# (614689) 2020 XL5

(614689) 2020 XL$_5$ (provisional designation 2020 XL$_5$) is a near-Earth asteroid and Earth trojan discovered by the Pan-STARRS 1 survey at Haleakala Observatory, Hawaii on 12 December 2020. It oscillates around the Sun–Earth L$_4$ Lagrangian point (leading 60°), one of the dynamically stable locations where the combined gravitational force acts through the Sun's and Earth's barycenter. Analysis of 2020 XL$_5$'s trojan orbit stability suggests it will remain around Earth's L$_4$ point for at least four thousand years until gravitational perturbations from repeated close encounters with Venus destabilize its trojan configuration. With a diameter about 1.2 km (0.75 mi), 2020 XL$_5$ is the second Earth trojan discovered, after (706765) 2010 TK7, and is the largest of its kind known.

## Discovery

2020 XL$_5$ was discovered by the Pan-STARRS 1 survey at Haleakala Observatory, Hawaii on 12 December 2020. It was first observed in the constellation Crater at an apparent magnitude of 21.4. The asteroid was moving at an on-sky rate of 3.02 arcseconds per minute, from a distance of 0.68 AU (102 million km; 63 million mi) from Earth.
The asteroid was subsequently listed on the Minor Planet Center's Near-Earth Object Confirmation Page (NEOCP) as P11aRcq. Over two days, follow-up observations were carried out by the Višnjan Observatory (L01), ESA Optical Ground Station (J04), and Cerro Tololo Observatory (807). The asteroid was identified in earlier Mount Lemmon Survey (G96) observations from 26 November 2020. The listing was confirmed and publicly announced as 2020 XL$_5$ on 14 December 2020.

## Name and numbering

This minor planet was given the permanent number 614689 by the Minor Planet Center on 28 March 2022 and is now eligible for naming. In accordance with the International Astronomical Union's naming conventions for near-Earth objects, 2020 XL$_5$ will be given a mythological name.

## Orbit and classification

The orbit of 2020 XL$_5$ is well known with an uncertainty parameter of 0 and a long observation arc over 8 years. The asteroid has been identified in several precovery observations by various sky surveys, including Pan-STARRS, from dates as far back as December 2012.
2020 XL$_5$ orbits the Sun at an average distance of 1.001 AU once every 365.8 days, or approximately 1 Earth year. Its orbit has a high eccentricity of 0.388 and an inclination of 13.8° with respect to the ecliptic plane. Over the course of its orbit, its distance from the Sun ranges from 0.61 AU at perihelion to 1.39 AU at aphelion, crossing the orbits of Venus and Earth. Since its orbit crosses Earth's while having a semi-major axis greater than 1 AU (by a small margin), 2020 XL$_5$ is classified as an Apollo asteroid.

### Trojan orbit

Trojan objects are most easily conceived as orbiting at a Lagrangian point, a dynamically stable location (where the combined gravitational force acts through the Sun's and Earth's barycenter) 60 degrees ahead of (L$_4$) or behind (L$_5$) a massive orbiting body, in a type of 1:1 orbital resonance. In reality, they oscillate around such a point.
On 26 January 2021, amateur astronomer Tony Dunn reported that 2020 XL$_5$'s nominal trajectory appears to be librating about Earth's leading L$_4$ Lagrangian point, suspecting it to be an Earth trojan. Subsequent analysis confirmed modeling stability for at least several thousand years into the future based on existing orbital parameters. This would make 2020 XL$_5$ more stable than the prototype L$_4$ Earth trojan 2010 TK7, which is potentially unstable on timescales of less than 2,000 years. Additional follow-up observations and precoveries confirmed 2020 XL$_5$'s trojan nature, and showed that it will leave trojan orbit at least 4,000 years into the future. Numerical simulations indicate that 2020 XL$_5$ has likely been captured into the L$_4$ Langrangian point since the 15th century.
2020 XL$_5$'s high orbital eccentricity results in wide, tadpole-shaped oscillation paths in a corotating reference frame with Earth and its Lagrangian points. Although the asteroid crosses Venus's orbit with a minimum orbit intersection distance (MOID) of 0.0273 AU (4.1 million km; 2.5 million mi), perturbations by the planet are currently negligible since its nominal orbit brings it either too high or too low from the plane of Venus's orbit. Venus's influence on 2020 XL$_5$'s orbit will become greater over time as their longitudes of the ascending node precess over hundreds of years, lowering 2020 XL$_5$'s Venus MOID and eventually destabilizing its trojan orbit by sending it to Earth's L$_3$ point in several thousand years.

## Physical characteristics

Photometric measurements of optical observations from 2020 to 2021 show that 2020 XL$_5$ exhibits a color resembling that of carbonaceous C-type asteroids. Assuming that 2020 XL$_5$'s phase curve behaves similarly to those of C-type asteroids, the absolute magnitude (H) of the asteroid is 18.6, which corresponds to a mean diameter about 1.18 km (0.73 mi) for a typical C-type asteroid's geometric albedo of 0.06. This makes 2020 XL$_5$ the largest Earth trojan asteroid known to date, being up to three times as large as the 0.3 km (0.19 mi)-sized 2010 TK$_7$.
Because 2020 XL$_5$ is only visible at low altitudes in the sky during twilight, atmospheric distortions and scattered light from the Sun hinder accurate photometry of the asteroid's light curve, thus information about its rotation could not be determined.

## Exploration

Due to 2020 XL$_5$'s high orbital inclination, a rendezvous mission to the asteroid from low Earth orbit (LEO) would require a minimum total delta-v of 10.3 km/s (6.4 mi/s)—too high to be considered an ideal target for a low-energy trajectory. On the other hand, a flyby trajectory to 2020 XL$_5$ from LEO could be more feasible with a minimum total delta-v of 3.3 km/s (2.1 mi/s).