Do you really want to know which object in the solar system is the most difficult to reach?
Do you know what the most frequently asked questions are by enthusiasts
to space exploration experts? Well, the first one is: "How long
does it take to get to that particular planet?" The second, more general, is:
"But why is it so difficult to reach that particular planet?"
When I happen to be asked the first question during a conference, I usually respond with
the famous metaphor of the old man and the traveler. Are you familiar with it?
In a faraway land, an old sage was sitting on the edge of a road. After a while,
a traveler passes by and stops to ask him if he knows how far the nearest city is.
The old man barely looks at him and just says, "Keep walking!"
The traveler gets a little offended and continues his journey disgruntled...
After a while, though, he hears the old man calling him and saying, "Boy... with your
brisk pace, you'll take less than two hours. I couldn't have told you before seeing you walk!"
Just like we can't say how long it will take someone to cover 10 km without seeing them walk,
similarly, it's impossible to know the duration of a space journey without knowing certain details.
Let me give you an example. To reach Pluto, then 4.6 billion kilometers away, the New Horizons
probe took just over 9 years, from 2006 to 2015. But if, instead of an automatic mission, it was
intended to send a crewed spacecraft to Pluto, the travel time would have had to be significantly
shorter. No human crew could spend so much time in space unless traveling in an enormous
spacecraft equipped with all the comforts. Even so, I strongly doubt that anyone would
be willing to spend 10 years of their life to reach Pluto... Perhaps to reach Alpha Centauri,
but certainly not to remain confined within the solar system.
To send humans to Pluto, perhaps an acceptable travel time would be one year with
a decidedly large spacecraft, but this would mean transitioning from chemical to nuclear propulsion!
In short, there is no standard travel time to go to Pluto;
everything depends on the nature of the mission, the mass of the spacecraft, and the propulsion
method. Besides the distance, of course. As for the second most popular question,
namely, "But why is it so difficult to get to that particular planet?" for Pluto, we can answer that
there are no problems, except those related to distance. Pluto is an object so slow in orbiting
that there's hardly a need to take its movement into account: you can almost aim straight,
without resorting to curved trajectories like those defined as "Hohmann transfer orbits."
And this generally holds true for all the distant planets in the solar system.
However, there are celestial bodies, even much closer to Earth, that require very long travel
times to be reached, regardless of all other factors. One example is the comet
67P/Churyumov-Gerasimenko! “Roll intro”
Perhaps this name won't mean much to most people, yet it represents a milestone in the history of
space exploration. It is, in fact, the only comet where a human artifact has ever landed.
On November 12, 2014, the lander Philae, a small lander released from the Rosetta probe, touched
down on the celestial body... an achievement never accomplished before and never repeated since. This
feat allowed the collection of an enormous amount of data on these peculiar astronomical objects
that have puzzled cosmologists for centuries. Earlier, the Giotto probe in 1986 and Deep
Impact in 2005 initiated the exploration of these celestial bodies by approaching
Halley's comet and impacting the surface of Comet Tempel 1, respectively. However,
this time the mission's purpose was to reach a comet, orbit it, and then release a lander
onto the surface to gather crucial data for understanding the origin of these objects.
Comets, indeed, have always been considered particularly important in unraveling the mysteries
of the birth and evolution of the solar system. These celestial bodies, formed from various icy
substances, including water, along with a mix of dust and minerals, are believed to be ancient
remnants of the condensation of the nebula from which the entire solar system developed.
The probe was named Rosetta in Latin precisely for this reason, in memory of the famous Rosetta
Stone, an ancient artifact that bore the same text in ancient Greek and Egyptian hieroglyphs. This
allowed the French archaeologist Champollion to translate the ancient Egyptian language,
which had remained incomprehensible until then. Similarly, in the scientists' intentions,
the Rosetta probe was supposed to help decipher the hitherto unknown language of
the matter that gave rise to our planetary system. The lander, on the other hand, was named Philae,
after the Latin name of an islet on the Nile, where an obelisk was found
with inscriptions in both Greek and Egyptian. Interestingly, the initial target of the mission
was not supposed to be Comet 67P. Originally, Rosetta was set to launch on January 12,
2003, to reach Comet 46P/Wirtanen in 2011. However, plans changed when the Ariane 5,
the chosen carrier for launching Rosetta, failed on December 11, 2002. The entire system
experienced a slowdown and was then rescheduled with a launch planned for February 26, 2004,
reaching Comet 67P/Churyumov-Gerasimenko in 2014. After two canceled launches, the Rosetta mission
finally took off on March 2, 2004. Although the launch date had changed, the mission's purpose
remained the same: to reach an orbit that allowed the probe to chase and accompany the comet,
be captured by its extremely weak gravity, and then lower the Philae lander onto the surface.
All of this, easy to put on paper, would lead the probe to embark on a sort of
planetary odyssey in the next ten years... Reaching a comet and landing on its nucleus
is quite a complex maneuver, not within everyone's reach. In the nineties, such a project presented
seemingly insurmountable difficulties: no one had ever attempted such a feat before then. No
one had any idea how to do it. In fact, it wasn't even known if it was possible with the technology
available at the end of the second millennium. Comets, in fact, are not only very distant but
also move at very high speeds, and there were no rockets capable of providing the necessary
velocity to reach and accompany them in their orbital journey around the sun. Rosetta was
thus launched into space by an Ariane 5, among the most powerful launchers available. However,
to give it the required speed to head toward the comet, it was necessary to resort to one of the
most common tricks of interplanetary flight, which involves flying the probe close to a planet and
using its gravitational force to accelerate it (a technique called "gravitational slingshot").
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Comet 67P/Churyumov-Gerasimenko has a highly elliptical orbit, bringing it to reach a
maximum speed of about 135,000 km/h when it is at perihelion, 186 million kilometers from the Sun,
and a minimum speed of about 18,000 km/h when it is at aphelion, 850 million kilometers
from the Sun and just beyond Jupiter's orbit. To reach the comet, the probe couldn't do what
New Horizons did with Pluto but had to synchronize its orbit with that of the comet. To achieve this,
it had to use gravitational assist maneuvers, exploiting the attraction of some planets to
modify its trajectory and speed. As can be seen from the animation that
reconstructs Rosetta's entire path, which we are pleased to show you in this same video,
this allowed the probe to position itself behind the comet and then gain speed to accompany it
when the comet was racing at 55,000 km/h at a distance of 525 million kilometers from the Sun.
Another significant issue was that 67P couldn't be reached at its closest approach to Earth. When a
comet comes within 450 million kilometers of the Sun, its nucleus becomes active, heating
up and producing the famous tail or coma—an emission of gas and dust that makes it highly
advisable not to be in its orbit, let alone try to descend to the surface. Therefore, Rosetta had to
approach the comet at a much greater distance than necessary to reach a common asteroid, for example.
These are, however, the main phases of the mission that we
present to you below. It's surprising to note how the flight planners found
a way to have the probe make two close encounters with two different asteroids!
In practice, things unfolded like this: after exactly one year from launch, in March 2005,
the probe had its first gravitational assist with Earth. Two years later, in February 2007,
there was the flyby with Mars, and in November 2007, the second assist was received from Earth.
In September 2008, there was the encounter with the asteroid Steins, and in November 2009,
the last gravitational push from our planet. Yes, you understood correctly! At that point, 5
and a half years had passed since launch, and the Rosetta probe was still in the vicinity of Earth!
Only from that moment onwards did the actual deep-space journey begin...
Rosetta took a very precise direction, and in July 2010, on its way, it encountered
the large asteroid Lutetia, then continued on the route that, two and a half years later,
would bring it on the trail of the comet. That long chase in outer space was probably one of
the most delicate phases of the journey. At those distances, the Sun is seen and felt very little:
it's dark and extremely cold. This practically means that the solar panels that ensured the
operation of Rosetta and communication with Earth would no longer receive sufficient energy
from the sun. It was decided to put the probe into "hibernation" for two and a half years,
essentially turning it off and putting it to sleep in a long cosmic winter during which scientists
would not receive any signals and would have no chance to intervene with corrective maneuvers.
All you can do in such cases is cross your fingers and hope that the probe is
spared from impacts, malfunctions, and any other cosmic accidents and that it can be
reignited once it gets close enough to the sun. And on January 20, 2014, in a press room at ESA
trembling and unbelievably packed, a burst of enthusiasm welcomed a faint radio signal coming
from about 700 million kilometers from Earth. Rosetta had survived. And it had awakened.
In August of that same year, the probe finally reached the comet. At that point,
the really difficult part began. It was necessary to decide where to release the
Philae lander module. Comets are such small bodies that their surface characteristics
cannot be observed from Earth. In the case of 67P, we are talking about a peanut-shaped object
just over 6 km long... so, some decisions, such as the choice of the landing point,
cannot be made well in advance. You navigate by sight, with the data provided by the probe.
If reaching the core of a comet resembles, as we've mentioned, a true space odyssey,
landing on it is a genuine gravitational puzzle. On comets, gravity is, of course, extremely low,
almost nonexistent, and especially for an irregularly shaped body like 67P,
the gravitational field generated by such a small mass is so weak and distorted that maintaining
a stable orbit by a probe becomes problematic. It's a matter of continuously correcting orbital
parameters, all this despite the distance, which allowed intervening with a delay of
almost an hour for the correction to be made. In short, a real nightmare!
An even more terrifying nightmare when you consider that to release the lander toward
the chosen landing point, one also had to take into account the fast rotation of the comet's
nucleus (about 12 hours) and the continually changing orientation of the rotation axis.
The good thing was that for landing, no parachutes, retrorockets, or other
systems to cushion the fall were needed, but a good damping system was sufficient.
The problem, if anything, was the opposite: avoiding the lander from bouncing on the surface
and drifting away forever into deep space, as there was not enough gravitational attraction
to hold it. For this purpose, Philae was equipped with a small nitrogen thruster at the top, which
should have kept it pressed to the surface long enough to fire two harpoons to anchor it securely.
But for unknown reasons, neither of these two systems worked. On November 12, 2014,
the lander ended up bouncing for real... but one should imagine a slow-motion bounce of a
few hundred meters, followed by a descent of exasperating slowness - about two hours - and
then by a second bounce, this time lower, which made it end up in a kind of crevice (as was
discovered two years later when Rosetta managed to photograph it). There, its solar panels could not
function. Fortunately, Philae was equipped with a battery with a three-day autonomy, allowing
its onboard instruments to perform most of the planned scientific operations and send valuable
information. Then, it went into hibernation. It would wake up briefly in 2015, thanks to better
exposure to sunlight, but without being able to maintain stable communication with the probe.
Rosetta, now stably positioned in the comet's orbit, continued for another two years to
collect an enormous amount of data. Their study by scientists is not yet entirely concluded, but they
have already allowed understanding the structure and evolution of comet nuclei in a way previously
unimaginable, which will likely provide new perspectives on the formation of the solar system.
In September 2016, ESA decided to end the Rosetta mission, now tested by twelve years
of space travel. But even in this case, it was not a simple matter. Rosetta was a tough one,
programmed to withstand the worst situations, reactivate in the presence of the slightest
sunlight, and attempt to contact Earth. To prevent its desperate radio signals from continuing to
spread indefinitely into space, technicians had to disable it through a software modification.
On Friday, September 30, 2016, the probe ended its mission by impacting the comet's
surface near a pit called Deir el-Medina. A long and challenging mission: first,
the chase of the comet lasting over 3,800 days and culminating on August 6, 2014,
with the orbital insertion around the celestial body, then the close-up study of the comet's
surface, and in November 2014, the exciting and partly unfortunate descent of the Philae
lander. A total of 780 days were spent near the comet's nucleus, studying its smallest
details. Clues that, on their own, outline the incredible value of this fantastic mission.
Believe me, guys, going to Pluto, Neptune, Uranus, or Saturn is as easy and restful as
going on a photographic safari on the tracks of a hippopotamus. Reaching a comet is,
instead, a headache... like chasing a quirky gazelle across the savannah!