When it comes to sharing its orbit with asteroids, Earth lags behind many of its fellow solar system planets. Trojan asteroids – bodies accompanying Earth along its path around the Sun – have been known to exist for decades alongside other planets. Mars has 9 known Trojan asteroids, Neptune 28 and Jupiter over 7 000. However, Earth’s one and only known Trojan was found in 2011. One and only, that is, until 2020, when astronomers discovered our planet’s second Trojan asteroid, 2020 XL5. Their findings were published in the journal ‘Nature Communications’. The newly identified Trojan is over 1 km in diameter, larger than the first-known Earth Trojan asteroid, 2010 TK7, whose diameter is roughly 300 m. It is a common C-type asteroid, carbon-rich and with an almost coal-black surface. The new object was first spotted in December 2020 in a Pan-STARRS sky survey. Partly supported by the EU-funded NEO-MAPP project, the research team then sought to confirm that it was in fact an Earth Trojan. To study 2020 XL5’s orbit, they used archival data from 2012 to 2019 and observed the object from three ground-based observatories in 2021. Asteroid 2020 XL5 is currently in a stable orbit around the Sun, at Sun-Earth Lagrange point 4 (L4). A Lagrange point is a point in space at which a small body, when influenced by the gravitational forces of two large bodies, remains at rest relative to them. Although there exist five theoretical Lagrange points in each system of two large bodies, only two – L4 and L5 – are stable, retaining small bodies despite minor perturbations by outside gravitational forces. Because of this stability, asteroids tend to accumulate there. According to the study, 2020 XL5 will remain in L4 for at least 4 000 years.
Not easily observable
Trojan asteroids are hard to spot. The reason for this difficulty “is related to the unfavorable viewing geometry of an object orbiting Earth-Sun’s L4 or L5 points as seen from our planet,” write the study authors. In other words, such asteroids are visible very close to the Sun and under large phase angles, meaning that a big part of the object is shadowed – and therefore faint – as seen from Earth. “Under such geometries, observations must be acquired at high airmass, where seeing is typically worse, which, together with higher background from zodiacal light, further increases the difficulty of these searches,” the study explains. To find 2020 XL5, the Southern Astrophysical Research Telescope in Chile – one of the 3 telescopes observing the asteroid – pointed just 15 degrees from the horizon, peering through a thicker blanket of Earth’s atmosphere to spot it than if the object had been overhead. Under these challenging conditions, the Trojan was observable only a few minutes before dawn. “If we are able to discover more Earth Trojans, and if some of them can have orbits with lower inclinations, they might become cheaper to reach than our moon. So they might become ideal bases for an advanced exploration of the solar system, or they could even be a source of resources,” remarks study co-author Dr Cesar Briceño of United States-based National Optical-Infrared Astronomy Research Laboratory in an article posted on ‘EarthSky’. The NEO-MAPP (Near Earth Object Modelling and Payloads for Protection) project ends in 2023. For more information, please see: NEO-MAPP project website
NEO-MAPP, asteroid, Earth, Sun, Trojan asteroid, 2020 XL5, orbit, Lagrange point