Why NASA Shut Down Systems on Voyager 2 as It Soars Into Interstellar Space

We're not giving up on the nearly 42-year-old spacecraft just yet.

Star Trek: The Motion Picture

Launched in 1977, engineers at NASA have managed to keep the Voyager 1 and 2 spacecraft flying for nearly 42 years – longer than any other spacecraft in history. But as NASA’s Jet Propulsion Laboratory announced July 8, it’s time to shut parts of Voyager 2 down.

Voyager 2 is over 11.2 billion miles from Earth, but it’s traveling at 34,391 miles per hour — that’s 10 miles per second — so that distance is increasing at a rapid rate. In December 2018, it became the second man-made object ever, after Voyager 1, to enter what’s considered interstellar space. It did so as it broke through the heliosphere, the bubble of ionized particles that envelops the solar system.

And it’s still sending back data that are picked up by radio telescopes in Australia.

But to ensure that these vintage probes continue to return the best science data possible from the frontiers of space, mission engineers have been forced to implement a new plan to manage them. It involves making difficult choices, particularly about instruments and thrusters.

Voyager 2 continues to return data from five instruments as it travels through interstellar space
Voyager 2 continues to return data from five instruments as it travels through interstellar space

One key issue with both Voyager vehicles is that they have less and less power available over time to run their science instruments and the heaters that keep them operational in the coldness of deep space. Engineers have had to decide what parts get power and what parts have to be turned off on both spacecraft.

But those decisions must be made sooner for Voyager 2 than Voyager 1 because Voyager 2 has one more science instrument collecting data — and drawing power — than its sibling.

Some stats on Voyager 2 as left the heliosphere and officially entered interstellar space
Some stats on Voyager 2 as left the heliosphere and officially entered interstellar space

After extensive discussions with the Voyager science team, mission managers announced on Monday that they turned off a heater for the cosmic ray subsystem instrument (CRS) on Voyager 2 as part of the new power management plan. The cosmic ray instrument played a crucial role last November in determining that Voyager 2 had exited the heliosphere. Ever since, the two Voyagers have been sending back details of how our heliosphere interacts with the wind flowing in deep space.

Why Did It Have to Be the Cosmic Ray Subsystem Instrument?

According to the JPL, mission team members can now preliminarily confirm that Voyager 2’s CRS is still returning data, despite dropping to a chilly -74°F (-59°C). That’s lower than the temperatures at which the CRS was tested more than 42 years ago (down to -49°F, or -45°C). So, it’s already surpassing expectations.

But running heaters and instruments requires power, which is constantly diminishing on both Voyagers.

Each of the probes is powered by three radioisotope thermoelectric generators (RTGs), which produce heat via the natural decay of plutonium-238 radioisotopes and convert that heat into electrical power. Because the heat energy of the plutonium in the RTGs declines and their internal efficiency decreases over time, each spacecraft produces about 4 fewer watts of electrical power each year. That means the generators produce about 40% less than what they did at launch nearly 42 years ago, limiting the number of systems that can run on the spacecraft.

So, for example, if fuel lines powering the thrusters that keep the spacecraft oriented were to freeze, the Voyagers’ antennae could stop pointing at Earth. That would prevent engineers from sending commands to the spacecraft or receiving scientific data.

The mission’s new power management plan explores multiple options for dealing with the diminishing power supply on both spacecraft, including shutting off additional instrument heaters over the next few years. Engineers from JPL will try to ensure that both spacecraft can continue to collect data from interstellar space for years to come. It’s hoped that Voyager 2 will continue to keep transmitting weak radio messages until the mid 2020s.

The journey of Voyager so far (distances are in Astronomical Units, or AUs)
The journey of Voyager so far (distances are in Astronomical Units, or AUs)

What Does the Universe Have in Store for Voyager?

Voyager 2 is not headed toward any particular star, although in roughly 42,000 years it will pass 1.7 light-years from the small red dwarf star Ross 248, which is in the northern constellation of Andromeda. If the probe is undisturbed for 296,000 years, Voyager 2 should pass 4.3 light-years from the star Sirius, the brightest star in the night sky. It’ll brush by two stars, Delta Pavonis and Gliese 754, about 100,000 years later.

When you consider that kind of time frame, it’s entirely possible that Voyager 2 could outlive the human race.

Having left the heliosphere and entered interstellar space, the next cosmological phenomenon Voyager 2 will encounter is the Oort cloud, in 300 years. Sadly, that will be long after it has run out of power.

The Oort cloud is a theoretical cloud of predominantly icy planetesimals believed to surround the Sun at distances ranging up to 200,000 AU (3.2 light-years). The outer limit of the Oort cloud defines the boundary of the Solar System as it’s believed this is where the gravitational influence of our sun ends. But it’s a big cloud: and will take Voyager 2 over 30,000 years to get through it.

All Alone, Up There, in the Dark

Both spacecraft have captured the imagination of scientists and sci-fi writers alike, which is not surprising when you take a moment and imagine them hurtling through deep, utterly uncharted space. Possibly forever.

One of the most well-known examples is in Star Trek: The Motion Picture, which was released in 1979. In this, the first of the big-screen features of Star Trek, the crew of the Enterprise must determine the purpose of an giant alien spacecraft threatening to extinguish all life on Earth. And whaddaya know, at the heart of this enormous alien vessel is a Voyager probe. (It’s actually a fictional one called Voyager 6, but that’s beside the point.)

Voyager 6 amassed so  much knowledge that it achieved consciousness itself and became self-aware
Voyager 6 amassed so much knowledge that it achieved consciousness itself and became self-aware CREDIT: Paramount Pictures

Kirk, McCoy, Spock, et al. discover to their astonishment that during its mission, this Voyager probe disappeared into a black hole and emerged on the other side of the galaxy, where it fell into the gravitational field of a planet populated by a race of living machines.

The inhabitants found the probe to be one of their own kind — primitive, yet kindred. Discovering its simple, 20th-century programming, which directed Voyager to collect all data possible and return that information to the creator, the machines interpreted it literally and constructed a massive vessel around the probe in order to facilitate that directive.

On its journey back to Earth, it amassed so much knowledge that it achieved consciousness itself, becoming a living thing.

Until that happens, you can check on Voyager 1 and 2’s speed and trajectory in real time on NASA’s mission status page.