[Image above] An artist’s concept of Voyager 2. The probe reached interstellar space in November, six years after its twin, Voyager 1. Credit: NASA/JPL-Caltech
NASA was busy these last few weeks. First, astrophysicists held a collective breath on November 26 until the InSight lander safely touched down on Mars, the eighth such spacecraft to do so. The lander is already sending back new insights, like the sound of Martian wind. Then, spacecraft OSIRIS-REx reached asteroid Bennu on December 3 and discovered water! But most surprisingly, two impressive milestones happened within hours of each other: the same day the Parker Solar Probe made its first close approach to our sun, Voyager 2 entered interstellar space.
The twin spacecraft Voyager 1 and Voyager 2 launched in the late 1970s with their five-year mission to complete Jupiter and Saturn flybys. More than 40 years later, Voyager 1 and Voyager 2 not only did flybys of Jupiter, Saturn, Uranus, and Neptune (Voyager 2 did the latter two), these spacecraft kept going until they reached interstellar space.
What is interstellar space? In the universe, there are billions of galaxies, and each galaxy contains millions to billions of planetary systems (sets of planets, asteroids, and other non-stellar objects gravitationally bound in orbit around a star or star system). Between each planetary system exists interstellar space, or the space that is not affected by magnetic fields generated by stars at the center of each planetary system.
In the Milky Way galaxy, our Solar System is one of many planetary systems. When NASA says Voyager 1 and Voyager 2 reached interstellar space, that means the probes left the area of magnetic influence of our sun, but have not entered the area of magnetic influence of a different star yet.
With the Voyager probes so far away from us—Voyager 2 is slightly more than 11 billion miles (18 billion kilometers) from Earth—how do we know when each probe reached interstellar space? The answer to that is the heliosphere.
Each star sends out a constant flow of charged particles called the solar wind, and this solar wind forms a giant protective bubble around that star’s planetary system. This bubble—called the heliosphere—protects the planets in the system from galactic cosmic radiation. If you think astronauts going to Mars need to worry about radiation, just imagine what astronauts going to interstellar space would need to worry about!
When Voyager 1 and Voyager 2 left the protective heliosphere bubble and entered interstellar space, their onboard instruments picked up two main things: an increase in galactic cosmic radiation, and disappearance of the solar wind. However, Voyager 2 picked up these signals much more cleanly than did Voyager 1 because one instrument—the Plasma Science Experiment (PLS)—broke aboard Voyager 1 before the probe reached the heliosphere; PLS still worked on Voyager 2.
Even though Voyager 1 and Voyager 2 reached interstellar space, NASA still reports the probes as being in our Solar System. How can Voyager 1 and Voyager 2 still be in our Solar System if interstellar space is defined as the space between planetary systems? The devil is in the details.
Interstellar space is defined as the space outside a star’s magnetic influence—not a star’s gravitational influence. If you measure the edge of a planetary system based on magnetic influence, then the system ends where interstellar space begins. However, if you measure the edge based on gravitational influence—which extends farther than magnetic influence—then interstellar space overlaps with the outer edge of a planetary system.
In our Solar System, the edge of our sun’s gravitational influence extends to the Oort Cloud, a ring of comets around our Solar System. Voyager 2 would take about 300 years to reach the Oort Cloud and approximately 30,000 years to fly beyond.
This week’s video pays homage to Voyager 2’s journey thus far, from its launch on August 20, 1977 (16 days before Voyager 1 launched), to pictures from its flybys of different moons, to finally reaching interstellar space.
Communications with Voyager 1 and Voyager 2 will continue until the Voyagers’ power sources can no longer supply enough electrical energy to power critical subsystems, so may they continue to voyage for many years to come!
Credit: NASA/JPL-Caltech, YouTube