When the first spacecraft to journey outside the Solar System were built, NASA included plaques in case aliens ever found them. For the Voyager missions, these were upgraded to golden records, subsequently the inspiration for many memes. Such messages serve less purpose for objects that will never escape the Sun's pull, so one craft will feature a different version, a message not for an alien civilization, but future human spacefarers.
Lucy is a mission to the Trojan Asteroids. These sit at the L4 and L5 of Jupiter's orbit, gravitationally confined to orbit the Sun either 60° ahead or behind the giant planet. The Trojans are thought to have undergone less alteration through collisions with other asteroids than those in the inner solar system or main asteroid belt. Consequently, they should provide insight into the pieces from which the planets formed.
Lucy will visit one main belt asteroid and seven Trojans, on six orbits around the Sun, including representatives of at least three Trojan whose appearances suggest differing compositions. Lucy's initial mission is set to end in 2033, although it may be extended. Nevertheless, it will continue to journey on an elongated orbit that will bring it close to the Earth, and out beyond Jupiter, until it has an unfortunate encounter with a space rock, or some future astronauts collect it.
Should the latter occur, NASA wants to send them a message, and last week installed a plaque on the under-construction space probe. To decide the content, NASA consulted Nobel Prize winners in literature, rather than science, as well as Poet Laureates, musicians, and science writers.
All going well, Lucy will launch on October 16, 2021, and the plaque includes a representation of the Solar System with the locations of the planets on that day, and the craft's initial orbit. Most missions to the outer Solar System take many years to plan and build – one of the reasons the idea of a voyage to the recently discovered megacomet never got far. Having been approved only in 2017, Lucy will be one of the quickest deep space missions to come to fruition.
The first Trojan found was named Achilles, starting a tradition of names from Homer's epic tale. With two exceptions, the Greek camp of Trojans (named after the war's victors) lead Jupiter around its orbit, with the Trojan camp (named after those on the war's losing side) trailing behind, safely separated. Although no Trojans are close to the size of Ceres or Vetsta, it is thought there are as many between the two locations as in the main asteroid belt.
There are competing theories about how the Trojans came to have their current orbits. If Lucy can resolve this question, it could tell us a lot about other aspects of the Solar System's formation, including the journeys the giant planets went on to reach their current locations.
The name Lucy comes from the Beatles' song, via the famous fossil of an early human ancestor named after the song playing at the campsite after discovery. The name was chosen, not because the Trojans are expected to contain diamonds, but because they too represent a sort of fossil, one from the very dawn of the solar system.