Paper ID: 823 The 16th International Conference on Space Operations 2020

Mission Design and Management (MDM) (1) MDM - 4 ”International Cooperation & Mission Architecture - Oral Presentation” (4)

Author: Dr. Carol Polanskey NASA Jet Propulsion Laboratory, United States, carol.polanskey@jpl.nasa.gov

Dr. Steven Joy University of California, Los Angeles (UCLA), United States, [email protected] Dr. Marc Rayman NASA Jet Propulsion Laboratory, United States, [email protected] Dr. Carol Raymond NASA Jet Propulsion Laboratory, United States, [email protected]

GOOD TO THE LAST DROP: DAWN’S DARING USE OF REMAINING HYDRAZINE TO INVESTIGATE CERES’ MOST FAMOUS FEATURES

Abstract By the summer of 2018, the Dawn spacecraft was almost out of hydrazine. After launching in 2007, Dawn had first spent 14 months in 2011 and 2012 orbiting the protoplanet (4) Vesta, and then began orbiting the dwarf planet (1) Ceres in March of 2015. The operations lifetime at Ceres had been signif- icantly constrained following the loss of the spacecraft’s second reaction wheel on departure from Vesta. Substantial replanning of the Ceres spacecraft operations and science data acquisition plans enabled the mission to meet all of its prime-mission science requirements without a full set of wheels. The fact that Dawn was still operating in mid-2018 was due primarily to the development of new flight software that enabled the spacecraft to maintain attitude using only two reaction wheels, supplemented by the hydrazine-based reaction control subsystem – a capability that was created as a contingency after the first reaction wheel failed prior to Vesta. This mode was reserved for use at the end of the primary mission when Dawn was in its lowest-altitude orbit at Ceres, consuming hydrazine at the highest rate due to the high gravity gradient torques at low altitude and rapid turn rate with a short orbit period. To prolong the mission life, additional hydrazine conservation measures were taken with both the spacecraft configuration and operations, providing an opportunity for two mission extensions. The first extended mission spent a significant period at high altitude, where it consumed little hydrazine, but by the time of the second extended mission, only 16% of the hydrazine that was available at the start of the Ceres mission was left for the final phase of science operations. Moreover, a third reaction wheel had failed by then, and Dawn had no single-wheel control mode. For the second extended mission, the project was faced with the challenge of determining how to optimize its use of the remaining resources. Two options were proposed by the project. The first was to leave Ceres for (145) Adeona, a 150-km-diameter carbonaceous asteroid, to perform a low-velocity (< 1 km/s) flyby. Adeona is the parent of an asteroid family with over 500 members, so this option would have provided brand new science from an important target. The second option was to remain at Ceres to build upon the new discoveries made previously. Achieving new science at Ceres would be challenging because without reaction wheels, hydrazine consumption rapidly increases with decreasing orbit altitude, so it was impossible to spend significant time in a circular orbit lower than the 385-km altitude achieved earlier in the prime mission. To improve the spatial data resolution for any of the science instruments, the spacecraft would need to be placed in a highly elliptical orbit, sampling very low altitudes for short periods of time. NASA ultimately approved the Ceres mission option so the Dawn team quickly developed the science goals and optimal characteristics for elliptical orbits within the constraints of an aging spacecraft.

1 From the earliest Ceres approach observations, Occator Crater’s bright features of Cerealia Facula and Vinalia Faculae beckoned for further investigation. By the end of Dawn’s prime mission, these sodium- carbonate evaporite features and their surroundings had been imaged from 385-km altitude with the Dawn framing camera at 30-to-40 m/pixel and observed spectroscopically with the visible and infrared mapping spectrometer. Their high contrast in albedo with the surrounding terrain and the ground track uncertainties both contributed to the difficulty of obtaining high-quality data on these features earlier in the mission. With the goal of improving on this data set, the spacecraft was placed in a 3:1 resonant orbit with a 35-km altitude periapsis designed to migrate south along the longitude of Cerealia Facula. The orbits started with the periapsis 25 degrees north of Cerealia Facula, moving towards it at 1.9 degrees per 27-hour orbit. This approach increased the probability of placing several periapsides within Occator Crater, with the goal of placing one directly over Cerealia Facula. At these altitudes the gamma-ray and neutron detector was expected to be close enough to isolate the elemental signature of Occator Crater from the Ceres background material, providing elemental mapping at a geologic scale. After the trajectory was selected, there were a number of technical challenges to achieving these unique observations. Increased thruster activity at low altitude near periapsis drove the ground track and periapsis placement uncertainty, requiring frequent spacecraft ephemeris and pointing updates. A short-turn-around trajectory correction maneuver was needed to target Cerealia Facula. New tools were developed to help the operations team assess the state of the ground track and make decisions about future updates. After many years of operating in circular orbits, the flight team needed to develop different intuition for the states the spacecraft would experience in elliptical orbits which sometimes translated into different operational procedures. The ground speed of the elliptical orbit made it impossible for a single camera to obtain contiguous images at periapsis, so the mission’s two cameras were used simultaneously to improve coverage. Although Dawn carried two redundant framing cameras, except for two observation sessions in the first extended mission, only one camera had been operated at a time. The primary camera had become more susceptible to single event upsets as the mission progressed, so operating the two cameras together provided robustness in addition to better coverage. To further mitigate the impact of camera resets, the onboard sequences were seeded with proactive power cycles to refresh the camera software on a regular basis between periods of observing. The first low elliptical science orbit began on June 9, 2018 with the three periapsides closest to Cerealia Facula occurring on June 22-24, 2018 (orbits 12-14). The spacecraft and all instruments executed flawlessly during these orbits and even more science data than anticipated was returned, providing spectacular views of the bright material in Occator Crater that ultimately advanced our understanding of the processes responsible for forming these enigmatic features. The team estimated that the spacecraft would have sufficient hydrazine to continue collecting science data through mid-September (approximately 90 orbits), several weeks after the orbit periapsis completed migrating along the lit portion of the Cerealia longitude and continued onto the dark side. Rather than halting science operations prematurely to evaluate the engineering subsystems during end-of-life conditions, the spacecraft productively collected science data until the hydrazine was exhausted on October 31, 2018 (127 orbits). While remote sensing was limited on the dark side, the gravity investigation and gamma-ray and neutron detector’s collection of elemental composition data continued until the end of the mission because they did not require a lit target to observe. The wealth of new science data returned in this phase of the mission surpassed even the most optimistic projections.

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