jupiter trojans can escape and new objects with semi-major axes similar to Jupiter's new semi-major axis are captured. Following its last encounter the ice giant can pass through one of the libration points and perturb their orbits leaving this libration point depleted relative to the other. After the encounters end some of these Jupiter trojans are lost and others captured when Jupiter and Saturn are near weak mean motion resonances such as the 3:7 resonance via the mechanism of the original Nice model.[32]
The long-term future of the Jupiter trojans is open to question, because multiple weak resonances with Jupiter and Saturn cause them to behave chaotically over time.
Exploration
On 4 January 2017 NASA announced that Lucy was selected as one of their next two Discovery Program missions.[36] Lucy is set to explore seven[37] Jupiter trojans. It was launched on October 16, 2021, and will arrive at the L4 Trojan cloud in 2027 after two Earth gravity assists and a fly-by of a main-belt asteroid. It will then return to the vicinity of Earth for another gravity assist to take it to Jupiter's L5 Trojan cloud where it will visit 617 Patroclus.[38]
See also
- Comet Shoemaker–Levy 9
- List of Jupiter trojans (Greek camp)
- List of Jupiter trojans (Trojan camp)
- List of Jupiter-crossing minor planets
- List of objects at Lagrangian points
Notes
- ^ The three other points—L1, L2 and L3—are unstable.[10]
- ^ The Maxwellian function is , where is the average rotational period, is the dispersion of periods.
References
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- ^ a b "Trojan Minor Planets". Minor Planet Center. Archived from the original on 29 June 2017. Retrieved 14 October 2018.
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- S2CID 35721399. Archived from the original(PDF) on 12 April 2020.
- ^ "NASA's WISE Mission Finds First Trojan Asteroid Sharing Earth's Orbit 27 July 2011". Archived from the original on 2 May 2017. Retrieved 29 July 2011.
- S2CID 205225571.
- ^ a b c d e f g h i j k Marzari, F.; Scholl, H.; Murray C.; Lagerkvist C. (2002). "Origin and Evolution of Trojan Asteroids" (PDF). Asteroids III. Tucson, Arizona: University of Arizona Press. pp. 725–38.
- ^ S2CID 119450236.
- ^ a b Brian G. Marsden (1 October 1999). "The Earliest Observation of a Trojan". Harvard-Smithsonian Center for Astrophysics (CfA). Archived from the original on 14 November 2008. Retrieved 20 January 2009.
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- ^ Bibcode:1938ASPL....3..113W.
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- ^ "List of Jupiter trojans". Minor Planet Center. Archived from the original on 12 June 2018. Retrieved 14 October 2018.
- ^ "Trojan Asteroids". Cosmos. Swinburne University of Technology. Archived from the original on 23 June 2017. Retrieved 13 June 2017.
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- ^ Merline, W. J. (2001). "IAUC 7741: 2001fc; S/2001 (617) 1; C/2001 T1, C/2001 T2". Archived from the original on 19 July 2011. Retrieved 25 October 2010.
- ^ S2CID 4416425.
- ^ "IAUC 8732: S/2006 (624) 1". Archived from the original on 19 July 2011. Retrieved 23 July 2006. (Satellite Discovery)
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- ^ a b c d e Barucci, M.A.; Kruikshank, D.P.; Mottola S.; Lazzarin M. (2002). "Physical Properties of Trojan and Centaur Asteroids". Asteroids III. Tucson, Arizona: University of Arizona Press. pp. 273–87.
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- ^ Northon, Karen (4 January 2017). "NASA Selects Two Missions to Explore the Early Solar System". NASA. Archived from the original on 5 January 2017. Retrieved 5 January 2017.
- ^ "Tour". Lucy Mission Website. NASA. Retrieved 5 October 2021.
- ^ Dreier, Casey; Lakdawalla, Emily (30 September 2015). "NASA announces five Discovery proposals selected for further study". The Planetary Society. Archived from the original on 2 October 2015. Retrieved 1 October 2015.
External links
- "Minor Planet Center's List of Trojan Minor Planets".
- Sheppard, Scott. "The Trojan Page".
- Lykawka, P. S.; Horner (2010). "The Capture of Trojan Asteroids by the Giant Planets During Planetary Migration". S2CID 54084401.
- NASA's WISE Colors in Unknowns on Jupiter Asteroids (NASA 2012-322 : 15 October 2012)
- NASA's New Discovery Missions: Psyche and Lucy on YouTube
- 3D Gravity Simulation of the Ten Largest Jupiter Trojan Asteroids Archived 11 June 2020 at the Wayback Machine