Lucy in the Sky With Diamonds – Getting to know the Lucy Mission

Article November 11, 2023
Artist’s impression of the Lucy spacecraft. Credit: NASA
Artist’s impression of the Lucy spacecraft. Credit: NASA

We’ve reached the end of NASA’s Asteroid Autumn—a few months that were coincidentally packed with exciting asteroid-related news stories, specifically the recovery of the OSIRIS-REx sample return container, the launch of Psyche, and the Lucy spacecraft’s flyby of the asteroid Dinkinesh.

I’ve talked about OSIRIS-REx and Psyche in this blog before, so it’s time to take a look at Lucy: what it’s doing, where it’s going, and why that flyby of Dinkinesh turned out to be way more than anyone expected.

 

Meet the Trojans (and Greeks)

Lucy launched in October 2021 on a very ambitious mission to visit several of Jupiter’s Trojan asteroids. The Trojans are an interesting group—they’re asteroids that started out in many other places, but fell victim to the quirks of gravity and now live in Jupiter’s orbit, despite being quite far away from the planet itself (somewhere on the order of 50,000 miles away, on average).

I’ve teased Lagrange points in this blog before, and one of these days I will get around to explaining them a bit more fully, but the relevant bit here is that the way Jupiter’s gravity interacts with the Sun’s gravity makes two special spots in Jupiter’s orbit, one 60 degrees ahead of Jupiter and one 60 degrees behind, that are gravitationally stable points in space. 

A map of the major asteroid sites in the solar system, with Jupiter’s Greek and Trojan groups of asteroids highlighted in green. Credit: Wikimedia commons
A map of the major asteroid sites in the solar system, with Jupiter’s Greek and Trojan groups of asteroids highlighted in green. Credit: Wikimedia commons

This means anything that drifts into those areas is likely to stay there, orbiting the Sun more or less as Jupiter does. Over the age of the solar system, a lot of asteroids have drifted into these spots, meaning Jupiter is actually sharing its orbit with thousands of rocks, over 9,800 that we know of. 

The term “Trojan” came about because the very first one ever discovered was named for a famous figure of the Trojan War, Achilles. The naming convention stuck and all of these rocks that have been given names get them from the saga of the Trojan War. In fact each camp from that war is represented, with the group of asteroids moving 60 degrees ahead of Jupiter being named after Greek characters and the ones moving 60 degrees behind Jupiter being named after characters from Troy.

That said, each camp has a spy—the asteroids Patroclus and Hektor, named for a Greek and Trojan respectively, got their names before the current convention was in place. As a result the Trojan hero Hektor is in the Greek camp while the Greek hero Patroclus is in the Trojan camp.

 

How the Beatles Come Into This

These asteroids make an inviting target for a space mission. There are a lot of them in a relatively small amount of space (I know most diagrams of the asteroid belt tend to show a bunch of closely-crowded dots, but those asteroids are actually hundreds of thousands of miles apart at best). And they hold information that can be hard to find elsewhere.

Scientists really want to know more about the formation of the solar system—how it formed, where it formed, what materials were already there when it formed and what must have been added since, how different parts of it might have formed differently, etc. The combined answers to all of this may help us better understand the origins of Earth and the life that that teems on its surface.

And the Trojans are an excellent place to go for some of these answers! They represent the primeval solar system, having undergone very little change in the last 4.5 billion years or so. And while we suspect that some of them formed with the main asteroid belt, others originated elsewhere, so they represent a concentrated sample from many parts of the early solar system. And that makes them a sweet target.

Since these rocks more or less represent the fossils of the solar system, the Lucy mission was named for one of Earth’s most famous fossils, the early hominid skeleton dug up in Ethiopia in 1974 and given the name “Lucy” after the Beatles song “Lucy in the Sky with Diamonds”, which played loudly and repeatedly in the excavation camp. 

 

The Rocks Are Multiplying

The Lucy mission is arguably one of the most ambitious solar system missions we’ve ever launched. When it was designed, it was going to fly past no less than seven asteroids—one main belt asteroid, four asteroids in the Greek camp, and a binary pair in the Trojan camp—using a long, looping path that will repeatedly swing it back into the inner solar system. 

Diagram showing Lucy’s looping path relative to Jupiter, Earth, and the two sets of Trojan asteroids. Credit: NASA
Diagram showing Lucy’s looping path relative to Jupiter, Earth, and the two sets of Trojan asteroids. Credit: NASA

In the years since Lucy was officially given the green light for development, two of those Greek asteroids have been discovered to have moons, bringing the total number of rocks Lucy would visit to nine. Then, in January 2023, mission operators added a small, nameless main belt asteroid, designated 1999 VD57, to the itinerary, since it was more or less in Lucy’s way, bringing the number to ten.

Once 1999 VD57 was going to get a visit from a spacecraft, it needed a real name. It was appropriately dubbed Dinkinesh, which is the Ethiopian name for the Lucy hominid fossil. It was also pretty immediately nicknamed Dinky, which was both predictable and inevitable. Dinky wound up being first on Lucy’s target list, with the flyby occurring on November 1, 2023.

 

The Wonder of Dinky

If you read the Spacing Out newsletter, you know what happened when Lucy flew past Dinkinesh to test its instruments. The flyby went down without a hitch, and the data and images taken by the spacecraft began to be downlinked to Earth, only for them to immediately reveal a surprise: Dinky was Dinkies! The asteroid has a moon (I call it Dinkymoon, which will never be seriously considered as a name, but come on, it’s perfect)I Lucy’s asteroid count ticked up to eleven. 

mages from Lucy showing the asteroid Dinkinesh and its unexpected, contact-binary moon. Credit: NASA/Goddard/SwRI/Johns Hopkins APL/NOIRLab
mages from Lucy showing the asteroid Dinkinesh and its unexpected, contact-binary moon. Credit: NASA/Goddard/SwRI/Johns Hopkins APL/NOIRLab

Then, as the pictures continued to come in, a new surprise was discovered. When viewed from a slightly different angle than in the first images, Dinkymoon turned out to be special. It’s actually a contact binary, something that started out as two separate objects that drifted together and now act as one object.

We’ve never seen a contact binary orbiting another asteroid! This is a first! And it was an accident, just a handy chance for Lucy to test out its stuff as it moved towards its true targets. So Lucy has already casually dumped something brand new in our laps. No big deal.

As to whether or not the contact binary counts as one object or two, opinion seems to vary. I personally am going to count it as two different things until I am officially informed otherwise, which would bring Lucy’s asteroid count to twelve. So far. Somehow I doubt that number is done changing.

 

What’s Next?

Lucy has a ways to go, since it can’t head straight out to Jupiter. It doesn’t have enough momentum. It’s going to swing by Earth in December 2024 to pick up the final momentum it will need to make it back out to Jupiter’s orbit. As it passes back through the main belt it will fly past the asteroid Donaldjohanson (yes, it’s one word), named for the scientist who discovered the Lucy hominid.

It will encounter its first Trojan asteroids, Eurybates and its moon Queta, in 2027. It will spend roughly one year cruising through the Greek camp, meeting Polymele and its so-far unnamed moon, Leucus, and Orus before swinging back into the inner solar system. It will pass Earth by again in 2030 to grab the momentum to get it back out to Jupiter’s orbit and the Trojan camp, where it will visit the binary pair of Patroclus and Menoetius in 2033.

Following this final flyby, the Lucy mission will officially be over, but the spacecraft will spend the next 100,000 years in a stable orbit that swings it close to Earth’s orbit and then back out to Jupiter’s. Mission planners realized this meant the spacecraft had the chance to act as a time capsule and designed things accordingly. 

An image of the golden plaque being carried by the Lucy spacecraft. Credit: NASA
An image of the golden plaque being carried by the Lucy spacecraft. Credit: NASA

Lucy carries a golden plaque. Other missions, like the Voyagers and Pioneers 10 and 11, have carried messages for whomever finds them. Unlike these, however, Lucy’s message isn’t meant for other potential civilizations—it’s meant for future humans. The plaque contains the spacecraft’s launch date, the positions of the planets at launch time, and twenty snippets from speeches, poems, and songs from folks ranging from Albert Einstein to Martin Luther King, Jr to, you guessed it, the Beatles.

It’s entirely possible that the reams and reams of data on the many primordial fragments of the solar system that Lucy will visit will not wind up being its biggest legacy. If our far future descendants wind up recovering the golden plaque, Lucy may wind up reaching farther into the future than any mission we’ve ever built.

In the meantime though, we’ve got some space rocks to ogle. I do love me a nice space rock.