Backup power for NASA’s Voyager Space Probe, and the intricacies of RTG-based power systems

Voyager 1 and 2 probes were launched in 1977, and have been working non-stop for more than 45 years, making their way from Earth to the outer planets of our solar system and beyond. Courtesy of the radioisotope thermoelectric generators (RTGs) that provided 470 watts at launch, they are able to operate in the darkness of deep space as they did within the confines of our illuminated solar system. However, since nothing in the universe is truly infinite, these RTGs also wear out over time, whether from the natural decay of their radiation source or from the degradation of thermocouples.

Despite this gradual decrease in strength, Nasa Recently announced That Voyager 2 has an as yet unknown source of backup power that would put off shutting down more science instruments by a few more years. The change essentially bypasses the normal circuit voltage and associated backup power system, freeing up the power this consumes for scientific instruments that would have started shutting down years earlier.

While that’s good news in and of itself, it’s also noteworthy because Voyager’s 45-year-old Multi Hundred Watt (MHW) is the progenitor of the RTGs that still power the New Horizons probe 17 years later, and the Mars Science Laboratory (Curiosity) For more than 10 years, the value of RTGs has been shown in long-term exploration missions.

Although the basic principle behind the RTG is quite simple, its design has changed dramatically since the US SNAP-3 RTG was put on Crossing Satellite 4B in 1961.

The need for power

Even on Earth, it can be difficult to find a reliable source of energy that will last for years or even decades, which is why NASA’s Auxiliary Nuclear Power Systems (NASA)popThe development program has produced RTGs intended for ground and space use, with SNAP-3 being the first to reach space. The given RTG only produced 2.5 watts, and the satellites also had solar panels and NiCd batteries. But as a space RTG testbed, SNAP-3 laid the groundwork for successive NASA missions.

SNAP-19 provided power (about 30 watts per RTG) for Viking landers 1 and 2, as well as Pioneer 10 and 11. Five SNAP-27 modules provided power for the Apollo Lunar Surface Experiments packages (ALSEP) that were left on the lunar surface by Apollo astronauts 12, 14, 15, 16 and 17. Each SNAP-27 provided approximately 75 watts at 30 V DC of power from a 3.8 kg plutonium-238 fuel rod rated for 1250 watts ther. Ten years later, SNAP-27 is still producing more than 90% of its rated electrical power, allowing each ALSEP to transmit data on moonquakes and other information recorded by its instruments for as long as the power budget allows.

By the time support operations for Project Apollo ceased in 1977, the ALSEPs were left with only their transmitters operational. Apollo 13’s SNAP-27 module (attached to the outside of the lunar module) has made its return to Earth, where it remains – intact – at the bottom of the Tonga Trench in the Pacific Ocean.

The relative inefficiency of RTGs was readily apparent even at the time, though Snap-10A Experiment demonstrating an integrated 500-watt fission reactor in an ion engine-powered satellite that easily outperformed SNAP RTGs. Although more powerful per unit volume and nuclear fuel, thermocouple-based RTGs have the advantage of having absolutely no moving parts and only passive cooling requirements. This allows them to be literally stuck on a space probe, satellite, or vehicle with heat radiation and/or convection providing the cool side for thermal.

These thermocouples are used Seebeck effect, the Peltier effect in reverse, to convert the temperature gradient between two dissimilar electrically conducting materials into essentially a generator. A large part of the challenge for RTGs based on thermocouples is finding the most efficient and durable mount. Although Rankine, Brayton and Stirling cycle RTGs have also been tried, they have the obvious disadvantage of moving mechanical parts, which require seals and lubrication.

When considering the estimated 45-year lifespan of Voyager MHW-RTGs With relatively old silicon-germanium (SiGe) thermocouples, the drawbacks of adding mechanical components should be apparent. Especially when considering the two generations of MHW RTG successors to date.

Not your 1970s RTG

While Voyager’s MHW-RTG was developed specifically for the mission by NASA, its successor, creatively titled General purpose heat source (GPHSThe RTG was designed by General Electric’s space division and has subsequently been used on the Ulysses (1990-2009), Galileo (1989-2003), Cassini-Huygens (1997-2017) and New Horizons (2006-) missions. Each GPHS-RTG produces approximately 300 watts of electrical power from a 4400 watt thermocouple, using similar silicon and germanium thermocouples.

An interesting aspect here is that even the solar-powered Mars rovers include a radioisotope module, although in the form of an radioisotope heating module (RHU), with the Sojourner Rover The presence of three such RHUs, f Spirit and opportunity Eight RHUs each. These RHUs provide a constant heat source that allows scarce electricity from the solar panels and batteries to be used for tasks other than running the heaters.

Meanwhile, the currently active Mars rover, curiousity And its dual resistor, it gets electrical energy and heat from one device A multifunctional radioisotope thermoelectric generator (mmrtg) lonliness. These RTG sets use PbTe/TAGS ferroelectric pairs, meaning a lead/tellurium alloy for one side and tellurium (Te), silver (Ag), germanium (Ge) and antimony (Sb) for the other side of the pair. The MMRTG is rated for up to 17 lifetimes, but is likely to outperform the design specifications by a significant margin as the MHW-RTGs and others. The plutonium-238 fuel with MMRTG is contained in General Purpose Heat Source Units (GPHS), which serve to protect the fuel from damage.

Main failure mode Of the SiGe thermocouples the germanium was migrating over time, causing sublimation. This was prevented in later designs by coating SiGe thermocouples with silicon nitride. PbTe/TAGS thermocouples should provide further stability in this regard, and MMRTGs in Curiosity and Persistence have served as real-world duration tests.

fuel issue

The Voyager 1 and 2 probes are quite out of the way for a major service and maintenance session, so NASA had to get creative to optimize energy use. Although a backup power circuit may have been considered a necessity in the 1970s in the event of power fluctuations from any of the three RTGs on each space probe, there is enough real-world observational data to support the suggestion that it may not be necessary, barring exotic effects. .

With approximately 46 years of data from Voyager RTGs, we can now see that the stability of thermocouples is necessary to maintain a constant power output, as decay plutonium-238 The fuel source is much easier to model and predict. Now that with MMRTGs we can address many of the issues that cause thermocouples to deteriorate over time. The only component missing is plutonium-238 fuel.

Most of the plutonium-238 that the United States had had came from the Savannah River site (SRS) before closing this facility and its own reactors in 1988. Then, the United States was importing Pu-238 plutonium from Russia before the latter’s stockpiles also began to run out, leading to the embarrassing position of the United States running out of what is one of the best radioactive isotopes to use in RTGs for long-duration missions. With a short half-life of 87.7 years and only an alpha decay, plutonium-238 is fairly benign to surrounding material, while providing large amounts of heat energy.

With only enough plutonium-238 left for two MMRTGs on current Mars rovers and two more beyond, the United States has Restart now Plutonium-238 production. Although plutonium-238 can be created through several different methods, the preferred method seems to be to use a stock of neptunium-237 and expose it to neutrons in fission reactors or similar neutron sources, to generate plutonium-238 by neutron capture. According to NASA, about 1.5 kg of plutonium-238 per year should be enough to meet the demand for future space missions.

Small spaceship in the dark

Voyager 1 is currently at a distance of 159.14 AU (23.807 billion km) from Earth, and Voyager 2 It is only marginally closer at 133.03 AU than Earth. As a project that has its roots in Space Race and has ended up living after not only many of its creators, but also the geopolitics of the time, it’s perhaps one of the few man-made constants we can all identify with in some way. fashion.

As carriers of the golden discs that contain the essence of humanity, these spacecraft’s life extension goes beyond the mere science they can do, in the darkness of deep space. With each additional year, we may learn a little more and see more of what awaits humanity beyond the reach of this more or less ordinary solar system.