is a sub-kilometer binary asteroid, classified as a near-Earth asteroid and potentially hazardous object of the Apollo group. It was discovered on 27 January 2020 by the Asteroid Terrestrial-impact Last Alert System survey at the Mauna Loa Observatory during its approach to Earth of . The Arecibo Observatory took radar images of on 4 February 2020, which led to the discovery of a small natural satellite orbiting away from the asteroid. With a diameter of around , is among the smallest 10% of known binary asteroids. It was the last binary asteroid discovered by the Arecibo Observatory.

Discovery was discovered on 27 January 2020 by the Asteroid Terrestrial-impact Last Alert System (ATLAS) survey at the Mauna Loa Observatory in Hawaii. The ATLAS survey was designed for detecting near-Earth asteroids on approach to Earth, particularly those that may be considered potentially hazardous under circumstances where they can approach Earth from close distances. was identified by a team of astronomers consisting of Larry Denneau, John Tonry, Aren Heinze, and Henry Weiland, who were chiefly involved in the ATLAS project. The asteroid was discovered during its approach to Earth and was at nominal distance of about from the planet. At the time of discovery, was located in the constellation of Puppis at an apparent magnitude of 17.6.

The discovery of was subsequently reported to the Minor Planet Center's Near-Earth Object Confirmation Page (NEOCP), where a preliminary orbit was calculated from additional observations conducted at multiple observatories. Follow-up observations of spanned three days since its discovery, and the asteroid was formally announced in a Minor Planet Electronic Circular issued by the Minor Planet Center on 30 January 2020.

Nomenclature Upon discovery, the asteroid was given the temporary internal designation A10jUnf. After follow up observations confirming the object, it was then given the provisional designation by the Minor Planet Center on 30 January 2020. The provisional designation signifies the object's discovery date and year. Once had a sufficiently long observation arc for its orbit to be accurately determined, the asteroid received the permanent minor planet number 809875 from the Minor Planet Center, making it eligible for naming.

Orbit and classification orbits the Sun at an average distance of approximately , taking 2.02 years to complete one full orbit. The orbit of is highly eccentric and inclined to the ecliptic plane: it has an orbital eccentricity of 0.757 and inclination of 40 degrees, with its orbit extending from 0.76 AU at perihelion to 2.44 AU at aphelion. As it approaches perihelion, moves above the ecliptic and comes closer to the Sun than Venus, whereas at aphelion, moves below the ecliptic and recedes from the Sun farther out than the orbit of Mars. The orbit of crosses that of Earth; thus it can occasionally make close approaches to Earth, making it a near-Earth object. With a semi-major axis (average orbit distance) greater than 1 AU and a perihelion distance within that of Earth, is formally classified under the Apollo group of near-Earth asteroids.

The asteroid's minimum orbital intersection distance (MOID) from Earth is approximately , or about 0.78 lunar distances. Having such a small Earth MOID, is considered a potentially hazardous asteroid (PHA) by the Minor Planet Center, under the definition that PHAs have Earth MOIDs less than 0.05 AU and absolute magnitudes under 22. Despite this, will not make any close Earth encounters within 15 lunar distances or over the next 200 years, and the asteroid has not been listed by the JPL Sentry Risk Table .

On 12 February 2020, a team of astronomers identified in several precovery images taken by the Pan-STARRS 1 survey, with the earliest images dating back to 5 January 2014. The observation arc of these precovery images spanned over six years, long enough for astronomers to refine and calculate 's orbit with accuracy. This greatly reduced uncertainties in the asteroid's orbit, reducing its uncertainty parameter from 8 to 1. the observation arc of spans 6.09 years or 2,224 days, with an orbit uncertainty parameter of 0 according to the JPL Small-Body Database.

2020 Earth approach On 3 February 2020 at 18:56 UTC, passed , or 11.35 lunar distances, from Earth. During its close approach to Earth, the asteroid approached Earth at a rate of and its apparent visual brightness peaked around magnitude 15.7, which is too faint to be seen with the naked eye. At closest approach the asteroid's apparent motion in the sky was 1.2 degrees per hour, and it was in the constellation of Cetus, with an apparent magnitude of 16 and an angular separation of 20 degrees from the Moon.

The February 2020 encounter by provided an opportunity for radar observatories to study the asteroid's characteristics in detail. was the first radar target observed by the Arecibo Observatory in Puerto Rico since the one-month shutdown of observatory operations due to a series of earthquakes in the southern region of Puerto Rico in December 2019 and January 2020. Radar observations of were conducted on 4 February 2020 by a team of astronomers led by Luisa Zambrano-Marín. Over a two-day observation period, astronomers measured the asteroid's size, shape, and rotation, and discovered a small satellite.

After the February 2020 encounter, passed perihelion on 21 March 2020. During its egress from perihelion, the asteroid made its closest approach to Mars on 28 June 2020, at a distance of .

Future approaches Over the course of its orbit in the next 200 years, will continue to pass by Earth, though it will not make any approaches as close as the February 2020 encounter that would otherwise warrant attention. The last Earth encounter by from a closer distance was on 1 February 1931, when the asteroid approached Earth from a distance of , or 3.5 lunar distances. An Earth encounter by occurred in February 2022 and another occurred in 2024, with approach distances of 0.18 AU and 0.34 AU, respectively.

Physical characteristics High-resolution delay-Doppler radar imaging by the Arecibo Observatory in February 2020 has shown that is a spheroidal object with a diameter of . This spheroidal shape is commonly seen in other near-Earth objects, such as and 101955 Bennu. Based on the asteroid's estimated diameter and the bandwidth of radar signals reflected from the asteroid, the rotation period of is estimated to be between 2 and 3 hours—close to the 2.2-hour spin barrier at which asteroids tend to break apart due to excessive centrifugal forces. Like other binary near-Earth asteroids, the rapid rotation of suggests that it was sped up by the YORP effect and may have undergone rotational breakup, which led to the formation of its satellite. The axial tilt or direction of 's rotational pole is unknown and could not be constrained by Arecibo's radar observations, however.

Astronomical spectroscopy by the Gran Telescopio Canarias in February 2024 has shown that the visible and near-infrared spectrum of most closely matches those of Q- and Sq-type asteroids, indicating that it is predominantly made of silicate minerals. In particular, appears reddish in the visible spectrum until the wavelength of , and exhibits a spectral absorption band from wavelengths of 0.75 to , indicating the presence of olivine and pyroxene. The silicaceous composition and spectral type of is commonly seen in near-Earth asteroids. The geometric albedo of has not been directly measured, but is inferred to be around 0.3 based on its spectral type.

Satellite

The satellite of was discovered in Arecibo radar observations conducted by a team of astronomers consisting of Luisa Zambrano-Marín along with other members of the Planetary Radar Science Group. With the discovery of a satellite around , the mass and density of the primary body can be determined from the satellite's orbit. The satellite may have formed as a result of rotational fission or mass ejection of the primary body, since along with other binary near-Earth asteroids have been observed to have rapid rotation periods and spheroidal shapes.

Physical characteristics With a diameter of , the satellite is less than one third of 's diameter. The satellite's shape is unknown as it was not resolved in Arecibo's radar imagery.

Orbit and rotation From radar images taken on 5 February 2020, the separation distance between the satellite and the primary body is estimated to be about , or 4.4 times the radius of the primary. The orbital period of the satellite is thought to be around 45–50 hours, with the best-fit period being 47 hours. However, another possible orbital period of 15–16 hours has not been yet been ruled out, due to projection effects of radar images. Radar observations by Arecibo suggest the satellite is likely tidally locked to the primary body, with its rotation period being synchronous with its orbital period. However, uncertainties remain in measurements of the satellite's rotation period, thus an upper limit to its period was placed at 49 hours.

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References

External links

Arecibo Observatory Discovers Moon Orbiting Near-Earth Asteroid, Tracy Becker, University of Central Florida, 12 Feb 2020 An Asteroid Totally Just Mooned Earth, Michelle Starr, ScienceAlert, 11 Feb 2020 Discovery Announcement of Binary System 2020 BX12 , Planetary Radar Science Group, 10 Feb 2020 2020BX12 – Summary, ESA Space Situational Awareness, 10 Feb 2020