Want a chance to win a Tesla
with Omaze? Stay tuned for the sponsor message at the end of the video. In the summer of 1977, NASA’s Jet Propulsion
Laboratory launched a pair of Titan-Centaur rockets containing nearly identical spacecraft.
Known as Voyager 1 and Voyager 2, these twin probes were built to last for 5 years with the
intention of studying Jupiter and Saturn and their larger moons. Incredibly, nearly 45 years later,
NASA is still in constant communication with both probes, which take routine commands and transmit
collected data back to Earth’s deep space network, making Voyager the longest-running mission
in the history of space exploration. After completing all of its initial objectives
within 4 years, NASA extended Voyager’s mission to include the two outer giants, Neptune and Uranus,
before embarking on the even more ambitious Voyager Interstellar Mission, with the purpose of
exploring the outer limits of the sun’s sphere of influence and beyond. Voyagers 1 and 2 have
now travelled 22 billion kilometres and 18 billion kilometres from Earth, respectively,
so far that they have left the heliosphere and entered interstellar space. But time, even for
the long-lived Voyager probes, is running out. Perhaps as soon as 2025, the probes will lose
their remaining power supply and go dark forever. In preparation for this, NASA has begun
taking the probes’ instruments offline, in the hope of extending the life of the mission
for a few more years. But when the Voyager probes inevitably do go dark, they will leave
behind a wealth of data that is unprecedented in size and scope. So, now that we are entering
the Voyager Mission’s final days, we can ask: what did Voyagers 1 and 2 discover out there? Did
the probes and their instruments hold up under the rigours of interstellar travel? And why are NASA
scientists so surprised by what they learned? I’m Alex McColgan, and you are watching
Astrum. Join me today as we look at the most stunning discoveries of the
nearly 45-year Voyager Mission and relive the remarkable journey the
probes took after they left Neptune’s orbit, travelling from the solar system’s icy outer
giants to the brink of interstellar space. While Voyagers 1 and 2 were supposed to be on a
5-year mission, their team of forward-thinking scientists and engineers made a number of design
choices that enabled the probes to hold up over a much longer journey. Each probe is equipped
with a long-lasting radioisotope thermoelectric generator, which converts heat from the decaying
plutonium 238 isotope into electric power. The probes also have redundancies of most
of their 11 scientific instruments in case of machine failure, as well as 16 hydrazine
thrusters, including 8 backups. Most importantly, the launch happened at the perfect time. When
the Voyager Planetary Mission launched in 1977, NASA took advantage of a once-in-176-year
alignment of the planets, which not only allowed for flybys of Neptune and Uranus with minimal
course adjustment, but gave the probes a gravity assist from each of the giants they visited,
thereby increasing their effective velocity beyond what they could get from their own rocket
propulsion. This idea was relatively new at the time, having been only attempted previously on
NASA’s Pioneer missions to Jupiter and Saturn. In 1981, Voyager 1 escaped the ecliptic, which is
Earth’s plane of orbit around the Sun, heading 35 degrees to the north. Voyager 2 later went under
the ecliptic, heading 48 degrees to the south. After the Voyager Planetary Mission
was extended to become the Voyager Interstellar Mission, the cameras on both probes
were deactivated in order to conserve power. The last image taken by Voyager 1 is the famous
pale blue dot photograph of Earth, seen from a distance of around 6 billion kilometers –
the most remote image of Earth ever taken! However, this was barely the
start of the Voyagers’ journeys. To reach interstellar space, the probes
would have to traverse the termination shock, a region in which hypersonic solar winds run into
fierce resistance from the interstellar wind. Beyond the termination shock, the
Voyagers would encounter the heliosheath, where slowing solar winds pile up, becoming denser
and hotter, followed by the heliopause – the final boundary between the heliosphere and interstellar
space. But, in spite of what you may think, the start of the interstellar medium doesn’t
actually mark the end of our solar system. Indeed, it will be another 300 years
until Voyager 1 reaches the Oort Cloud, the vast region of billions of icy planetesimals
that surrounds our solar system like a bubble, and another 30,000 years until it exits the
cloud, leaving our solar system forever. When the Voyagers travelled through the heliosheath,
they made an incredible discovery. Because the Sun’s magnetic field spins in opposite
directions on its north and south poles, the spins create a ripple where they meet
called the heliospheric current sheet, sort of like the rings created by dropping a
stone in water. However, when this sheet reaches the termination shock, it compresses, as though
the ripples were hitting the edge of a pool. The Voyager probes discovered that after
the termination shock, these stacked-up ripples form magnetic bubbles. This means the
boundary of the heliosheath is not as smooth and clear-cut as scientists thought. Instead, it is a
fluctuating and magnetically bubbly environment. This messy finding has prompted a complete
revision of our model of the heliosheath! On July 25, 2012, the Voyager 1 space probe
became the first manmade object to leave the Sun’s heliosphere and enter interstellar
space. It was travelling at an incredible speed of 540 million kilometres per year, or 3.6
Astronomical Units, an astronomical unit being the distance between Earth and the Sun
(approximately 149.6 million kilometres). The distance at which Voyager 1 crossed the heliopause
was about 120 Astronomical Units from the Sun, which itself was a revelation: it was unknown
where, exactly, the heliopause occurred. Funnily enough, some early models put it as close
as Jupiter, and others much farther. Remember: the heliopause is the boundary where the Sun’s
solar wind is stopped by its collision with the interstellar medium, kind of like the crashing of
two powerful bodies of water against each other. Solar wind is the steady stream of charged
particles, such as electrons, protons and alpha particles, that come from the Sun’s outer layer.
The interstellar medium, by contrast, consists of charged particles, gases and cosmic dust left over
from the Big Bang and from ancient supernovae. When these charged streams hit each other, they
change course and form a region of equilibrium, called the heliopause boundary. At
first, NASA wasn’t sure if Voyager 1 had truly crossed the heliopause and entered
interstellar space. As models predicted, the probe’s plasma wave detector found
a massive increase in plasma density, 80 times what it had registered in the outer
heliosheath, and a spike in galactic cosmic rays. But something strange didn’t happen that left
scientists baffled. After crossing the heliopause, Voyager 1 detected no change in the ambient
magnetic field. Why was that so surprising? Well, theoretical models assumed that the
ambient magnetic orientation would change in a region dominated by the magnetic fields
of other stars. But remarkably, Voyager 1 detected no discernible change in the ambient
magnetism. NASA was so confused that they waited nearly a year before announcing that Voyager
1 had, in fact, entered interstellar space. On November 5, 2018, Voyager 2, travelling at the
slightly slower speed of 490 million kilometres (or 3.3 Astronomical Units) per year, joined
Voyager 1 in becoming the second human-made object to enter interstellar space. The crossing
also occurred 120 Astronomical Units from the Sun, and like the Voyager 1 six years earlier, the
probe detected no change in the ambient magnetic field. But something else surprised scientists.
You see, the Sun goes through 11-year solar cycles, during which its activity waxes and wanes.
Voyager 2’s crossing occurred at a time when solar winds were peaking. Models predicted that the size
of the heliosphere would fluctuate with the solar cycle, meaning it should have been expanding when
Voyager 2 made its crossing. Yet Voyager 2 crossed the heliopause at exactly the same distance
Voyager 1 had six years prior, meaning our models were wrong. Like the magnetometer finding,
this demonstrated the value of testing theoretical models with field data. We now suspect that the
boundary between the heliosphere and interstellar medium is much more twisted and filled with
fluctuations than prior models proposed. One leading idea is that our Sun emerged billions
of years ago from a hot and heavily ionized region following the explosion of one or more supernovae,
and that magnetic turbulence persists to this day near the heliopause. If so, the probes will likely
encounter a different magnetic orientation as they travel farther away, but their instruments
will probably be long dark by that time. Although the historic Voyager Mission will soon
be ending, the twin probes are just beginning their cosmic journeys. In 40,000 years, Voyager
1 will likely drift toward AC+79 3888, a star in the Camelopardalis constellation, while Voyager
2 will pass 1.7 lightyears from the star Ross 248. In 296,000 years, it will pass 4.3 lightyears
from Sirius. These small, intrepid probes will likely outlast the Earth itself as they continue
their solitary wanderings across the Milky Way. And if by chance they encounter intelligent
life in one of the far reaches of our galaxy, they will be a testament to
humankind’s ingenuity and resilience. On each of the probes is a golden audio-visual
disc called the Golden Record. These records carry photographs of Earth and its many lifeforms:
the sounds of whales and of babies crying; music by Mozart and Chuck Berry and dozens of
indigenous peoples; and greetings in 55 languages. They would offer a distant stranger a glimpse of
who we are, and what life on Earth is like. As for us, we must say goodbye to these old familiar
friends and continue our own lives here on Earth. Hopefully, the Voyager Mission will not be our
last brush with the stars, but only the beginning. Incredibly, we haven’t just sent probes into
space, but SpaceX has even sent a Tesla beyond the orbit of Mars. It made its closest approach of
Mars in 2020, at a distance of only 5 million km away. Although it probably won’t take you to Mars,
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