S-band Spotlight

Discovery Announcement of Binary System 2020 BX12

Series of range-Doppler radar images of 2020 BX12 observed February 4, 2020.

Discovery Announcement of Binary System 2020 BX12

Arecibo, Puerto Rico - February 10th, 2020

Radar images obtained by the Arecibo Observatory on the 4th and 5th of February revealed that near-Earth asteroid 2020 BX12 is a binary asteroid. The detection of a satellite was made during the first planetary radar observations conducted at the observatory following a month-long shutdown of telescope operations caused by a series of earthquakes striking the southern part of the island of Puerto Rico. The primary asteroid was discovered on the 27th of January by the ATLAS survey on Mauna Loa in Hawaii and fits the definition of a potentially hazardous asteroid (PHA) due to its size and minimum orbit intersection distance (MOID) of 302,000 km (188,000 miles) from the Earth. While this means it could conceivably come closer to the Earth than the Moon, 2020 BX12 poses no danger at this time and is currently receding from Earth.

The Arecibo delay-Doppler radar images (2380 MHz, 12.6 cm) showing the asteroid and its satellite have a resolution of 7.5 meters/pixel in the vertical dimension and 0.075 Hz/pixel in the horizontal dimension (~5 mm/s). Preliminary analysis suggests that the primary asteroid is a round object at least 165 meters in diameter rotating approximately once every 2.8 hours or less. The satellite has a diameter of approximately 70 meters and rotates once every 49 hours or less. The distance between the two bodies is at least 360 m, as observed on February 5th. The movement of the satellite between the two observations, which were made ~23 hours apart, suggests a mutual orbital period of 45-50 hours and would be consistent with a tidally locked satellite. Due to projection effects, uncertainties remain on the rotation periods, and a shorter mutual orbital period of 15-16 hours has not yet been ruled out.

Range-Doppler radar images of binary near-Earth asteroid 2020 BX12 with range resolution of 7.5 m/pixel (vertical axis) and Doppler frequency resolution of 0.075 Hz/pixel (horizontal axis).

The secondary appears brighter than the primary body in the radar images, which is common for radar images of binaries. The horizontal axis in delay-Doppler images shows, in fact, the echo power per Doppler-shifted frequency, or in other words the surface area that has a specific radial velocity relative to the observer. This makes objects that rotate slower appear narrower in delay-Doppler images than objects that rotate faster. Because the secondary body rotates much slower than the primary, its echo power is distributed to fewer Doppler frequency bins than the echo power from the primary body, making the amount of echo power per pixel appear more intense.

The initial detection of the satellite was made during radar observations on 4 February 2020 conducted by Luisa Fernanda Zambrano-Marín, Sean Marshall, Anne Virkki, Dylan Hickson, Anna McGilvray, Johbany Lebron, and Israel Cabrera of the Arecibo Observatory (University of Central Florida/Yang Enterprises, Inc.).

The Arecibo Planetary Radar program and ATLAS are projects of the Near-Earth Object Observations Program in NASA’s Planetary Defense Coordination Office.

About Us


The Planetary Radar Science group is a department of the Arecibo Observatory, which is an NSF facility operated under cooperative agreement by the University of Central Florida (UCF), Yang Enterprises, Inc. (YEI), and Universidad Ana G. Mendez (UAGM). The Arecibo Observatory Planetary Radar program is fully funded by NASA's Near-Earth Object Observations program.

For more information, follow us on Twitter at and !

Recently Observed Objects


For media inquiries, please contact our communications lead, Ricardo Correa

All observations in 2020 - most recent first

2003 OC3
2020 CG2
2020 CY1
163373 (2002 PZ39)
137924 (2000 BD19)
200577 (2005 AC)
2020 BX12 - BINARY!
Comet 289P/Blanpain
2020 AB
2019 YM3
2019 YF4

Past Observations

Upcoming Radar Targets

The following tables list: (1) targets with submitted observing proposals, including those from our annual high and medium priority proposals and (2) possible radar targets, often newly discovered asteroids, that may be detectable, but have not yet been scheduled for observation.

These tables are not comprehensive lists of all asteroids visible from Arecibo. For more complete lists of possible targets of opportunity and long-term future radar targets, updated daily, see: all asteroids visible today (SNR > 2/day), this month (SNR > 2/day), and in the next five years (SNR > 100/day). Note that these tables inevitably include objects that are currently "lost," often suggested by "1d" in the Type column, which stands for "one apparition with a days-long arc," so observer beware!

Requests for Optical/Infrared Observations

Astrometry: optical astrometry is specifically requested for objects with three-sigma plane-of-sky pointing uncertainties (from the JPL Horizons database) of tens of arcseconds or more. The plane-of-sky uncertainties refer to the first date of the radar observing window listed. Often these are lower limits as the pointing uncertainty often grows with time, especially for newly discovered objects; however, in some cases the pointing uncertainty is much lower prior to the object entering the Arecibo field of view. Optical observers are asked to submit astrometry to the Minor Planet Center as soon as possible after observations.

Lightcurves: optical lightcurves and period estimates are specifically requested for objects that will produce high-resolution images (i.e., possible shape models), for targets of IRTF thermal-infrared observations, for candidate binary asteroids, and for potential human-exploration targets. Lightcurve observers are asked to relay period estimates to the radar team to help with the planning of radar observations. During the analysis and modeling process, the radar team may request to use available lightcurve data.

Spectra: spectra refers broadly to optical or infrared spectra and colors, which are most important for completeness of the strongest radar targets, for candidate binary asteroids, and for potential human-exploration targets. Observers are asked to relay probable spectral-class information to the radar team to compare with characteristics suggested by radar.

Object Dates Expected
H mag Request
363599 (2004 FG11) Apr 07-08 Imaging 21.0 Y Y BINARY, PHA, Yarkovsky?
52768 (1998 OR2) Apr 10-23 High-res Imaging 15.8 Y Y PHA, 4.1-h period
2019 HM Apr 16 Astrometry 25.8 NHATS, period unknown
388945 (2008 TZ3) Apr 27-May 02 High-res Imaging 20.4 Y Y PHA, 39-h period?
438908 (2009 XO) Apr 29-May 02 High-res Imaging 20.5 Y Y PHA, VLBA?, period unknown
2016 HP6 May 05-06 Imaging 25.3 Y NHATS, period unknown
136795 (1997 BQ) May 11-17 High-res Imaging 18.1 Y Y PHA, period unknown
2000 KA May 14-17 High-res Imaging 21.7 Y Y PHA, period unknown

The following may be detectable if scheduled for radar observations. Requests for optical astrometry are indicated where the pointing uncertainties are a significant fraction of, or larger than, our 2 arcmin pencil beam. Priority level (high, medium, low) is noted: high = likely to propose urgent radar observations, medium = may observe as a target of opportunity during other scheduled observations, low = unlikely to pursue. Priority level is subjective and is based on how detectable the object is, whether it is potentially hazardous or NHATS-compliant, and how likely it is to be scheduled. Objects may move from this table to the table above as they are scheduled on the telescope.

Object Dates Expected
H mag Request
Priority Level Notes
2020 DT3 Mar 31-April 14 Imaging 21.5 Medium PHA, 13.4-h period?
2019 UO9 Apr 26-27 Astrometry 19.9 45 arcsec Medium PHA, period unknown

Quick Links