NASA’s Lucy in the sky with Trojans

Even as one NASA mission is readying to rendezvous with a near-Earth asteroid this December, another is preparing to launch deeper into the asteroid belt in October 2021. 

Called Lucy, this mission will rendezvous with not one asteroid, but seven, six of them resident in two large clusters called the Jupiter Trojans. 

The Trojans circle in the same orbit as Jupiter, where a delicate balance of gravitational forces between the sun and the planet ensure that they will never collide with their host. More than 6000 are known, with one cluster preceding Jupiter in its orbit and the other lagging behind.

Lucy’s 12-year mission will begin with a launch into a low-velocity orbit not all that different from Earth’s. That helps keep the mission budget suitable for NASA’s low-priced Discovery-class missions. 

“We can use a small launch vehicle,” says Harold Levison, a planetary scientist from the Southwest Research Institute in Boulder, Colorado, US, who is the mission’s principal investigator.

Successive encounters with Earth will then give it gravitational boosts that will bend its orbit ever farther out into the solar system until, in 2027, it will make a slow looping pass through one of the Trojan swarms, having close encounters with four of its asteroids. 

The spacecraft will then fall back toward the sun, where a final encounter with Earth will bend it outward again, this time toward the other Trojan cloud.{%recommended 1245%}

One of the mission’s goals, Levison says, is to investigate why the asteroids collected in the Trojan clusters don’t all have the same surface brightness and colour. This indicates that they may have been formed in different parts of the solar system. 

Some might have been formed in about their present locations, while others might have formed much farther out, before gravitational interactions with the outer planets swept them together like a cosmic broom, four billion years or so ago. 

“These asteroids represent objects that formed in a very interesting region of the solar system and are now trapped in a place where it’s easy for us to send a spacecraft to go and study them,” Levison says.  

The process by which such a diverse array of asteroids was swept together is also important, because it is related to the migration of the giant planets early in the solar system’s history, before they settled into their present orbits. 

Thus, the Lucy team hopes, studying the Trojans up close can teach not only about the asteroids themselves, but about how that entire process worked. 

“Those are the two major goals of Lucy,” Levison states.

But several of the asteroids being visited by the spacecraft are interesting in and of themselves. 

First up is a small one, which the team has dubbed 52246 Donaldjohanson, to be visited in 2025 as the spacecraft is en route to its first encounter with the Trojans.

The name, Levison says, relates to the mission’s own, which refers a 3.2-million-year-old Australopithecus fossil nicknamed Lucy, discovered in Ethiopia in 1974. 

Lucy-the-fossil played an important role in unlocking our understanding humanity’s origins. Lucy-the-spacecraft will do the same for understanding the solar system’s origins. Donald Johanson was the paleoanthropologist who discovered the original Lucy.

52246 Donaldjohanson isn’t very big. When pressed, Levison guesses it might be about five kilometres in diameter. “Plus or minus five kilometres,” he adds.

More interestingly, it is a remnant of a collisional break-up that occurred only 100 million to 150 million years ago. Not that the asteroid’s rock is that young, but until the collision exposed it, it was tucked inside a larger body, away from the eroding effects of space weather. 

As a result, its surface is extremely young, offering a glimpse not only into the heart of an ancient asteroid, but at material which has passed through billions of years unaltered. 

Out in the Trojans proper, Lucy will encounter larger bodies, ranging from 20 to 100 kilometres in diameter. Two, Levison says, would be “worthy targets even if they weren’t Trojans”. 

One of these is 3548 Eurybates. Like 52246 Donaldjohanson, it appears to be part of a “collisional family”, meaning that it too is the result of a break-up, albeit not as recently. 

“It is the largest remnant of an asteroid family — the only major collisional asteroid family in the Trojans,” Levison explains. 

“We’ve never seen an object like that. It’s going to teach us how collisions work, particularly on primitive bodies.”

2930 Euripides is also interesting, he says, because its colour is different from most of the other Trojan asteroids. 

The third such asteroid, which won’t be visited until Lucy’s second Trojan pass-through, in 2033, is 617 Patroclus/Menoetious. This one is interesting because it’s a binary, with each of its two components very close in size. 

That close to the sun, such equal-size binaries are rare, but further out, beyond Pluto, they comprise about 40% of the known objects. 

“One of the main explanations is that when the first macroscopic objects in the solar system formed, they formed as binaries,” Levison says. 

Close in, such objects would have had a high chance of being ripped apart by gravitational encounters with planets. 617 Patroclus/Menoetious, Levison speculates, might be a primordial pair that just happened to survive. 

Humberto Campins, a planetary scientist from the University of Central Florida, US, who is not part of the Lucy team, agrees that going to the Trojans is a good way to peer back to the early solar system. 

The findings from the mission, he says, can also be compared to those from OSIRIS-REx, the mission that will be rendezvousing with a near-earth asteroid 101955 Bennu shortly after Christmas and returning a sample to Earth in 2023. 

It might even turn out that Bennu is related to the Trojans, he says, in the sense that it too might prove to be a primitive asteroid injected into the inner asteroid belt long ago by the migration of the giant planets.