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 us, well please
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