NASA'S Lucy Mission to Trojan Asteroids: Unraveling the History of the Outer Solar System

Abstract

Lucy is a NASA Discovery-class mission to send a highly capable and robust spacecraft to investigate primitive bodies near both the L₄ and L₅ Lagrange points with Jupiter; the Jupiter Trojan asteroids. This heretofore unexplored population of planetesimals from the outer planetary system holds vital clues to deciphering the history of the Solar System. Due to an unusual and fortuitous orbital configuration, Lucy will perform a comprehensive investigation that visits eight Trojans, including all the recognized taxonomic classes, a collisional family member and a near equal-mass binary. It will visit objects with diameters ranging from roughly 1 to 100 km. In particular, Lucy will perform flybys of (3548) Eurybates and its satellite Queta (L₄, C-type), (15094) Polymele and its currently unnamed satellite (L₄, P-type), (11351) Leucus (L₄, D-type), (21900) Orus (L₄, D-type), and the (617) Patroclus-Menoetius binary (L₅, P-types). This diverse array of targets will supply invaluable constraints on the formation and early dynamical evolution of the giant planets. In addition, Lucy will visit two main-belt asteroids, (152830) Dinkinesh and (52246) Donaldjohanson, in order to practice its encounters. Lucy's payload suite consists of a color camera and infrared imaging spectrometer, a high resolution panchromatic imager, and a thermal infrared spectrometer. Additionally, two spacecraft subsystems will also contribute to the science investigations: the terminal tracking cameras will supplement imaging during closest approach and the telecommunication subsystem will be used to measure the mass of the Trojans. Lucy launched on October 16, 2021 and will have encounters with the Trojans from August 2027 until March 2033.

Fig. 1

A schematic of the effects of planet migration on the small body reservoirs in the outer Solar System (i.e., the Trojans and Kuiper belt). Objects initially in a disk are gravitationally scattered by the migrating planets. Most are ejected from the Solar System, but a small fraction are captured in the Trojan swarms and Kuiper belt. Any compositional gradients in the original population (represented by arbitrary colors) are mixed in the Trojans. The degree of mixing is model dependent

Fig. 2

Top: The fraction of different spectral types as a function of size in the Trojan swarms DeMeo and Carry (2014). Bottom: The idealized reflectance spectra of primitive asteroids according to the Bus–DeMeo classification scheme (DeMeo et al. 2009). This taxonomy is based on the spectra of asteroids from 4500 Å to 2.45 microns. C-type asteroids are gray in color. D-type are very red, while P-types are moderately red. Reproduced from Levison et al. (2021b)

Fig. 3

The visible color slopes of Trojans are bimodal, as shown in this histogram (Wong et al. ; Schemel and Brown 2021). The Lucy targets, which will be discussed in detail in Sect. 4, fall as indicated, sampling this color range. The black symbol is the combined, unresolved color of the Patroclus–Menoetius binary

Fig. 4

An infrared color-color plot of Trojan asteroids with the Lucy targets identified (Emery et al. 2011). Reproduced from Levison et al. (2021a)

Fig. 5

The distribution of Trojan albedos in two wavelength bands: visible and 3.4 microns (W1). The small symbols represent all Trojans with well-measured values (Grav et al. 2012), while the large symbols represent the Lucy targets. Note that our targets cover the same range as the population as a whole. They will be described in more detail in Sect. 4

Fig. 6

The trajectory of Lucy in a frame that rotates with Jupiter as it orbits the Sun. Lucy’s orbit is shown in green, while the orbits of Jupiter and Earth are shown in orange. Since this is not an inertial frame, the trajectory of Lucy does not appear Keplerian. The approximate region of space that the Trojans occupy are shown in brown. Major mission events are labeled alphabetically in chronological order. For each encounter, the relative velocity of the spacecraft with respect to the target, $v_{\mathrm{rel}}$, the approach phase angle, $\varphi$, and the close approach distance, $d_{C/A}$, are listed. The (P) indicates that $d_{C/A}$ is for Patroclus

Fig. 7

Photos leading up to the Lucy launch on October 16, 2021

Fig. 8

The distribution of L₄ Trojans in a space defined by the pseudo-proper semi-major axis ($a_P$), pseudo-proper eccentricity ($e_p$) and pseudo-proper inclination ($I_p$). These pseudo-proper elements are defined in Brož and Rozehnal (2011). The sizes of the crosses represent the relative diameters of bodies. The asteroid (3548) Eurybates, around which a significant cluster is visible, is shown in red. This figure is reproduced from Brož and Rozehnal (2011)

Fig. 9

The view of Polymele as seen from the spacecraft during its encounter. The time relative to the instant of close approach is listed in each panel. The sunlit region of Polymele is shown in red, while the shadowed region is in black. Polymele is assumed to be a ellipsoid with principal axis of [27.0×24.4×10.4] km. Polymele’s north pole can be seen as the cyan dot in the left-most panel. The possible locations of Shaun are shown in green, assuming an orbital semi-major axis of 204.4 km

Fig. 10

The observed rotational period distribution of Trojan asteroids in the Asteroid Light Curve Database (Warner et al. 2021). The location of Leucus, which has a rotational period of 446 hours, is shown

Fig. 11

The cumulative number of craters per unit area on Trojan asteroids for surfaces of 4.4 Gyr (red dashed line) and 100 Myr (blue solid line) assuming the current impact probability and velocity of the Trojans, and using the impactor size-distribution Bottke et al. (2015a) and impact rate temporal evolution O’Brien et al. (2014) from the Main Belt. We also adopt the scaling law of Holsapple and Housen (2007) for a target strength of ${10}^6\text{ N}/{\text{m}}^2$. The horizontal dashed lines indicate the crater density at which there should be one crater per hemisphere for each Lucy target

Fig. 12

The spectral range of the two components of the L’Ralph instrument (MVIC and LEISA) and spectra of species of ices, tholins and minerals which maybe visible on Trojan asteroids. Reproduced from Levison et al. (2021a)