“The potential to really open a new window into the history of our solar system when Lucy gets to the Trojan asteroids is immense,” commented Tom Statler, NASA program scientist for the Lucy mission, following the flyby of the main-belt asteroid Donaldjohanson by the spacecraft. This meeting, which was carried out at a scorching pace of more than 30,000 mph, was much more than a brief photo op. It was a carefully rehearsed dress rehearsal, proving the spacecraft’s instrument suite and perfecting the operational choreography necessary to meet the mission’s most challenging targets: the mysterious Jupiter Trojans.
Lucy’s April 20 flyby of Donaldjohanson provided a unique opportunity to confirm its imaging and spectroscopic capabilities under actual mission conditions. The L’LORRI imager, which is modified from the LOng Range Reconnaissance Imager aboard New Horizons, took high-quality images at two-second intervals as Lucy flew within 600 miles of the asteroid. The data obtained exhibited Donaldjohanson’s “strikingly complicated geology”, such as an extended shape and a well-defined neck separating two lobes, characteristics that point towards an turbulent collisional history. Hal Levison, Lucy’s primary investigator, stressed the scientific return: “As we study the complex structures in detail, they will reveal important information about the building blocks and collisional processes that formed the planets in our Solar System”.
The technical success of the instruments on Lucy during this flyby is critical. L’LORRI’s sharp imaging, the L’Ralph color imager, and the L’TES thermal spectrometer together demonstrated their ability to capture data from faint, distant, and rapidly moving targets. This is essential as Lucy heads deeper into the outer solar system, where sunlight is scarce and targets are dim. The next encounters of the spacecraft will require accuracy: between August 2027 and late 2028, Lucy will make flybys of at least six Jupiter Trojans including Eurybates and two newly found satellites within 15 months.
The scientific interest could not be greater. Jupiter’s Trojan asteroids, which are concentrated around the planet’s L4 and L5 Lagrange points, are believed to be remnants from the formation of the solar system. Their orbits, makeups, and physical characteristics store information on the turbulent period of planetary migration and delivery of volatiles and organics to the inner planets. Earth and Mars, whose geologic activity destroyed much of their early history, differ from Trojans, which have not changed much over billions of years, retaining a history of the solar nebula’s makeup and the planetary system processes.
New studies emphasize the heterogeneity and richness of the Trojans. They show a bimodal color population “less-red” and “red” subpopulations with differences lying in the likely origins in their initial formation sites or later surface processing. Spectral observations indicate that, although their low albedos might suggest otherwise, Trojans’ surfaces are covered with featureless reddish material with no obvious signs of water ice or hydrates. However, mid-infrared spectroscopy indicates the existence of fine-grained crystalline silicates, and laboratory experiments indicate that the color bimodality could be caused by the presence or absence of hydrogen sulfide ice, which darkens upon irradiation.
Lucy’s instrument set is optimally placed to solve these enigmas. The LEISA mapping spectrometer will search for diagnostic features of ices, organics, and silicates, while MVIC’s color filters are tuned to detect subtle variations in mineralogy and possible hydrated phases. L’LORRI’s high-resolution imaging will provide geological context, mapping craters and surface features that reveal the collisional and resurfacing history of each target. These measurements will illuminate whether Trojans contain interior water ice, what their organic chemistry is like, and how their surfaces have been formed by subsequent processes since they were taken from the outer solar system.
The Donaldjohanson flyby has already shown how Lucy can image intricate geology and fine surface details from a rapidly approaching, far-away target. With each rehearsal and flyby, the mission presses on, sharpening the skills and anticipation for the big event: unraveling the origin and history of the Trojan asteroids, and by proxy, the early solar system as a whole.
