GPHS mounted to New Horizons spacecraft, image NASA

Exploring the Outer Solar System

Despite the dull-sounding name, the General Purpose Heat Source RTG (GPHS-RTG) is perhaps the most successful of all astronuclear systems, enabling the return of an immense amount of scientific data – more than any other RTG. This power source has powered four major scientific missions to the inner and outer solar system: Ulysses, Galileo, Cassini, and New Horizons.

It also truly revolutionized RTG design, being the first RTG to be built around a modular, scalable radioisotope heat source: the GPHS. For more information on the GPHS, check out the Radioisotope Fuel Element page here.

Let’s begin by looking at the history behind the units themselves, then look at the subsystems and missions. If you want to jump to a particular section, click here:

The Origins and History of the GPHS-RTG

The GPHS-RTG story begins shortly after the use of the MHW-RTG, with the development of the Ulysses mission to map the polar magnetosphere of the Sun. Originally conceived as the International Solar Polar Mission, ISPM, it needed more power than was available with the currently-off-the-shelf MHW-RTG. Selenite-based thermocouples were showing promise, so the plan was to go with a new material for the convertors. Sadly, long-term testing showed unacceptable degradation for either the ISPM or the Jupiter Orbiter (later Galileo) missions, and a solution needed to be found using the current thermocouple design from the MHW-RTG.

This required a greater heat output, and more surface area to maintain the optimal thermocouple thermal gradient, since more advanced thermocouples weren’t ready for deployment at the time.

At the same time, the fuel form used in the heat source was another major focus. It was possible to improve the safety of the heat source, a major focus of NASA and JPL, and at the same time create a truly standard fuel form for a wide range of not just this RTG, but a whole suite of different RPSs, both past and future. Almost every RTG or other RPS proposal at NASA and JPL (except the micro-RTGs) uses the GPHS-RTG as its heat source to this day. With GPHS-RTG, it would be the most flown as a single unit to date.

The program began in 1991, with a goal of providing five RTG units: two each for the Cometary Rendevous and Flyby (CRAF) mission, two for Cassini, one for Galileo/Ulysses, and one unfueled, electrically heated spare and test article.

1992 was a bad year for the entire space program, to put it mildly. The Challenger explosion cancelled many programs, delayed many others, and the GPHS-RTG program’s missions were no different. CRAF was cancelled, and Cassini was delayed – bad news for an RTG powered mission, even with as long-lived an isotope as 238Pu. Sadly, it was a sign of things to come, as old fuel would plague missions for the lifespan of this power source.

However, the total number of units ordered increased, with two more electrically heated spares with conversion capability ordered, as well as fabricate the parts but not assemble another unit, for an assembled total of seven and a total system number of eight.

The flight units used by mission, with power levels at launch, were:

New Horizons ran into special difficulties. The RTG assembly itself was from the unassembled unit #8, but the fuel was a mix of 52 fuel clads for the GPHS being those for the Cassini and Ulysses backup RTG (Flight Unit 5, never flown), and 20 new ones being purchased from Russia – this being the time that the US had no domestic 238Pu production capability, but rather purchased it from Russia.

The GPHS has been supplanted by the newer MMRTG, eMMRTG, and the upcoming ASRG and Next Generation RTG. Each of these bears the legacy of the GPHS-RTG, though, as the last RTG to use the silicon-germanium thermocouples of the SNAP27/MHW-RTG era and the first to use the General Purpose Heat Source. It helped explore the Sun’s polar magnetosphere, the Jovian and Saturnian systems, gave us our first glimpse of Pluto and Ultima Thule, and may yet show us more wonders.

Heat Source

Fueled GPHS unit, image INL

This was the first RTG to use the General Purpose Heat Source, the next (and latest) generation of 238Pu fuel form in use by NASA.

More on the General Purpose Heat Source is available here.

The GPHS-RTG used a total of 18 GPHS modules for heat, producing a total power of 4215 Wt at beginning of design life.

Due to delays in launch dates, and in the case of New Horizons a significant portion of old fuel in the GPHS modules, the power output of each heat source varied more widely than is the norm on RTGs. However, the closing of the Mound Laboratory and the loss of domestic 238Pu production made it impossible to launch all of the units with perfectly fresh fuel – even if the relatively short delays had necessitated it.

Thermoelectric Generator

GPHS-RTG and MHW-RTG thermocouple, image JPL

The thermopile on the GPHS-RTG was another silicon-germanium based generator, using the same design as the MHW-RTG due to long term failures being discovered in duration testing of the proposed advanced material. Using 572 unicouples, the system gained more power than the MHW-RTG due to the larger surface area available, not improvements in the basic generator. More information will be available on the upcoming Thermoelectric Generators page, coming soon.

During ground handling, an inert gas, xenon, was used during testing and handling as a blanket material and heat removal system from the RTG. During launch this was replaced with krypton, which was vented into space for normal operations.

Exterior Casing and Thermal Management

GPHS mounted to Cassini, image JPL

The overall casing size is 1.14 m in length and 0.422 m in diameter, with six radiator fins. Made out of beryllium, each casing was slightly modified for each mission to accommodate different hard points, launch configurations, and ground handling necessities.

The GPHS-RTG was never designed to be operated in an atmosphere, unlike the follow-on MMRTG.



Galileo spacecraft artist impression (unknown artist), image NASA

One of the jewels in the deep space exploration crown, Galileo orbited Jupiter and made a tremendous number of discoveries during its eighteen years in space. This spacecraft was powered by two GPHS RTGs: Flight Units 1 and 4, which continued to function with only minor difficulties until the end of the mission.

Galileo being launched from Atlantis, image NASA

Launched aboard the Space Shuttle Atlantis on 29 October 1989, it performed a flyby of Venus, as well as the asteroids Gaspra and Ida en route to Jupiter. It also observed the Shoemaker-Levy 9 impact with Jupiter while approaching the planet, the only spacecraft to have observed a cometary impact of another body from outside Earth’s orbit.

It finally arrived in Jovian orbit on 7 December 1995, after a more than six year transit, and released an atmospheric probe into the Jovian atmosphere. While the original mission only lasted for two years once in orbit (until 1997), the mission would be extended three times, until 2003. It would perform flybys of five of Jupiter’s moons, map the magnetosphere of Jupiter, provide atmospheric data of Jupiter directly for the first time, and complete 34 Jovian orbits during its mission.

In order to protect Europa from any potential contamination, Galileo was deorbited into the Jovian atmosphere on 21 September, 2003. The Galileo mission truly earned the outsized impact it has had on understanding the solar system, and laid the groundwork for Juno to collect the data it is currently gathering and sending back to Earth.

References and Further Reading

JPL Galileo mission webpage:

NASA Galileo webpage:

Galileo Mission to Jupiter, Ih 1996


Originally conceived as the canceled International Solar Polar Mission, with two spacecraft flying over opposite poles for a 3d view of the solar magnetosphere and plasma interactions, this NASA/ESA collaboration used a single spacecraft powered by a single GPHS-RTG (Flight Unit 3).

RTG being mounted to spacecraft, image ESA

Launched by the Space Shuttle Discovery on 6 October 1990, and used solid rocket motors to perform a slingshot maneuver around Jupiter to place it in a polar heliocentric orbit, it studied various properties of the heliosphere, galactic cosmic radiation, and other magnetospheric phenomena.

It orbited the sun a total of three times, and observed almost two complete solar cycles, interacted with comet tails, measured solar wind levels on an orbital plane that is generally inaccesible, and provided an immense science return.

Due to its orbital progression taking the spacecraft further away from Earth and data harder to transmit, and onboard thermal management issues (hydrazine for the thrusters freezing), the spacecraft wasn’t able to maintain the functionality that the agencies involved needed to maintain support.

The spacecraft was ordered to shut off its transmitter at 20:15 UTC on 30 June 2009.

References and Links

Ulysses in Depth, NASA


Cassini in stowed configuration. RTGs on right. Image NASA

Cassini was initially designed to be the first of a new set of outer space probes, which would be able to use a common bus but different instrumentation and power supplies, similar to the way the Mariner program was run in the days of Gemini and Apollo – minimal changes to the spacecraft for the benefits of mass production and iteration. Cassini would gain on the experience that was earned during the design and manufacture of Galileo, and mate it to a larger power supply of three RTGs (Flight Units 2, 6, and 7) and a suite of instruments geared toward its ultimate destination: Saturn.

Launched on 15 October 1997 on board a Titan IVB booster, Cassini carried a lander that it delivered to Titan: the ESA Huygens probe. Following two Venus flybys, an Earth flyby, and on 30 Dec 2000 a flyby of Jupiter, allowing simultaneous sensing by two of NASA’s flagship missions in the Jovian system at the same time. Finally, on 1 July 2004 Cassini entered orbit around Saturn. This was followed six months later – 23 December 2004 detachment, 14 January 2005 for landing – by the successful deployment of the battery-powered Huygens probe onto the surface of Titan.

Cassini would continue to orbit Saturn for the next thirteen years, until – like Galileo before it – it was deorbited into the gas giant it had been studying all this time to protect another water moon from Earthly contamination: Enceladus. It deorbited and burned up on 15 September 2017, ending a 20 year long mission of discovery and exploration.

Further Reading and References

Cassini mission page, NASA:

Cassini mission page, ESA:

An Introduction to the Design of the Cassini Spacecraft, Henry, C. 1998

RTG Waste Heat System for the Cassini Propulsion Module, 1994

Analytical Thermal Model Validation For Cassini Radioisotope Thermoelectric Generator Lin, Edward I. 1997

Adequacy of Simulator Hardware and Analytical Model for Cassini Radioisotope Thermoelectric Generator, Lin, Edward I. 1995

A Summary of the Cassini System-Level Thermal Balance Test: Engineering Subsystems, Mireles, V.; Tsuyuki, G. 1997

Analytical Thermal Model Validation For Cassini Radioisotope Thermoelectric Generator Lin, E. I. 1997

Cassini mission short circuit anomalies investigation, Carr 2012

New Horizons

New Horizons flyby artist’s impression, image NASA

New Horizons was an idea that had been kicking around on a lot of drawing boards over the years, mostly as the Pluto Express mission, but funding for interplanetary spacecraft is always smaller than the field would like, and especially in the early 90s until very recently there was very little fuel available for RTGs. This made them a precious commodity.

New Horizons RTG mounted to spacecraft, image NASA

In fact, there really was only one: Flight Unit 8, the unassembled final GPHS-RTG unit. There was fuel as well, left over from the Galileo and Ulysses missions – the Flight Unit 5 was completely fueled and ready to be exchanged for a faulty unit, but was never needed. A number of new fuel pellets were obtained from the Russian supplier, but that supply chain was never strong, and less than a third of the fuel pellets in the GPHS modules were fresh. The rest dated from the late 80s – almost a decade ago by the time New Horizons had its final name and a launch date set.

New Horizons launched on an Atlas V 551 on 19 January 2006, and flew by Jupiter for a gravity assist in February 2007. The long-anticipated flyby of Pluto and its moons occurred on July 14th 2015. A further flyby of another trans-Neptunian object, Ultima Thule, occurred on 1 January 2019. Further studies are being conducted to spot any other potential flyby targets in the future, but due to the age of the fuel in the GPHS modules the spacecraft wont have sufficient power for communications after about 2022.

Further Reading and References

New Horizons NASA mission page

New Horizons Johns Hopkins Univ. APL mission page

Spacecraft Power for New Horizons fact sheet, NASA 2005

Final environmental Impact Statement for the New Horizons Mission, Vol 1, 2005
Volume 2

A Power Subsystem for a Pluto Fast Flyby Mission, Underwood, M.L.; Shirbacheh, M. 1993

Advanced Radioisotope Power Source Options for Pluto Express, Underwood 1995

Further Reading and Sources:

Highly Recommended: Mission of Daring: The General-Purpose Heat Source Radioisotope Thermoelectric Generator, Bennett et al

Magnetostatic Cleanliness of the Radioistope Thermoelectric Generator Narvaez 1999