2020 BX₁₂ 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 0.02915 AU (4.361 million km; 11.34 LD). Radar observations of the asteroid were carried out by the Arecibo Observatory on 4 February 2020, revealing a natural satellite orbiting 360 m (1,180 ft) from the primary body.

2020 BX₁₂ 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. 2020 BX₁₂ 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 0.11 AU (16,000,000 km; 10,000,000 mi) from the planet. At the time of discovery, 2020 BX₁₂ was located in the constellation of Puppis at an apparent magnitude of 17.6.

The discovery of 2020 BX₁₂ 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 2020 BX₁₂ 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.

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 2020 BX₁₂ by the Minor Planet Center on 30 January 2020. The provisional designation signifies the object's discovery date and year. Although 2020 BX₁₂ has a sufficiently long observation arc for its orbit to be accurately determined, the asteroid has not yet been issued a permanent minor planet number by the Minor Planet Center. Once the Minor Planet Center assigns a minor planet number for 2020 BX₁₂, it will be eligible for naming.

2020 BX₁₂ orbits the Sun at an average distance of approximately 1.60 astronomical units (0.239×10^ km; 0.149×10^ mi), taking 2.02 years to complete one full orbit. The orbit of 2020 BX₁₂ 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, 2020 BX₁₂ moves above the ecliptic and comes closer to the Sun than Venus, whereas at aphelion, 2020 BX₁₂ moves below the ecliptic and recedes from the Sun farther out than the orbit of Mars. The orbit of 2020 BX₁₂ 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, 2020 BX₁₂ is formally classified under the Apollo group of near-Earth asteroids.

The asteroid's minimum orbital intersection distance (MOID) from Earth is approximately 0.002 AU (0.30 million km; 0.19 million mi), or about 0.78 lunar distances. Having such a small Earth MOID, 2020 BX₁₂ 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, 2020 BX₁₂ will not make any close Earth encounters within 15 lunar distances or 0.04 AU (6.0 million km; 3.7 million mi) over the next 200 years, and the asteroid has not been listed by the JPL Sentry Risk Table as of 2020.

On 12 February 2020, a team of astronomers identified 2020 BX₁₂ 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 2020 BX₁₂'s orbit with accuracy. This greatly reduced uncertainties in the asteroid's orbit, reducing its uncertainty parameter from 8 to 1. As of February 2020 the observation arc of 2020 BX₁₂ spans 6.09 years or 2,224 days, with an orbit uncertainty parameter of 0 according to the JPL Small-Body Database.

On 3 February 2020 at 18:56 UTC, 2020 BX₁₂ passed 0.02915 AU (4.36 million km; 2.71 million mi), or 11.35 lunar distances, from Earth. During its close approach to Earth, the asteroid approached Earth at a rate of 25.3 km/s (57,000 mph) 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 2020 BX₁₂ provided an opportunity for radar observatories to study the asteroid's characteristics in detail. 2020 BX₁₂ 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 2020 BX₁₂ 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, 2020 BX₁₂ 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 0.042 AU (6.3 million km; 3.9 million mi).

Over the course of its orbit in the next 200 years, 2020 BX₁₂ 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 2020 BX₁₂ from a closer distance was on 1 February 1931, when the asteroid approached Earth from a distance of 0.009 AU (1.3 million km; 0.84 million mi), or 3.5 lunar distances. An Earth encounter by 2020 BX₁₂ occurred in February 2022 and another will occur in 2024, with approach distances of 0.18 AU and 0.34 AU, respectively.

In Arecibo delay-Doppler radar observations from 4 January to 5 January 2020, 2020 BX₁₂ was resolved at a resolution of 7.5 meters per pixel, allowing for direct measurements of the asteroid's physical properties. Radar images show that 2020 BX₁₂ is at least 165 m (541 ft) in diameter, implying a geometric albedo or reflectivity of 0.30 given its absolute magnitude of 20.6. 2020 BX₁₂ appears to have a nearly spheroidal shape, which is commonly observed in other near-Earth objects such as 2005 YU₅₅ and 101955 Bennu. The rotation of 2020 BX₁₂ has not been fully observed in detail due to radar projection effects, thus only constraints on its rotation period can be made. From radar observations spanning two days, the rotation period of 2020 BX₁₂ is likely at most about 2.8 hours.

S/2020 (2020 BX₁₂) 1

Two radar images of 2020 BX12 and its satellite taken by the Arecibo Observatory in February 2020
Discovery
Discovered by	Luisa F. Zambrano-Marín et al. Planetary Radar Science Group
Discovery site	Arecibo Obs.
Discovery date	4 February 2020
Orbital characteristics
Semi-major axis	>0.36 km
Orbital period (sidereal)	1.96±0.08 d
Satellite of	2020 BX12
Physical characteristics
Mean diameter	>0.07 km >70 m
Synodic rotation period	47.04±1.92 h (synchronous) <49 h (constraint)
Absolute magnitude (H)	≈22.5

The satellite of 2020 BX₁₂ 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. Under the satellite naming conventions of the International Astronomical Union, the satellite would be provisionally designated S/2020 (2020 BX₁₂) 1. With the discovery of a satellite around 2020 BX₁₂, 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 2020 BX₁₂ along with other binary near-Earth asteroids have been observed to have rapid rotation periods and spheroidal shapes.

With a diameter of at least 70 m (230 ft), the satellite is less than half the size of 2020 BX₁₂, the primary component of the binary system. The magnitude difference between the satellite and primary is about 1.9, implying an absolute magnitude of about 22.5 for the satellite, given an absolute magnitude of 20.6 for the primary. Excluding delay-Doppler effects on the satellite's brightness in radar images, the satellite's albedo may be slightly higher than that of the primary (~0.3), likely around 0.36 based on a generic magnitude-to-diameter conversion using an absolute magnitude of 22.5 and a diameter of 70 m.

From radar images taken on 5 February 2020, the separation distance between the satellite and the primary body is estimated to be about 360 m (1,180 ft), or 4.4 times the radius of the 2020 BX₁₂ 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.

• 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 Archived 12 February 2020 at the Wayback Machine, Planetary Radar Science Group, 10 Feb 2020
• 2020BX12 – Summary, ESA Space Situational Awareness, 10 Feb 2020
• 2020 BX12 at NeoDyS-2, Near Earth Objects—Dynamic Site
  • Ephemerides · Observation prediction · Orbital info · MOID · Proper elements · Observational info · Close approaches · Physical info · Orbit animation
• 2020 BX12 at the JPL Small-Body Database
  • Close approach · Discovery · Ephemeris · Orbit viewer · Orbit parameters · Physical parameters