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.