Thanks to Geologie for sponsoring today’s video.
Pretty much anyone that has any interest in space has heard of the asteroid belt, the
large belt of asteroids that have fairly stable orbits between Mars and Jupiter.
There are hundreds of thousands that we know of, and likely many more still that we haven’t
discovered yet.
However, this is not the only place in the solar system where asteroids are found, in
fact, there are high concentrations of asteroids that aren’t just gravitationally bound to
the Sun, but also to the planets, known as trojans.
These are not moons, they don’t orbit around the planets, but rather they follow the orbits
of the planets, either just ahead of or trailing behind them in a position known as the planet’s
Lagrange points.
And while we don’t have close up photos of them yet, that will soon change with the
launch of NASA’s Lucy probe on the 16th October 2021.
I’m Alex McColgan, and you’re watching Astrum, and today we will look into the Lucy
mission, what we know so far about these trojan asteroids, investigate how they got stuck
in the positions they are in, and find out a very interesting characteristic about them
that you wouldn’t associate with most belt asteroids.
I hope by the end of this video to have earned your like and subscription!
Firstly, a small definition for an asteroid.
Asteroids can be found from around Venus all the way to beyond Pluto, however as their
compositions tend to get quite different depending on their distance from the Sun, it is more
common nowadays to only call small solar system objects asteroids if they are roughly within
the orbit of Jupiter.
Between Jupiter and Neptune, they are called Centaurs, and beyond Neptune they are called
trans-Neptunian objects, or Kuiper belt objects, named after the other asteroid belt in the
solar system.
Objects beyond Jupiter are icier than asteroids close to the Sun, meaning they are more akin
to comets.
When we talk of trojans, we are generally talking about the asteroids that share Jupiter’s
orbit around the Sun.
However, some of the other planets also have varying numbers of trojans.
One was recently discovered following Earth.
Mars, Uranus and Neptune all have more.
Jupiter, being the most massive planet, has by far the most, with the swarms in front
of and behind it estimated to have populations comparable in number to the entire asteroid
belt, around 1 million asteroids.
However, they are harder to observe, so this number may change as studies improve.
This is because they are likely smaller in general compared to asteroid belt asteroids,
a lot less massive, and are further out, so some scientists think there aren’t as many
as initially thought.
In this video, I will focus on Jupiter’s trojans.
The trojans following Jupiter are trapped in what is known as Lagrange points, namely
the L4 and L5 Lagrange points.
These are points in space where the pull of the Sun and the pull of the planet are comparable
in strength, meaning an object orbiting the Sun that gets too close to these points may
end up getting locked in.
The L4 point is ahead of the planet, and the L5 point is behind.
As a side note, not much gets naturally locked into the other Lagrange points, although sometimes
they can come in handy for our own artificial satellites.
The James Webb Space Telescope, for instance, will utilise Earth’s L2 point, this is to
allow constant communication with Earth, while being far enough away that Earth’s bright
day and night cycle doesn’t interfere with observations.
Back to Jupiter.
The objects orbiting ahead of Jupiter are known as the “Greeks”, and the objects
trailing Jupiter are known as “Trojans”, although they are all classified as trojans
in a broad sense.
It would be wrong to think though that all trojans orbit along the same plane as Jupiter,
some actually have orbital inclinations of up to 40°, way above or below Jupiter.
A few of these trojans are really big.
The largest, named 624 Hektor, is over 200km wide.
It’s so big, it actually has a 12km wide moon called Skamandrios.
While most other trojans don’t get quite that large, the biggest ones all share a naming
scheme, all based on names associated with the Trojan legend, like Achilles and Paris.
As I mentioned, the really interesting thing about these trojans is that a lot of them
have different compositions to what we are used to seeing on a lot of other asteroids.
They seem to be dark orange or red in colour, covered in something known as tholins.
This is the same substance that gives Pluto its red patches.
However, because the trojans are so small, and so distant, there’s not much more we
know about them.
Which is why NASA is imminently due to launch the $1 billion dollar Lucy spacecraft, on
a huge 12-year journey.
It will visit 7 of these bodies (plus a bonus belt asteroid) and image and investigate them
up close.
Lucy is a really exciting mission.
While the shape of it may look different from anything you’re used to seeing before, its
payload is actually a hybrid of previous, successful missions.
The onboard camera is a derivative of the one onboard New Horizons, and from what we’ve
seen from the Pluto and Arrokoth flybys, we know it can capture some stunning images.
Additionally, it has the Ralph spectrometer and infrared imager, again from the New Horizons
probe.
But New Horizons isn’t the only probe it has scientific equipment from.
There’s an infrared spectrometer onboard, based on the one that was used on OSIRIS-REX.
It also has radio science equipment for determining the mass of the trojans it approaches.
And while it shares a lot of similarities with other NASA missions, there is a big difference
too.
Namely, compared to New Horizons that was powered by a RTG, this one will have big solar
panels, unlike anything seen on a probe before.
Jupiter is roughly 5 times further away from the Sun than Earth is, and using current technology,
it really is about the maximum range where solar panels can still be used.
Even then, these panels have to be huge, each one being 6m in diameter.
As New Horizons went beyond Jupiter, solar panels weren’t a viable option for it.
Lucy’s trip to the various trojans will take it on a rather unusual route.
It will get two gravity assists from the Earth, that will help slingshot it towards its first
targets.
As it passes through the asteroid belt, it will just so happen to be passing by an asteroid
called Donaldjohanson.
It is a C-type, or a common carbonaceous asteroid.
Next will be another carbonaceous asteroid called Eurybates, this time found in the Greek
camp.
Scientists are interested in this 60km wide trojan because it has a tiny 1km wide moon
called Queta.
Scientists believe Eurybates was once involved in a big collision, and this is the largest
fragment of it that remains.
Queta could be a fragment too that ended up in orbit around its bigger neighbour.
Polymele is the next target, another Greek camp trojan and 21km across, but this time
it is one of the mysterious P-Type reddish asteroids I mentioned earlier.
It is believed to be covered in tholins, a bit like Arrokoth, although scientists think
the spectra is a bit different, so we’ll be interested to know what the difference
is when we see it up close.
Next is Leucus, a 34km wide D-type asteroid, the most common trojan type, again covered
in tholins.
What’s unusual about this one is that it rotates very slowly, only once every 466 hours,
and scientists want to see why.
The last one in the Greek camp will be Orus, another D-Type at 51km across.
It may well also have a moon.
After that flyby, Lucy will take the unusual step of swinging back around into the inner
solar system, which will allow it to catch up with the Trojan camp.
There, it only has one target, but this target is actually a binary system, called Patroclus
and Menoetius, at 113km and 104km across respectively.
This spacecraft has set a record for the number of planned targets in one mission, and they
seem like very promising targets.
Understanding asteroids helps us understand how the solar system formed, as often they
are the pristine primordial remnants of the early solar system.
Unfortunately, the first one won’t be reached until 2025, so we do have a bit of waiting
to do, but I’m excited for what’s to come, and I hope you will be too!
Fingers crossed everything goes well with the launch!
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Thanks for watching.
If you want to know more about asteroids, check out this collaboration I did with Scott
Manley here.
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All the best, and see you next time.