Pre- and post-impact observations of Didymos in support of NASA’s DART mission

On 26 September 2022, as NASA’s first planetary defense test mission, the DART spacecraft will intentionally impact the smaller member (named Dimorphos) of the binary near-Earth asteroid Didymos. In addition to energy from the spacecraft, ejecta removed by the impact will impart momentum to Dimorphos and change its orbital period around Didymos. The baseline requirement for the mission is to change the orbital period by at least 73 seconds. This change will allow direct measurement of the so-called beta parameter, or the momentum enhancement caused by escaping ejecta.

For this experiment to be successful, the pre-impact orbital state of Dimorphos must be well constrained. Measuring successive mutual events (eclipses and occultations) has served as the primary method to derive these constraints (Pravec et al. 2006, 2022). We will present a synthesis of mutual event detections from multiple apparitions that have contributed to understanding the pre-impact state of the system (Naidu et al. 2022, Scheirich & Pravec 2022). We will show that the most recent observations from July 2022 confirmed the reference orbit of Dimorphos and reduced the formal uncertainty of its orbital phase at the time of impact to 3.9 degrees, which is more than 10x smaller than the baseline requirement set by the mission.

Following impact, the ejecta may obscure the system for days or even weeks, and thus preclude measurement of mutual events. As such it is unlikely that a post-impact orbit solution based on new mutual event detections will be available at the time of the meeting. However, monitoring of the ejecta will be an important post-impact activity. We will present preliminary results from a coordinated ejecta monitoring campaign at the 4.1-m Southern Astrophysical Research (SOAR) telescope, Lowell Observatory’s 1.1-m Hall telescope, and the 4.3-m Lowell Discovery Telescope (LDT). This monitoring campaign will include measuring photometric colors and morphology of the ejecta cloud as a means to constrain key properties such as particle size and the dynamics of released particles. These observations are planned to probe a range of timescales (hours, days, weeks) and will directly complement other observations carried out by the DART investigation team.