Many of us might have heard about asteroids from the news and how they may pose a threat
to Earth.
But have you heard before of Trojan asteroids?
What are they?
And what’s the difference between them and other asteroids?
The name “Trojan” may sound frightening, but don’t worry, these asteroids aren’t
particularly dangerous to us on Earth.
Short answer is that the name refers to any asteroid that is trapped in the lagrange points
of a planet in the solar system.
But what does that mean?
Let's take a step back to better understand what Trojan asteroids are.
We’ll actually go as far back as 1906 to take a look at a german astronomer named “Max
Wolf” who observed an asteroid that seemed to always be ahead of Jupiter.
Later on, it was announced that this asteroid was actually trapped in one of Jupiter’s
lagrange points, these are points that exist around planets’ orbits at which a small
body could exist in a stable state.
Later on, more asteroids trapped in Jupiter’s lagrange points were found, and astronomers
gave some of them names of Trojan war survivors.
And by habit, all asteroids found trapped in lagrange points of planets in the solar
system fell under the category “Trojan Asteroids.”
Scientists think that Trojan asteroids are leftovers from the same material that formed
the outer planets in our solar system, that’s why studying them is very important in understanding
the formation of not only gas planets, but also the whole of the solar system.
All this talk about Trojan asteroids is to discuss how NASA is now planning a mission
called “Lucy” that’s going for the first time to visit and flyby some Trojan asteroids.
The mission’s spacecraft is an orbiter targeting four Trojan asteroids, in addition to a binary
Trojan asteroid, which makes a total of six, in addition to an ordinary asteroid as well.
And as “Lucy” travels between asteroids, it will also give us perspective into other
extraterrestrial objects from afar.
This will mark “Lucy” as the first mission to be launched with this many targets around
the Sun.
Before we talk about these targets, let us first talk about the story behind the name.
The mission “Lucy” is named after a fossil hominid skeleton that was found in 1974 in
Ethiopia and the name was chosen because the “Lucy” mission will give us insights into
the evolution of the solar system just as much as the lucy fossil gave us insight about
human evolution.
Now let’s begin with the first target named “(52246) Donaldjohanson” which is actually
the name of the scientists who co-discovered the “Lucy” fossil.
This asteroid however, is not a Trojan asteroid.
Rather, it’s a small main belt asteroid that Lucy will meet along the way and will
serve as a test object for Lucy to practice its instruments.
The asteroid is 4 kilometers (2-3 miles) in diameter which makes it the smallest of all
of Lucy’s targets.
The asteroid is small in size, but big in status as it is believed to be a remnant survivor
of a 130-million-year-old collision which makes for a very interesting case study of
the history of asteroids.
Up next we have “(3548) Eurybates” which is the first Trojan asteroid “Lucy” will
flyby and it’s located on the Lagrange point 4 (L4) in Jupiter’s orbit, the L4 point
is also called the “Greek swarm” because most Trojan asteroids in this point are named
after Greek characters.
“(3548) Eurybates” is a big asteroid with a 64 kilometer (40 miles) diameter, and observations
made by the “Lucy” team back in January of 2019 revealed that it has another body
orbiting it that is only 1 kilometer (0.5 mile) in size.
The next target is also located in the L4 point, called “(15094) Polymele” it is
a 21 kilometer (13 miles) Trojan asteroid which makes it the smallest among all of Lucy’s
Trojan targets.
Scientists believe that this asteroid is rich in organic materials
Afterwards, “Lucy” will flyby the red and dark asteroid “(11351) Leucus” which
is interesting in more than one way, first of all, a day on this asteroid is 466 Earth
hours!
And that’s because it rotates around itself very slowly.
Due to that slow rotation, the asteroid is believed to have higher temperature than other
asteroids during the day and lower temperature during the night, which makes it a very new
and interesting geology to study.
Additionally, scientists suggest that the asteroid is elongated in nature, not round
nor circular, and what leads to that belief is the extreme variation in its brightness
as seen from Earth, but nothing is for sure until “Lucy” makes its flyby!
The final L4 Trojan target is “(21900) Orus” which is a 51 kilometer (31.6 miles) asteroid.
By studying this asteroid, the “Lucy” mission will compare it to two of its previous
targets: “Leucus” and “Eurybates”, to draw similarities and differences.
This concludes the journey around the L4 point.
Next, “Lucy” will meet its final two targets around the Lagrange point 5 (L5) which is
also named the “Trojan swarm” because most Trojan asteroids in this point are named
after Trojan heroes.
The next two targets are the binary asteroid system (617) Patroclus and Menoetius, but
you would notice that these two names aren’t Trojan war heroes!
They’re actually Greek characters, and nicknamed by astronomers as “Greek spies” among
the “Trojan swarm” and the reason behind that is that these two asteroids were named
after the naming tradition was proposed.
More specifically, “Patroclus” is only the 2nd Trojan asteroid to be ever discovered
and that was back in 1906 and it’s also the largest of Lucy’s targets with 113 kilometers
(about 70 miles) in diameter and Menoetius is a little smaller at 104 kilometers (65
miles.)
These precise measurements of the asteroids’ sizes are possible because of occultation
analysis where the asteroids come between the Earth and a star allowing astronomers
to measure their shape and sizes.
What makes this binary system so special is that it is believed to be a very old system
from the time of the solar system formation about 4.5 billion years in the past.
Which gives scientists a time leap opportunity to study the formation of planets and to have
an insight into the properties of other primordial asteroid pairs that are believed to be numerous
in the Kuiper belt.
The orbit of the “Patroclus and Menoetius” pair often makes it very hard for it to be
reached by spacecrafts, but Lucy will catch them in a special time when they’re easily
accessible.
So what is “Lucy” going to do passing by these asteroids?
What are its scientific objectives?
The mission has objectives that has to do with four main scientific concepts:
Surface Geology: the mission is supposed to run a full geological survey of the asteroids.
It will make a profile of the surface shape and crater sizes, as well as measure how much
sunlight the surface reflects.
Surface Color and Composition: Lucy will determine the color of the asteroids’ surfaces as
well as report on the distribution of ice and organic materials.
Interiors and Bulk Properties: Lucy will not only study the surface of the asteroids, it
will also look deeper into sub-surface properties and measure the asteroids’ mass and density.
Satellites and Rings: finally, the mission is also supposed to be on the lookout for
any bodies orbiting around the Trojan asteroids.
Helping “Lucy” in these intense scientific goals are different high tech scientific instruments
that include 3 cameras.
One of the cameras is called the “Multi-spectral Visible Imaging Camera” or “MVIC” for
short, and it’s a visible color camera covering wavelengths from 0.4 to 0.85 microns.
Another camera is a wide-field “terminal tracking camera” or “T2CAM” for short,
and it’s going to especially capture wide images of the asteroids.
The “LOng Range Reconnaissance Imager”, or “L’LORRI” for short is the final
camera and it covers the visible range from wavelengths of 0.35 to 0.85 microns.
It’s also a very high quality camera and will deliver the most detailed images of the
asteroids.
The mission is also equipped with 2 spectrometers: the first is an infrared spectrometer covering
wavelengths from 1 to 3.6 microns, and called the “Linear Etalon Imaging Spectral Array”,
or “LEISA” for short.
it’s going to search for evidence of organic material and ice on the surface of the asteroids.
Another spectrometer is also covering the infrared spectrum from 6 to 75 microns and
is responsible for measuring properties regarding the bodies of the asteroids.
Called the “Thermal Emission Spectrometer”, or “L’TES” for short and will measure
the thermal inertia of the asteroids among other properties.
The spacecraft itself is measuring at more than 13 meter (~42 feet) length with a high
gain antenna to communicate back with Earth that is 2 meter (6.5 feet) high.
The high gain antenna will also be useful in doing some science because “Lucy” will
be using radio signals from the antenna to measure the mass of some targets on the asteroids.
It’s important to note that “Lucy” will not orbit the asteroids, it will only flyby
them with the closest approach lasting a mere few hours, so the onboard instrument will
have to act extra fast to perform all the science needed in such a short amount of time.
Regarding the Lucy mission timeline, it is divided on 6 main phases: phase A was when
the mission was still just a concept and a proposal, and then it was selected by NASA
back in 2017, which made it go on to phase B. In which, the mission engineers developed
the preliminary designs and requirements of the spacecraft and the instruments on board.
After phase B was done, phase C began which is the current phase of the mission when the
science team approves the final designs of the mission components.
This phase also marks the beginning of the fabrication process and the software design,
followed by countless testings to ensure that the spacecraft is fault free.
Phase D has to do with assembling all parts of the spacecraft and making sure they all
work together perfectly before launch that is scheduled for October of the year 2021
with a 21-day launch window.
Lucy is supposed to launch from Cape Canaveral in Florida, and if the mission unfortunately
missed all launch windows, another opportunity will be available the following year.
If all goes according to plans, and Lucy launches on time, phase E will begin, in which the
mission will be on its way to do great science and meet the spectacular Trojan asteroids.
On its journey (or Tour, as the official mission website likes to call it), Lucy will first
perform two gravity assists by the Earth to give the spacecraft the acceleration needed
to go on its way to meet the first target.
By April 2025, Lucy will reach the main asteroid belt and meet “(52246) Donaldjohanson”,
then fast forward to the year 2027, when it will reach the orbit of Jupiter and flyby
“(3548) Eurybates” in August then “(15094) Polymele” in September.
Fast forward again to the year 2028 when Lucy will encounter “(11351) Leucus” in April,
then “(21900) Orus” in November.
“Lucy” will then flyby Earth again before meeting the last targets; “(617) Patroclus
and Menoetius” in March of the year 2033.
The entirety of the mission travelling from one target to another, will take 12 years
excluding any extended time, after which, the final phase will start.
Phase F is the long dead end of the mission!
When “Lucy” has completed its duty, it will be shut off, but will remain in an orbital
dance between the Earth and Jupiter for well after 600 thousand years in the future!