Lucy In The Sky: the most beautiful space mission ever is about to start!
Jupiter is by far the largest and most massive planet in the solar system.
And as befits even the mightiest of gods, it surrounds itself with something of an impressive
"court of miracles," consisting of rings, 79 moons buzzing around it, comets and asteroids
frequently sinking into its atmosphere, and... two swarms of asteroids preceding and following
it in its orbit... the Trojan asteroids!
Despite all that we have discovered about Jupiter, its moons, and even its wafer-thin
rings, we know very little about the Trojans.
Pioneers 10 and 11, the two Voyagers, Galileo and Juno have all returned a wealth of data
on the Jovian system.
Until now, however, the only way to study the Trojans has been from afar, with ground-based
and Earth-orbiting telescopes.
But all that is about to change.
In just a few days, a new Discovery-class robotic mission is scheduled to launch in
2021.
The space probe will visit and explore six different Trojans of Jupiter and "in passing"
also an asteroid in the main belt.
So little is known about the Trojans that the data that the probe will send us will
surely revolutionize our understanding of their origin and that of the entire solar
system.
We're used to thinking that the objects that populate our solar system all revolve around
the sun, each of them neatly in its own orbit.
But there is one class of objects, the "Trojan" asteroids, that prefers to go their own way
in good company.
The "Trojans" are small planets that in astronomy are immediately distinguished from all others.
In fact, they revolve around the Sun occupying (without disturbing) the orbit of a planet,
preceding it or following it along its celestial circuit; with the same period of revolution,
but moved on average of an angle of about 60° ahead or behind it.
This particular configuration tends to remain stable, in the sense that a slight advance
or a slight delay, instead of accumulating over time, are compensated by a dynamic effect
that tends to bring the planet "on track".
If this dynamic effect (discovered by Turin mathematician Giuseppe Lagrange in 1772) did
not act, sooner or later the Trojan would come close to the planet to the point of receiving
a gravitational push that would send it on a different orbit, a fate that sooner or later
(over tens or hundreds of thousands of years), touches all asteroids that cross the orbits
of the major planets, except for the Trojans.
But how long have we been aware of the existence of these unique hitchhikers in the sky?
Well... the German astronomer Max Wolf accidentally discovered the first Trojan - later called
Achilles - with the help of the emerging photographic technique.
It was February 22, 1906, and Wolf was just exploring the region corresponding to Jupiter's
Lagrangian Point L4.
Eight months later, one of his students, August Kopff, discovered a second in L5, and in the
following February a third, this time in L4.
Almost all asteroids at that time were given a feminine gender name taken from Roman and
Greek mythology, but Austrian astronomer Johann Palisa suggested that all asteroids in Jupiter's
orbit be named after the heroes of the Trojan War.
And the first three were in fact named Achilles, Patroclus, and Hector.
However, the number of discoveries was increasing, and to make order it was decided to give to
asteroids discovered in the point L4 (the "Greek Field"), the names of Greek heroes,
and the names of Trojan heroes to those found in L4 (the "Trojan Field").
The curious fact is that Patroclus (in L5) and Hektor (in L4) were named before this
convention took hold... so that each camp still houses an "enemy spy"... (Patroclus
in the Trojan camp and Hektor in the Greek camp).
Meanwhile, the number of discoveries grew, yes, but very slowly.
In 1961, more than half a century later, the Trojans amounted to just 14 members.
Then, as instrumentation progressed, the two regions on either side of Jupiter began to
crowd together.
Slowly at first, and then faster and faster.
To date, about 9000 Trojans have been cataloged: two-thirds of the total in the "Greek Field".
A quantity so large as to make it impossible to continue to assign names taken from the
Iliad.
So the International Astronomical Union (the official Keeper of Space Names) allowed naming
them after Olympic athletes.
A decision in our opinion very questionable.
By extension, asteroids that are located at points L4 and L5 of other planets in the Solar
System are also called "Trojans".
At present, only for Mars and Neptune, 4 and 6 Trojans are known, respectively.
To avoid confusion with those of Jupiter, the adjective Trojans should be followed by
the name of the planet.
When nothing is specified, it is implied that they are the Trojans of Jupiter.
The origin of the Trojans has always been a very controversial subject, and still is.
The hypotheses are diverse.
They range from the capture of fragments of a Jovian satellite destroyed by tidal interaction
with the planet, to the entrapment of planetesimals orbiting near proto-Jupiter, to the capture
of short-period comets.
However, according to the results of recent numerical simulations on the early evolutionary
phases of the Solar System, the Trojans would not be bodies captured by Jupiter during its
planetary accretion phase, but asteroids from the outer Solar System... captured and then
trapped forever in the L4 and L5 Lagrangian points of the Jupiter-Sun system.
Planet formation and evolution models suggest that the Trojan asteroids are likely remnants
of the same primordial material that formed the outer planets, and thus serve as time
capsules from the birth of our solar system over 4 billion years ago.
These primitive bodies hold vital clues to deciphering the history of our solar system
and may even tell us about the origins of organic materials - and even life - on Earth.
That is to say that over billions of years Jupiter has collected and preserved, in two
vaults sealed by its gravitational field, thousands of objects that can be considered
the fossils of the solar system in formation.
For astronomers, it would be a dream to get their hands on those treasures, and finally,
understand how it all began...
And that's where Lucy comes in.
About 3.2 million years ago, in what is now the Awash River Valley in Ethiopia, a small
bipedal creature ceased to live.
It is not known how it happened... death took it on the banks of a swamp, probably from
exhaustion.
Fortunately, no predator touched its remains, so that the body, submerged in mud, earth,
and volcanic dust, fossilized over the millennia to become rock.
Then, in 1974, a team of paleoanthropologists led by Donald Johanson had the good fortune
to bring to light about 40% of its skeleton.
Never had such an ancient bipedal hominid been found, and our distant ancestor was recognized
as the first representative of a new hominid species, Australopithecus afarensis, becoming
the most famous pre-human fossil in history.
Her scientific name is AL 288-1, but everyone knows her as Lucy.
A name that comes from the equally famous Beatles song, "Lucy in the Sky With Diamonds,"
which Johanson's team was listening to in camp the night of the discovery.
Over the years, Lucy has proven to be a veritable mine of information about the physical constitution
of the hominids of that period, helping us to reconstruct the evolutionary history of
our species.
And in the same way, a probe bearing her name and about to be launched in these very days
will attempt to wrest the secrets of the formation of the solar system from fossils of another
kind: the Trojans of Jupiter!
"Hey, guys, just a moment before we continue...
BE sure to join the Insane Curiosity Channel...
Click on the bell, you will help us to make products of ever-higher quality!"
Lucy will launch on October 16 from Cape Canaveral on an Atlas V rocket, with a launch window
that will remain open for no more than 23 days.
After that, the necessary alignment of the planets will fail.
And that brings us to the wonderful thing that is Lucy's trajectory.
Getting to Jupiter's orbit is difficult.
It takes a lot of thrust to lift something so far from the Sun, and Lucy is a very heavy
spacecraft (1550 kg, including the onboard fuel needed for correction maneuvers).
Yes, because by the way, the spacecraft is solar powered.
And this is an extraordinary progress.
Only recently, in fact, solar cells have become efficient enough to power a spacecraft that
must move so far from the Sun.
When fully deployed, the solar panels will reach a total diameter of 14 meters!
After launch, Lucy will enter a high orbit around the Earth; then, a few months later,
it will descend back to our planet to benefit from a gravitational assist that will push
it beyond the orbit of Mars, until it meets the main asteroid belt where on April 20,
2025, it will have its first encounter with a 4 km asteroid called Donaldjohanson (yes,
that's right, the discoverer of Lucy's skeleton!).
This will be the smallest of the mission's targets, which Lucy will use as a testbed
for all of her instrumentation.
Not to mention the fact that Donaldjohanson is also an interesting object in itself, as
it has been identified as a fragment of a massive collision that occurred about 130
million years ago that produced the asteroid family Erigone.
On August 12, 2027, the spacecraft will reach its first Trojan target, Eurybates, about
64 km in diameter.
Eurybates is much larger than Donaldjohanson, but it does share some similarities.
It is also member of a collisional family (the only known in the Trojans).
In January of 2019, the Lucy team learned that Eurybates has a satellite (named Queta)
that is likely around 1 km in size.
So this flyby will be two for the price of one!
While Lucy will continue to fly through the Greek field, in L4, her next target will be
Polymele, which at 21 km in diameter will be the smallest of Lucy's Trojan targets.
And this will be the first time a spacecraft will have a chance to closely examine such
an object: very dark and reddish, thought to be rich in organics.
Lucy will fly close to this asteroid on September 15, 2027.
Lucy's next target will be Leucus.
A 40 km diameter asteroid that rotates very slowly.
Its day is in fact 446 hours long!
Also, as it rotates its brightness observed from Earth varies greatly, suggesting that
it possesses a rather elongated shape.
Lucy will know for sure when she passes by it on April 18, 2028.
Only a few months after the flyby of Leucus, exactly on November 11, 2028, Lucy will approach
Orus, a 51 km diameter Trojan also very dark.
And this will be the last flyby of this first part of the Mission.
Then Lucy - and this maneuver will be something never seen before in the history of space
exploration - will drop back down through the asteroid belt and back to Earth's orbit,
to take in December 2030 another gravitational assist able to launch her this time towards
the asteroids of the Trojan field in L5.
Arrived in L5 in 2033, on March 3 of that year it will fly over the double asteroid
formed by Patroclus (113 km) and Menoetius (105 km), two objects separated only by a
distance of 680 km.
Will this be the end of the mission?
Not even close!
The coolest part of the Mission is that this amazing cycle of departures and returns will
repeat every six years!
Lucy will descend back to Earth and then head into Jupiter's orbit, cycling between asteroids
L5 and L4 each time.
As long as the spacecraft stays healthy and NASA wants to continue the mission, the goings-on
can continue for a long time to come.
And even after the eventual final shutdown, Lucy will continue to orbit between Earth
and the Trojan asteroids for at least 600,000 years.
Until, perhaps a million years from now, gravitational forces will either crash her into the Sun
or propel her out of the solar system.
But there's more.
The first spacecrafts to leave the Solar System, Pioneer and Voyager... carried with them messages
from Earth for any intelligent life that may one day encounter them.
The Lucy Mission continues this tradition, but the plaque it carries is not for unknown
aliens, but for our own descendants.
As we have just said, after the mission is over the Lucy spacecraft will remain on a
stable orbit - traveling between the Earth and the Trojan asteroids for hundreds of thousands
of years.
It is not hard to imagine that someday in the distant future our descendants may retrieve
the Lucy spacecraft as a relic of the early days of humanity’s exploration of the Solar
System. and that’s why the spacecraft carries with
it a plaque as a time capsule, including messages from prominent thinkers of our time
We, therefore, find it very appropriate to close by quoting the words of the writer Dava
Sobel: “We, the inquisitive people of Earth, sent
this robot spacecraft to explore the pristine small bodies orbiting near the largest planet
in our solar system.
We sought to trace our own origins as far back as evidence allowed.
Even as we looked to the ancient past, we thought ahead to the day you might recover
this relic of our science.”