Of the varying types of celestial objects humans have sent spacecraft to investigate, there remains a select group of asteroids, located in two separate locations in our solar system, that we have yet to visit — the Trojan asteroids.
Trojan asteroids are located at Jupiter’s L4 and L5 Lagrange points, 60 degrees ahead of and 60 degrees behind Jupiter, respectively. The asteroids are present at the Lagrange points in groups, sometimes referred to as “swarms” or “camps.”
Although telescopes have allowed scientists to research asteroids from Earth, the nature of the Trojans remains largely unknown. However, this is set to change.
No earlier than October 16, the Lucy mission will start a 12-year journey to visit six of these mysterious asteroids and an additional main-belt asteroid. During its time with the Trojans, Lucy will uncover the history behind these bodies and study the Trojans in depth using a suite of cameras, spectrometers, and antennas.
But why Trojan asteroids?
“Asteroids are the fossils of planet formation. They’re left over from early on, and they’ve remained relatively unchanged, at least relative compared to the planets since they formed,” said Dr. Hal Levison, Principal Investigator for Lucy at the Southwest Research Institute (SwRI) in an interview with NASASpaceflight.
“I’m actually a theorist by training. This is the first space mission I’ve been involved in. My background is to do large-scale planet formation models and try to figure out the small body reservoirs because of what the fossils of planet formation tell us about how the planets formed,” added Dr. Levison.
“If you look at the major small body reservoirs that are accessible by spacecraft, the only ones yet to be visited are the Trojans. We’re sort of putting a bow around the initial exploration of the small body reservoirs with this mission.”
Despite going to the Trojans, Lucy will first visit the main-belt asteroid Donaldjohanson, performing a flyby on April 20, 2025. This target will largely serve as a dress-rehearsal flyby for Lucy’s teams; however, they are still interested in seeing Donaldjohanson’s surface features.
And this relates to the type of asteroids Lucy will visit in the Trojan camps.
“[The Trojans] occupy a very small region of orbital space. They’re very different from one another, particularly in their colors. They go from gray (C-type) to actually pretty red (D-type). That diversity, I think, is telling us something about their history.”
Dr. Levison added that the color is likely an indication of where the asteroids now trapped in Jupiter’s L4 and L5 points originally formed in relation to the Sun.
So what are these Lagrange Points anyways and why should we explore the asteroids trapped there? Check out the most recent Episode of Lucy Goes to Space!https://t.co/0JnBeEHUZ7 pic.twitter.com/lSDUCMW8a7
— Lucy Mission (@LucyMission) September 22, 2021
“The Kuiper Belt, for example, is a lot redder than the Trojans, which are a lot redder than the asteroid belt. Exactly how that works, we don’t know.”
“So [this is] an opportunity for several reasons. First of all, if you imagine you were to go to the gray objects in the asteroid belt and the red objects in the Kuiper Belt, it’s going to be almost impossible to detangle the evolutionary differences they experienced from their intrinsic differences.”
“The beauty of the Trojans is that they’ve been hanging out in the same place on very similar orbits for long periods of time. Therefore, the differences that we see are going to be intrinsic differences rather than evolutionary differences.”
And Lucy will be able to help sift through part of that mystery.
“We have instrumentation, particularly the Ralph instrument, half of which is a near-infrared spectrograph. Hopefully, we’re going to be able to figure out the chemistry that makes these things red as we fly by because we can’t do it from the ground.”
Moreover, the teams have several working hypotheses based on data gathered elsewhere in the solar system. Dr. Levison continued: “I think whatever makes these things red is only skin deep, and when you break them open, they’re gray inside. Lucy will be able to directly test the second hypothesis because if that’s true, when we get to our red objects, young craters should be gray.”
One of the surface processing features that could be in play with the redder, D-type Trojans is that they were exposed to ice that is only stable beyond 20 AU distance (with a single AU being approximately 149 million km) from the Sun. The Trojans captured at Jupiter are located an average 5.2 AU distance from the Sun, far too close for the known process to work. Unless they formed farther out.
Of course, as Dr. Levison points out, “That’s one thing you can imagine. Hopefully, the data we collect will help us understand if an idea like this is true. Because if you can say, ‘Ah, the red guys all formed beyond 20 AU,’ for example, then you look at the mixing ratio in the Trojans between red and gray and that puts constraints on how the giant planets moved around.”
This potential bounding of how the giant planets migrated in the early solar system is a key part of Dr. Levison’s work as one of the authors of the Nice model of solar system formation. This theory postulates that — among other things — the giant planets first formed closer to the Sun and then migrated out to their current positions due to gravitational perturbations.
Part of the Nice theory also holds that Neptune and Uranus originally formed in opposite locations as the seventh and eighth planets, respectively, before gravity flung Neptune outward and made it the eighth planet from the Sun.
Under this model, Neptune would have greatly perturbed the scattered disk and flung objects — redder D and P-type asteroids — into the solar system.
Back to the idea of Lucy’s mission being able to help understand how the giant planets migrated, “[The red Trojans] tell you that you got almost all the stuff from beyond 20 AU captured, but almost none of the stuff inside of 20 AU. That’s the game we can eventually play to answer these pretty fundamental questions of how the giant planets moved around … by measuring colors on the surface of Trojans.”
“Now … if we find red craters on gray objects, we don’t know [what’s going on out there]. We’ll need data in order to answer this.”
And that last statement is key to Lucy’s mission. “The reason why I went from being a theorist to doing a spacecraft mission was basically the realization that when I started in this field, we had more problems than we had ideas. Now we have more ideas than we have data. We can’t tell which of our ideas are right with the data we have. It became clear to me that developing more theories isn’t going to be as useful as going off and trying to collect the data that will help us determine which of our ideas are right.”
Another target for Lucy that could hopefully shed light on early solar system formation is Patroclus. This will be the last Trojan to be visited and is a near-equal mass binary with Menoetius.
“They’re exactly the same size with a circular orbit … which is, if you think about it, really huge,” said Dr. Levison. “It turns out [the odds of this are] high because it’s a very unusual object in what I’ll call the inner parts of the solar system. And, in this case, I mean inside the orbit of Neptune, in the planetary region.”
“If you go out into the Kuiper Belt, there’s a part of the Kuiper Belt called the Cold Classical Kuiper Belt, which is believed to be undisturbed from the original accretion of the first stuff. When we look with [the Hubble Space Telescope], almost all of them are binaries like this.”
“It’s biased, but at least half, let’s say, are binaries. I think what that’s telling us is the first macroscopic objects to form in the solar system were these binaries, and they’re now models that would actually predict that. If that’s true, then Patroclus is one of the remnants of that really early population that just happens to survive. It’s probably extremely primordial, and that’s why I think for somebody who’s interested in planet formation, seeing one of these things close up — it’s going to be cool.”
When asked about potential surprising finds from Lucy at the Trojans, Dr. Levison replied that finding red D-type Trojans with red craters (instead of gray) would be surprising given how the reddish color is thought to form.
Another surprise would be if Patroclus and Menoetius are different from each other. “All these theories I was talking about with the Cold Classical Kuiper Belt models, [they] say that they form together. If they’re very different from one another, I’d be very surprised because that is an argument that they didn’t form together. I could think of a couple of things, but we just have to go and find out. That’s why they call it exploration.”
Lucy is currently set to begin its exploration of the Jupiter Trojans with a launch no earlier than October 16, 2021, at 5:34 am EDT (09:34 UTC) from SLC-41 at the Cape Canaveral Space Force Station on a United Launch Alliance Atlas V 401 rocket.
The launch period remains open for 21 days, with an approximately 60-minute launch window each day.
(Lead image: Artist’s depiction of Lucy performing a Trojan encounter. Credit: Southwest Research Institute)