The Next-Generation Ground-Based Planetary Radar

Planetary radar observations have a laudable history of “firsts” including determining the astronomical unit with the precision sufficient for interplanetary navigation, water ice distribution at the Moon’s south pole, water ice indications in the permanently shadowed regions at Mercury’s poles, determining Venus’ rotation state, polar ice and anomalous surface features on Mars, indications that the asteroid (16) Psyche is an exposed metallic core of a planetoid, establishing the icy nature of the Jovian satellites, and the initial characterizations of Titan’s surface. In many cases, these discoveries made by planetary radar systems have motivated missions and mission radar instruments. This W. M. Keck Institute of Space Studies study was intended to identify the compelling science and potential technical developments required for a next-generation, ground-based planetary radar. As new discoveries have occurred since the first generation of planetary radar observations, our understanding of the Solar System has improved, and new questions have emerged. One of the study’s motivations was to identify what discoveries might be enabled or how a next-generation planetary radar might address fundamental questions. The study found that there are three compelling science drivers for a next-generation planetary radar—Venus, near-Earth asteroids, and the icy moons (“ocean worlds”) of the outer Solar System. For Venus (“Earth’s evil twin”), long-term measurements of surface geology obtained with a planetary radar would provide context within which to interpret measurements from a suite of spacecraft planned to explore that planet over the next decade or more. For near-Earth asteroids, an improved characterization of the population (or populations) would result from the increase in both the quantity of near-Earth asteroids that could be tracked and the quality of the data obtained. A planetary radar would provide precise orbit determinations for planning future spacecraft missions and assessing planetary defense hazards. For the outer Solar System, much like for Venus, observations of icy moons/“ocean worlds” over unparalleled durations could be obtained, even enabling investigations of seasonal changes. Multiple additional science cases would be enabled, including potentially the tracking of interstellar objects, thereby bridging the fields of Planetary Science and Astrophysics.A second motivation for this study was the two major ground-based planetary radar facilities were approaching their half-century anniversaries in 2023, with the Arecibo Observatory planning to celebrate its 60th anniversary of operations and NASA’s Deep Space Network planning to celebrate the 50th anniversary of the start of construction of its Deep Space Station-14 (DSS-14) antenna, which hosts the Goldstone Solar System Radar (GSSR). Notably, and unfortunately, between the two workshops held as part of this study, the Arecibo Observatory collapsed. This study found that a next-generation, ground-based planetary radar could be implemented as an antenna array, analogous to those already used in multiple radio astronomy facilities, which would provide greater resilience than a monolithic antenna. Much of the technology for such a planetary radar array is maturing. A planetary radar array could be implemented with solid-state transmitters at each antenna, leveraging considerable commercial investments in solid-state technology and offering the promise of (much) more reliable performance than the traditional vacuum-tube klystrons. Solid-state transmitters have been prototyped and, in one case, deployed for spacecraft telecommunications. Considerable promise exists to develop automation and new algorithms, even within existing systems, for both scheduling observations and processing planetary radar data. Following this study, two more specific concept studies were conducted: “Cross-Disciplinary Deep Space Radar Needs Study” and “A Ground-Based Planetary Radar Array”. Both reached similar conclusions: an array of 15-25 m diameter antennas equipped with 50-80 kW transmitters would be technically feasible and could address all compelling science cases identified in this report. A planetary radar array might even be capable of undertaking a survey designed to find near-Earth asteroids, a capability not currently available.