1 Introduction

The NASA Lucy mission will explore Jupiter’s Trojan asteroids, a class of bodies co-orbital with Jupiter. The Trojan asteroids are found in two large reservoirs located at about ± 60° relative to Jupiter in its orbit around the Sun, close to the L4 and L5 Lagrangian Points. The L4 and L5 Lagrangian Points are dynamically stable locations for a restricted three-body problem (in this case, the Sun, Jupiter, asteroid), and Trojan asteroids are predicted to be stable over the age of the Solar System (Levison et al. 1997; Holt et al. 2021). While the origin of the Trojan asteroids is still debated (see Levison et al. 2023, this collection), there is consensus that they must have been dynamically captured early in Solar System evolution, perhaps within a few tens of Myr after formation (e.g., Nesvorny et al. 2013). Planetesimal formation models and dynamical models also indicate that Trojan asteroids probably formed within a few Myr of the origin of the Solar System and originally spanned a broad range of heliocentric distances (∼15–30 AU), but then were displaced into their current positions (∼5 AU) during the migration of the giant planets (e.g., Nesvorny et al. 2013). If such planetary migration models are correct, the Trojan asteroids could provide us with a close-up view of pristine materials that formed in the outer Solar System.

This hypothesis opens an interesting comparison with other stable classes of small bodies, such as Near-Earth objects (NEOs), Main Belt asteroids (MBAs) and Kuiper Belt objects (KBOs; Fig. 1). MBAs have heliocentric distances between the orbits of Mars and Jupiter (∼2–5 AU). Many large MBAs, especially those of the S-complex spectral type (related to ordinary chondrite meteorites), are thought to have formed at heliocentric distances within Jupiter’s orbit (e.g., Bermingham and Kruijer 2022). Dynamical models also indicate that a significant fraction of MBAs, such as those with C-complex spectral types, could have formed close to the giant planets (Jupiter and Saturn) or even beyond Neptune’s orbit, but were subsequently displaced into the Main Belt due to dynamical perturbations (e.g., Levison et al. 2009; Vokrouhlický et al. 2016; Raymond and Izidoro 2017). A fraction of these C-complex asteroids are the likely parent bodies of carbonaceous chondrite meteorites (e.g., Marchi et al. 2022).

Fig. 1
figure 1

Proper elements for numbered Main Belt asteroids (gray), numbered Trojan asteroids (orange), and numbered Kuiper Belt objects (green). Data from AstDyS (https://newton.spacedys.com/astdys/index.php?pc=5). Small MBAs and KBOs visited by spacecraft are indicated by their initials (Gaspra, Steins, Mathilde, Lutetia, Ida, Arrokoth). Similarly, Lucy’s targets (Polymele, Eurybates, Orus, Leucus, Patroclus-Menoetius) are shown (note Leucus and Polymele overlap). Comets and near-Earth asteroids visited by spacecraft are not shown

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KBOs are located at heliocentric distances larger than ∼30 AU. They are thought to be more volatile-rich than MBAs, including those belonging to the C-complex (e.g., Barucci et al. 2008, 2011; Brown et al. 2012), and have remained relatively unchanged since their formation. A fraction of KBOs, the so-called scattered disk and hot classical bodies, achieved their current orbits thanks to the dynamical interaction with Neptune (e.g., see review by Nesvorny 2018). The scattered disk is the source of Jupiter family comets (JFCs), objects that sporadically can reach the inner Solar System and offer opportunities for up close investigations (e.g., Nesvorny et al. 2017), such as comet 67P/Churyumov–Gerasimenko, the target of the ESA Rosetta mission. The cold classical disk, in contrast, is a reservoir of objects that only marginally interacted with Neptune and have remained on relatively stable orbits since their formation (e.g., Nesvorny 2018).

Space missions have explored a variety of NEOs, MBAs, and KBOs (e.g., NEAR, Rosetta, Dawn, New Horizons, Galileo, and others) revealing intriguing geological properties. Volatile-poor MBAs (e.g., (4) Vesta, (951) Gaspra, (243) Ida) are characterized by heavily cratered surfaces with overall morphologies resembling those of lunar terrains. The smaller sizes of MBAs compared to the Moon, however, result in distinctive features such as irregular shapes (e.g., (21) Lutetia, Ida, Gaspra), or global-scale ridges and grooves (e.g., Vesta, (433) Eros). (253) Mathilde is the only large (∼50 km) MBA C-complex object with resolved images from a spacecraft (Veverka et al. 1999) and exhibits surface morphology distinct from that typically seen on S-complex bodies. Specifically, Mathilde has multiple large craters whose diameters approach the body radius. In conjunction with the low estimated bulk density (∼1.3 g/cm3, Veverka et al. 1999), Housen and Holsapple (2003) proposed that a relatively high bulk porosity explains the ability to survive these large impact events. In comparison to the NEOs and MBAs thus far explored by spacecraft, KBOs and former KBOs (i.e., JFCs) encountered so far have been seen to exhibit some surface features and compositional characteristics indicative of volatile-related processes. Examples include morphology and activity of comet 67P/Churyumov–Gerasimenko (e.g., Thomas et al. 2015) and troughs and pit chains potentially related to volatile release observed on (486958) Arrokoth (Spencer et al. 2020).

Trojan asteroids occupy intermediate heliocentric distances with respect to MBAs and KBOs. As such, they are of particular interest as they may represent a class of bodies that is not sampled in either population (e.g., distinct compositions; see Emery et al. 2023, this collection), or that have gone through radically different evolutionary processes (e.g., different collisional evolution; see Bottke et al. 2023, this collection). The comprehensive set of investigations to be performed by Lucy’s Geology Working Group (GWG) and broader Science Team will shed light on the origin of the Trojan asteroids, and their possible relationships with MBAs and KBOs.

In particular, the Lucy Level 1 science requirements have been carefully crafted to address major mission science goals, including the study of the crater populations of the targeted Trojans and their collisional evolution (Levison et al. 2021). Craters also offer an opportunity to study the near-surface stratigraphy, with implications for the composition and internal properties of Trojans (see Emery et al. 2023, this collection). For these reasons, the Lucy mission was designed to fly-by a set of Trojan asteroids spanning a wide range of physical properties (see Levison et al. 2023, this collection). Because of the differences in their physical properties (mass, shapes, spectral types), it is expected that their surface geology could also exhibit considerable variations. For instance, ground-based occultation data indicate that the best-fit ellipsoidal shape for Lucy Trojan target (15094) Polymele has major axes 27.6 km × 23.8 km × 10.0 km, with indications of a very complex, non-convex shape (Levison et al. 2023, this collection). At this time, it is not clear if this shape is due to the presence of large cavities (e.g., craters) or reflects a rubble pile nature. Notably, Polymele has a ∼5 km diameter satellite, whose origin is also unclear (Levison et al. 2023, this collection). Lucy Trojan targets (3548) Eurybates and (21900) Orus have similar and more regular shapes, with average diameters of 69.3 and 60.5 km, respectively (Mottola et al. 2023, this collection). Eurybates has a ∼1 km diameter satellite (Noll et al. 2020), Queta, enabling an estimate of Eurybates density to be 1.1±0.3 g/cm3 (Brown et al. 2021). This density suggests that either Eurybates is largely made of ices, has a high porosity, or both. These properties could result in complex surface geology, affecting cratering morphology and creating features indicative of ice-loss (e.g., sublimation) processes. The Lucy Trojan target (11351) Leucus has a very elongated shape (major axes 60.8 km x 39.2 km x 27.8 km) and extremely long rotation period (445 hours), but we have no constraints yet on its surface geology (Mottola et al. 2020). Both elongated shape and slow rotation could be explained if Leucus was once a binary system now collapsed to a contact binary, however, several cords gathered during five occultation events did not reveal (or resolve) a contact binary shape (Mottola et al. 2020). The binary Trojan system (617) Patroclus and its companion Menoetius have a system density of ∼0.8 g/cm3, thus again suggesting high ice content and/or high porosity. Patroclus and Menoetius’ large diameters (∼113 and ∼104 km, respectively; Marchis et al. 2006), and the fact that they form a roughly equal-mass binary pair similar to many found in the classical Kuiper Belt, suggest that they are primordial objects (Nesvorny et al. 2018) and thus may have accumulated significant cratering on their surfaces. In the next sections, we will review the geology of relevant objects (e.g., MBAs and KBOs), and discuss in detail Lucy’s planned geology-related observations.

2 The Geology of MBAs and KBOs

Spacecraft exploration of small (i.e., diameter smaller than ∼100-200 km) asteroids, comets, and natural satellites has revealed an impressive diversity of geologic processes on these bodies, which is perhaps surprising given their low gravity, lack of significant internal heat, and atmosphere-less surface environments. Impact cratering is generally the dominant surface modification process on such worlds, providing a way to potentially assess relative ages of surface regions (via crater areal density) as well as local relative stratigraphic relationships of geologic units. Evidence for both erosional and tectonic processes (e.g., ridges, grooves) has also been found on many of these bodies, providing additional clues about the evolution of their surfaces over time. While no specific evidence of (cryo)volcanic processes has yet been observed on the ∼12 small rocky asteroids or any of the small rocky planetary satellites so far encountered by spacecraft, evidence for volatile release processes has been observed on all ∼6 comet nuclei so far encountered. Additionally, significant geologic and compositional evidence for volcanic or cryovolcanic processes has been observed on a number of somewhat larger asteroids and planetary satellites (larger than ∼500 km diameter; e.g., Vesta, Ceres, Enceladus, Dione, Miranda), suggesting that the role of internal heating and potentially even differentiation begins to be much more important in the geologic evolution of bodies above that size. The relevance of these processes to smaller bodies such as the Trojan asteroids remains to be investigated.

The observed surface geology of a number of relevant small bodies can help to set expectations for the kinds of features that could be encountered during the Lucy mission’s exploration of the Trojan asteroids. For example, spacecraft imaging of the Main Belt asteroids Ida, Mathilde, and Gaspra – all comparable in size to Lucy’s first four Trojan asteroid targets – reveals surface morphologies dominated by the effects of impact cratering (e.g., Fig. 2a; Sullivan et al. 2002). Other small bodies such as Lutetia, Eros, Saturn’s moon Phoebe, and Arrokoth – the latter two of which might have compositional similarities to Trojan asteroids – also show significant evidence for downslope movement and even “ponding” of loose, fragmental regolith materials (e.g., Fig. 2b; e.g., Robinson et al. 2002; Porco et al. 2005; Spencer et al. 2020). Evidence for throughgoing structures like grooves and ridges on Eros and the Martian moon Phobos (Fig. 2c) suggest tectonic processes and intact (but likely fractured) rocky interiors, and also point to the potential role of tidal forces in modifying small body surfaces (e.g., Bottke et al. 1999; Robinson et al. 2002; Hurford et al. 2016). Finally, evidence for internal stratigraphic layering has been observed in spacecraft imaging of the nuclei of comets 9P/Tempel and 67P/Churyumov–Gerasimenko (Fig. 2d; e.g., Thomas et al. 2007; Giacomini et al. 2016; Birch et al. 2017). Detection of similar interior stratigraphic markers and relationships on potentially compositionally-analogous Trojan asteroids could provide unique clues about their interior structures as well as the relationship between Trojans and pristine outer Solar System cometary and KBO populations.

Fig. 2
figure 2

A summary of surface properties of selected minor bodies. (a) Typical cratered surfaces of MBAs, NEOs, and planetary satellites generally comparable in size to many of the Lucy Trojan asteroid targets. Clockwise from upper left: Ida and its tiny moon Dactyl, Mathilde, Deimos, Phobos, Eros, and Gaspra (Sullivan et al. 2002); (b) Evidence for downslope regolith motion within the eastern wall of ∼45 km wide Jason crater on Saturn’s ∼213 km diameter moon Phoebe. Typical streamer lengths are ∼10 km (Giesea et al. 2006); (c) Various morphological features: ridges (c1), shallow troughs (c2), pit chains (c3), flat-floored troughs (c4), fractures (c5), grooves (c6), on \(34 \times 11 \times 11\) km diameter NEO Eros (Buczkowski et al. 2008; Barnouin et al. 2013), all scale bars are 250 m; (d) Evidence for stratigraphic layering and potential flow features on the nucleus of ∼6 km diameter comet 9P/Tempel (Thomas et al. 2007). Figure 2a is from “Asteroid Geology from Galileo and NEAR Shoemaker Data” by Sullivan et al. in Asteroids III, edited by W.F. Bottke et al. © 2002 The Arizona Board of Regents. Reprinted by permission of the University of Arizona Press

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As noted above, cratering is one of the dominant geological processes on small bodies observed up-close so far, and we present below theoretical expectations for cratering on Trojan asteroids. The collisional environment of Trojan asteroids is similar to that of MBAs (Marschall et al. 2022). Collisional models compute an average intrinsic probability of collisions \(P_{i}\) = 7x10−18 and 3x10−18 km−2 yr−1, respectively for Trojans and MBAs. At a reference size of 1 km, the Main Belt has about 4 times more objects than the combined L4 and L5 Trojans. Average impact velocities are also similar, about 5 km/s for both populations (Marschall et al. 2022). Note that these parameters are significantly different from KBOs; for instance, the impact velocities for Arrokoth are in the range 0.5–2.5 km/s, depending on the source of impactors (Morbidelli et al. 2021).

In light of these similarities between Trojan and MBA collisional properties, we derive here theoretical predictions for the Trojans’ cratering history. For this purpose, we use a collisional model developed for MBAs (Marchi et al. 2009, 2012, 2016) and adapted to Trojans using the input parameters above.

The crater size is a complex function of impactor size, mass, velocity, and the mechanical properties of the target. Here we use the so-called Pi-group scaling law (e.g., Hosapple and Housen 2007) that allows computation of the transient crater diameter (\(D_{t}\)) as a function of impactor size (\(d\)), impact conditions and material properties:

$$\begin{aligned} D_{t}=kd \left[\frac{gd}{2v_{\bot}^{2}}\left(\frac{\rho}{\delta}\right)^{2v/\mu}+\left(\frac{Y}{\rho v_{\bot}^{2}}\right)^{(2+\mu)/2}\left(\frac{\rho}{\delta}\right)^{v(2+\mu)/\mu}\right]^{-\mu/(2+\mu)} \end{aligned}$$
(1)

where \(g\) is the target gravitational acceleration, \(v_{\bot}\) is the perpendicular component of the impactor velocity, \(\delta \) is the projectile density, \(\rho \) and \(Y\) are the density and “cratering strength” of the target, \(k\) and \(\mu \) depend on the cohesion of the target material and \(\nu \) on its porosity. Therefore, the nature of the terrain affects the cratering efficiency and the functional dependence of the crater size with respect to the input parameters (e.g., impactor size and velocity).

Equation (1) models the so-called transient crater size resulting from the direct excavation and removal of target material. Large craters on planetary surfaces are expected to undergo a phase of crater modification in which highly fractured materials flow back toward the cavity. The resulting final crater (\(D_{f}\)) is typically from 20% to 50% larger than the transient crater in rocky targets. Here we assume a similar relationship for small bodies, and use 30% (Marchi et al. 2015). Figure 3 shows the computed crater size vs impactor size for average impact velocity for a generic 100-km Trojan asteroid. We implemented three different formulations of Eq. (1), namely a cohesive soil case for \(Y\) = 10, 100 kPa, and a porous case for \(Y \) = 10 kPa. We stress, however, that these relationships are poorly understood for highly porous materials. In addition, the strength of Trojan asteroids is not known. For reference, the strength of terrestrial basalt is of the order of 10 MPa, while dry alluvium is of the order of 50 kPa. Of note is the observation that the surface of small bodies (either comets or asteroids) are considered to be very low strength. For instance, Ballouz et al. (2020) concluded that meter-sized boulders on Bennu have a strength 0.5-1.7 MPa. These strength values are inferred from meter-scale properties, and the applicability to craters from hundreds to several km in scale is not clear.

Fig. 3
figure 3

Cratering scaling laws relevant for Trojan asteroids. The computed craters assume a 100-km target with a 1 g/cm3 density (impactors have the same density). Scaling laws are from Eq. (1) for cohesive soils (CS) and porous materials (P). The material parameters are: \(\nu \) = 0.4, \(\mu \) = 0.41, \(k \) = 1.03 (CS); and \(\nu \) = 0.4, \(\mu \) = 0.4, \(k \) = 0.725 (P) are from Holsapple and Housen (2007). Gray lines indicate craters with diameters equal to 1x and 10x the impactor size

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Next, we derive a prediction for the cumulative number of craters. This requires knowledge of the evolution of the Trojan asteroid population over time, and whether other populations of objects have contributed to cratering of Trojan asteroids (for instance, interlopers from the outer Solar System). These are important details not yet fully worked out by dynamical models, and so here for simplicity we adopt a Main Belt-like impact rate over time (i.e., known as “chronology”) for illustrative purposes (Fig. 4). The Main Belt chronology used here is from O’Brien et al. (2014) and has an increase on the MBA population number at 4.5 Ga of about a factor of 10 with respect to the present. We also compare our results with the observed craters on Mathilde (Chapman et al. 1999). Among the asteroids visited by spacecraft at close range, Mathilde likely has bulk properties closest to the Trojans based on its C-type spectral class, and density ∼1.3 g/cm3. Mathilde’s shape and geology is dominated by impact craters (Veverka et al. 1999; Thomas et al. 1999; Chapman et al. 1999). Figure 5 provides a view of Mathilde acquired by NEAR during approach. Large portions of the object from both views are hidden in the depressions of large craters. The crater population seen in the illuminated surfaces is close to saturation (Chapman et al. 1999; Marchi et al. 2015), except for the walls of the giant craters. The relative paucity on the walls suggests downslope movement erases craters that form in those regions. Given the very slow rotation rate of Mathilde (17.4 days, Mottola et al. 1995), the ∼1 cm/s2 surface acceleration for Mathilde (Thomas et al. 1999) is not modulated by rotation, and slope failure of material under such low acceleration implies weak cohesive strength. The low strength of the surface material is countered by high bulk porosity (Veverka et al. 1999; Thomas et al. 1999), which is expected to give Mathilde its ability to survive multiple large impacts (Housen and Holsapple 2003) whose resulting craters have diameters comparable to Mathilde’s mean radius.

Fig. 4
figure 4

Model crater production functions for a generic 100-km Trojan asteroid. All curves are for 4 Ga. We assumed a MBA-like decay in the impact rate over time, intrinsic probability and impact velocities as described in the text. The crater size-frequency distribution of asteroid Mathilde (gray points) is also reported for comparison (see text)

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Fig. 5
figure 5

Main belt asteroid Mathilde, as seen by NEAR. The large depressions are interpreted to be impact craters. Credit: https://photojournal.jpl.nasa.gov/catalog/PIA02477

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From this comparison, we find that if the Trojans have cratering similar to Mathilde, then either the Trojan population was much more numerous back in time as we assumed for the Main Belt, or other impactor populations contributed to cratering. Alternatively, if the primordial Trojan population was less numerous than assumed, then Trojans likely have significantly fewer craters compared to Mathilde. We stress that this analysis is speculative and based on a Main Belt-like chronology, and it will be updated in preparation for Lucy’s flybys. For further details on this topic, we refer to Marchi et al. (2023).

Another interesting comparison is with KBO Arrokoth, the target of the New Horizons mission (Spencer et al. 2020). As discussed by Morbidelli et al. (2021), cratering on Arrokoth indicates that the impactor SFD has a cumulative slope \(q\ \sim -1.1\) in the size range ∼0.2–2 km, similar to MBAs. Arrokoth cratering shows that a similar slope is likely to hold down to impactor sizes ∼0.03 km, while MBAs are much steeper (\(q\ \sim -2.7\)) in the size range ∼0.03–0.2 km. Thus, a prediction of Morbidelli et al. (2021) is that cratering on Trojans may be significantly shallower than Mathilde for craters diameter in the size range ∼0.6–4 km (based on the scaling cratering scaling laws discussed above; see Fig. 3). As we shall discuss next, this is well within the observable resolution of the Lucy mission.

3 Lucy’s Geology Investigation

The bulk of Lucy’s science investigation is carried out by five Working Groups (Geology, Surface Composition, Interior and Bulk Properties, Satellites and Rings, Activity), while two additional Working Groups coordinate Lucy’s cruise science and Earth-based observations. Here we report the Level 1 mission science requirements that are relevant to the Geology Working Group:

Requirement 4.1.2, Global Imaging for Shape Models and Geological Units:

Lucy shall obtain panchromatic images, spaced by 1/25 to 1/13 of a rotation, over a full target rotation. The target shall not overfill the field of view of the imagers during these observations. The resulting images shall be used to determine the global distribution of geological units and create shape models.

Requirement 4.1.3, Global Imaging for Shape Models and Geological Units:

Lucy shall obtain, during encounter, panchromatic images at a series of phase angles covering the sunlit surface in total, and separated by between 15° and 25°. The target shall not overfill the field of view of the imagers during these observations. The resulting images shall be used to determine the global distribution of geological units and create shape models.

Requirement 4.1.4, Stereoscopic Imaging for Elevation Models:

Lucy shall obtain panchromatic images of an area > 100 km2 with a sub-spacecraft characteristic resolution ≤ 200 m at two emission angles (angle at the surface between the surface normal and the spacecraft) that are appropriate for the construction of stereoscopic imagery. The resulting images shall be used to produce stereoscopic images and elevation models.

Requirement 4.1.5, Landform Degradation at Different Latitudes:

Lucy shall obtain panchromatic images that cover a total area ≥ 500 km2 or 50% of the available sunlit surface (whichever is less) and sample regions from the equator to latitudes of at least 60° from the equator, at a characteristic resolution ≤ 100 m. The resulting images shall be used to study how the structure of craters varies with latitude.

Requirement 4.1.6, Impact Crater Size Distribution:

Lucy shall obtain panchromatic images of an area ≥ 700 km2 or 80% of the available sunlit surface (whichever is less) that are capable of resolving craters larger than 7 km in diameter. The resulting images shall be used to determine the surface number density of the observed craters.

Requirement 4.1.7, Impact Crater Size Distribution:

Lucy shall obtain panchromatic images of an area ≥ 10 km2 that are capable of resolving craters larger than 70 m in diameter. The resulting images shall be used to determine the surface number density of the observed craters.

The surface geology Level 1 mission requirements will be accomplished by a combination of imaging data taken by different instruments, and in particular panchromatic imaging from L’LORRI, MVIC panchromatic and color imaging, and panchromatic imaging from the TTCam tracking camera (Olkin et al. 2023; Bell et al. 2023; Emery et al. 2023, this collection). Note that all the spatial requirements are meant to be based on resolution elements, that is, 3 native pixels for L’LORRI, 2 pixels for MVIC, 2 pixels for TTCam.

For each Lucy target, we have developed a robust and comprehensive timeline for data acquisition. As an example, we report here the simulated panchromatic imaging data to be acquired at Polymele, the smallest of Lucy’s primary targets (Fig. 6). We stress that this is based on current flyby trajectory planning, and the final imaging sequence is subject to change. For this simulation, we assumed a close approach distance of 434 km, a flyby speed of 6 km/s, and an approach phase angle of ∼80°. We approximated Polymele’s shape using an ellipsoid with major axes 28 km × 28 km × 11 km (about 784 km2 hemispheric area), a rotation period of 11.5 hours (Mottola 2023, this collection), and we assumed a pole orientation perpendicular to the plane defined by the Sun and spacecraft velocity vector.

Fig. 6
figure 6

Lucy’s panchromatic imaging of Polymele. The resolution coverage map shows the higher resolution hemisphere imaged near close approach, while the rest of the imaged surface is from a larger range. Note that the northern pole is not visible for the assumed pole orientation

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In this specific example, Requirement 4.1.2 is achieved by L’LORRI observations during the inbound trajectory from a range from 160,000 to 33,000 km, or from 7 to 1.5 hours prior to close approach. These data, in addition to selected TTCam images near close approach, will be used to achieve Requirement 4.1.3. TTCam images near close approach (range \(-13{,}000\) to \(+13{,}000\) km) will be used to achieve Requirement 4.1.4, while MVIC panchromatic images with a similar range will be used to achieve Requirement 4.1.5. TTCam images at a range of about +10,000 km will achieve Requirement 4.1.6, while LORRI imaging at a range of about −1200 km will achieve the resolution and coverage requirement for Requirement 4.1.7.

Additional data will be used to support and expand Lucy’s geological investigations, in particular MVIC color imaging. Planning the acquisition of these data is the responsibility of the Surface Composition Working Group, and includes the goal of obtaining a global color map of each target’s surface (Emery et al. 2023, this collection).

The general structure of the data acquisition plan presented here for Polymele will be repeated for all of Lucy’s targets, with modifications as needed to accommodate specific flyby conditions. For instance, the planned close approach distance of about 434 km for Polymele is needed to measure the mass of the object with the required accuracy (Mottola et al. 2023, this collection). Because Lucy’s other targets are significantly more massive than Polymele, the close approach distance can be larger. A larger close approach distance (e.g., 1000 km) allows for a more relaxed timeline for additional observations to be performed, while still achieving all the Level 1 mission requirements.

An alternative example is provided by the largest of Lucy’s targets, the Patroclus-Menoetius binary system (Fig. 7). The nominal close approach distance is 1075 km (with respect to Menoetius), a flyby speed of 8.8 km/s, and an approach phase angle of about 56°. The binary orbital period is 103 hours (Marchis et al. 2006), assumed to be the same as each object’s rotation period, and we adopted a pole orientation perpendicular to the mutual orbital plane, with a declination of −23° with respect to the plane defined by the Sun and spacecraft velocity vector. Similar to the Polymele encounter, the Level 1 requirements are met with a combination of L’LORRI, MVIC panchromatic and TTCam imaging. A peculiarity of the binary system is that the highest resolution observations (Requirement 4.1.7) taken near close approach require Lucy to slew its instrument platform a few times back and forth between Patroclus and Menoetius, adding significant complexities to the observation sequence.

Fig. 7
figure 7

Lucy’s panchromatic imaging coverage of Patroclus and Menoetius (bottom and top panel, respectively)

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4 Conclusions

Lucy’s observations are carefully planned for maximum scientific return. Not only the nominal observational sequences guarantee fulfillment of the Level 1 science requirements, but the science return is also significantly expanded by adding more observations when possible.

The investigations pertinent to the Lucy mission’s Geology Working Group cover a broad range of topics that include terrain morphologies, stratigraphic relationships, cratering, and overall asteroid shapes. Lucy’s diversity of targets provides unique opportunities for comparative studies among Trojan asteroids, and with other populations of small bodies visited at close range, including near-Earth asteroids, Main Belt asteroids, and Kuiper Belt objects. Of particular interest is the range of physical properties of Lucy’s targets, across different spectral types (C-, D- P-types), masses (a factor of 100), and rotational periods (a factor of 50).

While the physical properties of Trojan asteroids are poorly understood, Lucy observations offer a unique opportunity to study at close range objects that may have formed in the outer Solar System and that could be volatile-rich. Because collisional rates within Trojans are similar to MBAs (Marshall et al. 2022) the Lucy encounters will provide a multi-body data set to compare with MBAs and to assess the similarities and differences of the two populations. A fundamental aspect of the Lucy mission is to address whether its targets are primordial or collisional fragments, thus helping to significantly constrain models of the origin of the Jupiter Trojans.

An interesting application of the methods described in this article is offered by Eurybates, the largest remnant of a collisional family. While the age of the family is not well constrained (Marshall et al. 2022), Eurybates’ surface could be significantly less cratered than other similarly sized Trojans, including Orus. For this reason, the comparison of crater SFDs between Eurybates and Orus may reveal whether the shape of the impactor population SFD has evolved over time. In addition, Patroclus and Menoetius could inform us about the timing of their formation. Our expectation is that this binary system is primordial (e.g., Nesvorny et al. 2018), and their cratering ages could confirm or reject this expectation.

The investigations described here by the Lucy mission’s Geology Working Group will directly address some of these fundamental open questions in planetary science, and synergies with other working groups will provide an unprecedented understanding of Jupiter’s Trojan asteroids.