In 1974, paleoanthropologist Donald Johanson came across an unusual discovery. While working in the field in Ethiopia, he noticed a small bone poking out from one of the many slopes in the region that had the distinct shape of an arm. But it wasn’t just any arm; Johanson quickly came to realize that it likely belonged to an ancient member of a hominin species.
Within weeks, scientists excavated nearly 40% of a complete skeleton, which they named “Lucy”, after the Beatles’ song “Lucy in the Sky with Diamonds”. What was peculiar about the fossilized remains was that they didn’t seem to match with any other discoveries in the region. Over time, as more skeletons emerged, scientists identified Lucy as a member of a brand new species: Australopithecus Afarensis.
In the decades since, Lucy has come to revolutionize our understanding of human evolution. The current estimate for her age is just over 3 million years, which places her species just ahead of the earliest known members of the genus Homo. However, just because Lucy isn’t a direct member of our family doesn’t mean that she doesn’t display behaviours commonly associated with the genus Homo. In fact, the largest revelation that has come from Lucy’s discovery has to do with something we would identify as very “human”: walking. Analysis of her skeleton has proved that our ancestors were walking as early as 3 million years ago. This combination of bipedal motion with ape-like characteristics led her to be labelled by some as the “missing link” in human evolution. While there is still much to learn about our origins, there is no doubt that Lucy has played a critical role in the little that we do know.
So, it is fitting that NASA’s latest interplanetary mission, which seeks to unravel the origins of our solar system, has also been named “Lucy”.
Within weeks, scientists excavated nearly 40% of a complete skeleton, which they named “Lucy”, after the Beatles’ song “Lucy in the Sky with Diamonds”. What was peculiar about the fossilized remains was that they didn’t seem to match with any other discoveries in the region. Over time, as more skeletons emerged, scientists identified Lucy as a member of a brand new species: Australopithecus Afarensis.
In the decades since, Lucy has come to revolutionize our understanding of human evolution. The current estimate for her age is just over 3 million years, which places her species just ahead of the earliest known members of the genus Homo. However, just because Lucy isn’t a direct member of our family doesn’t mean that she doesn’t display behaviours commonly associated with the genus Homo. In fact, the largest revelation that has come from Lucy’s discovery has to do with something we would identify as very “human”: walking. Analysis of her skeleton has proved that our ancestors were walking as early as 3 million years ago. This combination of bipedal motion with ape-like characteristics led her to be labelled by some as the “missing link” in human evolution. While there is still much to learn about our origins, there is no doubt that Lucy has played a critical role in the little that we do know.
So, it is fitting that NASA’s latest interplanetary mission, which seeks to unravel the origins of our solar system, has also been named “Lucy”.
All interplanetary missions launched to date have contributed to our understanding of the formation of the solar system in one way or another. However, we still know very little about this period of time largely because the solar system has evolved significantly since then. As a result, many scientists have turned their attention towards bodies that have been largely untouched for billions of years. Instead of learning about the origins of planets by studying the planets themselves, these scientists are focusing on the building blocks the planets grew from.
In the early days of the solar system, the sun was surrounded by a protoplanetary disk of gas and dust, which eventually coalesced into the planets and moons that exist today. However, not all of the available material was used up in this process; otherwise, planets would be significantly more massive than they actually are.
You can think of it as trying to assemble a particular structure from a box of assorted Legos. While many of the blocks might come in useful, you will likely have some leftovers. In the solar system, these cosmic “leftovers” are found in many forms: asteroids, comets, and other pieces of debris. As a consequence of the enormous gravitational forces produced by the sun and the planets, much of this debris has bunched up in certain areas of the solar system.
Take, for instance, the asteroid belt. It’s not a coincidence that a bunch of asteroids just happened to fall into the space between Mars and Jupiter. Instead, we can attribute this to the massive gravitational influence of Jupiter, which has essentially acted as a shepherd for stray asteroids over billions of years.
And it’s not just the asteroid belt either. Remnants of the solar system can also be found in many other places, like the Kuiper Belt and Oort Cloud. Because each of these regions has been shaped by unique conditions, they all contain different perspectives on the origins of the solar system. That’s why missions into space have been distributed across a wide variety of targets from the asteroid belt, all the way out to the distant Kuiper Belt, which exists beyond the orbit of Neptune. However, in all of this exploration, one group of interesting cosmic leftovers has been left out: the Trojan asteroids.
Generally speaking, a Trojan asteroid is any asteroid that exists in the same orbit as a planet. This is possible because of a gravitational phenomenon known as a Lagrange point. Every planet has two Lagrange points in its orbit where the gravity of the sun and the planet exist in a delicate harmony that allows objects located at those two points to maintain their positions indefinitely. These two points, known as Lagrange points 4 and 5 (or L4 and L5) always exist 60 degrees ahead of and behind the planet (L1, L2, and L3 also exist, but they are less stable than L4 and L5).
So far, scientists have spotted Trojan asteroids at the Lagrange points of quite a few planets. Mars has at least 4, and both Saturn and Neptune are also known to have Trojan swarms of their own. Even Earth has at least 1 confirmed trojan asteroid that is always leading us in our orbit. But by far, Jupiter has the most Trojan asteroids at nearly 5000 confirmed bodies.
What makes the Jupiter Trojans so compelling is that they are direct remnants from the formation of the outer planets. While we often call these planets the gas giants, they are thought to have rocky cores, and by studying Trojan asteroids we may be able to know how these planets came to be. Additionally, these asteroids haven’t experienced much in the 4 billion years of their existence, so they are as pristine as celestial bodies can get. You could even think of them as time capsules from the formation of the solar system!
Scientists have also identified three separate subgroups with the Trojans, called the C, D, and P-type asteroids, which have varying characteristics. Unfortunately, our understanding of the Trojan asteroids has been limited by the sheer distance separating us, and no spacecraft has visited them for an up-close look yet. However, the Lucy spacecraft is on a mission to change that.
First conceived more than a decade ago, the Lucy mission experienced an unexpected road to its launch last week. Initially, the plan was to have the spacecraft visit two unique Trojan asteroids. However, in 2014, trajectory planners noticed that Lucy would pass by an additional asteroid, which encouraged them to continue experimenting with different orbits and targets. Eventually, the initial scope of 2 asteroids expanded to a portfolio of 7 Jupiter Trojans and an additional main-belt asteroid (which happens to be named 52246 Donaldjohanson, after the paleoanthropologist who discovered the Lucy fossil in 1974).
Each of these 8 bodies is unique in its own way. For instance, 4 of the asteroids are thought to exist in binary pairs, and altogether, the 7 Trojan targets include members from all 3 known subgroups of Jupiter Trojans. No single spacecraft has ever visited so many bodies before, but that isn’t the only record that Lucy will be setting either! It will also become the farthest solar-powered spacecraft from the sun thanks to its massive twin solar arrays.
To visit so many asteroids on a limited fuel tank, Lucy will make use of multiple gravity assists over its 12-year main mission. Simply put, a gravity assist is when a spacecraft uses the gravitational field of a planet like a slingshot to gain some free velocity. The first gravity assist for Lucy is expected in the fall of next year, when the spacecraft will make a close fly-by of Earth. Lucy will return home again in 2024 before it shoots out to the asteroid belt for its first target encounter in 2025. From there, it will venture out to the vicinity of Jupiter for its first few fly-bys of the Trojans which exist at Jupiter’s L4 Lagrange point.
However, the mission won’t end there! To get an up-close look at a Trojan asteroid located in Jupiter’s L5 Lagrange point, Lucy will return to Earth one final time for a gravity assist before heading out to its final target, which might be its most grand destination of all: a binary pair of Trojans that are each over 100km in diameter.
By the time this final fly-by occurs in 2033, mission planners may have discovered even more potential targets, so there’s no telling how long Lucy will keep going. But one thing’s for sure: our understanding of the early solar system is about to be changed forever.
Sources & Further Reading
Lucy the Skeleton
Australopithecus Afarensis
Lagrange Points
Trojan Asteroids
Lucy Mission