The Cosmos

From NASA: A little over 4 billion years ago, the planets in our solar system coexisted with vast numbers of small rocky or icy objects orbiting the Sun. These were the last remnants of the planetesimals – the primitive building blocks that formed the planets. Most of these leftover objects were then lost, as shifts in the orbits of the giant planets scattered them to the distant outer reaches of the solar system or beyond. But some were captured in two less-distant regions, near points where the gravitational influence of Jupiter and the Sun balance, and have remained trapped there, mostly untouched, for billions of years.

Three years from now, a spacecraft named Lucy, inspired by the famous fossil, will begin its exploration that could help determine the early history of the Solar System.

NASA’s Lucy mission will fly by six of those trapped planetesimals – the Jupiter Trojan asteroids – giving humanity its first glimpse of these ancient objects. By studying these fossils of planet formation, the Lucy mission could reveal as much about the development of the solar system as the Lucy fossil did about human evolution. And on the way to the Trojans, Lucy will visit an asteroid that the team has named Donaldjohanson, after the anthropologist that discovered the fossilized skeleton of our ancestor.

The Trojans orbit the Sun in synchrony with Jupiter, following almost the same path, but leading the giant planet by about one-sixth of the way around the orbit, or trailing by the same amount. This keeps them near one of two gravitationally stable “Lagrange” points, L4 and L5, positioned at the apex of an equilateral triangle with Jupiter and the Sun, where they are protected from being perturbed onto different orbits or out of the solar system entirely. The areas around Jupiter’s L4 and L5 points each contain a swarm of objects billions of years old that hold information about the history of our solar system.

Earth-based observations have enabled astronomers to classify the Trojan asteroids by subtle variations in color and likely composition. “We see variation in the properties we can measure from the Earth and we would like to know the physical basis behind this variation,” Lucy Project Scientist Keith Noll said. “A mission to a single object would not have allowed that kind of comparison – by sampling a diverse set of objects, Lucy will provide a better basis for understanding what we are seeing in the broader population.” By visiting six Trojans spanning all of the major types, two of which make up a binary system (two objects that orbit each other), Lucy will gain a wealth of information about the objects that made up the solar system’s original planetesimal disk. Noll works at NASA’s Goddard Space Flight Center in Greenbelt, Maryland which is a key partner in the Lucy mission.

Some of the same processes that trapped the Trojans in their present orbits sent other leftover planetesimals farther from the Sun, and we now find them in the Kuiper Belt, the icy region beyond Neptune that is home to Pluto and other dwarf planets. (NASA’s New Horizons spacecraft explored Pluto following a 9-year journey, and will fly past another Kuiper Belt Object on New Year’s Day 2019.)

Data collection will be vital to Lucy’s success. The mission will carry four instruments in its payload: L’Ralph, consisting of MVIC (Multi-spectral Visible Imaging Camera), a multi-color imager, and LEISA (Linear Etalon Imaging Spectral Array), a spectrograph that will provide information on surface composition; L’LORRI (Long Range Reconnaissance Imager), a high-resolution camera; and L’TES (Thermal Emission Spectrometer), which will measure the surface temperatures of the Trojans. And in addition to the scientific instruments, Lucy’s communications (radio) and target acquisition system (TTCam) will contribute to the science mission. L’Ralph will analyze the Trojans’ surfaces to look for the presence of different silicates, ices and organics on these asteroids. L’LORRI will take high-definition pictures of the Trojans supplemented by TTCam at closest approach. L’TES will investigate the physical state of the Trojans’ surfaces, and the radio data will be used jointly with the imaging data to determine their masses and densities.

Lucy is scheduled to launch in October 2021, flying by more targets in different orbits around the Sun than any other mission in history. Answers to key questions about the solar system’s distant past will be now within reach, thanks to the Lucy mission.