On November 5th 2018, Voyager 2 left the protective bubble of the Sun and joined
its sister ship Voyager 1 on an interstellar mission
41 years after it launched. Welcome back to Launch Pad, I'm Christian Ready, your
friendly neighborhood astronomer. Voyager 2 became just the second spacecraft ever
to reach interstellar space. But to really put its achievement into
perspective, we need to wind the clock back all the way to 1977 when Voyagers 1
and 2 were launched from Cape Canaveral. Their mission was unprecedented; a grand
tour of the outer solar system. Interestingly, Voyager 1 actually
launched about a month after Voyager 2. But because Voyager 1 by that time was
on an inside orbital track, it was able to overtake Voyager 2 and reach Jupiter
first, hence the name "Voyager 1". Both spacecraft flew past Jupiter and its
moons in 1979, then Saturn and its moon system in 1980 and 1981. Following the
Saturn encounter, Voyager 1 accelerated upwards and out of the plane of the
solar system while Voyager 2 would continue on to fly past Uranus in 1986
and Neptune in 1989. As both spacecraft raced toward the outer reaches of the
solar system, they were still well inside of the heliosphere. The heliosphere is
like a giant protective bubble created by charged plasma particles blowing out
from the Sun. This stream of particles is called the solar wind, and it surrounds
the Sun and most of the planets of the solar system. The solar wind races out at
2 million kilometers per hour, far beyond the orbit of Pluto, until it slams into
the oncoming interstellar wind. This forms a boundary called the heliopause,
which separates the heliosphere from the interstellar medium. As luck would have
it, both spacecraft happened to be racing toward the leading edge of a heliopause.
Along the way, Voyager 2 was able to monitor the solar wind using its plasma
science experiment, or PLS. Voyager 1 also carried its own
PLS system, but it failed back in 1980. So Voyager 2 gave scientists an
opportunity to directly monitor the solar wind all the way out to the
heliopause. On November 5th 2018, Voyager 2 detected a sharp drop-off in solar
wind particles, while at the same time detecting a sudden increase in galactic
cosmic rays. Cosmic rays are highly energized protons that move at extremely
fast speeds and pack a lot of energy. The cosmic rays reaching the Voyager
spacecraft were forged and accelerated in the supernova explosions of massive
stars 10 to 20 million years ago. The sudden increase in cosmic rays, coupled
with the sudden decrease in the solar wind, was proof that Voyager 2 had joined
its sister ship in interstellar space. Now both spacecraft find themselves in
the local interstellar cloud, which is about a hundred thousand times larger
than the heliosphere. But this cloud is really thin - about a hundred trillion
times less dense than the clouds in our atmosphere. Our Sun is moving through
this cloud and that's why the heliosphere is shaped much like a comet.
But with Voyager 2's working plasma detector, we may be able to learn whether
or not the Sun has already left the local cloud or if it has in fact
transitioned into the neighboring G cloud. This will help us to literally
understand our place in the galaxy! Both spacecraft are still functioning and
will continue to work for as long as they have the power to do so. Both
spacecraft are powered by radioisotope thermal generators, which is simply a block of
radioactive plutonium that decays over time. As the plutonium decays, it
generates heat which can then be used to power the rest of the spacecraft systems.
However, as the plutonium decays, its power output weakens. Today, both
spacecraft are transmitting with a power output of about 20 watts. That's the same
amount of power generated by your refrigerator's light bulb. By the time
that signal reaches Earth, it is already weakened to one ten-trillionth of a
billionth of a watt. To conserve power, Voyager 2's onboard camera systems
were deactivated following its flyby of Neptune
in 1989. Still, the overall power output of both spacecraft continues to diminish
by about 4 watts every year. This means that mission managers are gonna have to
make some decisions as to what other instruments they can turn off in order
to extend the Voyager interstellar mission. Even though both spacecraft have
reached interstellar space, they have a long way to go before they actually
leave the solar system. Voyagers 1 & 2 are 144 and
120 astronomical units from the Sun. One astronomical unit is
the distance from the earth of the Sun, and Neptune is about 30 astronomical
units from the Sun. So both spacecraft are 4 and 4.8 times
farther from the Sun than Neptune. That's far, really far! But starting at
1,000 astronomical units is the Oort cloud, a giant sphere of small icy comet
nuclei that silently orbit the Sun over millions of years. It will take the
spacecraft about 300 years to reach the inner edge of the Oort cloud, and then
another 30,000 years to emerge from the other side. By that point, both
spacecraft will be more than a light-year from Earth, and will have left the
outermost fringes of the solar system behind. Their ultimate destinies are to orbit
the Milky Way among its 400 billion stars.
But even then, Voyagers 1 & 2 will carry on with one final ultimate mission. Each
spacecraft carries a golden record with music, sounds, images, and greetings in 55
languages from the people of planet Earth. They are humanity's message in a
bottle, carrying with them a record from a long-extinct
civilization that was once curious enough to explore the universe
surrounding it. Now if you'd like to explore more of the universe surrounding
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watching the skies.