mission control A bountiful harvest on Ceres In its second extended mission at Ceres, the Dawn spacecraft returned a harvest of high-resolution data on the intriguing Occator crater, a landmark for understanding the role of impacts in shaping ice-rich bodies, explain Project Scientist Julie Castillo-Rogez and Chief Engineer Marc Rayman.

he Dawn mission, NASA’s ninth increase spatial resolution, the flight team Discovery project, launched in designed a highly elliptical orbit with a T2007 and concluded operations in peridemeter altitude of ~35 km, 11 times October 2018 after orbiting and exploring lower than in the prime mission (Fig. 1). the two largest objects in the asteroid belt: Because of the astrobiological interest, protoplanet Vesta (2011–2012) and dwarf Dawn was subject to planetary protection planet Ceres (2015–2018). Both bodies are requirements and required to remain on a considered fossils from the dawn of the stable orbit for at least 20 years. That phase Solar System, hence their exploration was presented many challenges, especially motivated by the goal to constrain early with the absence of reaction wheel control Solar System history. One key result is the following the failures of two wheels in strong evidence for the origins of the two the prime mission and a third in the first objects in different reservoirs. While Vesta extended mission. Hydrazine had been probably formed near where it is now, Ceres carried only for desaturating the wheels (and probably migrated from the outer Solar some safe modes), but it became a critical System, as indicated by its high carbon and resource without wheels. Optimized use of nitrogen content (M. C. De Sanctis et al., hydrazine enabled the low-altitude orbit to Nature 528, 241–244; 2015). run from June to October 2018, for a total Dawn achieved many interesting firsts, of 127 orbits, and significantly exceeding some of which are the combined result of Fig. 1 | Representation of Dawn’s low elliptical baseline plans. As the orbit precessed, high- the spacecraft’s design and the innovative trajectory for a few example orbits. Peridemeter resolution observations progressed from operations principles to take advantage of of ~35 km and apodemeter of ~4,000 km. Credit: Occator crater southward at a rate of 1.9° that design. It is the only mission ever to NASA/JPL-Caltech/UCLA/MPS/DLR/IDA. per 27-hour revolution. Visible images were orbit two extraterrestrial destinations, an returned with a ground sampling as small accomplishment enabled by solar electric as 3.3 m per pixel over part of the faculae propulsion (SEP). SEP has ten times places like Occator, a ~90-km-diameter and the rest of Occator’s floor. GRaND data the efficiency of conventional chemical crater at ~20°N latitude. Highly reflective were acquired with a footprint of ~50 km, propulsion. Without SEP, a mission to orbit material in Occator contrasts with the dark and gravity data led to a local gravity field only one of Dawn’s targets would have been surface so strikingly that it was even visible up to degree 45 (versus 18 globally at the unaffordable in the Discovery Program, and as a bright spot in Hubble images. As Dawn end of the prime mission). Infrared spectra a mission to orbit both would have been approached Ceres, that one spot turned and colour images were acquired for selected impossible. This innovative system also into two distinct regions: the sharply regions focused on the faculae. The final allowed extensive manoeuvring in orbit to outlined Cerealia Facula at the crater centre extended mission’s harvest added to Dawn’s optimize the science return. SEP will be used and the Vinalia Faculae, a set of more extensive datasets and represents a fine on NASA’s upcoming mission to Psyche and diffuse deposits to the east. Both areas ending to a grand adventure of exploration is now routinely considered for other deep display a significant amount of sodium and discovery. ❐ space missions as well. carbonate, a marker of warm, alkaline Dawn’s sensing instruments included waters. Bright material on an otherwise Julie C. Castillo-Rogez and a pair of identical German-contributed low-albedo surface should darken rapidly Marc D. Rayman cameras with a panchromatic and seven as a result of micrometeorite gardening. Jet Propulsion Laboratory, California Institute of colour filters, a visible (0.25–1 μm) and Hence, the high albedo of these sites (up Technology, Pasadena, CA, USA. infrared (1–5 μm) mapping spectrometer to 0.6 at Cerealia) and low crater count e-mail: julie.c.castillo@jpl.nasa.gov; contributed by Italy, a gamma-ray and indicate their material was emplaced as [email protected] neutron detector (GRaND), and gravity recently as a few million years ago, much science obtained through Dawn’s telecom more recently than the ~20-Mya formation Published online: 17 March 2020 subsystem and the Deep Space Network. of the crater itself. https://doi.org/10.1038/s41550-020-1031-5 Global mapping of Vesta and Ceres from Better understanding the source of multiple altitudes provided an extensive the sodium carbonate and drivers for dataset, the largest ever returned for an that recent exposure became the core Acknowledgements This work was carried out at the Jet Propulsion Laboratory, ice-rich body. motivation for Dawn’s second and final California Institute of Technology, under a contract with Ceres’s surface is globally homogeneous, extended mission (2017–2018), the results the National Aeronautics and Space Administration cratered and dull, except in a handful of of which are featured in this issue. To (80NM0018D0004).

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