A large comet is announced coming from the true edge of the Solar System: the Oort Cloud!
In 50,000 years or so, the human species will have mutated, died out, or transmigrated.
But on a distant planet orbiting a distant star, a sufficiently evolved alien civilization
might find itself listening to the first movement of Bach's Brandenburg Concerto No. 2 or Chuck
Berry's Johnny B. Goode.
The aliens might in fact have captured a strange ancient machine, a stray spacecraft, still
in orbit or crashed on their planet, and found hidden in it a bizarre metallic contraption,
a gold-plated disk.
They may have realized, with wonder, that it was an ancient analog-type recording.
And after improvising a device to decode it, they may have stayed for entire revolutions
of their planet trying to understand the meaning of sounds such as the cry of a child, the
song of our whales, and the greeting in over 70 languages from those who once lived in
an incredible place called Earth.
But where is this romantic oddity now?
You already know... aboard two spacecraft that are currently roaming in deep space;
over 20 billion miles away from us, where space is empty, dark, silent, dark, and downright
cold.
These two small probes are now the farthest man-made objects from Earth.
With a fantastic history behind them.
Like all NASA exploration vehicles of the 1970s, Voyager 1 and 2 were named after the
vessels of the first exploratory ocean crossings.
It all began with a rare and extraordinary event in our solar system.
Every 178 years, more or less, the mysterious dance of the planets around the Sun proposes
in fact the alignment of the four "outer" planets (the gas giants Jupiter and Saturn
and the icy twins Uranus and Neptune).
"Eureka", someone thought: why not take advantage of the unusual opportunity to send a probe
able to visit them one after the other?
So began, it was 1977, the planetary tour.
As in a miniature golf course, the twin probes were launched to the four planets aligned.
They visited, one after the other using the new technique of gravitational assist.
The journey lasted 12 years.
The Voyagers sent to Earth the almost unreal images of the four "outer" planets, with their
moons, spectacular rings, and incredible colors.
The tour ended in 1989 with a flyover of Neptune.
With the mission over, it was thought that the end was near for the twin probes.
Soon they would wear out.
And they would shut down altogether.
They would stop sending us data and photographs.
Alone and silent, lifeless, they would wander for millennia with their cargo of sound and
footage.
But instead... the Voyagers, disproving the technicians, persisted in not dying.
And so they were entrusted with yet another mission, that of arriving at the extreme boundary
of the Solar System, or the edge of the heliosphere, the "bubble" that protects us all from the
effects of stellar wind blowing in the opposite direction to the motion of the Sun.
It was assumed that its limit came to more than 18 billion kilometers from Earth, and
at that distance, Voyager 1 arrived in 2012, and Voyager 2, which followed a longer path,
in 2018.
Of course, all the media emphatically titled it "reaching interstellar space"... pointing
out that the two "spaceships" had "left the solar system" and set off "to other stars".
But it was not true... in fact, they would not have done it before thirty thousand years,
when and if they would have been able to cross unharmed the last obstacle, the so called
Oort Cloud: a sphere of dust, ice, and rocks even a thousand times bigger and farther than
the heliosphere!
That would be like comparing a millimeter with a meter!
Oort Cloud?
Someone will ask... but what is it... does it really exist or is it just a theory?
Well, let's go find out together!
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The Oort cloud is a huge spherical cloud of comets and dust that extends three light-years
from the Sun in all directions.
The fact that the cloud is spherical rather than disk-shaped distinguishes it from other
collections of debris in the solar system, such as asteroids and Kuiper belts.
The sphere is so large that its edge is closer to our nearest star than to the sun itself.
Almost every star is thought to have its own Oort cloud, either larger or smaller.
These clouds certainly overlap, and our sun's Oort cloud probably overlaps with Alpha Centauri's
cloud.
To appreciate the distance to the Oort Cloud, it’s helpful to set aside miles and kilometers
and instead use the astronomical unit, or AU — a unit defined as the distance between
Earth and the Sun, with 1 AU being roughly 150 million kilometers.
For comparison, Pluto’s elliptical orbit carries the planet between 40 and 50 astronomical
units from the Sun.
The inner edge of the Oort Cloud, however, is thought to be located about 2,000 AU, with
the outer edge being located somewhere between 100,000 and 200,000 AU from the Sun.
If those distances are difficult to visualize, you can instead use time as your ruler.
At its current speed of about 1.5 million kilometers a day, Voyager 1 spacecraft won't
enter the Oort Cloud for about 500 years.
And it won’t exit the outer edge for maybe 40,000 years.
Even if you could travel at the speed of light (about 1 billion kilometers per hour), a trip
to the Oort Cloud would mean engaging in a journey of several months.
When light leaves the Sun, it takes a little over eight minutes to reach Earth, and about
4.5 hours to reach Neptune’s orbit.
Just under three hours after passing Neptune’s orbit, the Sun’s light passes beyond the
outer edge of the Kuiper Belt, the region ruled by dwarf planets Pluto and Eris, averaging
50 AU from the Sun.
After another 12 hours, the sunlight reaches the heliopause, where the solar wind — a
torrent of charged particles flowing away from the Sun at about 400 kilometers per second
— smooshes up against the interstellar medium.
Beyond this boundary is interstellar space, where the Sun’s magnetic field holds no
sway.
The sunlight has now been traveling away from the Sun for about 17 hours.
Less than one Earth day after leaving the Sun, the sunlight has already traveled farther
from the Sun than any human-made spacecraft.
Yet somehow it will be another 10 days before that same sunlight reaches the inner edge
of the Oort Cloud, and perhaps as much as a year and a half before the sunlight passes
beyond the Cloud’s outer edge.
Have I been able to get an idea of how terribly large the Oort Cloud is compared to the heliosphere,
which many felt was the limit of the solar system reached by the Voyagers?
The difference lies only in the fact that the heliosphere has been observed and measured,
while the existence of the Oort Cloud is just a conjecture, although, as we will see in
a moment, based on very solid clues.
But now we’re trying to figure out where this incredible shell of dust, ice, and rock
that envelops our solar system could come from.
The leading idea for the formation of the Oort Cloud says that these icy objects were
not always so far from the Sun.
After the planets formed 4.6 billion years ago, the region in which they formed still
contained lots of leftover chunks called planetesimals.
Planetesimals formed from the same material as the planets did.
The gravity of the planets (primarily Jupiter) then scattered the planetesimals every which
way.
Some planetesimals were ejected from the solar system entirely, while others were flung into
eccentric orbits where they were still held by the Sun’s gravity, but were far enough
out that galactic influences also tugged on them.
Likely the strongest influence was the tidal force from our galaxy itself.
In short, gravity from the planets shoved many icy planetesimals away from the Sun,
and gravity from the galaxy likely caused them to settle in the borderlands of the solar
system, where the planets couldn’t perturb them anymore.
And they became what we now call the Oort Cloud.
Again, that’s the leading idea, but the Oort Cloud could also capture objects that
didn’t form in the solar system.
Unlike the planets, the main asteroid belt, and many objects in the Kuiper Belt, objects
in Oort Cloud do not necessarily travel in the same direction in a shared orbital plane
around the Sun.
Instead, they can travel under, over, and at various inclinations, around the Sun as
a thick bubble of distant, icy debris.
Hence, they’re called the Oort Cloud rather than the Oort Belt.
The existence of the Oort Cloud is the answer to a question that man has asked himself since
ancient times: what are comets and where do they come from?
Since the time of Halley, comets have been divided into two major groups: those of long
period, with orbital periods longer than 200 years, and those of short period.
This subdivision is to some extent arbitrary, but it also reflects real differences.
Comets of long period, in fact, enter the inner regions of the Solar System coming from
all directions at random, while the others are generally coplanar to the ecliptic, or
at least have orbits little inclined.
It is thought that long-period comets come from the Oort Cloud, which is a spherical
region, while the others from the Kuiper Belt, which is a flat ring lying on the ecliptic.
Dutch astronomer Jan Oort proposed the existence of the cloud to explain (among other things)
where long-period comets come from, and why they seem to come from all directions rather
than along the orbital plane shared by the planets, asteroids, and by the Kuiper Belt.
There may be hundreds of billions, even trillions, of icy bodies in the Oort Cloud.
Now and then, something disturbs the orbit of one of these icy worlds, and it begins
a long fall toward our Sun.
Some have orbits so long that the last time they passed through the inner solar system,
our species did not yet exist.
Others have never ventured close to the Sun in the billions of years since they formed.
Based on the number of long-period comets known today, it is estimated that the Oort
Cloud contains 6000 billion cometary nuclei, corresponding to a total mass of about 40
times that of the Earth.
Numerical simulations carried out in recent years have shown that stellar perturbations
are sometimes anything but mild.
Occasionally, a star comes so close to the Sun that it crosses the Oort Cloud, and then
the perturbations are very violent.
Statistically, a star is expected to pass within 10 thousand UA of the Sun every 40
million years, and within 3000 UA every 400 million years.
Comets close to the star's trajectory are hurled into interstellar space, while the
orbits of others are heavily modified.
The next expected close pass will occur in 1.4 million years and the star will be the
red dwarf Gliese 710, which will cross the outer part of the Oort Cloud at a distance
of 70,000 UA from the Sun.
This passage could increase the frequency of comet occurrence in the inner Solar System
by about 50%.
And almost to exemplify everything we have said so far, just in these days there has
been confirmation of the discovery of a large comet directed towards the Sun, coming from
the very Oort Cloud.
The object, officially designated a comet on June 23, is called Bernardinelli-Bernstein
after its discoverers, University of Pennsylvania graduate student Pedro Bernardinelli and astronomer
Gary Bernstein.
First spotted in archival images from the Dark Energy Survey taken in 2014, Comet Bernardinelli-Bernstein
is now located at the equivalent distance of Uranus, roughly 20 astronomical units from
the Sun.
The comet shines at magnitude 20...
That’s 400,000 times weaker than the weakest star you can see with the naked eye
Despite receiving 400 times less sunlight than Earth’s surface at its current location,
the comet is bright enough to be seen by telescopes, which hints that its size must be somewhere
between 100 and 370 kilometers.
The uncertainty arises because of the object’s unknown reflectivity and shape.
But at either end of the scale, this estimate would still make it much bigger than any previously
known comet.
The next largest in terms of its nucleus - Hale-Bopp, which wowed stargazers in 1997 - measured
a relatively paltry 60 kilometers across.
The Bernardinelli-Bernstein comet is “certainly the largest comet we’ve seen in the modern
astronomical era.
When the comet swings closer to Earth, in 2031, it will still be at 11 AU, which is
a little more distant than Saturn's average orbit from the sun.
But... what makes Comet Bernardinelli-Bernstein so special, aside from its size, is the fact
it hasn't visited the inner solar system in three million years, roughly the same era
that the famous human ancestor "Lucy" was walking the Earth.
The comet, in fact, is going through a very long orbit, which has its aphelion at more
than 40,000 astronomical units away.
The classic example of an icy body from the very confines of the Solar System.
And who knows that before its passage around the orbit of Saturn it is not possible to
send a probe to meet it, able to reveal its secrets…
A new Voyager probe... that would be great, wouldn't it?