The Lucy Mission’s Upcoming Flyby of Asteroid (52246) Donaldjohanson

The Lucy mission is a NASA Discovery-class mission, the first mission to study asteroids in the Trojan population. It launched October 21, 2021, and will fly past Trojan asteroids Eurybates (Aug. 2027), Polymele (Sep. 2027), Leucus (Apr. 2028), Orus (Nov. 2028), and the Patroclus – Menoetius binary (Mar. 2023). In preparation for the Trojan flybys, Lucy flew past the main-belt asteroid Dinkinesh in Nov. 2023, discovering its contact binary moon Selam (Levison et al. 2024). That flyby was mainly intended as a test of the terminal tracking system used by Lucy to track its target. Lucy has another test flyby coming up on Apr. 20th, 2025 – the main-belt asteroid (52246) Donaldjohanson. Donaldjohanson (DJ) is a small (~4 km diameter) C-type asteroid in the inner main belt (semimajor axis of 2.38 AU). It belongs to the Erigone collisional family, which has a mean albedo of 0.051 (Masiero et al. 2013). Its lightcurve is unusual, with high amplitude and a long rotational period of 251 hours. It is possible that DJ may be a tumbling object or a binary system. Given our ignorance of many aspects of this asteroid, the upcoming flyby should provide interesting science. The observing plan at Dinkinesh was kept simple. However, the encounter sequence for DJ is designed to be a test of the full complexity of the planned Trojan encounter sequences, packed with more observations and taking more data volume. The current Polymele encounter sequence was used as the basis for the DJ sequence. Lucy will fly past DJ at 13.4 km/s with a C/A distance of 960 km, giving the same angular rate of motion as the Polymele flyby. We will move the Instrument Pointing Platform away from the asteroid at about 40 seconds before close approach, to prevent pointing the instruments too close to the sun. Additionally, due to DJ’s small heliocentric distance, we will point the instruments away from DJ from E-9 hours to E-40 minutes to keep the near-IR imaging spectrometer (LEISA) cool so that it can provide good data near close approach. During the encounter, the high-resolution L’LORRI camera will obtain panchromatic imaging of DJ down to ~5.5 m/pixel and the MVIC camera will obtain color imaging down to ~83 m/pixel. The LEISA instrument will obtain spectroscopic images covering a 1.0-3.8 µm spectral range down to 345 m/pixel and covering 4-2.3 µm at about 88 m/pixel. The TES instrument will take disk-integrated thermal measurements.