Voyager 1, launched on September 5, 1977, is still transmitting data, although it recently faced communication challenges. After going silent in November 2023, the spacecraft successfully resumed contact with Earth on April 20, 2024, following a series of troubleshooting efforts by NASA’s Jet Propulsion Laboratory (JPL) engineers. They identified a malfunctioning memory chip that had rendered the spacecraft’s outgoing communications unintelligible. By relocating the affected code to different memory slots, they were able to restore communication and receive engineering data from Voyager 1, which is currently about 15 billion miles from Earth[2][4][5].
Voyager 1 has been continuously transmitting data since its launch and is the most distant human-made object, having entered interstellar space in August 2012. The spacecraft is equipped with several scientific instruments that continue to collect and send data back to Earth, although the rate of transmission has significantly decreased over the years due to the vast distance[1][3].
How does Voyager 1 launched Sept 5, 1977 still have electrical power? In 2010 JPL stated that the spacecraft’s plutonium power source would continue up until at least 2020 and it did[6] .
Voyager’s Power Generation
The Voyager spacecraft have been able to operate for nearly five decades in the far reaches of space thanks to their radioisotope thermoelectric generators (RTGs). These ingenious power sources convert heat from the decay of plutonium-238 into electricity, providing a steady and long-lasting energy supply.
How RTGs Work
RTGs on Voyager consist of three key components:
- Radioisotope heat source: Plutonium-238 in the form of plutonium dioxide (PuO2) serves as the fuel.
- Thermoelectric converter: This device transforms the heat from radioactive decay into electrical energy.
- Heat rejection system: Excess heat is dissipated to maintain optimal operating temperatures.
The alpha particles released during the decay of plutonium-238 bombard the inner surface of the container, generating heat that is then converted to electricity by the thermoelectric converter.
Long-Term Power Supply
Each Voyager probe is equipped with three Multi-Hundred Watt (MHW) RTGs. At launch in 1977, these RTGs collectively provided about 474 watts of electrical power. Due to the gradual decay of the plutonium fuel, the power output has decreased over time:
| Year | Total Power Output |
|---|---|
| 1977 | 474 watts |
| 2022 | 230 watts |
The consistent power supply from the RTGs has allowed the Voyager probes to continue their scientific observations far beyond the reaches of solar power. It is projected that both Voyager spacecraft will have sufficient power to operate their radio transmitters until at least 2025, marking over 48 years of continuous operation since their launch.
How Long Might it Go?
Plutonium-238 Fuel: The RTGs use Plutonium-238 as fuel, which has a half-life of 87.7 years, allowing for long-term power generation.
The fuel itself will be half as strong in 87.7 years, but as you can see from the power output in the table above, the power has dropped to less than 1/2 in 45 years. That is curious. It got me wondering how long the plutonium used had been sitting around.
474 watts / (0.57 watts/gram) ≈ 831.58 grams
Voyager 1 launched with approximately 832 grams of Plutonium-238, but that assumes the Pu-238 was created the same year or even month as it was launched. Is that true?
- Given the planning and preparation required for such missions, it’s reasonable to assume the fuel was produced at least a year or two before launch, possibly earlier.
- Pu-238 has a half-life of 87.7 years, so a few years of storage before launch would not significantly impact its power output.
It’s worth noting that this calculation assumes 100% efficiency in converting thermal power to electrical power, which is not realistic. The actual amount of Pu-238 would have been higher to account for conversion inefficiencies.
The Specs Say: Each Voyager spacecraft, Voyager 1 and Voyager 2, is equipped with three Radioisotope Thermoelectric Generators (RTGs) that utilize Plutonium-238 (Pu-238) as their fuel source. Each RTG contains approximately 4.5 kilograms of Pu-238 oxide, making the total amount of Pu-238 in each Voyager spacecraft about 13.5 kilograms (or 29.7 pounds) when considering all three RTGs[13][15][16].
How Much Does It Cost?
The estimated cost to produce 30 pounds of Pu-238 would be (approximately) a staggering $816 million based on current production costs and rates. In 1975, the cost of producing Plutonium-238 (Pu-238) was much lower, approximately $1,000 per gram, similar to the cost reported in 1969. This would mean that the cost for producing 30 pounds (or about 13,600 grams) of Pu-238 would have been around $13.6 million. The total cost for both missions, including spacecraft development, launch, and mission operations through the Neptune encounter, was approximately $865 million.
Neat, but is it safe to launch plutonium into space?
The Apollo 13 mission in April 1970 experienced a critical failure that prevented the lunar landing, resulting in the Lunar Module carrying a Radioisotope Thermoelectric Generator (RTG) reentering Earth’s atmosphere and burning up over Fiji. The SNAP-27 RTG contained 44,500 curies (1,650 TBq) of plutonium dioxide, which was designed to survive reentry intact[6][7]. The trajectory was carefully planned to ensure the RTG would plunge into the Tonga trench in the Pacific Ocean, at a depth of 6-9 kilometers[8].
Subsequent atmospheric and seawater sampling confirmed the absence of plutonium-238 contamination, validating the assumption that the cask remained intact on the seabed[9]. The cask is expected to contain the fuel for at least 870 years, equivalent to 10 half-lives of plutonium-238[10]. The U.S. Department of Energy conducted seawater tests, concluding that the graphite casing was stable and no plutonium release should occur[7].
Mobility and Bioaccumulation: The environmental mobility of Pu-238 is limited. When released, plutonium tends to bind to sediments rather than remaining in the water column. This binding significantly reduces its potential to affect marine life or ecosystems. Studies indicate that the bioaccumulation of plutonium in marine organisms is variable, with some species showing higher uptake than others, but overall, the levels are not high enough to cause widespread ecological harm[18].
Safety Measures and Design
To minimize the risk of radioactive material release, RTGs incorporate several safety features:
1. Modular fuel storage units with individual heat shielding
2. Iridium metal layer surrounding the fuel
3. High-strength graphite block encasement
4. Corrosion- and heat-resistant materials
5. Aeroshell protection against reentry heat
6. Ceramic fuel form to minimize vaporization and aerosolization risks
Other RTG Incidents
The Russian Mars 96 probe launch failure on November 16, 1996, involved two RTGs carrying a total of 200 g of plutonium. These are believed to have survived reentry and are thought to be located in a region near Iquique, Chile[8].
Soviet-produced Beta-M RTGs, used in lighthouses and beacons, have become orphaned radiation sources. Some have been illegally dismantled, fallen into the ocean, or have defective shielding[10].
Safety Record
NASA reports that 28 U.S. space missions have safely flown radioisotope energy sources since 1961[4]. The Apollo 13 incident, despite its extreme circumstances, has served to validate the safety design of later-generation RTGs[6].
While the Voyager spacecraft’s power systems are indeed impressive, there are a few examples of power supplies that have operated for longer periods or provided more sustained power:
Does Voyager I Get the Longest Power Supply Record?
Not really. Some nuclear power plants have been in continuous operation for over 50 years, generating far more than 200 watts consistently:
- The Beznau Nuclear Power Plant in Switzerland has been operating since 1969, making it over 55 years old.
- The Nine Mile Point Nuclear Generating Station in New York began operation in 1969 as well. A BWR-2 (Boiling Water Reactor) that went online in 1969 with a rated capacity of 644 megawatt is still operational in 2024.
These facilities generate hundreds of megawatts continuously, far exceeding Voyager’s power output.
Radioisotope Thermoelectric Generators (RTGs) in Space
While Voyager’s RTGs are remarkable, other space missions have employed similar technology for extended periods:
- The Cassini spacecraft operated for about 20 years (1997-2017) using RTGs.
- The New Horizons mission, launched in 2006, is still operating using an RTG and is expected to continue functioning into the 2030s.
Voyager’s RTGs are the longest in operation in space, as far as is publicly known.
Terrestrial RTG Applications
RTGs have been used in remote locations on Earth for decades:
- The longest-running still operational Radioisotope Thermoelectric Generator (RTG) is believed to be the Beta-M RTG used in Russian lighthouses. These RTGs were deployed in the late 1960s and early 1970s, and many have reportedly been operational for over 40 years.
- Another source says: Some Russian lighthouses used RTGs for power, with some units operating for over 50 years before being decommissioned.
- Remote weather stations and seismic monitoring equipment have used RTGs for extended periods, sometimes exceeding 30-40 years of operation.
While Voyager’s power system is undoubtedly an impressive feat of engineering, especially considering its distance from Earth and harsh operating environment, there are other examples of power supplies that have operated for longer periods or provided more sustained power output. The longevity of Voyager’s mission is particularly noteworthy given the challenges of deep space exploration.
Read More
[1] https://science.nasa.gov/mission/voyager/voyager-1/
[2] https://www.latimes.com/science/story/2024-04-23/after-months-of-silence-voyager-1-returns-earths-call
[3] https://voyager.jpl.nasa.gov/frequently-asked-questions/fact-sheet/
[4] https://www.space.com/voyager-1-communications-update-april-2024
[5] https://www.kxan.com/news/science/voyager-1-spacecraft-resumes-sending-data-from-beyond-our-solar-system/
[6] https://www.jpl.nasa.gov/news/news.php?release=2010-360
[7] https://nssdc.gsfc.nasa.gov/planetary/lunar/ap13acc.html
[8] https://www.foxweather.com/earth-space/apollo-13-launch-april-11-1970
[9] https://www.nasa.gov/missions/apollo/apollo-13-the-successful-failure/
[10] https://airandspace.si.edu/explore/stories/apollo-missions/apollo-13
[11] https://www.space.com/17250-apollo-13-facts.html
[12] https://www.reddit.com/r/explainlikeimfive/comments/1bbqas0/eli5_what_is_powering_the_voyager_1_it_has_been/
[13] https://voyager.jpl.nasa.gov/mission/spacecraft/instruments/rtg/
[14] https://www.acs.org/education/whatischemistry/landmarks/plutonium-238-production.html
[15] https://www.reddit.com/r/explainlikeimfive/comments/1bbqas0/eli5_what_is_powering_the_voyager_1_it_has_been/
[16] https://www.funtrivia.com/askft/Question151269.html
[17] https://inl.gov/integrated-energy/national-labs-resume-plutonium-production-for-space-exploration/
[18] https://solarsystem.nasa.gov/system/downloadable_items/2623_appendc.pdf