This sequence of images shows asteroid 2016 AJ193 rotating as it was observed using Goldstone's 70-meter antenna. This 1.3-kilometers-wide object was the 1,001st near Earth asteroid measured by planetary radar. It was captured on Aug. 22, 2021. Credit: NASA/JPL–Caltech
A small, near-Earth asteroid (NEA), designated 2021 PJ1, passed our planet on Aug. 14, 2021 at a distance over 1,000,000 miles (about 1.7 Million kilometers). The recently discovered asteroid was between 65 and 100 feet (20 to 30 meters) in width, and wasn't considered a threat. This asteroid's historic approach to Earth was nevertheless significant, as it marked the 1,000th NEA that planetary radar had observed in less than 50 years.
Seven days later, the planet radar spotted the 1,001st such object. However, this one was larger.
This powerful technique has been used since 1968 when the first radar observation was made of 1566 Icarus, an asteroid. These radar detections increase our knowledge about NEO orbits and provide data that can be extended calculations of future motion by decades or centuries. They also help to predict whether an asteroid will hit Earth or just pass by. Recent radar measurements of Apophis, a potentially dangerous asteroid, helped to eliminate the possibility of it hitting Earth in the next 100 years.
They can also provide detailed information about physical properties, which scientists could only match by sending a spacecraft to observe these objects close up. Based on the size and distance of an asteroid, radar can be used for detailed imaging its surface. It can also determine its shape, size, spin rate and whether it is accompanied with one or more small lunars.
This animation shows asteroid 2016 AJ193 rotating as it was observed on Aug. 22, 2021 by Goldstones 70-meter antenna (230 feet). The object was three-quarters mile (3.35 kilometers) in diameter. It was observed by Goldstones 70-meter (230 foot) antenna on Aug. 22, 2021.
2021 PJ1 was a case in which the asteroid was too small to be photographed and the observing period too short. The milestone of the 1,000th NEA detected via planetary radar highlights the effort to study NEAs close to Earth.
Lance Benner, NASA's asteroid radar researcher at NASA's Jet Propulsion Laboratory, Southern California, said that 2021 PJ1 was a small asteroid. "We couldn't get detailed radar imagery because it passed us at a distance over a million feet." "But even at this distance, planetary radar can detect it and measure its velocity with very high precision. This greatly improved our understanding of its future motion."
Benner and his team were the ones who led the effort. They used the Deep Space Station 14 (DSS-14), a 70-meter antenna located at Deep Space Network's Goldstone Deep Space Complex, near Barstow in California to transmit radio waves to the Asteroid and to receive radar reflections or "echoes" from the Asteroid.
This 360-degree video shows NASA's huge 70-meter (230 foot) DSS-14 antenna located at the Goldstone Deep Space Communications Complex, Barstow. DSS-14 and other DSN antenas can be used for radio science. Credit: NASA/JPL-Caltech
Listening to the Radio
Over half of all asteroids detected by planetary radar were seen by the large telescope at Arecibo Observatory in Puerto Rico. It was able to see 305 meters (1,000 feet) of them before they were destroyed and decommissioned in 2020. Soon after, the antenna fell to pieces. Goldstone's DSS-14 antenna and the 34-meter (112-foot tall) DSS-13 antenna have so far observed 374 near Earth asteroids. 14 NEAs were also observed in Australia, using antennas at Canberra Deep Space Communication Complex. These antennas transmit radio waves to the asteroids. Parkes Observatory in New South Wales and the CSIRO Australian Telescope Compact Array can also receive radar reflections.
Nearly three quarters of all NEA radar observation have been made since NASA’s NEO Observations Program (now a part NASA’s Planetary Defense Program) increased funding for this work ten years ago.
Only a week after 2021 PJ1, the closest asteroid ever observed by radar came within a week of Earth's approach. Goldstone photographed 2016 AJ193 passing our planet between Aug. 20-24 at a distance 2.1 million miles (or 3.4 million km). This asteroid was 40 times closer than 2021 PJ1. Its diameter is approximately three-quarters mile (1.3 km) and its radar echoes were much stronger. Radar images showed a lot of detail, including concavities and boulders.
This is the radar echo of asteroid 2021PJ1 on Aug. 14, 2021. The horizontal axis is the difference between the predicted Doppler frequency (and the new radar measurement). Credit: NASA/JPL-Caltech
Shantanu Naidu (a scientist at JPL) said that the 2016 AJ193 approach gave us an opportunity to study the object and increase our understanding of its future movement around the Sun. He was the one who made the Aug. 22 observations in 2016 AJ193. It has a cometary orbit which suggests it could be an inactive camet. We knew very little about it, except its size and the amount of sunlight that its surface reflects. This is why we started this observation campaign many years ago."
NASA's NEOWISE spacecraft had previously measured 2016 AJ193 in size. However, the Goldstone observations revealed that it is a complex and fascinating object that rotates for 3.5 hours.
These new observations of 2016 AJ193, the 1,001st NEA, will be used by scientists to better understand its structure, shape, and makeup. Similar to 2021 PJ1, data from its speed and distance during this approach provided information that will help reduce uncertainty in computing its orbit.
Kelly Fast, NEO Observations Manager at NASA Headquarters in Washington, stated that planetary radar, along with the ground- and space-based optical telescopes that detect and track almost 27,000 NEOs, is an important tool to monitor asteroids that are close to Earth. "Reaching this milestone, now just over 1000 radar detections of NEAs, highlights the significant contribution that has been made to characterizing this dangerous population, which is fundamentally important for our planetary defense efforts."
