10:45 〜 12:15
[MZZ45-P06] Overview of Earth Trojan Asteroids
キーワード：地球近傍小惑星、トロヤ群小惑星、鉱物資源探査
This paper provides an overview of near Earth asteroids (NEAs), especially Earth Trojan asteroids (ETAs). Trojan asteroids are known as objects orbiting around the point of an equilateral triangle consisting of the Sun, a planet and the asteroids, and such positions are called the L4 (60° ahead) and L5 (60° behind) Lagrangian points. J. L. Lagrange theoretically expected the existence of such asteroids in 1772 [1]. The first Trojan asteroid was found around the Sun-Jupiter system in 1906 and the asteroid 588 Achilles is orbiting around L4 Lagrangian point. As of February 23, 2023, we have known 12,403 Trojan asteroids around the Lagrangian points of Earth, Mars, Jupiter, Uranus, and Neptune [2].
NEAs have been searched for a long time and the total number of known NEAs is 31,320 [3]. Classifying these by size, the estimated number of NEAs with diameters of 0-30m, 30-100m, 100-300m, 300-1000m, >1000m are 8,446, 10,314, 6,937, 4,767, and 856, respectively, indicating that the smallest group is difficult to find, while the largest group is almost to be discovered.
Two ETAs orbiting around L4 Lagrangian point have been found, whose provisional designation are 2010 TK7 and 2020 XL5. 2010 TK7 was discovered by NEOWISE mission in 2010 [4] and the semi-major axis a, the eccentricity e, and the inclination i are 1.001 au 0.387, and 13.8°, respectively. The absolute magnitude is observed as 20.78, the geometric albedo is estimated at 0.059, and therefore the diameter is calculated to be 379m. On the other hand, 2020 XL5 was discovered by Pan-STARRS 1 at Haleakala in 2020 [5] and the values of a, e, i are 0.999, 0.190, and 20.9°, respectively. The absolute magnitude is 20.29, and if the geometric albedo is the same as 2010 TK7 and the diameter is estimated to be 480m.
Observing ETAs, the L4 and L5 Lagrangian points of the Sun-Earth system is closer to the direction of the Sun, and it makes difficult for us to search them. As a test, if we plot the daily apparent expected positions of 2010 TK7 and 2020 XL5 at 5am JST in the Tokyo sky, we can see that 2010 TK7 is distributed in an ellipse and 2020 XL5 in a triangle shape between east and south. Since it is dark at 5am in winter and blight in summer, we understand that there are very few opportunities to observe ETAs compared to other Lagrangian point surveys.
We obtained daily geocentric distances for two ETAs from January 1, 2023 and January 1, 2030 [6], and the plots show that the maximum and minimum ranges approximately are between 0.3 au and 0.9 au for 2010 TK7 and 0.5 au and 1.9 au for 2020 XL5. Due to the large minimum geocentric distance, it is unlikely that the two ETAs will have the opportunity to come close to the Earth, so it is not easy to explore the ETAs by a spacecraft at present. On the other hand, ETAs are expected to be mineral resources, and widespread and continuous survey observations may lead to the discovery of ETAs that can be explored by a spacecraft.
[1] e.g., F. R. Moulton 1942, An Introduction to Celestial Mechanics, Second revised edition , 277.
[2] IAU Minor Planet Center, https://www.minorplanetcenter.net/iau/lists/Trojans.html .
[3] NASA JPL CNEOS, https://cneos.jpl.nasa.gov/ .
[4] Connors et al. 2011, Nature 475, 481.
[5] Santana-Ros et al. 2022, Nature Communications 13, 447.
[6] NASA JPL Horizons System, https://ssd.jpl.nasa.gov/horizons/app.html#/ .
NEAs have been searched for a long time and the total number of known NEAs is 31,320 [3]. Classifying these by size, the estimated number of NEAs with diameters of 0-30m, 30-100m, 100-300m, 300-1000m, >1000m are 8,446, 10,314, 6,937, 4,767, and 856, respectively, indicating that the smallest group is difficult to find, while the largest group is almost to be discovered.
Two ETAs orbiting around L4 Lagrangian point have been found, whose provisional designation are 2010 TK7 and 2020 XL5. 2010 TK7 was discovered by NEOWISE mission in 2010 [4] and the semi-major axis a, the eccentricity e, and the inclination i are 1.001 au 0.387, and 13.8°, respectively. The absolute magnitude is observed as 20.78, the geometric albedo is estimated at 0.059, and therefore the diameter is calculated to be 379m. On the other hand, 2020 XL5 was discovered by Pan-STARRS 1 at Haleakala in 2020 [5] and the values of a, e, i are 0.999, 0.190, and 20.9°, respectively. The absolute magnitude is 20.29, and if the geometric albedo is the same as 2010 TK7 and the diameter is estimated to be 480m.
Observing ETAs, the L4 and L5 Lagrangian points of the Sun-Earth system is closer to the direction of the Sun, and it makes difficult for us to search them. As a test, if we plot the daily apparent expected positions of 2010 TK7 and 2020 XL5 at 5am JST in the Tokyo sky, we can see that 2010 TK7 is distributed in an ellipse and 2020 XL5 in a triangle shape between east and south. Since it is dark at 5am in winter and blight in summer, we understand that there are very few opportunities to observe ETAs compared to other Lagrangian point surveys.
We obtained daily geocentric distances for two ETAs from January 1, 2023 and January 1, 2030 [6], and the plots show that the maximum and minimum ranges approximately are between 0.3 au and 0.9 au for 2010 TK7 and 0.5 au and 1.9 au for 2020 XL5. Due to the large minimum geocentric distance, it is unlikely that the two ETAs will have the opportunity to come close to the Earth, so it is not easy to explore the ETAs by a spacecraft at present. On the other hand, ETAs are expected to be mineral resources, and widespread and continuous survey observations may lead to the discovery of ETAs that can be explored by a spacecraft.
[1] e.g., F. R. Moulton 1942, An Introduction to Celestial Mechanics, Second revised edition , 277.
[2] IAU Minor Planet Center, https://www.minorplanetcenter.net/iau/lists/Trojans.html .
[3] NASA JPL CNEOS, https://cneos.jpl.nasa.gov/ .
[4] Connors et al. 2011, Nature 475, 481.
[5] Santana-Ros et al. 2022, Nature Communications 13, 447.
[6] NASA JPL Horizons System, https://ssd.jpl.nasa.gov/horizons/app.html#/ .