If the Nice Model or “Great Migration” applies, traces of this planetary migration can be read from the Trojans. Because once Jupiter’s Lagrange points take hold, the orbits of its peers are actually pretty stable. Only when Jupiter developed resonance with Saturn, the second heaviest planet, that is, their orbital times reached an integral multiple of each other, did the calculated disturbance forces appear. At this time, smaller bodies in the Solar System, especially including the distant Kuiper Belt, were likely to have mixed and then become trapped in today’s Lagrangian points of Jupiter. If this assumption is correct, then the Trojans must have been rich in ice and organic matter, like the objects in the Kuiper Belt.

Lucy searches for traces of chaos

Many of the basic assumptions of the Nice model are now accepted, but the interpretation of giant wandering planets poses new problems. One of them is the existence of Mars, Earth, and other minor rock planets that must have been destroyed or thrown out of their orbits by moving gas giants. Therefore, now even more complex models have been developed. Lucy can now test her original assumptions: “The idea is that the Trojans were caught during this chaotic phase,” says deputy mission director Cathy Olkin. If the Trojans visited are very different from each other, this indicates an early stage of chaos. If they are similar, on the other hand, they may have originated on-site.

When viewed with terrestrial telescopes, little can be learned about the Trojans, but spectroscopically they appear to be truly odd. Some have a completely contourless surface, most are very dark in color, and some have hints of ice and organic compounds. Perhaps, however, their surfaces have also been redesigned by space impacts, collisions, heating and outgassing in such a way that their true nature is only revealed when they fly by.

“The most important criterion in our selection was to fly as many different Trojans as possible,” says Kathy Olkin. Lucy is an exploration mission because it is the first time it enters a previously unknown region of a planetary system. If the probe survives twelve years in space, even more is likely: the researchers plan to search for further targets at Lagrange point L5 during the mission.