Ralph McNutt (pictured above) has had a long and successful career working as a scientist on a great many missions from Voyager through Cassini to New Horizons and his current role as project scientist for Mercury Messenger. Like previous guest David Southwood, his specialty is space physics, a field with broad applicability across the solar system.
However, when Dr. McNutt spoke with our co-host Raymond Francis at this year's LPSC, it was on a different, yet no less important topic. What Voyager, Cassini and New Horizons have in common is a reliance on nuclear power to provide electricity to their computers, scientific instruments and onboard systems. You just can't do deep space exploration without these nuclear batteries, powered by Plutonium 238. However, production of Pu-238 stopped many years ago in the United States and current stocks to run spacecraft have been purchased from the Russians.
However, the lack of Pu-238 is starting to approach crisis levels. This is an issue that the DPS has spoken out strongly about for several years. As such, the US Congress has sought ways to restart the production of this material domestically for spacecraft usage. Unfortunately, agreements between the DoE and NASA have kept falling through. For many years the language Authorizing NASA to restart production was set-out in law, but no money was Appropriated for that purpose (The US has a two-stage budget process in which not everything an agency is legally permitted to do is actually funded). This year, it looks like things might be changing with NASA expected to receive approximately $15 million for plutonium restart in the president's budget request.
In the meantime, the plutonium shortage has helped spur the interest of engineers to try to improve the efficiency of Nuclear Batteries. Interestingly, the decay energy of Pu-238 goes almost entirely into creating heat which means that they are particularly helpful when thermal energy is what is ultimately desired. But the main problem is that by moving around heat to keep components warm, or to increase the efficiency (such as with a sterling cycle) you introduce moving parts with non-infinite duty cycles. Just like the human heart, all such components will ultimately stop beating, bringing the missions they power to an end.
We don't always focus on the technical aspects here at WW, but this episode is a fascinating study of a critical technology for space exploration and is not to be missed. You can find a copy of the episode here and, as usual, my intro is under the cut.
I will say, categorically, we cannot effectively explore space without nuclear power and, in the longer run, nuclear propulsion. You’re listening to Western Worlds!
Hello and welcome back for another conversation here on Western Worlds, an AFM*Original show heard right here on Astronomy.fm. My name is Dr. John and I’m coming to you this week, as every week, from the Centre for Planetary Science and Exploration at Western University, located roughly a billion billion fission gamma ray wavelengths from the Douglas Point Demonstration Reactor, in London, Ontario, Canada.
Former NASA Administrator and author of our opening quote Dr. Michael Griffin had it right: no technology has enabled our exploration of the solar system and particularly our exploration of the outer solar system more than has a device called the Radioisotope Thermoelectric Generator or RTG for short. RTGs are very simple devices based on a special radioactive isotope called Plutonium-238 in which most of the stored decay energy is released as heat. The RTG then converts this heat into electrical energy. While RTGs only put out a few tens to hundreds of Watts, but can do so reliably for decades, making them ideally suited to solar system exploration.
As such, NASA has had a great interest in such devices since their inception and has long been closely associated with this kind of nuclear technology. The reason is a simple one – out past Mars, the amount of sunlight available to power solar cells falls off precipitously. Chemical batteries can’t help either since their charges would be depleted long before spacecraft ever reached their targets. In recognition of the importance of nuclear-powered spaceflight, which has just reached its 50th year, this year’s Lunar and Planetary Science Conference was held jointly with the Nuclear and Emerging Technologies for Space Meeting.
There is certainly plenty of cross-over between members of the two fields. No one exemplifies that better than this week’s guest, Dr. Ralph McNutt of the Applied Physics Laboratory of Johns Hopkins University in Baltimore, Maryland. Dr. McNutt has made enduring contributions as a science team member of the Voyager, Cassini, New Horizons and, most recently, as project scientist of the Messenger Mission. Simultaneously, he has been a tireless advocate for nuclear technologies that enable exploration goals. Our own Raymond Francis sat down with Dr. McNutt recently at the LPSC/NETS joint meeting. Transitioning to that interview is “Pulstar,” by Vangelis.