A mission in three numbers
- The distance
- 25.9 billion kilometers (16.1 billion miles) Voyager 1's approximate distance from Earth when it reaches the one-light-day milestone.
- The conversation
- 48 hours The minimum time required for a radio signal to travel to the spacecraft and back.
- The possibility
- 0.5 watts The power consumed by a motor engineers decided to keep running to preserve the possibility of reactivating a scientific instrument.
Billions of kilometers from Earth, Voyager 1 is still receiving instructions from the team operating it. Engineers can prepare and send a command in seconds, but finding out whether it worked is another matter. At that distance, even the simplest message takes almost an entire day to travel through space.
On November 18, 2026, Voyager 1 will reach a historic milestone: it will become the first human-made object to reach a distance of one light-day from Earth, approximately 25.9 billion kilometers (16.1 billion miles). At that distance, a radio signal will take 24 hours to reach the spacecraft and another 24 hours to return. Even if Voyager could respond immediately, its engineers would have to wait at least two days to learn the outcome of a command.
What makes this remarkable is not just how far the spacecraft has traveled. Voyager 1 was launched on September 5, 1977, to explore Jupiter and Saturn, and eventually became the first spacecraft to enter interstellar space. Nearly half a century later, it continues to collect information from a region of space we still know relatively little about, using technology developed long before modern personal computers became commonplace.
Keeping it operational has become an exceptional engineering challenge. The spacecraft is aging, its power supply is declining, and some of its components have stopped working. Every operation requires an understanding of how its systems interact, what resources they depend on, and what might happen when something changes. Sometimes, the future of the mission depends on a complex computer repair carried out from billions of kilometers away. Other times, it comes down to something as seemingly insignificant as keeping a small motor running on half a watt of power.
One day to send a message. Another to hear back.
We’re used to technology responding almost instantly. A video call can connect people on different continents in seconds, a device can receive a software update remotely, and an engineer can usually tell almost immediately whether a software change has fixed a problem. None of this is possible when working with a spacecraft billions of kilometers away.
A light-day is the distance light travels through a vacuum in 24 hours. Radio signals travel at the same speed, which means no communications technology can eliminate the time it takes to cover that distance. We can improve transmission systems, design more sensitive antennas, and build more powerful computers, but we cannot make a signal arrive faster than the speed of light allows.
For the team operating Voyager 1, this limitation fundamentally changes how problems are solved. Imagine one of the spacecraft’s systems starts behaving unexpectedly and a parameter needs to be adjusted. Before sending a command, engineers must study the spacecraft’s last known condition, investigate possible causes, and assess the potential consequences of the change. Once the command has been transmitted, they must wait for it to reach the spacecraft and for the resulting information to make the return journey.
The delay also means that every decision must account for what might happen while a signal is in transit. The spacecraft continues operating during that time, conditions may change, and it may not be possible to intervene before new problems arise. On a mission like Voyager, understanding system behavior and anticipating dependencies is therefore more than a matter of efficiency. It is an essential part of spacecraft operations.
INFOGRAPHIC · THE 48-HOUR SIGNAL
One day out. One day back.
One light-day Approximately 25.9 billion km
-
Command
24 hours
outbound · Earth → Voyager 1
-
Response
24 hours
return · Voyager 1 → Earth
48 hours
minimum for a round-trip exchange.
Half a watt to keep an option open
One of Voyager 1’s biggest challenges today is its power supply. Unlike spacecraft operating closer to the Sun, Voyager does not rely on solar panels. At such enormous distances, sunlight is too weak to provide a practical source of electricity. Instead, the spacecraft uses three radioisotope thermoelectric generators (RTGs), which convert heat produced by radioactive decay into electrical power.
This system has kept Voyager operating for decades, but it has an unavoidable limitation: the available electrical power gradually decreases. According to NASA, the Voyager spacecraft loses approximately four watts of available electrical power each year. That may sound like a small amount, but for a spacecraft that has been operating for nearly fifty years, the loss requires engineers to continually reassess how power is distributed.
The onboard computers need electricity to process commands and data. The attitude control system must keep the high-gain antenna pointed toward Earth to maintain communications. Scientific instruments also need power, while certain components must be kept within safe operating temperatures. Switching off one piece of equipment may save energy, but it can also affect other functions or allow temperatures to drop to levels that compromise the spacecraft’s operation. Every decision therefore needs to be evaluated in the context of the entire system.
Over the years, engineers have shut down heaters and scientific instruments to extend the mission. In February 2025, they turned off the Cosmic Ray Subsystem (CRS). On April 17, 2026, the Low-Energy Charged Particles experiment (LECP) followed. The instrument had been operating almost continuously since 1977, studying charged particles throughout Voyager’s journey across the solar system and, later, into interstellar space.
The decision to shut it down was not made on the spot. Years earlier, the science and engineering teams had agreed on a sequence of instrument shutdowns designed to save on power without giving up valuable scientific observations prematurely. When the command was transmitted, it would take approximately 23 hours to reach Voyager 1, and the shutdown process itself would require another three hours and fifteen minutes. Everything had to be carried out remotely, with no possibility of physically intervening if something went wrong.
There was, however, one particularly interesting detail about the operation. Although the instrument was switched off, engineers decided to leave one of its small motors running. The motor is used to rotate the sensor so it can take measurements in different directions, and it consumes just 0.5 watts. The team considered that a reasonable amount of power to reserve because keeping the motor running would improve the chances of reactivating the experiment in the future, provided they could free up enough energy.
This decision illustrates how much the nature of the mission has changed. Engineers are not only focused solely on getting the maximum scientific output from every instrument. They must preserve the most valuable capabilities for as long as possible. Sometimes that means switching off equipment that still works, accepting that certain measurements will no longer be available, or setting aside a small amount of energy to avoid permanently losing an option.
According to NASA’s April 2026 update, two scientific instruments remain operational aboard Voyager 1: the Magnetometer (MAG), which measures magnetic fields, and the Plasma Wave Subsystem (PWS), which studies plasma waves in interstellar space.
There is something paradoxical about the fact that Voyager has gradually had to give up some of its ability to study the universe in order to keep exploring it. Yet that ability to adapt is precisely what has allowed a mission originally designed to explore the outer planets to continue providing scientific data nearly five decades after launch.
Explore Voyager 1 from the inside
From Earth, Voyager 1 may look like little more than a large dish antenna traveling through space. Behind that familiar silhouette, however, lies a complex architecture of systems that have worked together for decades. Power generation, attitude control, communications, data processing, and scientific instruments form a network of interdependent elements whose combined operation makes the mission possible.
To help explain that architecture, we’ve created an interactive visualization of Voyager 1 that lets you view the spacecraft from different angles, separate its components, and explore some of the connections between its subsystems. Rather than attempting to reproduce every engineering detail of the original spacecraft, it provides an accessible, visual way to discover how its systems work together.
Inside the explorer, you can rotate the spacecraft and select individual components to learn about their functions. The exploded view separates the different elements, making it easier to see how they are arranged around the main structure. You can also display connections between subsystems, a particularly useful feature for understanding that a spacecraft is far more than a collection of independent pieces of equipment.
The explorer also includes a timeline covering the mission from 1977 to 2026. You can follow key events, see how the status of scientific instruments changes over time, and examine the evolution of estimated electrical power. The power curve helps illustrate why energy management has become such an important part of the mission over the past several decades.
Repairing a computer you can never touch
The importance of understanding relationships between systems became especially clear in November 2023, when Voyager 1 began transmitting information that NASA engineers could no longer interpret. The spacecraft was still receiving commands from Earth, but the scientific and engineering data it sent back had become unreadable. Communications had not simply stopped. Something was going wrong in the process that turned onboard information into unusable data for the teams on Earth.
The investigation led engineers to the Flight Data Subsystem (FDS), one of the spacecraft’s onboard computers, which is responsible for preparing science and engineering data for transmission. They discovered that a memory chip had stopped working, making a section of the software stored on it inaccessible. In a conventional computer, the most straightforward solution might have been to replace the faulty component. For Voyager 1, already more than 24 billion kilometers from Earth, that was out of the question.
The alternative was both clever and complex. Mission engineers decided to divide the affected code into smaller sections and distribute them across different areas of memory that were still working. The problem was that none of those areas had enough space to hold all the code, so they also had to update references between the different sections and make sure everything could continue running correctly.
On April 18, 2024, they sent the commands needed to relocate the portion of the software responsible for engineering data. Two days later, they received a response they had been waiting months for: Voyager 1 was once again transmitting readable information about its condition. The recovery of scientific data would come later, but this first result confirmed that the team had found a way around the failure without physically repairing the hardware.
What stands out about this story is how much the engineers accomplished under such severe constraints. A spacecraft built in the 1970s experienced a computer failure almost half a century later, and a team on Earth managed to restore an essential function by reorganizing software on one of its onboard computers. They did it without physical access to the machine, with extremely limited resources, and with long delays between each command and its result.
But the most important lesson is not just about the repair itself. The failure occurred in a specific component, yet the solution required an understanding of how memory, software, data processing, and communications worked together. Moving a section of code could affect other parts of the system, and a seemingly small change could have much broader consequences.
The failure was in a single chip, but fixing it required understanding the spacecraft's architecture.
It is a clear example of why, in systems engineering, knowing the individual components is not always enough. We also need to understand how they interact, which functions depend on one another, and what might happen when we change part of the system.
Seeing a spacecraft is not the same as understanding it
A 3D visualization can help us recognize Voyager 1’s components, understand where they are located, and see some of their connections. It offers an intuitive way to explore a complex technical architecture, particularly when the individual elements may be unfamiliar to the reader.
However, representing a system’s geometry is not the same as describing how it works. To analyze it from an engineering perspective, we need to know what each component does, how it communicates with others, what resources it uses, which constraints it must satisfy, and what requirements govern its operation. We also need to understand how changes affect other parts of the architecture, even when those dependencies are not physically visible.
This is one of the fundamental ideas behind Model-Based Systems Engineering (MBSE). Rather than relying exclusively on separate documents, spreadsheets, and diagrams, MBSE provides a structured way to represent a system’s elements and the relationships between them. The goal is to support analysis, technical communication, and architecture reviews as the system evolves.
To accompany the interactive visualization, we’ve prepared a simplified Voyager 1 example using SysML v2, a standardized systems modeling language. The example describes selected components, ports, connections, attributes, and illustrative requirements, showing how part of the spacecraft’s architecture could be represented formally.
This model does not reproduce NASA’s engineering documentation or include all the information needed to assess Voyager 1’s actual behavior. Its purpose is educational: to help us move from a visual representation of components toward a more precise description of their relationships and dependencies.
Think like a systems engineer
Imagine you’re part of the team responsible for extending Voyager 1’s operations. Available power continues to decline, and you must decide which functions to preserve. You cannot replace the hardware, increase the power produced by the RTGs, or assume that shutting down one component will have no effect on the rest of the spacecraft. Any change could affect other systems and, ultimately, the capabilities the mission still needs.
To get started, we can establish three simple conditions. The first is to maintain communication with Earth, since the spacecraft must be able to transmit scientific data and continue receiving commands. The second is to keep the antenna properly oriented, which requires certain parts of the attitude control system to remain operational. The third is to preserve at least one scientific instrument capable of collecting useful measurements.
Now return to the interactive explorer and select the Magnetometer (MAG) or the Plasma Wave Subsystem (PWS). Explore its connections to the rest of the spacecraft and discover what other systems need to keep working for its data to reach Earth. Then try to identify what additional information you would need to determine whether any of those components could be switched off without compromising the functions we want to preserve.
You would probably need to know each component’s power consumption, its dependencies, the temperatures at which it can operate, and the requirements associated with its functions. You would also need verified information about interfaces and operating conditions. A graphical representation can help identify the most important relationships, but evaluating the consequences of a decision requires an additional level of precision.
This is where a structured system model becomes particularly valuable. Not because it can replace engineering judgment, but because it helps make the relationships, constraints, and assumptions underlying engineering decisions more explicit.
This is a conceptual exercise and does not reproduce NASA's spacecraft operating procedures or validation processes.
A lesson that goes far beyond Voyager
Most engineers will never have to keep a spacecraft operating one light-day from Earth. Yet many will face less extreme versions of the same challenges Voyager has encountered over the decades: aging components, limited resources, changing requirements, and decisions whose effects can extend far beyond the part of the system being modified.
A railway system, a satellite, a medical device, or an industrial platform may look very different, but all require an understanding of the relationships that allow their components to function as a whole. When those relationships are not clearly represented, evaluating a change, reviewing a decision, or communicating an architecture to other professionals can become unnecessarily difficult.
That is why diagrams and models should not be treated merely as documentation accompanying a design. When properly integrated into engineering workflows, they can help teams analyze how a system works, identify dependencies, and maintain a shared understanding of its architecture. Their value lies not only in showing how something is built, but also in helping specialists understand how it operates and how it can evolve.
At Apricot, we’re working to make SysML v2 modeling more visual, intuitive, and collaborative. By combining textual and graphical editing, Apricot helps engineering teams explore an architecture from different perspectives, work with the relationships between its elements, and communicate more effectively without sacrificing the technical precision that systems engineering requires.
Voyager 1’s story offers an opportunity to reflect on why this matters. What began as a mission to explore Jupiter and Saturn has become a demonstration of the importance of understanding a system deeply, not only when it is designed, but throughout its operational life.
We’re still listening
When Voyager 1 reaches a distance of one light-day from Earth, it will have spent more than 49 years traveling through space. During that time, it will have completed the mission it was originally designed for, explored regions no spacecraft had visited before, and overcome challenges its creators could hardly have anticipated.
Its longevity cannot be explained by a single extraordinary decision. It is the result of generations of scientists and engineers who have come to understand the spacecraft better and better over time. Some of their interventions have been as complex as reorganizing software on a malfunctioning computer billions of kilometers away. Others have involved deciding that it was worth reserving half a watt of power to keep the possibility of using a scientific instrument again.
Perhaps this is one of Voyager 1’s most valuable lessons. Building a system capable of fulfilling its mission is an engineering achievement. Understanding it well enough to adapt, repair, and keep it operational as conditions change is a different challenge, and sometimes an even greater one.
After nearly half a century, Voyager 1 continues to send information from interstellar space. Its messages take longer and longer to arrive, its resources are increasingly limited, and no one knows exactly how much longer it will be able to operate.
But for now, we're still listening.