Lucy Mission to the Trojan Asteroids: Science Goals

Lucy will directly measure the masses, M, of its targets using radio-Doppler tracking of the spacecraft with the X-band telecommunications system. Mass determination by radio-Doppler tracking and ranging is a powerful and precise direct measurement (Anderson 1971; Pätzold et al. 2001), and has been performed at many flybys of small bodies and satellites (Anderson et al. 1992; Andert et al. 2010; Pätzold et al. 2011, 2014). In addition to accurately measuring the carrier frequency, it is also necessary to reconstruct the close approach distance during the flyby from optical navigation for an accurate mass determination. The estimated mass determination uncertainties for all Trojan targets are much smaller than the requirement of ±25% for the baseline bulk density of 1000 kg m⁻³ and the flyby distance of 1000 km with the exception of Polymele. Lucy must target Polymele at a close approach distance of 434 km to ensure that the mass determination meets the ±25% uncertainty allocation (R-16). The system masses for the Patroclus–Menoetius binary (M_sys = 1.41±0.03 × 10¹⁸ kg; Grundy et al. 2018) and Eurybates and its satellite Queta (${M}_{\mathrm{sys}}\,\approx$ 1.4 × 10¹⁷ kg; Noll et al. 2020c; Brown et al. 2021) have been determined from their orbits, but Lucy will validate and likely reduce the uncertainties of these determinations. With careful astrometric observation of the orbit of the individual components, Patroclus and Menoetius, relative to their barycenter, the mass ratio of the two components may be derived.

Volume determination from imaging is limited to the portion of the flyby when the target is resolved. As an example, this time period is about one week for Eurybates ($d\sim 64$ km, ${v}_{{rel}}\,=$ 5.7 km s⁻¹, time scales with d and v_rel ) when using the high-resolution imager, Lucy LOng Range Reconnaissance Imager (L'LORRI; see Table 1 in the companion paper, Olkin et al. (2021) for additional details). During this time, Lucy will obtain images of each target every 1/13 rotation and measurements of limb profiles and stereo-photogrammetric analysis will be assembled to construct an overall 3D shape model for volume determination (R-2,3). A fraction of each object, determined by the spin pole orientation, may not be illuminated during the Lucy flybys. Leucus' slow rotation further reduces the fraction of the object that will be sufficiently resolved by Lucy to constrain the volume. As a result, Lucy has no requirement to measure the volume to any particular level of accuracy. Instead, we require that the observations needed to determine the volume be taken (R-2,3).

Given the above risks to determining the volume with Lucy, it is beneficial to supplement the volume determination with Earth-based observations. Lightcurves are one method that can be used to estimate an object's shape. Lightcurves observed over time, as viewing and illumination geometries change, can be inverted to estimate a shape model (Ďurech et al. 2011; Mottola et al. 2020). Stellar occultations are another important tool for constraining the plane-of-sky shape of an object; with enough stations (see Figure 3), the resulting plane-of-sky shape can be quite accurate (Buie et al. 2015; Buie 2021). Finally, there is the observation of the object's thermal radiometry which is a function of its physical diameter and albedo, and the physical properties of its surface (Mainzer et al. 2011). Combining all these complementary observational techniques often produces excellent synergistic results that can significantly reduce the uncertainty inherent in any one method (Ďurech et al. 2011), see Figure 3.

Zoom In Zoom Out Reset image size

By measuring mass and volume of its targets, Lucy will determine the density of each body. We estimate that in most cases the density will be known to better than $\pm 33 \%$ for objects with densities of $\rho \geqslant 1000$ kg m⁻³. The current density data for Trojan asteroids is limited to three resolved binary systems (Merline et al. 2002; Marchis et al. 2006; Noll et al. 2020a) with well-determined orbits (see Section 6), two of which are Lucy targets. The bulk density of the near-equal-component (617) Patroclus is $\rho =800\mbox{--}1000$ kg m⁻³ (Berthier et al. 2020; Grundy et al. 2018). Eurybates' density is roughly 1100 kg m⁻³, assuming it is a sphere with a diameter of 64 km (Noll et al. 2020c; Brown et al. 2021). (624) Hektor's bulk density is $700\lt \rho \lt 2800$ kg m⁻³; it is poorly constrained, owing to differing estimates of the volume of the bilobed primary (Marchis et al. 2014; Descamps 2015). For Patroclus and Eurybates, the most significant improvement to knowledge of density is likely to come from an improved volume determination by Lucy.

More generally, the bulk densities (Baer et al. 2011; Carry 2012) and rotation rates of small bodies (Pravec et al. 2002), along with analysis of the density and structure of likely meteorite analogs (Consolmagno et al. 2008), indicate that many small bodies are relatively low-density objects (Britt et al. 2002). Composition and the level of internal porosity are the critical factors that determine the density of small bodies.

From cosmogonic considerations, it is usually assumed that small bodies that formed beyond the snow line started with roughly equal proportions of condensed ices and mineral-containing rock (Lodders 2003). The rock composition of small bodies can be inferred from meteorite analogs with similar spectral behavior. For Trojans, the closest analogs are CM, CI, and Tagish Lake carbonaceous chondrites that have visible-near-IR spectral signatures that mimic the spectra of C-, P-, and D-type asteroids (Gilmour et al. 2019). Ices constitute a lower-density component of asteroids. Water ice is the dominant component, but other condensed volatiles may also be present. Micron-scale microporosity in meteorites can range from a few percent to as much as 40% (Macke et al. 2011), but even after adjusting for meteorite microporosity and adjusting the relative ice/rock ratio, the bulk density of small bodies tends to be lower than their typical meteorite analogs. This suggests that, in addition to microporosity, asteroids have substantial large-scale porosity, or macroporosity. Although macroporosities have been inferred for only a modest number of small bodies, most of these range from 20% to more than 70% (Consolmagno et al. 2008; Carry 2012; Noll et al. 2020a). In the face of this large range of possible porosity, additional observational constraints, such as spatially resolved colors and spectra from Lucy's L'Ralph instrument, will be needed to supplement density when it comes to inferring bulk composition.

Digital terrain models (DTM) will provide valuable information to study the geology of Lucy's targets. The role of global shape models has been discussed in Section 3. Here we focus on the more localized, higher-resolution DTMs that will be obtained at variable spatial and vertical resolutions and coverage, depending on the specific target and flyby conditions including spacecraft velocity and close approach distance, phase angle, target pole position and spin rate. These investigations will rely on panchromatic imaging from multiple angles (R-4) making it possible to derive DTMs over at least 100 km² and at a horizontal spatial resolution less than 200 m. The anticipated vertical resolution is on the same order as the horizontal resolution.

DTMs will be derived from stereo imaging for each primary target to study in detail local geology, including crater shapes, exposed vertical layering in crater walls, mass wasting, lineaments, and other possible landforms, such as pits resulting from the sublimation of subsurface volatiles. All these features have been observed on small bodies (see Figure 4), and may also be present on Trojan asteroids. For instance, vertical layering has been observed on Vesta (Jaumann et al. 2014); mass wasting and ponding has been observed on the surface of Eros (Roberts et al. 2014); mass wasting and lineaments have been found on Lutetia (Massironi et al. 2012); and sublimations pits have been seen on Vesta and Ceres (Denevi et al. 2012; Sizemore et al. 2017).

Zoom In Zoom Out Reset image size

The ensemble of panchromatic imaging will be used to produce regional mosaics with various degrees of resolution, and larger coverage than local DTMs (R-5-7). These data will enable the assessment of crater morphology and landform degradation (e.g., due to ejecta blanketing or other mass wasting processes) over an area of at least of 500 km² and at a resolution better than 100 m (R-5).

Crater morphology is diagnostic of subsurface bulk properties (e.g., material strength and stratification), and overall degradation due to impact-induced seismic shaking and regolith displacement (Marchi et al. 2015). Additional mass wasting processes may be associated with subsurface volatiles (e.g., water ice) mobilized by impact excavation/heating as observed at various locations on Ceres (Sizemore et al. 2017). Lucy has a requirement to investigate the variation of crater morphology as a function of latitude (R-5). Unlike rocky asteroids previously observed by spacecraft, the Jupiter Trojan asteroids orbit at a solar range that results in low enough near-surface temperatures to permit water ice to be stable on billion year timescales, and the latitudinal dependence of surface temperature may drive latitudinal-dependent geological crater morphology and probe existence and distribution of subsurface volatiles (Combe et al. 2019).

Regional mosaics will also be used to derive the crater SFDs both at local scale (10 km², crater larger than 70 m in diameter; R-6), and at regional scale (low resolution, 700 km², craters larger than 7 km in diameter; R-7). Impact craters are a nearly universal feature on all small bodies observed by spacecraft to date (Marchi et al. 2015). Craters enable a suite of investigations: crater population statistics constrain the surface age from the crater retention age (e.g., Hartmann 1966), their morphologies are a function of target properties (Housen et al. 2018), and smaller craters are a proxy for the Trojan asteroid population that is too small to be observed by telescopes. Current telescopic measurements of the Trojan asteroid SFD extend to objects with diameters $d\geqslant 2$ km (Yoshida & Terai 2017). Assuming a crater-to-impactor diameter ratio of 10:1, craters smaller than $d\,\approx$ 20 km are derived from an unsampled portion of the SFD. Lucy's requirements that address the crater SFD by resolving craters across more than two orders of magnitude (from 70 m to 7 km in diameter) will provide constraints on surface crater retention ages, and past collisional evolution (Marchi et al. 2015). A preliminary collisional model (Figure 5) indicates that Trojan asteroids with a surface age as young as 100 Myr should have accumulated approximately two craters larger than 70 m per km² (that is, approximately 20 craters per 10 km²). Roughly 100× more craters are expected to have formed over the last 4.4 Gyr (not taking into account crater preservation and erasing processes). In addition, detection of 70 m craters would constrain Trojan asteroid SFD down to small and unobservable diameter limit of ∼7 m which will enable an evaluation of the importance of nongravitational effects on the stability of Trojans (Hellmich et al. 2019). Similar inferences of impactor populations from craters have been performed for MBAs and Cold Classical TNOs (Marchi et al. 2014; Singer et al. 2019) enabling comparisons of these distinct small-body populations.

Zoom In Zoom Out Reset image size

During the flybys of the targets, Lucy will use several instruments to acquire resolved imagery of the asteroids' surfaces from a wide range of illumination and observation geometries. Measurements of the brightness distribution on the surface under such varying conditions will enable us to establish a photometric model for the target and to look for correlations between the model parameters and the physical properties of the surface regolith (Li et al. 2019). Spatial variations of the photometric properties will be put in the context of the morphology and geology of the targets to identify possible evolutionary processes (Schröder et al. 2017). Near-closest-encounter images will encompass zero-phase illumination, which will enable the characterization of the opposition effect (Belskaya & Shevchenko 2000), while pre- and post-encounter far-field imaging at large phase angles will help constrain the degree of unresolved roughness of the surface. Furthermore, the establishment of a photometric model will represent a prerequisite for the analysis, interpretation, and comparison of heterogeneous multicolor and spectral data, by allowing their representation under a reference illumination geometry. An accurate photometric model will also allow us to provide precise estimates for the albedo and phase integral of the targets. In addition to being diagnostic of the composition of the surface, those quantities are crucial for establishing the thermal balance and assess the thermal evolution of the objects.

Color imaging is a proven technique for investigating compositional heterogeneity and regolith properties on planetary surfaces at relatively high spatial resolution (Chapman 1996; Olkin et al. 2017; DellaGiustina et al. 2020). The Multispectral Visible Imaging Camera (MVIC) will image the Lucy targets through five color filters (violet 377–489 nm; green 478–525 nm; orange 522–632 nm; phyllosilicate 627–758 nm; and near-IR 753–914 nm) (Olkin et al. 2021).

Observations will be made over a range of spatial resolutions. Low-resolution observations at different rotational phases will determine the global distribution of color units. Observations at the regional (≤1.5 km resolution) and local (≤600 m resolution) scales will be used to search for small regions of freshly exposed materials and correlations of color with geologic features.

Primary surface compositional units (minerals, ices, and complex organic molecules) can be delineated by color imaging (Thangjam et al. 2013; Nathues et al. 2016). A relevant feature for low-albedo asteroids is the absorption at 700 nm in Fe-bearing phyllosilicates (Hamilton et al. 2019; Vilas 1994). Lucy's "phyllosilicate" filter is designed specifically to search for this band (Figure 6).

Zoom In Zoom Out Reset image size

The space environment can modify the surfaces of airless bodies. The spectral effects of these interactions vary due to primary surface composition and style of interaction, but generally include changes in albedo and color (e.g., Brunetto et al. 2015; Pieters & Noble 2016). Color imaging can therefore be an effective means of investigating surface processing on Lucy's targets. Trojan asteroids have been hypothesized to have an icy or differently processed interior overlain by an ice-free irradiation mantle (Guilbert-Lepoutre 2014; Wong & Brown 2015). If so, color maps of the surface may reveal exposures of fresh, subsurface material that can be used to constrain bulk composition (Chapman 1996). Indeed, R-6 and R-7 were specifically designed to see craters younger than 100 Myr, assuming the impact model shown in Figure 5.

The colors of Trojan asteroids at both visible and near-infrared (NIR) wavelengths, as determined from telescopic observations, reveal the presence of two distinct color groups: a "red" group analogous to the asteroidal D-type (and comparable to "gray" Centaurs) and a "less-red" group analogous to the asteroidal P and C-types (Szabó et al. 2007; Roig et al. 2008; Emery et al. 2011; Grav et al. 2012); see Figures 2 and 7. In this regard, the Lucy targets Orus and Eurybates are a very interesting pair for understanding the diversity of Trojans. These two objects have similar sizes, orbital eccentricities, and inclinations (Tables 2, 3), but very different colors. The similar sizes and orbits guarantee that these objects have experienced nearly equivalent solar radiation and collisional environments, at least since their formation and/or delivery to their current orbits. Color differences therefore can be interpreted to mean that they represent different physical/compositional classes.

Zoom In Zoom Out Reset image size

It is notable that whereas red Trojans dominate the population at large diameters, the fraction of less-red objects increases at smaller sizes (Wong et al. 2014). Wong & Brown (2015) suggest that both color groups have similar interior colors, and that impacts on red Trojans expose their less-red interiors. If this scenario is correct, we might expect fresh surface units on the red Lucy targets (Orus and Leucus) to have a less-red color than surrounding terrain. If abundant subsurface ice exists (Guilbert-Lepoutre 2014), spectral maps of fresh exposures on the Lucy targets will test this hypothesis (R-10, 11).

Compositions of primitive objects reflect the conditions in the solar nebula in the region in which they formed providing clues to the structure of the protoplanetary disk. Determining the compositions of Trojan asteroids is, therefore, important for understanding solar system history. Mapping surface compositions enables investigation into potential surface processing from exposure to the space environment during an object's dynamical evolution, as well as the identification of exposed fresh material. The materials most important for understanding Trojan histories—H₂O and other ices, organics, anhydrous minerals, and hydrated minerals—have strong, diagnostic absorption features in the NIR.

Ground-based spectra of Trojan asteroids (Figure 8), including the Lucy targets, are featureless at $\lambda \lt 2.5\ \mu {\rm{m}}$ (Emery et al. 2015; Sharkey et al. 2019). The absence of features at these wavelengths enables limits to be placed on the abundances of ices, crystalline silicates, and hydrated minerals (Yang & Jewitt 2007; Emery et al. 2011; Sharkey et al. 2019). The Trojans show evidence of absorptions in the 3–4 μm spectral region, stronger for the less-red than the red (Emery & Brown 2003; Brown 2016), enabling even stronger constraints on abundances of ices and hydrated minerals, and suggesting the red slopes of Trojans are not due to organics (Emery & Brown 2004). Brown (2016) reports an absorption at $\sim 3.1\,\mu {\rm{m}}$ in several less-red Trojans, which may be due to H₂O frost or NH-bearing material. The spectral region between 2.5 and 2.95 μm contains strong absorptions of ices and hydrated silicates, but is completely inaccessible to ground-based telescopes, so no data currently exist of Trojan asteroids in this wavelength range. Fine-grained silicates have, however, been detected on Trojan asteroids from an emissivity feature at mid-infrared wavelengths (Emery et al. 2006).

Zoom In Zoom Out Reset image size

The Linear Etalon Imaging Spectral Array (LEISA) will measure NIR spectral images of Lucy targets from 1 to 3.6 μm with a sensitivity sufficient to detect absorptions deeper than 4% (Olkin et al. 2021) (R-12). The spectral resolution of 10 nm will resolve absorption features of ices, organics, anhydrous minerals, and hydrated minerals that occur in the LEISA spectral range (R-13; Figure 6). The center of the OH feature at 2.8 μm and the 3.4 μm organic absorption feature will be resolved with 10 and 20 elements, respectively, which are adequate for detection and characterization (De Sanctis et al. 2017). Observations when the targets do not fill the LEISA FOV will be used to search for global-scale compositional variations on the surfaces (R-14). Regional and local scale observations will be used to search for compositional anomalies resulting from freshly exposed materials, as a window into the interior compositions of the Lucy targets (R-15).

Surface temperatures provide important input for understanding the stability of materials detected (or not detected) on planetary surfaces (Spencer & Denk 2010; Capria et al. 2014; Tosi et al. 2014), and temperature anomalies often indicate interesting surface processes (Howett et al. 2010). Temperature maps are therefore valuable companions to compositional and geologic maps used to investigate correlations and anomalies. Observations in the thermal infrared, particularly at multiple times of day, enable determination of thermal inertia, which, in turn, gives insight into the grain size and porosity of the regolith (Rozitis et al. 2020).

Thermal inertia estimates have been published for a few Trojan asteroids, and they are comparable to or lower than the lunar value (≤100 J m⁻² K⁻¹ s^−1/2; Mueller et al. 2010). Spectral emissivity peaks in the 10 and 20 μm spectral regions indicate fine-grained (∼1 μm) silicates, in a high-porosity regolith (Emery et al. 2006; Vernazza et al. 2012), consistent with the low-thermal inertias. Compacted regions of the regolith and exposures of ices could lead to regions of different thermal inertias than their surroundings. Widespread ices do not appear to be present on the surfaces of Trojan asteroids, as indicated by the featureless NIR spectra described above. However, Guilbert-Lepoutre (2014) find that Trojans could support water ice ∼10 m below a dusty mantle at their equators, and as close as 10 cm to the surface in polar regions or even at the surface in isolated cold traps. If ice exists below the regolith, we may expect to see it in exposures of subsurface material on the Lucy targets.

The Lucy Thermal Emission Spectrometer (L'TES) is a point spectrometer that will measure thermal flux spectra of the Lucy targets over the range 7–100 μm (Olkin et al. 2021). From these spectra, surface temperatures will be determined with an accuracy $\leqslant 2$ K for temperatures ≥75 K. L'TES is required to observe the thermal flux at $\geqslant 4$ locations, including at least one nightside measurement and at least one observation within 30° of the subsolar point (R-17).

The discovery of (243) Ida's satellite Dactyl during the Galileo flyby in 1993 (Belton et al. 1996) was the first confirmed detection of a satellite for any small body other than Pluto. Since then, there has been an explosion in the number and variety of satellites that have now been found in every small-body population (Merline et al. 2002; Noll et al. 2008, 2020a; Margot et al. 2015). Asteroids are also known to exhibit activity (Jewitt et al. 2015) and rings have been found around the TNO Haumea, the Centaur Chariklo, and possibly the Centaur Chiron (Braga-Ribas et al. 2014; Ortiz et al. 2015, 2017). Taken together, the possibility of satellites and other near-asteroid material must be considered during any small-body flyby. Two of the Lucy mission targets, Patroclus and Eurybates, are already known to have satellites (Merline et al. 2001; Noll et al. 2020b), thus the need to plan for satellite observations as part of the Lucy mission flybys is essential.

Satellites are stable within a fraction of the Hill sphere of the system, with the stable inner third of the Hill radius sometimes referred to as the Szebehely radius (Szebehely 1967, 1978). The Hill radius is given by ${R}_{\mathrm{Hill}}=a{\left({M}_{1}/3{M}_{\odot }\right)}^{1/3}$ where a is the semimajor axis and M₁ and M_⊙ are the masses of the primary and the Sun respectively. The vast majority of known small-body satellites orbit at distances from the primary of just a few percent or less of the Hill radius. At Trojan distances from the Sun and for a density of $\rho =1000$ kg m⁻³, the Szebehely radius is equivalent to ${R}_{\mathrm{Sz}}\approx 210\,{r}_{1}$ (where r₁ is the asteroid radius). The region of stable orbits extends inwards to the Roche limit at ${R}_{\mathrm{Roche}}\approx 2.44\,{r}_{1}$. The known rings around three small bodies all have ${r}_{{ring}}\approx 3\,{r}_{1}$. Lucy will travel well within the Hill sphere of each Trojan target with close approach distances of ${s}_{\min }\geqslant 30\,{r}_{1}$.

Lucy's requirements include a search of the full Szebehely sphere for satellites with ${d}_{\mathrm{sat}}\geqslant 2\,{km}$. This search will be accomplished by Lucy using L'LORRI (R-17), but it is possible to supplement this with Earth-based observations from the Hubble Space Telescope (HST), Keck, and other observatories. Observations with HST can detect satellites with $d\geqslant 1\,{\rm{km}}$ over the Szebehely sphere outside of an inner region where scattered light from the primary dominates the background. Searches have been conducted for all of the Lucy Trojan targets yielding the detection of a previously unknown satellite of Eurybates shown in Figure 9 (Noll et al. 2020b, 2020c). This satellite, Queta, is a ${d}_{\mathrm{sat}}=1.2\pm 0.4$ km body in a relatively wide orbit, $a/{R}_{\mathrm{Hill}}=0.11$, near the limit of what is detectable using Earth-based observatories. Satellites of similar or larger size at angular separations of 05 or greater were not detected at the epoch of the HST observations (Noll et al. 2018). HST observations also constrain large companions at smaller angular separations, but with a complex trade off between relative size and orientation.

Zoom In Zoom Out Reset image size

Earth-based observations of lightcurves and stellar occultations can also, in principle, reveal the presence of satellites and rings, but a positive detection would require a combination of unlikely circumstances and larger aperture telescopes than are typically employed. The Lucy L'LORRI satellite search will occur on either inbound or outbound from close approach depending on where the solar phase angle is lowest. The search will occur in two separate phases, a distant search and a close-in search to cover both the full Szebehely sphere and satellites or rings that are at small separations from the primary. During the encounter, there will be many images obtained for other purposes that will also be available to search for and track any satellites or other orbital material that may be present.

Activity has not been observed in any Jupiter Trojan, although it has been observed in various closely related objects. Some Centaurs, thought to share an outer protoplanetary-nebular origin with the Trojans, display dust and gas that appears to be thermally driven and is of unknown (and likely diverse) origin (Jewitt 2009). However, Trojans' stable orbits reduce thermal forcing. Some main-belt asteroids (sometimes called main-belt comets) have also been observed to display episodic dust activity driven by collisions and other forms of mass shedding (Jewitt et al. 2015; Sanchez et al. 2019), which may be more relevant to Trojans. Lucy's science goal to understand the volatile inventory of Trojans will be indirectly tested by constraining current activity and searching for surface features indicative of outgassing.