The origin of these asteroid populations is a mystery, and its solution would tell scientists much about the dynamics of the young solar system. Planetary scientists now believe that the orbits of the giant planets migrated in toward the sun and then out again soon after their formation. In the process, they scattered asteroids and comets hither and thither. Jupiter’s Lagrangian orbits may have been sticky gravitational traps that caught a diverse sample of bodies that originated from throughout the outer solar system to its fringes. Another theory suggests that the Trojans originated in the same region as Jupiter and followed it in its movements and are therefore samples of conditions where Jupiter formed. Either way – and it’s possible that the present population represents a mixture of sources – these bodies hold clues to conditions and processes from the infancy of our solar system.

The most recent planetary decadal survey emphasized the importance of these bodies by prioritizing a mission to explore them. “Trojan asteroids, at the boundary between the inner and outer solar system, are one of the keys to understanding solar system formation. Originally thought to have been captured from the outer parts of the asteroid belt, Trojan asteroids are proposed in new theories to have been captured instead from the Kuiper belt during a phase of extreme mixing of the small bodies of the solar system. In-depth study of these objects will provide the opportunity to understand the degree of mixing in the solar system and to determine the composition and physical characteristics of bodies that are among the most primitive in the solar system.”

The report listed three key questions related to the study of Trojan asteroids in context with other bodies throughout the outer solar system:

What were the initial stages, conditions, and processes of solar system formation and the nature of the interstellar matter that was incorporated? Important objects for study: comets, asteroids, Trojans, and Kuiper belt objects.

“What governed the accretion, supply of water, chemistry, and internal differentiation of the inner planets and the evolution of their atmospheres, and what roles did bombardment by large projectiles play? Important objects for study: Mars, the Moon, Trojans, Venus, asteroids, and comets.

“What were the primordial sources of organic matter, and where does organic synthesis continue today? Important objects for study: comets, asteroids, Trojans, Kuiper belt objects, Enceladus, Europa, Mars, Titan, and Uranian satellites.”

The Lucy looks to the New Horizon Pluto mission for two of its instruments with near copies of that mission’s LORRI high resolution camera and the RALPH color camera and imaging spectrometer. The third instrument is a thermal emission spectrometer derived from an instrument on the OSIRIX-REx asteroid mission. Data from these instruments will provide information on the processes that shaped these worlds, their composition, and physical properties of the surface material such as the average size of particles. Tracking of the spacecraft’s radio signal will provide information on each asteroids mass and therefore density which provides clues to their composition and to whether they are solid objects or rubble piles.

The creativity behind the Lucy mission is that its proposers found a trajectory that over 12 years encounters seven asteroids (two in a binary system). The Lucy mission will encounter its targets using two large solar orbits that take it out to the orbit of Jupiter to encounter the Trojan swarms. In the first of these orbits, it will fly by a tiny main belt asteroid (DonaldJohanson, named after the paleontologist who led the team that found the Lucy fossil) and then four diverse asteroids in the Greek population. The next orbit takes it into the Trojan population for a single encounter with a binary asteroid system whose characteristics are similar to those of comets suggesting they may be refugees from the distant outer solar system. After this second long orbit, the spacecraft should have sufficient fuel for further encounters with main belt and Trojan asteroids in a third orbit if NASA approves funding for an extended mission. (Each of these extended orbits appear to take approximately six years, so any encounters from an extended mission seem likely to occur in the late 2030s.)

The Lucy mission will study a variety of asteroids through brief, but intense flybys. It will be something like photographing boulders along the roadside while speeding by on a freeway for later analysis. The second Discovery mission selected, by comparison, will be like parking your car next to one especially intriguing boulder for a nearly yearlong examination.

The single destination for the Psyche spacecraft will be the relatively large asteroid of the same name. This world is the largest of the rare (type M) metallic asteroids. Psyche could be unique remnant of a class of asteroids that formed so close to the sun that only metals could condense out of the early solar nebula and was later flung into the main belt of the asteroids. Or it could be the inner, metallic core of a once larger protoplanet that had its overlying layers of rock and possibly ice blasted off by impacts with other asteroids.

Telescopic observations reveal that Psyche’s surface is 90% metallic and 10% silicate rock. The spacecraft’s instruments should distinguish between these scenarios by measuring the composition in detail and looking at the arrangement of the silicate material. The mission’s principal investigator wrote me, “If the silicate material is primarily high-magnesian pyroxene or olivine, then these silicates are likely the remnants of a crystallizing magma ocean, and indicate that Psyche started as a differentiated planetesimal and had its mantle stripped, validating the mission’s prime hypothesis for this body. If the silicates are all primitive chondritic material, then they were likely added as later impacts, and Psyche may have started life as a highly reduced metallic body without a significant silicate mantle, or, the nature of impact flux and its consequences are far more significant than our current models indicate. The numbers and shapes of craters on Psyche’s surface may help decipher that story.” The spacecraft’s gamma-ray and neutron spectrometer (derived from an instrument on the MESSENGER Mercury orbiter) will help determine the asteroid’s bulk elemental composition.

Psyche the asteroid won’t be an unchanged relic. Its original surface will have been battered by numerous impacts over the subsequent billions of years. The hydrated materials recently discovered on its surface with telescopic studies, for example, are likely to have been delivered by impacts of other asteroids. It’s possible that by now, the body is a jumbled rubble pile. The cameras on the spacecraft (near copies of the cameras that the Mars 2020 rover will carry) will be tasked with taking the images that will allow geologists to reconstruct its history. By using filters tuned to specific wavelengths of visible and near-infrared light, the camera’s images also will help map the surface’s fine-scale composition.