Artist depictions of NASA's Psyche (upper left) and Lucy (lower middle) missions that will explore special asteroids to probe the earliest formation of our solar system. (NASA/JPL-CalTech/SwRI/Peter Rubin/SSL)
NASA has given the green light for two new robotic space missions, "Lucy" and "Psyche", which will launch in the early 2020s. And though they are separate missions bound for different destinations, their common goal is to investigate the distant past when the infant solar system was a mere 10 million years old.
Some of the big questions Lucy and Psyche may help answer include: what were conditions like when our solar system was in its infancy? Where in the solar system did the planets originate? And how did the planets form?
These missions and their retrospective goals take advantage of the fact that there are objects still flying about the solar system that have remained mostly unchanged since the solar system formed. Items including comets and in the case of Lucy and Psyche's destinations, asteroids, that are like celestial time capsules.
Meet Spacecraft Lucy
The Lucy spacecraft is bound for a special group of asteroids clustered in Jupiter's orbit, called "Trojans." Between 2027 and 2033, Lucy will visit as many as six different Trojan asteroids.
There are two groups of Trojan asteroids, which accumulate at special locations that lead and trail Jupiter in its orbit around the sun. At these locations, called "Lagrangian points," the gravitational pull of Jupiter and the sun are equal, and somewhat cancel out to create gravitationally stable "pockets" that asteroids get trapped in—kind of like how dust bunnies accumulate under couches protected from the air currents.
The dense concentration of asteroids in the Trojan groups not only offers a sort of "Mother Lode" of nuggets for Lucy to prospect in, but also a variety of materials that may have originated in widely different regions of the outer solar system, possibly far beyond Jupiter's orbit. This is not unlike how a geologist might explore and collect samples from the downhill end of a river wash, where rocks and soils from different places high in the mountains may have accumulated.
Since Lucy will be studying ancient, largely unaltered material that originated from Jupiter's orbit and beyond, we will get a first-hand analysis of the substances from which the gas giant planets and their moons formed long ago. A better understanding of the nature of the material might even tell us something about the interiors of those planets today.
Lucy was named after the famous 3.2-million-year-old Australopithecus fossil skeleton found in Ethiopia in 1974. Like those ancient bone fossils, the Trojan asteroids are a form of archaeological evidence that can help us reconstruct a picture of the solar system's past.
The Psyche Spacecraft
This spacecraft will target a singular destination: a very unusual and mysterious asteroid named 16 Psyche. To be launched in 2023, Psyche will arrive at its destination in 2030.
The asteroid 16 Psyche is three times farther from the sun than the Earth and measures about 130 miles across. What makes it so mysterious is that it isn't a typical asteroid composed of rocky or icy materials. 16 Psyche appears to be made of solid metal—iron and nickel.
This means that 16 Psyche is much more similar to Earth's iron-nickel core than a common asteroid, which has led some scientists to speculate that it might actually be the core of a primordial planet whose lighter rocky mantle layers were stripped away long ago, perhaps by a series of violent collisions with other objects.
Scientists believe that the planets were formed around 5 billion years ago in a process of accretion, where smaller pieces of material came together to form larger chunks which in turn were the building blocks of bigger hunks. But they do not know how this "snowballing" process actually took place.
At some point in the process a young planet like Earth, hot from the bombardment of material that formed it, would have been largely molten. The heavier liquid materials like iron and other metals would have sunk to the center to form a dense metallic core, while lighter silicate materials floated upward to form the mantle and crust.
The problem with exploring this idea is that the interiors of today's planets are buried deep, hidden from direct view, and we cannot see their core and mantle layers exactly as they are. We have achieved a fuzzy picture of what Earth's inner layers may be like by tracking the motion of seismic waves that echo and bounce around inside, but the picture lacks much detail.
But if 16 Psyche is a naked iron planetary core as some suspect, then the Psyche mission will give us a window to peer directly to the core of what was once a planet—a planet maybe as large as Mars, judging by the dimensions of this iron asteroid. This would give us an unprecedented glimpse into how that planet initially formed during its molten phase long ago.
In the field of archaeology, processes like weathering and decomposition and tectonic activity tend to erase, consume, or bury evidence that could tell us something about the past. That's why archaeologists seek out places where conditions are best for preserving that evidence, like ancient ice, fossil beds, bogs and the like.
Lucy and Psyche's expeditions will dig up clues from as far back as five billion years ago, when the solar system was less than 10 million years old. It's a great time to be alive when we can send robots millions of miles into space to explore the solar system in the distant past!