NASA made a resolution for the new year to go back to our roots.
On 4 January, NASA selected two Discovery missions to explore the earliest stages of our solar system. Lucy will be launched in October 2021 to journey toward the Trojans, an asteroid swarm that leads and trails Jupiter as it swings through its orbit. Psyche will launch in 2023 toward the only metal asteroid in our solar system: potentially the frozen core of a long-dead planet.
These two missions came out on top of a competition that started in March of 2014 with a field of 28 proposed Discovery missions. Teams of scientists spent months preparing proposals for NASA to consider. The winners, chosen in a peer-review process, get funding, mission management, and systems engineering support from NASA.
NASA Planetary Science Division director Jim Green admits that the process is extremely rigorous, putting proposals through “the toughest scrutiny you can possibly imagine. These teams are really put through the wringer.”
“It was like a cross between the thesis defense from hell and some sort of Hollywood superproduction,” commented Lindy Elkins-Tanton, principal investigator for the Psyche mission and director of the School of Earth and Space Exploration at Arizona State University. The proposals are evaluated by a team of science, industry, and technical experts for the rigor of their science and their grounding in the National Academy’s decadal questions, the academy’s list of broad issues to explore in planetary science over the next decade.
The panel evaluated the initial 28 proposals and selected 5 for the second round, which examined each proposal’s implementation plan. Two proposals came out on top. “That whole process separates doing good science from doing the top science,” Green said, “from going after the top questions that really move our field ahead.” Lucy and Psyche were the best prepared to move ahead.
Lucy in the Sky
The Lucy mission is named after the fossil that revolutionized our understanding of human origins. The mission team hopes to do the same for the origins of our solar system by exploring the Trojan population of asteroids, remnants from the solar system’s formation. Hal Levison, principal investigator for the Lucy mission and planetary scientist at the Southwest Research Institute, likened this goal to deciphering a crime scene. “Sometimes the blood splatter on the walls tells you more about what happened than the bodies on the floor,” he said, “and in this case the splatters on the wall are the [asteroid] populations.”
We know little about the Trojans, Levison said, but we do know that they exhibit great diversity. This variation likely stems from a wide range of origins: The asteroids developed in different parts of the solar system, then migrated to their present positions. “By studying those differences,” Levison continued, “we’re going to try to untangle how the planets moved around.”
“Basically, the planets are aligning for us to do this mission,” Levison said. Lucy’s trajectory will carry it past six Trojan asteroids, including the Patroclus-Menoetius binary, a pair of asteroids whose high-inclination orbit prevents them from being studied easily. “It just so happens to be traveling through the plane of the solar system at the time the spacecraft is going by,” said Levison, referring to the asteroid pair. “We have this unique list of targets that would be very hard to reproduce in the future.”
Psyche: A New World of Metal
The Psyche mission will look at a feature that is the only one of its kind in our solar system: an asteroid made completely of metal. “This is not just a unique object,” Elkins-Tanton said of the Psyche asteroid, “it’s not just the only object like it in the solar system, but it’s also an improbable object.”
Psyche is most likely the core of a forming planet that early in the solar system’s history was so pounded by other objects that it lost its outer layers. This sort of occurrence could happen once or twice in a solar system but often doesn’t happen at all. If it is a core, it will give insight into the early solar system as well as the cores of our own planets.
Elkins-Tanton said the first job of the mission is to determine whether Psyche is, in fact, a naked core. “If it’s not a core, then it’s something so exotic that it actually hasn’t even been thoroughly hypothesized about,” she remarked, “and that would be even more exciting.”
In the course of the mission, the spacecraft will examine Psyche’s magnetic field, composition, and surface topography. Elkins-Tanton is most interested in that final point. We don’t know what metal craters look like, she said. On impact, molten metal could freeze into tall spires before it has the chance to fall back to the ground, or the surface could shatter like glass. It will be an unexplored metal world. “Everything we measure will be new,” she said.
A Snapshot of Our Beginnings
Both missions will trace the earliest stages of our solar system’s formation. “It just so happens the two [missions] we picked go after completely different regimes in our solar system, but in the first 10 million years,” said Green. Lucy will give us insight into the formation in the outer part of the solar system, whereas Psyche will tell us about early terrestrial planet formation.
As in summer 2015’s New Horizon’s flyby of Pluto, the scientists have a basic idea of what to expect—but they also anticipate surprises. These missions will examine objects that have never been studied in such detail, and they intend to live up to the program’s name: Discovery.
Elkins-Tanton hopes that the missions will move beyond even our scientific knowledge of the solar system to affect people around the world at a deeper level. “If it can inspire people to stand up and start solving problems that they are not now solving,” she said, “that would be the greatest outcome.”
—Elizabeth Jacobsen, Staff Writer