Earth trojan
An Earth trojan is an asteroid that orbits the Sun in the vicinity of the Earth–Sun Lagrangian points L4 (leading 60°) or L5 (trailing 60°), thus having an orbit similar to Earth's. Only two Earth trojans have so far been discovered. The name "trojan" was first used in 1906 for the Jupiter trojans, the asteroids that were observed near the Lagrangian points of Jupiter's orbit.
Members
L4 (leading)
- (706765) 2010 TK7: A 300-metre diameter asteroid, discovered using the Wide-field Infrared Survey Explorer (WISE) satellite in January 2010.[1][2][3]
- (614689) 2020 XL5: Discovered by the Pan-STARRS survey in December 2020 and later recognised as an Earth trojan in January 2021.[4] It is 1.2 km in diameter.
L5 (trailing)
- No known objects are currently thought to be L5 trojans of Earth.
Searches
An Earth-based search for L5 objects was conducted in 1994, covering 0.35 square degrees of sky, under poor observing conditions.[5] That search failed to detect any objects:
- "The limiting sensitivity of this search was magnitude ~22.8, corresponding to C-type asteroids ~350 m in diameter, or S-type asteroids ~175 m in diameter."[5]
In February 2017, the OSIRIS-REx spacecraft performed a search from within the L4 region on its way to asteroid Bennu.[6] No additional Earth trojans were discovered.[7]
In April 2017, the Hayabusa2 spacecraft searched the L5 region while proceeding to asteroid Ryugu,[8] but did not find any asteroids there.[9]
Significance
The orbits of any Earth trojans could make them less energetically costly to reach than the Moon, even though they will be hundreds of times more distant. Such asteroids could one day be useful as sources of elements that are rare near Earth's surface. On Earth, siderophiles such as iridium are difficult to find, having largely sunk to the core of the planet shortly after its formation.
A small asteroid could be a rich source of such elements even if its overall composition is similar to Earth's; because of their small size, such bodies would lose heat much more rapidly than a planet once they had formed, and so would not have melted, a prerequisite for differentiation (even if they differentiated, the core would still be within reach). Their weak gravitational fields also would have inhibited significant separation of denser and lighter material; a mass the size of 2010 TK7 would exert a surface gravitational force of less than 0.00005 times that of Earth (although the asteroid's rotation could cause separation).
Giant-impact hypothesis
A hypothetical planet-sized Earth trojan the size of Mars, given the name Theia, is thought by proponents of the giant-impact hypothesis to be the origin of the Moon. The hypothesis states that the Moon formed after Earth and Theia collided,[10] showering material from the two planets into space. This material eventually accreted around Earth and into a single orbiting body, the Moon.[11]
At the same time, material from Theia mixed and combined with Earth's mantle and core. Supporters of the giant-impact hypothesis theorise that Earth's large core in relation to its overall volume is as a result of this combination.
Continuing interest in near-Earth asteroids
Astronomy continues to retain interest in the subject. A publication[12] describes these reasons thus:
The survival to the present day of an ancient [Earth Trojan] population is reasonably assured provided Earth's orbit itself was not strongly perturbed since its formation. It is therefore pertinent to consider that 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.
Such chaotic evolution may at first sight appear unfavorable to the survival of a primordial population of [Earth trojans]. However, during and after the chaotic assembly of the terrestrial planets, it is likely that a residual planetesimal population, of a few percent of Earth's mass, was present and helped to damp the orbital eccentricities and inclinations of the terrestrial planets to their observed low values, as well as to provide the so-called "late veneer" of accreting planetesimals to account for the abundance patterns of the highly siderophile elements in Earth's mantle.
Such a residual planetesimal population would also naturally lead to a small fraction trapped in the 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 NEOs that originate from more distal reservoirs of small bodies in the solar system like the main asteroid belt.
Other companions of Earth
Several other small objects have been found on an orbital path associated with Earth. Although these objects are in 1:1 orbital resonance, they are not Earth trojans, because they do not librate around a definite Sun–Earth Lagrangian point, neither L4 nor L5.
Earth has another noted companion, asteroid 3753 Cruithne. About 5 km across, it has a peculiar type of orbital resonance called an overlapping horseshoe, and is probably only a temporary liaison.[13]
469219 Kamoʻoalewa, an asteroid discovered on 27 April 2016, is possibly the most stable quasi-satellite of Earth.[14]
Name | Eccentricity | Diameter (m) | Discoverer | Date of Discovery | Type | Current Type |
---|---|---|---|---|---|---|
Moon | 0.055 | 3474800 | ? | Prehistory | Natural satellite | Natural satellite |
1913 Great Meteor Procession | ? | ? | ? | 1913-02-09 | Possible Temporary satellite | Destroyed |
3753 Cruithne | 0.515 | 5000 | Duncan Waldron | 1986-10-10 | Quasi-satellite | Horseshoe orbit |
1991 VG | 0.053 | 5–12 | Spacewatch | 1991-11-06 | Temporary satellite | Apollo asteroid |
(85770) 1998 UP1 | 0.345 | 210–470 | Lincoln Lab's ETS | 1998-10-18 | Horseshoe orbit | Horseshoe orbit |
54509 YORP | 0.230 | 124 | Lincoln Lab's ETS | 2000-08-03 | Horseshoe orbit | Horseshoe orbit |
2001 GO2 | 0.168 | 35–85 | Lincoln Lab's ETS | 2001-04-13 | Possible Horseshoe orbit | Possible Horseshoe orbit |
2002 AA29 | 0.013 | 20–100 | LINEAR | 2002-01-09 | Quasi-satellite | Horseshoe orbit |
2003 YN107 | 0.014 | 10–30 | LINEAR | 2003-12-20 | Quasi-satellite | Horseshoe orbit |
(164207) 2004 GU9 | 0.136 | 160–360 | LINEAR | 2004-04-13 | Quasi-satellite | Quasi-satellite |
(277810) 2006 FV35 | 0.377 | 140–320 | Spacewatch | 2006-03-29 | Quasi-satellite | Quasi-satellite |
2006 JY26 | 0.083 | 6–13 | Catalina Sky Survey | 2006-05-06 | Horseshoe orbit | Horseshoe orbit |
2006 RH120 | 0.024 | 2–3 | Catalina Sky Survey | 2006-09-13 | Temporary satellite | Apollo asteroid |
(419624) 2010 SO16 | 0.075 | 357 | WISE | 2010-09-17 | Horseshoe orbit | Horseshoe orbit |
(706765) 2010 TK7 | 0.191 | 150–500 | WISE | 2010-10-01 | Earth trojan | Earth trojan |
2013 BS45 | 0.083 | 20–40 | Spacewatch | 2010-01-20 | Horseshoe orbit | Horseshoe orbit |
2013 LX28 | 0.452 | 130–300 | Pan-STARRS | 2013-06-12 | Quasi-satellite temporary | Quasi-satellite temporary |
2014 OL339 | 0.461 | 70–160 | EURONEAR | 2014-07-29 | Quasi-satellite temporary | Quasi-satellite temporary |
2015 SO2 | 0.108 | 50–110 | Črni Vrh Observatory | 2015-09-21 | Quasi-satellite | Horseshoe orbit temporary |
2015 XX169 | 0.184 | 9–22 | Mount Lemmon Survey | 2015-12-09 | Horseshoe orbit temporary | Horseshoe orbit temporary |
2015 YA | 0.279 | 9–22 | Catalina Sky Survey | 2015-12-16 | Horseshoe orbit temporary | Horseshoe orbit temporary |
2015 YQ1 | 0.404 | 7–16 | Mount Lemmon Survey | 2015-12-19 | Horseshoe orbit temporary | Horseshoe orbit temporary |
469219 Kamoʻoalewa | 0.104 | 40-100 | Pan-STARRS | 2016-04-27 | Quasi-satellite stable | Quasi-satellite stable |
DN16082203 | ? | ? | ? | 2016-08-22 | Possible Temporary satellite | Destroyed |
2020 CD3 | 0.017 | 1–6 | Mount Lemmon Survey | 2020-02-15 | Temporary satellite | Temporary satellite |
2020 PN1 | 0.127 | 10–50 | ATLAS-HKO | 2020-08-12 | Horseshoe orbit temporary | Horseshoe orbit temporary |
2020 PP1 | 0.074 | 10–20 | Pan-STARRS | 2020-08-12 | Quasi-satellite stable | Quasi-satellite stable |
(614689) 2020 XL5 | 0.387 | 1100-1260 | Pan-STARRS | 2020-12-12 | Earth trojan | Earth trojan |
2022 NX1 | 0.025 | 5-15 | Moonbase South Observatory | 2020-07-02 | Temporary satellite | Apollo asteroid |
2023 FW13 | 0.177 | 10-20 | Pan-STARRS | 2023-03-28 | Quasi-satellite | Quasi-satellite |
Gallery
- Illustration of Trojan Asteroid 2020 XL5[15]
See also
References
- ^ Reilly, M. (27 July 2011). "Earth stalker found in eternal twilight". New Scientist. Retrieved 2014-02-21.
- ^ Choi, C.Q. (27 July 2011). "First asteroid companion of Earth discovered at last". Space.com. Retrieved 2011-07-27.
- ^ "OSIRIS-REx searches for Earth-trojan asteroids" (Press release). NASA. 9 February 2017.
- ^ Hui, Man-To; Wiegert, Paul A.; Tholen, David J.; Föhring, Dora (November 2021). "The second Earth trojan 2020 XL5". The Astrophysical Journal Letters. 922 (2): L25. arXiv:2111.05058. Bibcode:2021ApJ...922L..25H. doi:10.3847/2041-8213/ac37bf. S2CID 243860678.
- ^ a b Whiteley, Robert J.; Tholen, David J. (1998). "CCD search for Lagrangian asteroids of the Earth–Sun system". Icarus. 136 (1): 154–167. Bibcode:1998Icar..136..154W. doi:10.1006/icar.1998.5995. article no. IS985995A. Received 24 November 1997; revised 13 April 1998.
- ^ "NASA mission to search for rare asteroids" (Press release). NASA. Retrieved 2017-03-01.
- ^ "OSIRIS-REx asteroid search tests instruments". NASA. Retrieved 2017-03-24.
- ^ "太陽−地球系のL5点付近の観測について". JAXA. 2017-04-11. Retrieved 2017-04-18.
- ^ Mission status of Hayabusa2 (PDF). 49th Lunar and Planetary Science Conference 2018. Retrieved 2018-08-10.
- ^ Knapton, Sarah (29 January 2016). "Earth is actually two planets, scientists conclude". The Telegraph.
- ^ "The Theia hypothesis: New evidence emerges that Earth and Moon were once the same". The Daily Galaxy. 2007-07-05. Retrieved 2013-11-13.
- ^ Malhotra, Renu (February 18, 2019). "The case for a deep search for Earth's Trojan asteroids". Nature Astronomy. 3 (3): 193–194. arXiv:1903.01922. Bibcode:2019NatAs...3..193M. doi:10.1038/s41550-019-0697-z. S2CID 119333756.
- ^ Murray, C. (1997). "The Earth's secret companion". Nature. 387 (6634): 651–652. Bibcode:1997Natur.387..651M. doi:10.1038/42585.
- ^ Agle, D.C.; Brown, Dwayne; Cantillo, Laurie (15 June 2016). "Small asteroid is Earth's constant companion". NASA / JPL. Retrieved 15 June 2016.
- ^ "Data from NSF's NOIRLab Show Earth Trojan Asteroid Is the Largest Found". Retrieved 27 January 2023.