For thousands of years, human beings have gazed at the glistening stars above
and wondered, what it would be like to travel to them.
Even today, the idea of sending astronauts beyond our solar system remains a distant dream,
but humanity have launched five unmanned robotic spacecraft that
are on a path to interstellar space, the region in between the stars.
Each of the five probes were primarily designed to explore the solar systems outer planets,
flying past the giant worlds at extreme speeds, while capturing data that has changed our
understanding of the solar system forever. But when they had finished their initial missions,
their momentum continued to carry them farther from the
sun, setting them on a path that will ultimately place them among the stars.
For this reason, four of the five probes carry a message for any extraterrestrial intelligence
that may one day come across them, a message that will introduce the reader, to the probes creator.
Reaching interstellar space is considered leaving the solar system. According to NASA,
this is where the Sun's constant flow of charged particles, known as solar wind,
is impeded by the interstellar medium, the matter and radiation that exists between the stars.
This forms an enormous bubble around the sun,
which ultimately travels past all of the solar system's planets and is called the heliosphere.
It is at the edge of the heliosphere, where a theoretical boundary named the heliopause is found
and according to the most recent measurements, interstellar space begins once you cross it.
Although this boundary is not easy to measure, as it is constantly shrinking and stretching,
making it very difficult to estimate when exactly a probe has passed through it.
Only five probes are currently set on a trajectory to cross this boundary,
and only two have been confirmed to have currently crossed it.
Launched in 1972, Pioneer 10 was the first NASA mission to the outer planets.
The ambitious probe achieved great success, including being the first to fly beyond Mars,
the main asteroid belt and Jupiter. The first to use an all nuclear power supply and
the first to be placed on a trajectory that would allow it to escape the solar system.
Originally designed for a 21-month mission Pioneer 10 lasted for more than 30 years,
until in January 2003 the distant probe sent back its last signal to Earth.
Pioneer 10 is now drifting, like a ghost ship through the cold,
darkness of space more than 11 billion miles away, which is around 18 billion
kilometres away in the opposite direction to the Sun's motion through the Galaxy.
This means that the probe is heading through the tail of the sun,
an extended region of the heliosphere that is shaped by the surrounding
interstellar medium, much like the wake of a boat travelling through water.
Although Pioneer 10 cannot be communicated with anymore,
it has been estimated that the distant probe will enter interstellar space by around 2057,
a journey that would have taken it more than 85 years to complete. Then in around
2 million years from now, it is expected to pass the giant red star, Aldebaran,
which forms the eye of the Taurus constellation and is currently around 68 light-years away.
Much like its twin, Pioneer 11 saw similar success.
It was launched just a year later in 1973 and after becoming the second NASA probe
to successfully pass Mars, the main asteroid belt and Jupiter, it became the first to fly by Saturn.
Unlike its twin, however, after leaving Saturn, Pioneer 11 headed out of the solar system
in the opposite direction, towards the centre of the Galaxy in the constellation of Sagittarius.
NASA received its last signal from Pioneer 11 in 1995, before losing contact forever,
a moment that marked the end of its incredible 22-year mission.
Currently, the dead probe is more than 9 billion miles away,
which is more than 15 billion kilometres away and is estimated to reach interstellar space, by 2027.
In around 4 million years, however, Pioneer 11 will drift into the neighbourhood of
a young star called Lamba Aquilae, which is currently around 125 light-years away.
Scientists were always aware that the probes would eventually travel
to new and exotic parts of the Galaxy and so they were equipped with a special message.
Attached to each is an identical gold anodised aluminium plaque designed to introduce the human
race to whoever or whatever may come across them. The plaques show a man and women,
along with a diagram of our solar system, revealing what we look like and where we are from.
Not long after the success of the Pioneer probes, NASA launched possibly the most
famous unmanned spacecraft to date, the Voyager probes.
Both were launched in 1977 to take advantage of a rare alignment of the outermost planets,
which only occurs once every 175 years. Jupiter, Saturn, Uranus and Neptune would be perfectly
placed for a flyby, allowing each probe to explore the gas giants like never before.
After encountering Jupiter and Saturn, however, NASA decided to change the trajectory of Voyager
one, so that it could closely fly by Titan, Saturn's mysterious, haze covered moon.
This placed Voyager one on a direct route out of the solar system and in 2012,
the historic probe became the first-ever human-made object to enter interstellar space,
the region in between the stars.
Meanwhile, Voyager two continued the grand tour of the planets,
becoming the first and only spacecraft to encounter the Ice Giants, Uranus and Neptune.
After its magnificent grand tour was complete, Voyager two, like its twin,
carried enough momentum to escape the solar system, and In 2018, it became the second
probe to pass through the heliopause and join its partner in interstellar space.
Incredibly, even after more than 40 years of travelling through deep space,
both Voyager probes are still communicating with NASA,
as they study a bizarre region of the cosmos that has never been explored by humans before.
The objects of our solar system will not be the last encounters for these probes, however,
because, in approximately 40,000 years from now,
Voyager one will come within 1.6 light-years of an approaching star, called Gliese 445.
While Voyager two will pass by another star called Ross 248 within the same time scale.
After around 256,000 years, however, the now ancient Voyager two probe will drift past the
brightest star in our night's sky, Sirius, which is currently 8.6 light-years away.
Much like the Pioneer probes before them,
the Voyager probes carried a special message, a kind of time capsule,
intended to communicate with any extraterrestrial space-faring race that might come across them.
The message was recorded onto a phonograph record, a 12-inch gold plated copper disk
containing sounds and images that portray the diversity of life
and culture on Earth. A small part of our magnificent world, placed among stars.
The most recent spacecraft to be launched that will eventually cross the boundary
into interstellar space, was New Horizons, the only probe ever to visit the icy world, Pluto.
New Horizons arrived at Pluto in 2015, after a nine and half year journey,
revealing the demoted dwarf planets surface features in magnificent detail.
After passing Pluto, the probe continued speeding through deep space
and became the first to encounter a Kuiper Belt Object, named Arrokoth.
The New Horizons mission is still active and is expected to visit another Kuiper
Belt Object soon, before ultimately crossing the heliopause boundary,
into interstellar space in an estimated 30 years from now. According to NASA scientists, however,
the distant probe only has enough power to keep operating until the late 2030s, so it
is unclear whether it will still be functioning as it enters the space in between the stars.
Unlike those that came before it,
New Horizons isn't carrying a special plaque that introduces us to any extraterrestrials,
should they come across it, but it is still carrying items that are equally as interesting.
The main one is attached to the underside of the spacecraft in a small canister that contains some
of the ashes of the man that discovered Pluto, Clyde Tombaugh, the astronomer that unknowingly
opened the door to the vast "third zone" of the solar system we now know as the Kuiper belt.
The pioneer probes will be remembered as one of the most successful missions in
NASA's history as they drift like ghost ships through the darkness of space,
unable to communicate with Earth, a fate carried by all interstellar probes.
The Voyager probes, however, continue to surprise scientists
as they persist on the other side of the heliopause, exploring the unknown
and only time will tell how far New Horizons will travel before its power is depleted forever.
All five of these incredible probes may only be considered as tools used to further our knowledge
of the outer solar system and beyond, but they are so much more than that, they are reminders
of our ambition, sent out into the deepest darkest regions of space, to help us find our
place in the cosmos. Relics of a civilisation that dared to explore beyond their horizon.