This is Geology Bites with Oliver Strimpel.
In several previous episodes of Geology Bites, such as the ones
with Sarah Russell and Harold Connolly, I've talked to
researchers seeking to understand the early history of
the solar system through studies of asteroids.
Asteroids are especially valuable in this regard because
they preserve a record of the early solar system.
There are many unanswered questions about the early
history of the solar system. In particular, did the giant
planets form roughly at the distance from the sun they
presently occupy? Or, as some theories predict,
have they migrated to their present positions after they
formed? The discovery of other solar
systems with radically different configurations of planets has
made this question more pressing, since it appears that
the configuration of the solar system is highly atypical.
It turns out that certain asteroids called the Trojans
that are locked into the same orbit as Jupiter offer us the
best opportunity to discriminate between the various models of
solar system evolution. And that is why a spacecraft
called Lucy is now well on its way to a rendezvous with these
asteroids. Hal Leveson is principal
investigator of the Lucy mission.
He studies the dynamics of astronomical objects and in
particular the formation and long term behaviour of solar
system bodies. He is one of the original
proponents of the Nice model, a scenario that proposes the
migration of the giant planets from an initial compact
configuration closer to the Sun to their present positions.
He is chief scientist in the Department of Space Sciences at
the Southwest Research Institute in Boulder, Co.
Hal Leveson, welcome to Geology Bites.
Thanks for inviting me, it's a pleasure to be here.
Can you explain how it is that the small bodies in the solar
system have survived as fossils of the very early solar system?
Let's take a step back. Our understanding of how planets
form is that originally the solar system consisted of gas
and a bunch of dust, and it does slowly build up over time to
make first pebbles, then rocks, the mountains, then asteroid
sides, things all the way up to the planets.
So the planets are made of these things, but planet formation is
not 100% efficient. And as a result, we have a
population of small bodies that are basically leftovers, the
dregs of planet formation. And that's what asteroids and
comets are. And the reason why they're
interesting for somebody like me who's interested in how the
solar system formed is that while planets have done their
own thing, they have internal geology or they have weather
that have changed what the planets look like.
These small bodies are basically fossilized at the time that the
planets formed. And so if you really want to
understand how the planets formed, these are the best
places to go, I think. OK, so the Trojans are
asteroids, but they're not in the asteroid belt that lies
between Mars and Jupiter, but further out and actually in the
same orbit as Jupiter, with one group of Trojans leading Jupiter
in its orbit by about 60° and the other trailing Jupiter by
the same amount. How come they're herded into
that space, and how do they compare to the asteroid belt in
terms of their numbers and the volume they occupy?
So Trojans are in the special balance points gravitationally.
And the way to think about it, although it's technically not
really right, is that if you put an object in front of a planet
by 60° in its orbit, the gravitational attraction of the
planet and the sun plus the centripetal force of the orbit
balances out in such a way that the object put there stays there
forever. And that's how come we know
these objects are actually very ancient objects.
And what makes them interesting from a solar system formation
point of view is they're leftover from the outer planets.
While we think most asteroids in the asteroid belt are leftovers
from the rocky terrestrial planets, these guys come from
the outer solar system and that's what makes them
interesting. Now the population is
comparable, somewhat less, but comparable to what is in the
main asteroid belt today. And they occupy actually a
really small region of orbital element space.
They're actually more concentrated in a way than the
main asteroid belt is. I said in the introduction that
the Trojans offer us the best opportunity to distinguish the
various models of solar system evolution and in particular, to
test the NICE model. You were one of the original
proponents of that model. What motivated the development
of this model and can you explain how the solar system
evolved according to the model? Sure.
What motivated me to work on the model?
And there's a lot of reasons why people started questioning
whether the planets that we see in the outer solar system today
formed where we see them. So there's a lot going on there.
But the thing that motivated me is the realization that Uranus
and Neptune, which are the ice giants in the outer parts of the
solar system, could not have formed where we see them today
after Jupiter and Saturn formed. So the classical thinking of
planet formation is that the planets in the outer part of the
solar system formed more slowly because the orbital periods are
longer. So just things just take a lot
longer to get going out there. And we realized that that's not
true. Uranus and Neptune can't have
formed after Jupiter. Saturn formed.
So we looked at a way to allow the four giant planets to form
at the same time. And that led us to the idea that
the solar system formed in a much more compact configuration,
and that some dynamical event moved the planets around to
where we see them today. So the NICE model basically
hypothesizes that all the four giant planets formed in a much
more compact configuration, let's say within 12/13/14
astronomical units from the sun. So an astronomical unit is the
average distance between the Earth and the sun.
Currently Neptune, which is the outermost planet, is at 30
astronomical units. So we're hypothesizing that the
solar system of roughly half as big and then some dynamical
event threw them out. And what we think happened is
that this compact planetary system was surrounded by a disk
of small bodies that are extended just outside the orbit
of the ice giants to around 30 AU and contain basically 2530
Earth masses versus asteroids. And if you put that all on the
computer and you integrate the orbits, you find that the system
is not stable for long periods of time.
And eventually what happens is that the orbits of the ice
giants go unstable. They cross one another, they
start gravitationally interacting with Jupiter and
Saturn, which throws them out into this disk.
The disk just goes Kapluy, not because of physical collisions,
but because of the gravitational interaction with the planets.
And that disk of material gets scattered all over the place.
Most of it ends up in interstellar space, but some of
it ends up trapped in the Trojans.
And the gravitational interaction between the ice
giants and that disk evolves them onto the orbits we see
today. And actually, we can match the
orbits of the giant planets in our numerical simulations.
One of the premises I didn't quite follow is that you said we
know that Uranus and Neptune could not have formed much later
than Jupiter and Saturn and that was really one of the arguments
that they must have formed closer in initially.
How do we know that they wouldn't have formed much later
on? Well, it's a theoretical
understanding. Once you get a planet forming
and it's slowly accruing these small bodies, Once we get to the
point where the escape velocity from the planet is about the
same as the planet's orbital velocity around the sun, the
planets will scatter objects around rather than eating them.
And so eventually what happens is they'll hand them off to
Jupiter and Saturn if they're there, and Jupiter and Saturn
will just knock them out of the solar system.
So what we find is when you get to the point where the ice
giants are a couple times the mass of the Earth, they will
stop growing and all their food will be fed down to Jupiter and
Saturn. So let's get back to the
Trojans. Why do they in particular allow
us to distinguish the various models of solar system
evolution? And what does the NICE model
predict as far as they're concerned?
So one of the interesting aspects of Trojans when we
started studying them from the ground is while we expected them
to be a homogeneous population, basically all the same things
that went into building the core of Jupiter, we see that they're
very different from one another. In particular, the colors are
very different from things that are basically Gray to things
that are redder to Mars. And we think what that's saying
is that this is represents material that comes from the
many different regions of the solar system, this disk that
existed outside the orbits of the giant planets initially, and
that the different colors are due to different formation
temperatures because you're further from the sun.
And so the nice model mixes all that up and delivers them to the
Trojan swarms. And so by understanding, first
of all, what those different colors mean, we don't have a
good understanding of that. So looking at these things close
up will help us understand that we might be able to figure out
where in the disk they formed. And then looking at the
distribution of objects in the Trojans will allow us to
understand how the planets moved around.
That's the basic idea. Another thing that the NICE
model predicts is I said that most of this disc goes away
scattered into interstellar space, but some of it not only
ends up in the Trojan swarms, but they end up in the Kuiper
Belt and are the sources of comets that we see today.
So the NICE model basically predicts that Trojan should be
the same as comets. And we've sent spacecraft to
visit Comets before, so we have decent understanding of what
they are. So one of the things we're going
to test is how different the Trojans are from the Comets.
And does the model also predict that these small objects would
be sampled from a whole lot of different distances and
therefore you'd expect them to be very diverse rather than just
homogeneous? That's right, and that diversity
is the key. That's what Lucy is designed to
do. As I said, we see this diversity
from the ground, and understanding what that
diversity means will help us constrain how the planets moved
around. That's the basic story.
Lucy is going to visit more objects in orbit around the Sun
than any other spacecraft in history.
Because we need to get to a lot of them and all.
We're going to be studying 8 Trojans, 5 Trojan systems.
Two of our objects are known to have satellites.
We discovered the satellites as we were studying these things
and preparing for the mission. One of these objects, which is,
I must admit, my favorite. You know you're not supposed to
love one of your children better than the others, but this one I
actually do is an object called Petrocholis, which is a near
equal mass binary. It's a system that contains 2
objects that are almost exactly the same size, about 100
kilometers in diameter, in a circular orbit around one
another, and I think that that itself is an important clue to
how the solar system formed and evolved.
Well, that's fascinating. One of the things about the Lucy
mission also is that it's the so-called exploration mission
that has gone to places that we've never been before.
Yeah, Lucy is going to this population.
We've never seen them before. We've basically have sent
spacecraft to all the other small body reservoirs in the
solar system, the main asteroid belt, the Kuiper Belt, comets,
but not Trojans. So I'm a theorist, right?
So you start talking to me about Lucy and I'm going to be
concentrating on what it's going to tell us about the history of
the solar system. But in a way, more importantly,
we're going to a population we've never seen and it's really
the last small body population that we have to study.
And what's more, while other small body reservoirs are
contributing meteorites to the Earth, because of the Trojans
proximity to Jupiter, meteorites can't get toss from there.
So these objects are really, really a mystery.
Let's talk about what the spacecraft looks like and what
instruments it carries. Sure.
Because Lucy's job is to study the diversity.
We have to be moving really quickly to get from object to
object to object. So we're travelling really fast.
Our encounter velocities with our Trojans range between
roughly 6 and 9 kilometers a second.
As a result, we're not stopping, we're not orbiting, we're not
scratching and sniffing. Everything that we're doing is
remote sensing, but yet we're trying to do everything we
possibly can with those remote sensing instruments.
So in particular, we have panchromatic cameras which are
going to be able to allow us the study at high resolution, the
geology on the surface of these bodies, counting craters,
looking for maybe vents because we think these things are
related to comets. Maybe we'll see evidence that
they were one time active. We have a near infrared
spectrograph which is going to be studying the chemistry of
these objects. We have all far thermal infrared
spectrometer, which is going to tell us something about how
fluffy the surfaces are. And as we fly by these objects,
we're going to be able to measure the Doppler shift due to
the gravity. And so we're going to be able to
get their mass. And because we're going to know
their shape after the encounter, we're going to get an idea of
what their internal density is, which is going to tell us
something about their internal structure.
The interesting thing about the Trojans is that we have a few
densities for these objects, and the densities are always less
than one. So think about it.
These objects are made out of rock and ice.
We believe they're about half rock and half ice, and yet their
density is less than ice, which means there are a lot of void
spaces inside of it. And so we're going to be able to
understand that as well. And of course, that tells us
something about how they created, right, how fast they
came together. Because if their density is low,
whatever process formed them when these objects came together
to form them, they had to be going at really low velocities
or they would become more compact.
So understanding their density is critical to understanding
their history. You're going to be flying by at
these very rapid speeds, as you said.
So how long will you actually have upon approach and then as
you leave to capture all your data?
So one of the interesting things about Lucy and one of the
challenging things about Lucy is that it's a 12 year mission, but
all our important science is going to be collected within
about 24 hours. Two hours on either side of
close approach is where we're getting all our high quality
scientific data. So first of all, we can't blink.
Something can't go wrong because once an encounter is done, we
can never go back. We can't do a do over.
So we have to plan every second of what the spacecraft is going
to be doing during these encounters.
Also, we're not talking to the spacecraft during the encounter.
After all, the light travel time is long enough that we couldn't
say no, no, do this instead. So it's totally autonomous and
everything needs to be planned beforehand.
So although we launched in 2021, you might think we've had a long
time to wait and we're sitting and twiddling our thumbs, but we
have been very, very busy planning each encounter in order
to make sure that we're getting the best signs that we can.
So how close will you get and what kind of surface resolution
will you get with your instruments?
So we're flying at a distance of a few 100 kilometers, five 600
kilometers, and with the high resolution spectrograph that we
have, we'll get about 15 meters per pixel on the camera.
You mentioned that you're going to be flying by and going very
fast and that you launched in 2021.
So where has Lucy been travelling all that time and
what trajectory is it taking to get to the Trojans?
So Lucy's trajectory is really spectacular.
It's one of my favorite aspects of the mission, to be able to
get to these objects, and they're very special objects.
I said we're going by 5 Trojan systems to study the diversity,
but we've been lucky that the objects we've been able to get
to are interesting as individuals.
Several of them would be worth actually sending a spacecraft to
if that's the only place the spacecraft could get to.
I like to say is I'm a dynamicist.
I study how things orbit the sun.
So I've been worshipping at the feet of the celestial mechanics
gods for 40 years, and they're paying us back.
It's really, really a spectacular mission due to this
really unique trajectory. But one of the things that we're
doing in order to save money is loosely launched on a very small
rocket, Atlas four O 1. And as a result, when it first
left the vicinity to the Earth, its velocity with respect to the
Earth was really small. So we could use a small rocket.
And what we've been doing is using a series of Earth gravity
assists where we fly by the Earth and use the Earth's
gravity to slingshot Lucy out to slowly pump up the orbit of Lucy
so it can cross the orbit of Jupiter.
And so that's taking some time. And so that's what Lucy's been
doing, basically. But on two of those orbits, we
actually went through the main asteroid belt.
Although we didn't get quite far enough to get to the Trojans, we
did get to the point where we could get into the asteroid belt
and we did 2 flybys as tests of our system, right?
Again, there's no going back. So testing the system and making
sure we understand how to fly it was key.
And so we've studied 2 asteroids so far and they've turned out to
be particularly interesting. One in particular is really
interesting. Tell us about the afterwards you
encountered them. Let me take a step back and
point out that most asteroids that we know about are not
named. All the objects that we're
studying with Lucy, only two of them had names before we
started. So we got to name a lot of these
objects. For the main asteroid belt
objects, we flew by an object that we named Dinkanesh, which
is the Ethiopian name for the Lucy fossil.
So let me take a second to talk about how we came up with the
name Lucy. Lucy the mission is named after
Lucy the fossil because we're arguing that these objects are
the fossils of planet formation. And just like our ancestral
fossils will tell us about our history, these objects are going
to tell us about the history of the solar system.
So we named the Lucy mission after the Lucy fossil.
I should also point out the Lucy fossil was named after Lucy in
the sky with diamonds. So if you actually look at our
logo, we made sort of a connection to that, that our
logo was diamond shaped. So the first asteroid we
encountered on route, we named Dinkanash, which is the
Ethiopian name of the Lucy fossil.
Lucy was discovered in Ethiopia and it currently is in their
museums. Dinkanash turned out to be one
of the most fascinating objects I think I've ever seen.
Because remember I said at the beginning, these things are
interesting because they basically remained unchanged
over the history of the solar system, and that you need to
have big objects in order to make geology happen.
Turns out that's not right. If you have very small objects,
objects that are about a kilometer in diameter, radiation
forces can actually change the spin rate of these objects over
time. And as a result they can spin
up. And then centripetal forces
start putting stresses on the bodies and they can crack and
pieces can fall off and form satellites.
And all that can happen on really fast time scales, 10
million years or so. And that's what's happening on
Dickenish. Dickenish is only 700 meters in
diameter, and as a result it's been spinning up and slowing
down. And we see evidence of a big
crack that's opened up and in. And it has the satellite that we
named Salaam, which is named after another fossil.
And it is a contact binary of two objects that are roughly the
same shape that are stuck together.
And all these things are probably very young and form
because of this radiation forces.
So at a size scale of about a kilometer, really, really cool
things can happen. And I didn't quite really
appreciate that before we saw this object.
The second object we went to, we named Donald Johansson after the
discoverer of the Lucy fossil. Actually, one of the cool things
is that he was able to come to the encounter.
So I think this is the first time that a person for whom an
asteroid was named after, I actually was there to see it
when this thought of light in the sky became a world.
So that was pretty cool. Donald Johansson is shaped like
a bowling ball. Turns out that that's pretty
common. So it didn't really hold any
surprises when it came to its global structure, but it has
some weird things going on on the neck between the two
components. There seems to be some geology
going on that we don't quite understand.
So that's been interesting. Too.
Let's look forward then to your encounter with the first set of
Trojans that you're going to. And you mentioned that the gods
of celestial mechanics are rewarding you.
And is that because basically you have a limited supply of
fuel on board, so you can't really change course once you're
speeding along all that much? So they have to be kind of lined
up pretty much in a straight line for you to go from one to
the other. Did that actually leave you with
a choice as to which asteroids to visit?
And if so, how did you actually make that choice?
Yeah, our first goal when we put together the mission was to try
to run an experiment with as much control as possible.
I told you that Trojans have very different colors from one
another, so we tried to find 2 objects that are the same size
and on the same orbit. So they've had the same basic
history but yet have very different colors.
We did that so that we could say anything that we saw.
The in differences between these two objects are due to their
different compositions and not to say the different histories,
and we put together a list of about 1/2 a dozen.
That's all we could find candidate objects and then ask
the people that designed the mission to see which ones we can
actually get to within a reasonable amount of time and a
reasonable amount of fuel. And it turns out there was one
pair, OK. And that is the center of our
mission, which is going to an optic called Euribides, which is
a Gray type asteroid, which is the first one we encounter in
2027. And then Oris, which is a very
red object that we encountered in 2029.
And that's when we're going through the 1st loop, going
through the leading swarms of Trojans, the ones that lead
Jupiter. So that's how we started.
Everything else was just luck. And you might think it didn't
require much luck, but one of the things NASA cares about when
you put in a proposal to fly a mission through them is you need
to make an argument of why you want to do it now.
Why not wait to do the mission? Because after all, technology
gets better as time goes on. And if you waited a decade,
maybe you would get more signs out of it.
So we ran the experiment asking ourselves what would happen if
we were launching in 2030 instead of 2021, and tried to
put together a list of targets and we didn't find anything
anywhere near as interesting. And again, we were really lucky.
We maybe got three objects or 4 objects rather than the eight
that we're going to, and they were just uninteresting objects
compared to the ones we're actually visiting.
Well, that's really very fortuitous indeed then, now that
we know how unlikely this alignment is.
So how long will it take to fly by all these systems in the
first set of Trojans? And then what does the Path of
Lucy look like after you're done with those?
OK, so as I said, we rattled around the inner part of the
solar system for a while, using Earth gravity assist to pump up
the orbit of Lucy. Right now it's in an orbit with
a perihelion, a close approach distance to the Sun just inside
the orbit of the Earth, and an aphelion distance, the furthest
distance it gets from the Sun, just outside the orbit of
Jupiter. And that's where it'll be for
most of its life. We're doing 2 orbits.
The first orbit will take us into the leading swarm, and the
second orbit will take us into the trailing swarm.
And in the leading swarm, we're having 4 encounters in 15
months. So it's going to be very fast,
but we're going to be very busy. As a matter of fact, I'm worried
about it. Then we come back into the inner
part of the solar system, have another Earth gravity assist,
which is going to allow us to target our final Trojan, which
like I said, this is my favorite system of Patrocalis.
We're only going to one object in the trailing swarm, and
that's because we made the decision to go to a very low
density region of the Trojan swarms in order to get to
Petrocholis. We could have flown through the
center of the swarm and probably gotten a few more objects, but
Petrocholis is so cool that I decided that we needed to go
after it rather than more targets.
And so that's what we're going to do.
And the fact that it's an equal mass binary, does that make you
think that it's more likely to be like a really pristine
fossil, as it were? Yes.
So let's talk about this object and objects like it.
When we look around the solar system, the inner part, let's
call it the inner part of the solar system.
And by inner part, I'm talking inside the orbit of Neptune.
So in the region where planets actually form, seeing 2 objects
like this, these near equal mass binaries are incredibly rare.
I think we know of three or four of them.
But if you look out in the region of the Kuiper Belt, the
region beyond the orbit of Neptune, there's a place that we
call the cold classical Kuiper Belt.
And it is a place where we see asteroid type objects that we
believe to be pristine because they were far enough from
Neptune that they weren't affected by the planet formation
process themselves. When you're looking at that
area, it turns out they're all near equal mass binaries, almost
at 100%. So I think what that's telling
us is that the first macroscopic objects to form in the solar
system were formed as binaries. And indeed, modern theories of
planet formation and planetesimal formation actually
can explain that. And the idea is that the first
macroscopic objects form because small things, things the size of
a meter or the size of your fist, were concentrated due to
their interaction with gas. And when they got concentrated
enough, sort of like, think of them as like little star
clusters, they became gravitationally bound to one
another and then collapsed to form the big objects.
By the way, they collapse very slowly, which is probably why
these objects have very low density.
But when you model this, you find out is what's happening is
that these clumps actually have a little bit of rotation, and as
they collapse, angular momentum has to be conserved.
So they spin up faster and faster and faster, like a dancer
bringing in their arms. Angular momentum is conserved,
so they need to spin faster as their arms move in.
There's so much angular momentum in these systems that you can't
fit all of it into one object, it would fly apart.
So what nature naturally does, it turns out, is it makes 2
objects the same mass, but in orbit around one another.
And that's exactly what we see in the cold classic Kuiper belt.
So we really think that these are the first real big objects.
And if that's true, then Petrocholis is one of those rare
survivors that survived the planet formation process, which
is very violent. After all, things are hitting
each other. Things are gravitationally
flying by one another, which could RIP these binaries apart
gravitationally. That's why they didn't survive,
but Patroclus did. So it's going to be really cool
to look at that. So after you finished with the
second Trojan encounter with Patroclus, what's going to
happen to Lucy? Where will its orbit take it?
It's going to be in the orbit that it's on basically.
Now that takes it out to Jupiter's orbit and brings it in
the Earth's orbit. We think there should be fuel
enough and if the spacecraft is healthy, I'm sure NASA will find
A use for the spacecraft and do something cool with it.
One of the interesting aspects of the orbit that it's on is it
turns out it's very long lived. It'll last in this orbit if no
one does anything with it, for about on average 2,000,000
years. Though we did something I think
that was interesting for those of you who know about the
history of NASA. NASA, particularly early on, had
this idea of sending messages to aliens, right?
So the first spacecraft to leave the solar system, which were The
Pioneers and the Voyagers, all have messages to the aliens.
The Pioneers have these famous plaques.
The Voyagers have record player, the sounds of the Earth.
All that is focused on aliens because they're leaving the
solar system. Lucy, on the other hand, is
going to be in orbit in the solar system and it'll last for
a very long time. So we had an idea to put a
plaque on Lucy, not with messages to aliens, both to
messages to our descendants. You could easily imagine 100
years from now, maybe it won't be so interesting then.
But 1000 years from now, 10,000 years from now, hundreds of
thousands of years from now, our descendants want to learn about
us by going around the solar system and picking up our space
junk. So we left them a little
message. So take a look, it's online.
You can see a picture of our plaque.
I know this is probably a bad time to ask you this, but once
you have the data from a successful Lucy mission in hand,
what would be your next preferred destination for a new
space mission to help resolve further questions about solar
system evolution? Well, there are two places that
I think I would like to go. The first is send another
spacecraft out to the carper belt and study the cold
classical carper belt. As I said earlier, there are
regions of the carper belt that we think should contain the same
stuff as the Trojans. And so being able to compare 100
kilometer sized thing in the Trojan swarms to something in
the carper belt would be, I think, really interesting.
But there is another place that we could go that's much closer.
There's a population of asteroids just outside the main
asteroid belt called Hilda's. If the nice bottle is correct,
They were also captured at the same time the Trojans were.
So one of the really interesting places to go is to a Hilda.
And indeed, we've recently discovered that we can get Lucy
to a Hilda at the end of its main mission.
So maybe if NASA decides to extend the mission, we'll be
able to go to one of those objects, too.
Because comparing populations to one another, looking at
diversity, like I said, it's the key to figuring all this out.
Al. Levison, thank you very much.
My pleasure, it's fun. To see pictures and
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