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Science rationale: Our knowledge of the internal structure of asteroids entirely relies on inferences from remote sensing observations of the surface combined with theoretical modeling [1]. Is Apophis a rubble-pile, as expected, or a monolithic rock, and how high is the porosity? What is the typical size of the constituent blocks? Are these blocks homogeneous or heterogeneous? If Apophis is bilobed, how does the material differ between each lobe? After many asteroid rendezvous and fly-by missions from different nations, these crucial and yet basic questions remain open. Direct measurements of the deep interior structure and composition are needed to better understand the accretion and dynamical evolution of asteroids in general. These measurements at Apophis in particular will directly improve our ability to understand and predict stability conditions as well as to interpret the response of Apophis to the tidal forces induced by its close approach to the Earth. This information is also crucial to plan any interaction of a spacecraft with Apophis and other similar asteroids, especially for Planetary Defense purposes. Direct observations of asteroid subsurfaces in general are also required to better model the dynamics of granular materials in low gravity, and to determine material composition and mineralogy, while space weathering and thermal cycling alter surface properties as observed by optical remote sensing. DROID mission concept: Radar observation of Apophis from a spacecraft is the most mature technique capable of achieving these objectives, by providing a direct measurement of its interior. This is the goal of DROID – (Distributed Radar Observations of Interior Distributions), a mission concept developed in collaboration between NASA JPL and CNES [2] and discussed in more detail in the accompanying presentation [3]. The DROID mothership will release two CubeSats each carrying a low-frequency radar. The radar will be a version of JuRa (60 MHz) [4], modified to operate in a bistatic mode and using an inter-satellite link as a synchronization channel. The mothership and the two CubeSats (daughtercraft) will also have cameras for both science and navigation. Radar observation: Each daughtercraft radar can operate in a monostatic mode, or in a bistatic mode using the two platforms to measure the signal transmitted throughout Apophis, as CONSERT did onboard Rosetta orbiter and Philae lander [1,5,6,7]. Monostatic radar. A radar at 60 MHz offers a larger penetration (up to 100 meters or more) with a limited resolution (≈5 m). It corresponds to the instrument under implementation for the Juventas Cubesat on the Hera/ESA mission [4]. Furthermore, multi-pass processing allows us to build a 3D tomographic image of the interior to identify internal structure like layers, voids and sub-aggregates, to bring out the aggregate structure and to characterize its constituent blocks in terms of size distribution and heterogeneity at different scales (from sub-metric to global). Initial dynamics modeling of the two Cubesats orbiting Apophis at 3 body radii indicates that 20% full Doppler coverage is possible in 40 days [2,8]. Shallow subsurface characterization and radar images to support the shape modeling are also possible in this configuration, but with degraded performance due to a limited resolution. Bistatic radar. The bistatic radar will firstly measure the signal in transmission, allowing us to achieve a direct measurement of the dielectric permittivity, which is related to composition and microporosity [6]. This objective is less demanding in terms of data volume and operation compared to full bistatic coverage. Partial transmission coverage will provide slices of the body with average characterization and its special variability. With dense coverage, benefiting from a larger diversity of observation angles, the bistatic mode will allow a complete 3D tomography [8,9]. In general, multi-angular acquisition allows for a better decorrelation of the size effect and permittivity contrast in the return power. Ground-to-space: In addition to radar observation at close proximity, there is the possibility for joint ground-to-space radar observations at the epoch of the Apophis close approach. [10]. Such measurements would make use of high-power transmitters or sensitive radio astronomy observatories on Earth [11]. Ground-to-space configurations would be used to collect echoes in unique bistatic configurations or to collect echoes during spacecraft maneuvers at close approach (e.g., required spacecraft stand-off). Acknowledgement: The research was carried out in part at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004). References: [1] A. Herique et al. (2018) ASR 62, 2141‑2162., [2] R. Amini et al. (2022) Apophis T-7, #2012, [3] C. Raymond et al. (2022) this meeting., [4] A. Herique et al. (2022) JuRA radar, EPSC, [5] W. Kofman et al. (2015) Science 349, aab0639.,[6] Herique et al. (2016) MNRAS 462, S516‑S532., [7] A. Herique et al. (2019) A&A 630, A6., [8] M. Haynes et al. (2021) ASR 68 (9), [9] M. Haynes et al. (2021) LPSC #1295, [10] A. Herique et al. (2019) Apophis T-9 #2029, [11] M. Haynes et al (2022) Apophis T-7 #2020

Historically, Earth science investigations have been independent and highly focused. However, the Earth's environment is a very dynamic and interrelated system and to understand it, significant improvements in spatial and temporal observations will be required. Science needs to document the need for constellations to achieve desired spatial and temporal observations. A key element envisioned for accomplishing these difficult challenges is the idea of a distributed, heterogeneous, and adaptive observing system or sensor web. This paper focuses on one possible approach based on a LEO constellation composed of 100 spacecraft. A cost analysis has been done to indicate the financial pressures of each mission phase and conclusions are drawn suggesting that new technology investments are needed, directed toward lowering production costs; that operations costs will need to be reduced through autonomy; and that, of the on-board subsystems considered, advanced power generation and management may be the most enabling of new technologies.