Both of NASA’s Voyager spacecraft are now interstellar. Where to next?

NASA’s twin Voyager
spacecraft^[1] have been traveling the
interstellar road for more than 40 years, sweeping past the giant
planets of the outer solar system before heading to the very
fringes of our sun’s domain. Now, one probe has achieved a
milestone in exploration: On December 10, NASA announced that Voyager
2 has entered interstellar space^[2], six years after Voyager
1 first crossed the threshold. The twins are the only two
spacecraft ever to venture so far from home.

To confirm Voyager 2’s exit, scientists
analyzed data from the spacecraft’s Plasma
Science Experiment^[3], which measured the
heliopause—the boundary between the sun’s protective plasma bubble
and interstellar space. On November 5, the instrument recorded a
steep decline in particles from the sun’s plasma. At the same time,
Voyager 2 saw more high-energy particles from elsewhere in the
galaxy, confirming that the probe had left our cosmic
cul-de-sac.

“Working on Voyager makes me feel like an explorer, because
everything we’re seeing is new,” MIT researcher John Richardson,
the principal investigator for the plasma experiment, said in a statement^[4]. “Even though Voyager 1
crossed the heliopause in 2012, it did so at a different place and
a different time, and without the PLS data. So we’re still seeing
things that no one has seen before.”

But even at more than 11 billion miles from the sun, the
Voyagers’ story is just beginning. On their current paths, both
probes will still be heading outward across the galaxy eons after
they escape the gravitational tug of the sun, and perhaps long
after our star dies in four or five billion years. Should the
spacecraft ever be intercepted by an alien civilization, two
golden records^[5]
bearing the sights and
sounds^[6] of planet Earth could
play the last whispers of a long-dead planet.

Assuming they stay on course, it’s going to be a lonely trip for
the Voyagers, because even though our Earthly skies appear to
glitter with countless pinpricks of light, space is mostly empty.
Distances between the stars are so vast that even when our
galaxy smashes into the one next door^[7], not much will happen in
the way of stellar collisions.

Still, according to astronomers’ predictions, their cosmic treks
will bring the Voyagers close to several starry milestones and a
dust cloud or three over the next hundred thousand or so years.
Here are some of the known cosmic landmarks the Voyagers could meet
in their relatively near futures.

Launched from Cape Canaveral, Florida, on August 20, 1977, the
dairy cow-size Voyager 2 began its odyssey with a grand tour of the
outer solar system, making vital discoveries at Jupiter and Saturn
and returning some of the best views yet of Uranus and Neptune.

The probe is now dipping below the plane of the solar system at
roughly 34,500 miles an hour and heading celestially south, toward
the constellation Sagittarius. On November 5, 2018, it crossed the
heliopause, exiting the sun’s protective plasma bubble.

Voyager 2 may be freshly interstellar, but it won’t be anywhere
near another star until 40,000 years from now, when it will pass
within 1.7 light-years of the small red dwarf star Ross 248. At
that point, Ross 248 will be the closest star to our sun, briefly
eclipsing the Alpha Centauri system’s claim to fame as its path
through the galaxy brings it just 3.02 light-years away.

About 61,000 years from now, Voyager 2 will pass beyond the Oort
Cloud—a large, diffuse swarm of icy objects that is thought to
extend as far as 200,000 times farther from the sun than Earth,
says Wesleyan University’s Seth Redfield^[8].

Most scientists think the Oort cloud is the source of comets
that take thousands of years to complete one orbit around the sun.
It could also be the true boundary of the solar system, if you
consider that to be the point where the sun’s gravitational
influence drops off.

“The outer edge of the Oort Cloud is approximately where that
is,” says Redfield, who has used the Hubble Space Telescope to
study the Voyagers’ paths in detail for the next hundred thousand
years, and more crudely for the next several million.

Next, in the year 298,000 Voyager 2 will swing within about four
light-years of Sirius, the brightest star in Earth’s sky. It’ll
then brush by two stars, delta Pav and GJ 754, about a hundred
thousand years later.

Voyager 1 followed its twin into the sky on September 5, 1977.
It rendezvoused with Jupiter and Saturn in 1979 and 1980, returning
some spectacular views of volcanoes erupting on the Jovian moon Io,
then continued outward.

On February 14, 1990, it swiveled to capture the solar system
retreating from its view, including a series of images that
revealed our pale blue planet looking like “a mote of dust
suspended in a sunbeam,” according to Carl Sagan.

Now traveling more than 35,000 miles an hour in the direction of
the constellation Ophiuchus, Voyager 1 is the fastest human-made
object in space. In 2013, it became the first spacecraft to leave
the heliosphere and cross into interstellar space.

In the year 40,272, the spacecraft will sail within 1.7
light-years of the star
Gliese 445^[9] in the constellation
Camelopardalis.

In 56,000 years, Voyager 1 will exit the Oort cloud, then brush
by the stars GJ 686 and GJ 678 in 570,000 years.

Beyond that, both spacecraft will continue on their journeys
outward, passing through scattered dust clouds on their way through
the local bubble, a cavern of relatively empty space blown by
tempestuous dying stars. They’ll exit this bubble in 5.7 and 6.3
million years, respectively.

From there, things get murky. The peculiar motions of stars and
dust clouds orbiting the galactic center aren’t well known yet, and
the spacecraft could be slowed by sailing through dust clouds.
Plus, gravitational encounters with things like wandering, starless
planets could bounce the probes around like billiard balls.

“There may be a whole population of rogue planets out in
interstellar space,” Redfield says. “A mildly close encounter with
one of those could change the itinerary of the Voyagers.”

Ultimately, like the stars in the Milky Way, the Voyagers will
march to the galaxy’s tune and orbit its core for the rest of their
lives.

“All the stars in the solar neighborhood are in an orbit around
the center of the Milky Way, with orbital periods of 225 to 250
million years,” says Voyager project scientist
Ed Stone^[10]. “The Voyagers will
also be in such independent orbits, completing an orbit every 225
million years for billions of years until a collision with another
galaxy disrupts this orderly flow.”

How the Voyagers’ ultimate demise occurs—whether it’s death by a
thousand micrometeorite impacts, or one large improbable
collision—remains a mystery. It’s even possible the Voyagers will
outlive the solar system, surviving long after the sun’s death
sculpts our neighborhood into a dramatically different place.

“The universe tends to do a great job of preserving its
occupants,” Redfield says. After all, “the objects in our solar
system, even small objects, have been around for 4.5 billion
years.”