On July 24th 2019, an asteroid called 2019 OK passed by the Earth at only 1/5 the distance to the Moon, with a relative velocity of 15 miles per second (~55,000 miles per hour). 2019 OK was discovered by the SONEAR Observatory in Brazil just one day before it zipped past the Earth. It is estimated to be between 50 – 130 meters (165 – 430 ft) in size.
The orbit and position of 2019 OK at the time of the radar observations conducted at the Arecibo Observatory on July 25, 2019. The circle around the Earth is the moon’s orbital path. Figure from JPL Horizons.
Arecibo Observatory radar detection of asteroid 2019 OK. The offset of 30 Hz in the reflected radio signal from the asteroid is used to refine the predicted velocity and future trajectory of the asteroid. The width of the peak is used to determine the rotation period of the asteroid.
Near-Earth objects (NEOs) are asteroids and comets that pass within 30 million miles (125 lunar distances) of the Earth’s orbit. NASA established the Planetary Defense Coordination Office to coordinate its NEO-related activities, including providing early detections and characterizations of these objects to determine if any of them pose an impact threat. Through its Near-Earth Object Observations Program, multiple observatories and telescopic surveys are used to search, find, determine orbits for, and measure physical characteristics of NEOs. The Arecibo Observatory plays a critical role in characterizing near-Earth objects. The radar system transmits a radio signal from the observatory to the asteroid and then receives the echo, or reflected, radio signal. From the timing of the returned radio signal, scientists can establish the position of the asteroid and get a sense of its overall shape and size. The Doppler shift – a measure of the change in frequency of the reflected signal - can precisely determine the velocity and the spin rate of the targets. These measurements are critical for assessing the object and predicting the future positions and any potential encounters of the asteroid with Earth into the distant future. There are over 20,000 known near-Earth asteroids, with an average of 30 NEOs discovered each week. The radar system at the Arecibo Observatory characterizes approximately 100 of these objects each year and has already been used to study nearly 70 near-Earth asteroids in 2019, twenty of which are Potentially Hazardous Objects – larger asteroids passing particularly close to Earth’s orbit.
The Arecibo Observatory is operated by the University of Central Florida (UCF) in partnership with Universidad Ana G. Méndez and Yang Enterprises Inc., under a cooperative agreement with the National Science Foundation (NSF). The planetary radar program is supported by NASA’s Near-Earth Object Observations Program in its Planetary Defense Coordination Office.