Science Requirements for the Next Generation Arecibo Planetary Radar

For many decades, the Arecibo Observatory (AO) hosted the most powerful and sensitive planetary radar facility in the world, until the collapse of the 305-meter William E. Gordon telescope platform on December 01, 2020. The S-band (2.38 GHz, 12.6 cm, 1 MW) planetary radar system at AO provided incomparable support for planetary defense by observing more than 850 near-Earth asteroids (NEAs). In addition, it provided physical characterization of NEAs that is not only valuable for science, but also for planning space missions. Currently, the most active planetary radar facility is the Goldstone Solar System Radar in California, which is approximately 15 times less sensitive than the former Arecibo planetary radar system. We will discuss the science requirements in the planetary field with emphasis in planetary defense, which inspired the creation of the Next Generation Arecibo Telescope (NGAT) concept, a new instrument that could push forward the frontiers of the planetary sciences and other fields (Roshi et al, 2021).

Post-discovery detection of near-Earth objects (NEOs) with radar is an unparalleled technique for accurately determining their future trajectory and assessing whether they pose a real impact threat to Earth. Approximately 40% of NEOs with D ≥ 140 m have been discovered by asteroid surveys, and with the new upcoming instruments, such as the Vera C. Rubin Observatory (LSST) and the infrared Near-Earth Object Surveillance Mission (NEOSM), the number of asteroid discoveries will increase significantly. Radar can quickly rule out false impact alarms that otherwise would take months of observations using optical telescopes to reach the same orbit accuracy. Radar detectability depends on the instrument’s characteristics and the target’s physical and dynamical properties, the main factors: a) instrumentation: transmitter power, antenna gain, field of view/steerability, effective collecting area, system temperature; b) target: distance, diameter, rotation period, radar albedo, sub-radar latitude. Because the echo power is inversely proportional to the fourth power of the distance between target and the radar, a powerful system is needed to detect smaller and/or more distant objects. The proposed NGAT instrument is a compact phased array that would have 5 MW transmitter power, operating at 5 GHz frequency (C band). With the legacy Arecibo telescope, dozens of NEAs were observed every year, with a record of 126 NEAs detected in 2019. NGAT would be able to detect six times more NEAs, with its increased sky coverage, higher transmitter power, and greater sensitivity than the former Arecibo telescope.