Travelers 1 and 2 have the distinction of being in space for 42 years and still being active. And even though they are 18 billion kilometers from the Sun, they still have scientific value. But they are running out of energy, and if NASA wants them to continue to be even longer, they have decisions to make.
The problem of energy
The issue of energy is becoming more and more critical for travelers. Not only do their scientific instruments require energy, but they must also stay warm in the icy environment of space. The pair of spacecraft has no solar energy: it would not be possible so far from the Sun. They use thermoelectric generators with radioisotopes (RTG) for their energy.
Each of the Voyager probes has three RTGs and uses plutonium 238 as a fuel source. When this isotope disintegrates, it produces heat that is converted into electrical energy. Each Voyager generated 470 watts at 30 volts DC, but over time, it degrades. Not only does the fuel run out regularly, but the thermocouples used in the system degrade over time. In 2011, the two Voyagers produced just under 270 watts, which is about 76% of the power they started with.
Although 270 watts is better than expected when designing and launching the probes, it does mean that inevitable decisions must be made regarding the spacecraft systems that must be deactivated.
First, you must recognize that NASA has kept the probes as long. It's amazing in itself. Some parts of the probes have already been turned off and remarkably, they are still performing.
In 2011, in response to energy issues, NASA deactivated the Voyager 1 ultraviolet spectrometer heater. This instrument was designed to operate at temperatures as low as -35 degrees Celsius (-31 degrees Fahrenheit), but once his heat was off, he continued to operate at -79 degrees Celsius (-110 degrees Fahrenheit).
But that was in 2011 and since then, RTGs have lost even more power. In fact, they lose about 0.8% of their power each year. Now, NASA engineers are refining their calculation rules and putting in place a new energy management plan that allows probes to go even longer.
Turn off the heat to keep travelers
NASA recently decided to turn off the heater for another instrument, this time on Voyager 2. They turned off the heater for the Voyager 2 Cosmic Ray Subsystem (CRS) subsystem. That's a shame because in November 2018 , the CRS instrument was essential in determining that Voyager 2 had left the heliosphere and entered interstellar space. Since the probes have left the heliosphere, they are sending us unique and important information about how the heliosphere interacts with the interstellar wind. No other spaceship can do it, and putting another one in place would take decades.
However, even though Voyager 2's Cosmic Ray (SRC) subsystem instrument heater has been disabled, engineers have confirmed that the instrument still operates at -59 ° C (-74 ° F), even though they had only been tested at -45 ° C (-49F.)
"It's amazing that Voyagers' instruments have proven so robust," said Suzanne Dodd, Voyager's project manager, based at NASA's Jet Propulsion Laboratory in Pasadena, California. "We are proud that they have stood the test of time. The long life of the spacecraft means we are confronted with scenarios we never imagined. We will continue to explore all options available to enable travelers to make the best science possible. "
In the current state of things, Voyager 2 still returns data from five instruments, even though the CRS had the heater off. In particular, it continues to heat the low energy particle instrument. This is because it can return transition data out of the heliosphere in the same way as CRS. The CRS is unidirectional, while the low energy instrument is omnidirectional, which is one of the reasons why the CRS turned off its heat.
Delay the inevitable
The production of heat is an energy-consuming activity. As the current in the probes runs out, you may need to turn off more radiators to allow the remaining instruments to operate. There is no way to get around it. But reducing the energy budget of probes also affects other systems than scientific instruments.
The probes have small thrusters and are essential to the operation of the probes. Spacecraft must be oriented in such a way that their antennas face the Earth in order to receive commands and return data to the Earth. Each spacecraft has a hydropine monopropellated fuel tank that feeds its small thrusters, which operate by small pushes or puffs to steer it.
If thruster power lines freeze, engineers will not be able to direct spacecraft antennas or instruments. At this point, the spaceship would probably be useless. So, they need heat too.
However, there is another problem with thrusters. Inevitably, systems like these are deteriorating over time, and in 2017, the engineers saw a problem. Some of Voyager 1's thrusters needed to work harder to maintain the proper orientation to the Earth. So they turned to long unused boosters to see if they could do the job.
Believe it or not, this secondary set of thrusters has not been used for 37 years. But they were excited and did their job. It must be a kind of disk in itself.
Now, the main thrusters of Voyager 2 are starting to show problems. Building on the success of the old unused Voyager 1 thrusters, the engineers also decided to launch the old Voyager 2 thrusters. But they did not sleep as long as Voyager 1. They had been last used when Voyager 1 met Neptune in 1989, 30 years ago, and NASA plans to activate them later this month.
Thanks to intelligent engineering, careful planning and wise use of the remaining energy of both Voyageurs, the inevitable end of the spacecraft has been delayed. As a result, their permanent scientific contributions may continue in the future for some time to come.
Engineers and mission planners believe that there is still a few years of operational capability. This is important because all we learn about the area of the space in which they are is due to their unique position to observe it. This can not be underestimated, as new papers are still being written based on Voyager's data, not only of their current state, but also of that of years or even decades ago.
In 2017, Fran Bagenal, former president of NASA's Outdoor Planet Evaluation Group, was interviewed in Nautilus. In this interview, she said: "I still analyze Voyager's data, believe it or not. We have just published three articles on Voyager data that were collected 33 years ago, as we all celebrate the 40th anniversary of the launch. It was fun. I realized that some of the data on which I had written my thesis, from 1979, had not been reanalysed. "
This raises the question: how many future papers based on current data could be written in decades?
Nothing lasts eternally
The most famous photos of the Voyager program are the Pale Blue Dot photos (actually a series of photos). We no longer receive photos from Voyager cameras. There is nothing to take pictures of the road. But it is almost strange to see how these spacecraft still provide unique data and data, decades after their design, construction and launch. They are strangely similar to time capsules of the first space exploration technology.
"The two Voyager probes explore areas never visited before, so every day is a day of discovery," said Caltech-based scientist Ed Stone. "Traveling will continue to surprise us with new ideas about deep space."
Nothing lasts forever and one day it will be for the spaceship Voyager. For people who know the missions well and have a little idea of what they have brought to the knowledge of humanity, it is a sad day. It's strange to think of everything that happened here on Earth as the two spaceships make their way.
The good news is that the future spacecraft will rely on the work of the Voyager program.
NASA will launch the Interstellar Accelerator and Mapping Probe (IMAP) in 2024, which will rely on Voyagers observations. Their Interstellar Frontier Explorer (IBEX) already supports the work of Voyager 1 and 2 and provides us with more detailed information on the heliosphere.
In the same interview for Nautilus, Fran Bagenal said, "I predict that we will no longer be able to communicate with her about 15 years later." She is not an official spokesperson for the Voyager program, but if it is accurate, it means that communications can be completed by 2032.
Mark this date on your calendar.