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For now, the Planetary Defense Conference Exercise 2021's incoming fictitious(!) asteroid, 2021 PDC, seems headed for impact on October 20th, 2021, exactly 6 months after its discovery. Today (Monday, April 26th, 2021), the impact probability is 5%, in a steep rise from 1 in 2500 upon discovery six days ago. We all know how these things end. Or do we? Unless somebody wants to keep civil defense very busy very soon, the chance is 95% that it will not hit; instead fly by closely to Earth, swing by to a new orbit that takes it away essentially forever or back again sooner or later through a keyhole, for a re-play at different odds. This is where our story starts and the story sounds familiar: season's greetings from 2004 MN4, now better known as (99942) Apophis. One more thing is similar: the close fly-by is an easy launch opportunity to 'jump aboard' that potentially hazardous asteroid for planetary science and tracking of longterm Yarkovsky-shifted keyhole resonant return risks. Indeed, missions are currently being discussed to launch during the 2029 fly-by of Apophis to rendezvous and investigate it closely right after. Others strive for an earlier launch to rendezvous well before, to observe all of the close fly-by at Earth and what it might do to a likely delicate rubble pile asteroid. Presently, this is an unlikely if not impossible option for sudden encounters like 2021 PDC with a lead time of months. But when asteroid mining (...possibly the other ...-not-if of asteroids?) takes off in the same manner as low Earth orbit communications satellites, this option may become a reality. But for now, even if a suitable planetary mission were serendipitously ready atop a suitable launch vehicle, could you get it an asteroid lander within 6 months? Surprisingly, this option existed between late 2014 and late 2018 when the MASCOT Qualification Model turned Flight Spare was kept fully integrated and flight ready for on-ground testing to prepare for the Flight Model's brief but complete mission on Ryugu with JAXA's highly successful HAYABUSA2 probe. At the same time, the MASCOT2 detailed design study for ESA's former AIM mission within the common NASA-ESA AIDA mission to (65803) Didymos and its moonlet, Dimorphos (then affectionately known as 'Didymoon'), paved the way for long-life MASCOTs, many of which have been discussed and studied since. The thoughtful design of MASCOT’s hardware and software allowed for a very high degree of re-use and flexibility regarding scientific payloads. MASCOT2 was to investigate the interior of Didymoon by Low-Frequency Radar. Close encounters like Apophis' offer unique opportunities for Earth-based planetary radar assets to work with spacecraft near and landers on the passing asteroid. We present a range of options for radar- and composition-oriented long-life MASCOT variants - to be delivered to the surfaces of the respective asteroid bodies - for the presently most likely near miss of 2021 PDC and the most certain close fly-by of (99942) Apophis on Friday, April 13th, 2029.

The Arecibo Observatory (AO) is a multidisciplinary research and education facility that is recognized worldwide as a leading facility in astronomy, planetary, and atmospheric and space sciences. AO's cornerstone research instrument was the 305-m William E. Gordon telescope. On December 1, 2020, the 305-m telescope collapsed and was irreparably damaged. In the three weeks following the collapse, AO's scientific and engineering staff and the AO users community initiated extensive discussions on the future of the observatory. The community is in overwhelming agreement that there is a need to build an enhanced, next-generation radar-radio telescope at the AO site. From these discussions, we established the set of science requirements the new facility should enable. These requirements can be summarized briefly as: 5 MW of continuous wave transmitter power at 2 - 6 GHz, 10 MW of peak transmitter power at 430 MHz (also at 220MHz under consideration), zenith angle coverage 0 to 48 deg, frequency coverage 0.2 to 30 GHz and increased Field-of-View. These requirements determine the unique specifications of the proposed new instrument. The telescope design concept we suggest consists of a compact array of fixed dishes on a tiltable, plate-like structure with a collecting area equivalent to a 300m dish. This concept, referred to as the Next Generation Arecibo Telescope (NGAT), meets all of the desired specifications and provides significant new science capabilities to all three research groups at AO. This whitepaper presents a sample of the wide variety of the science that can be achieved with the NGAT, the details of the telescope design concept and the need for the new telescope to be located at the AO site. We also discuss other AO science activities that interlock with the NGAT in the white paper., Comment: 82 pages (executive summary 10 pages), 21 figures, Arecibo observatory white paper (Updated with the complete author list and minor edits)

The white paper discusses Arecibo Observatory's plan for facility improvements and activities over the next decade. The facility improvements include: (a) improving the telescope surface, pointing and focusing to achieve superb performance up to ~12.5 GHz; (b) equip the telescope with ultrawide-band feeds; (c) upgrade the instrumentation with a 4 GHz bandwidth high dynamic range digital link and a universal backend and (d) augment the VLBI facility by integrating the 12m telescope for phase referencing. These upgrades to the Arecibo telescope are critical to keep the national facility in the forefront of research in radio astronomy while maintaining its dominance in radar studies of near-Earth asteroids, planets and satellites. In the next decade, the Arecibo telescope will play a synergistic role with the upcoming facilities such as ngVLA, SKA and the now commissioned FAST telescope. Further, the observatory will be actively engaged in mentoring and training programs for students from a diverse background., Submitted as an Activities and Projects White Paper for the Astro2020 decadal survey (13 pages, 3 figures, 1 table)

Planetary radars have obtained unique science measurements about solar system bodies and they have provided orbit determinations allowing spacecraft to be navigated throughout the solar system. Notable results have been on Venus, Earth's "twin," and small bodies, which are the constituents of the Sun's debris disk. Together, these results have served as "ground truth" from the solar system for studies of extrasolar planets. The Nation's planetary radar infrastructure, indeed the world's planetary radar infrastructure, is based on astronomical and deep space telecommunications infrastructure, namely the radar transmitters at the Arecibo Observatory and the Goldstone Solar System Radar, part of NASA's Deep Space Network, along with the Green Bank Telescope as a receiving element. This white paper summarizes the state of this infrastructure and potential technical developments that should be sustained in order to enable continued studies of solar system bodies for comparison and contrast with extrasolar planetary systems. Because the planetary radar observations leverage existing infrastructure largely developed for other purposes, only operations and maintenance funding is required, though modest investments could yield more reliable systems; in the case of the Green Bank Telescope, additional funding for operations is required., Comment: 9 pages, 3 figures; Astro2020 Activities, Projects, or State of the Profession Consideration white paper