The Doppler Spike of Asteroid 2021 PJ1

This figure represents the 1,000th near-Earth asteroid (NEA) to be detected by planetary radar since 1968. Being only 65 to 100 feet (20 to 30 meters) wide, asteroid 2021 PJ1 was too small to be imaged in any detail, but the powerful 70-meter (230-foot) Deep Space Station 14 (DSS-14) antenna at the Deep Space Network's Goldstone Deep Space Complex near Barstow, California, was able to measure the Doppler frequency of the radio waves that reflected off the object's surface. The data was recorded for 16 minutes between 2:26 p.m. and 2:42 p.m. PDT on Aug. 14, 2021.

The figure shows radar echo signal strength on the vertical axis versus Doppler frequency (in units of hertz, or Hz) on the horizontal axis. The strong spike at a value of minus 70 Hz is the reflected signal (or "echo") from 2021 PJ1; the other, smaller spikes are receiver noise, which is like the static on an AM radio if there aren't any nearby stations.

Using this information, scientists at NASA's Jet Propulsion Laboratory in Southern California could more accurately calculate the asteroid's velocity, its distance from Earth and its future motion around the Sun.

These observations used a prediction – known as an "ephemeris" – that had relatively large uncertainties because the asteroid had been discovered only a few days earlier. If the original prediction had been perfect, the radar echo would appear at zero Hz. The fact that the radar echo is at minus 70 Hz indicates a correction could be made to the predicted velocity of minus 2.7 miles per hour (minus 1.2 meters per second). This measurement also reduced the uncertainty in the asteroid’s distance from Earth from 1,300 miles (2,100 kilometers) to 5.2 miles (8.3 kilometers) – a reduction of a factor of about 250.