Running out of fuel for navigation, NASA intentionally caused the Cassini spacecraft to burn up in Saturn’s atmosphere to prevent the possibility it might impact one of the planet’s moons that may support life. The spacecraft sent back data on its flight to the very last second of its existence.

Even as Cassini’s mission was ending in August NASA convened a working group at JPL to plan the next generation of Radioisotope Thermoelectric Generators.

Running on Empty – No Gas Stations in Space

The key thing to know about NASA’s decision to end the Cassini mission is that it didn’t run out of electrical power from its PU-238 power source. What did happen is that a separate system that provides navigational positioning with thrusters and a limited fuel supply reached its limits.

The thrusters are used for making small corrections to the spacecraft’s course, for some attitude control functions, and for making angular momentum adjustments in the reaction wheels, which also are used for attitude control.

There are two types of propellant on Cassini: mono-propellant and bi-propellant. The mono-propellant, hydrazine, is used for small maneuvers, dead-band control, and for reaction wheels momentum management. The bi-propellant, mono-methyl hydrazine and nitrogen tetroxide, is used for big maneuvers (those larger than 0.3 meters/second).

After nearly two decades in space and billions of miles traveled, Cassini’s journey is finally over. It has nearly exhausted its fixed supply of propellant and needed the remainder to insure a precise trajectory into Saturn’s atmosphere.

History of PU-238 Power Sources

When the NASA spacecraft met its end in Saturn’s upper atmosphere the morning of Sept. 15, 2017, burning up in a fireball of silicon and metal, the last components to disintegrate were some of its most essential: pods of exceedingly rare and vital nuclear fuel that made the mission possible.

Those components were the iridium-clad pellets of plutonium-238 developed by the Department of Energy and NASA decades ago. Contained within three cylindrical power sources called radioisotope thermoelectric generators (RTG), they produced the heat and electricity that kept Cassini powered up and its instruments warm throughout its long journey in the cold depths of space.

The outside temperature in orbit around Saturn was recorded as minus 288F. By comparison, the coldest temperature ever recorded on earth was minus 136F in Antarctica in 2010.

RTGs are lightweight, compact spacecraft electrical power systems that have flown successfully on 23 previous U.S. missions over the past 37 years. Cassini’s RTGs contained plutonium from the Department of Energy’s Savannah River Site in Aiken, South Carolina. The iridium cladding around that nuclear fuel was produced by Oak Ridge National Laboratory and the Y-12 National Security Complex. The New Horizons RTG was built at the Idaho National Laboratory’s Space and Security Power Systems Facility.

Cassini launched on Oct. 15, 1997, with the European Space Agency’s Huygens probe. Named after astronomers Giovanni Cassini and Christiaan Huygens, the pair of spacecraft reached Saturn in 2004 after a 2.2-billion-mile (3.5-billion-kilometer) voyage. In 2005, Huygens was deployed to the surface of Saturn’s largest moon, Titan.

Over 20 years and two mission extensions, Cassini produced countless key discoveries about Saturn, its rings, and its moons, like liquid methane seas on Titan and a global ocean with indications of hydrothermal activity on Enceladus. Its findings expanded our understanding of the kinds of worlds where life might exist.

Although the spacecraft may be gone, the treasure trove of data it collected about Saturn — the giant planet itself, its magnetosphere, rings and moons — will continue to yield new discoveries for decades.

And with the Department of Energy working to produce new nuclear fuel for future space missions, Cassini’s legacy of exploration will be carried on by new spacecraft for generations to come.

Next Generation RTG Requirements

NASA wants to determine the characteristics of a Next-Generation RTG that would “best” fulfill Planetary Science Division (PSD) mission needs. This study is limited to systems that convert heat to electricity using thermocouples. “Best” is defined as a confluence of the following factors:

• An RTG that would be useful across the solar system

• An RTG that maximizes the types of potential missions: flyby, orbiter, lander, rover, boats, submersibles, balloons

• An RTG that has reasonable development risks and timeline.

More details are available from NASA – NASA Plans for next generation RTGs which is a 78 page slide deck in PDFD file format. Note that slide 67 contains draft RFI requirements for a contractor to work with NASA to produce a next generation power source that meets these requirements. Slides 73 & 74, and 76 discuss planetary landing scenarios and uses along with potential operating environments and challenges to the technology.

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