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Power Shortage in the Outer Solar System?

As if we didn’t have enough trouble getting to the outer Solar System, now comes word that the US inventory of plutonium-238 is diminishing. That’s what NASA administrator Mike Griffin told a House appropriations subcommittee this past week, pointing out that after the Mars Science Laboratory launches in 2009, the agency will find itself running out of the plutonium needed to fuel radioisotope power systems. Even New Horizons, on the way to Pluto/Charon, is using Russian plutonium, the periodic purchase of which has been forced by NASA’s dwindling supplies.

New Horizons’ principal investigator Alan Stern told the committee that beyond the Mars Science Laboratory, NASA probably has enough plutonium on hand or on order to fuel the outer planets flagship mission targeted at 2017 and an interim Discovery class mission scheduled to fly a few years earlier, the latter intended, ironically enough, to test more efficient radioisotope power systems now under development.

Rings around Rhea

Meanwhile, the outer planets continue to surprise us with findings like the apparent ring system around Saturn’s moon Rhea. Or is it just a halo of dust and debris? Geraint Jones (University College, London) is lead author on a paper looking at the Rhea possibilities. In the current issue of Science, Jones and colleagues note that the small moon (some 1500 kilometers in diameter) is circled by a debris disk perhaps 5000 kilometers from end to end, with particles ranging from small pebbles to boulders. Simulations show that such rings could be stable for lengthy periods.

Image: An artist concept of the ring of debris that may orbit Saturn’s second-largest moon, Rhea. The suggested disk of solid material is exaggerated in density here for clarity. Due to a decrease in the number of electrons detected by NASA’s Cassini spacecraft on either side of the moon, scientists suggest that rings are the likeliest cause of these electrons being blocked before they reach Cassini. Credit: Jet Propulsion Laboratory.

Are we looking at a ring system composed of the remnants of an ancient asteroid or comet collision with the moon? Saturn’s lively system shows evidence of other catstrophic impacts of the sort that could have created the needed debris field. And just as we’re now looking at asteroids with moons (the Eugenia occultation is imminent — see yesterday’s story here), we also deal with the intriguing thought that, as Jones puts it, “…Rhea seems to have some family ties to its ringed parent Saturn.”

The paper is Jones et al., “The Dust Halo of Saturn’s Largest Icy Moon, Rhea,” Science Vol. 319, No. 5868 (7 March 2008), pp. 1380-1384 (abstract). See also this Los Alamos National Laboratory news release.

Comments on this entry are closed.

  • James M. Essig March 8, 2008, 18:44

    Hi Folks;

    Even though the comments below are not directly related to the issue of supplying space probes with Pu-238, I bring them up because of the practical significance to nuclear fission power and radio-nuclear powered space-craft. It occured to me that other planetary bodies within our solar system might be a source for radio nucleids.

    Perhaps, deposits of U-238 and U-235 and other long half life radionucleids might be found on our Moon, Mars, some of the moons orbiting Jupiter and Saturn and perhaps even within the asteriods. These fuels could be useful for power production back on Earth as well for nuclear fission powered space craft including nuclear electric propulsion, fission fragment drive, and nuclear charge driven pulsed vehicles such as those of the design of project Orion in the 1950s and 1960s.

    Perhaps if we are able to develope stable super heavy elements such as those proposed to exist within the metaphorical so-called “Island of Stable Super-heavies”, we will have a means to generate more intense nuclear fission reactions and an additional means to explore quantum-chromo-dynamics or qcd in order to learn more about the strong nuclear force. Such can only pay off in our research to come up with practical energy sources for manned interstellar travel.

    I personally find the field of low energy to medium low energy particle physics very interesting since such an energy region may enable us to eventually fabricate stable super-heavies with novel properties.

    I almost jump for joy every time I hear of the creation of yet another element or isotope. Part of my interest in high atomic mass radio-nucleids I am sure was derived from my father being an engineering duty officer involved in U.S. Navy’s Naval Reactors nuclear engineering program. When I was about 25 years old, he presented me with a publically available 3 feet by 4 feet paper chart of all of the known elements and their isotopes. Included where hundreds of radio-nucleids and thousands of isotopes. Theorectically, there should exist about roughly 1/2 an order of magnitude more isotopes based on known elements which have not yet been discovered. This I though was really cool.



  • Frank Smith March 9, 2008, 10:38

    Could someone tell us why Plutonium is the isotope of choice for space probes?

    Why not U-235 for instance?

  • Enzo March 9, 2008, 22:11

    Scientific American has an article about using Stirling engines that
    use only a fraction of the amount of isotopes as the current RTGs:

    The article is not public but it says that they are ready to be used
    in the next decade.


  • Adam March 10, 2008, 3:21

    Hi Frank

    Deep space probes use Radioisotope Thermal Generators (RTGs) – electrical power-units that use the heat of radioactive decay. Pu-238 produces about 550 Watts per kilogram of isotope as heat – lumps of the metal glow red-hot with the heat from its decay. Its lifetime is just 88 years, thus why it produces so much heat. But not too much – shorter-lived isotopes vanish before they give a space-probe a useful lifetime of power-supply. Its radiation is just high-energy electrons and that converts to heat very easily. Harder radiations would escape and also damage the generator. Plus Pu-238 is a byproduct of reprocessing spent reactor rods – something the US doesn’t do, to their loss and no great gain for anybody.

    Uranium-235 is a very mild radioactive material – its half-life is over 700 million years, which means its heat output is very low. It is fissionable, but fission reactors are more complex than RTGs and their radiation is more damaging. Hard to fit in a space-probe massing just a couple of hundred kilograms – the lightest Russian Topaz space-reactors mass over a ton with their shielding.

  • george scaglione March 10, 2008, 9:47

    enzo, i seem to recall seeing that article last week,shows alot of potential promise.thank you for pointing it out. george

  • Edg Duveyoung March 10, 2008, 10:56

    I think that putting Plutonium on a rocket — knowing that it could crash back upon us instead of reaching orbit — is a risk-taking that is unconscionable when we see that power plants that are almost ready to go, such as Bussard’s, are not being forthrightly funded.

    To me this is like our using oil and coal instead of investing in green energy. Plutonium is perhaps the most poisonous substance known, and a pound of it, vaporized by the fire of re-entry from space, could poison thousands to death by cancer if the vapor came to Earth over a population center. A mere speck is a lethal dose — lung cancer is assured.

    Abstractly, this is the same deal as sending the Beatles’ song to the North Star. It is a group of elitists making a decision for all of humanity. We all know about the controversy of citizens being riled up merely because a truck was carrying some radioactive substance on a major highway to a “safe site.” Their fears might be largely unfounded because of the strength of the packaging of the material, yet, because of secrecy, these same folks often have poisons traveling over their heads without any notification. It is simple abuse by those in power. An old story, but one that cannot be resisted too strongly.

    I’m all for exploration — even in the midst of world wide poverty that could use the money, but all it would take is one Plutonium disaster for heads to roll at NASA. Worst case scenarios can be imagined that would be horrific beyond anything history has yet recorded except for dinosaur-killing impacts, super volcanoes, etc. A cloud of vapor settling on New York City would veneer everything with poison and would kill that city as dead as dead can be for decades. That’s the worst kind of risk that is being taken for the sake of getting nice pictures of Mars.


  • dad2059 March 10, 2008, 12:51

    …yet, because of secrecy, these same folks often have poisons traveling over their heads without any notification. It is simple abuse by those in power…

    All too true and I agree, but until somebody does invest in something different, we either use that stuff or don’t send probes to the outer system at all.

    To you scientist and engineer types out there, how about lunar He3 being mined and using hydrogen fuel cell technology? Can you design a small, efficient cell to power a modified Stirling Engine? I’m a tech, so I can build one if you brainiacs can do the math and design one.

  • philw1776 March 10, 2008, 13:20

    First, Bussard’s powerplant is not “almost ready to go” as it is an unproven concept. That aside I would like to see it funded so as to ascertain if it is really a feasable concept. The beautiful people at Google touting the concept could fund it themselves and reap the investment return as VCs. The revenue from owning the patents to economical fusion electricity production would be huge.

    Obviously you’ve read Dr Kaiku’s pseudo-science plutonium hysteria. All US deep space probes with Pu are in containers that survive re-entry intact should the rocket blow up or fail to achieve orbit. We’ll still need Pu for deep space probes in the next decades (assuming any get funded) but thankfully, we’ll use far less of it.

  • philw1776 March 10, 2008, 15:00

    “To you scientist and engineer types out there, how about lunar He3 being mined and using hydrogen fuel cell technology?”

    Aside from the minor engineering and economics of mining Lunar He3, we haven’t the slightest clue how to build a He3 fusion reactor which is an order of magnitude or more more difficult than building a hydrogen fueled reactor, something that has been ’25 years away’ since the 1960s.

    And fuel cells need fuel and oxidizer, Helium is neither. O2 and every other oxidizing neighbor on the Periodic Table of the Elements shuns He and its neighbors.

    Imaginary fantasy schemes aren’t going to power ANY deep space or Mars rovers for the next couple decades. Beyond that ‘immediate’ window, we can fantasize away, although I caution everyone to remember that NASA’s 21st century spacecraft are all powered by schemes 1st deployed in the 60s and 70s, with ion propulsion (still rarely used) being the only latecomer to the party. Progress is slow. My favorite dark horse is VASIMIR engines powered by so-far politically unacceptable FISSION reactors, vigorously opposed by the same Luddite mentality that exaggerates the probability and resultant impact of a Pu disaster.

    In the real world, stirling engine Pu fueled heat engine generators are our best bet for the next generation.

  • Adam March 11, 2008, 2:50

    Hi All

    Edg, contrary to the fearful fantasies concocted by anti-nuke types the plutonium toxicity risk is very low – and is mostly make-believe by the deluded.

    Sure, the stuff is poisonous – it is a heavy metal – and it is radioactive – that’s why we’re using it on space-probes – but it is very difficult for it to get into a human body, which is the only place it can do harm. A few pounds of the stuff dispersed to the four winds would only slightly raise the back-ground radiation because it would be spread over a very wide area. Also – if it wasn’t already an oxide – burning up in the atmosphere turns it into an insoluble compound that is utterly inert biologically. No way it would enter the biosphere chemically.

    Don’t believe the hype. The stuff is dangerous and nasty – small lumps are red-hot – but it’s only a small amount on any space-probe at any one time. A far smaller carcinogenic risk than what comes out of the Solid-Rocket boosters on the Space Shuttle, for example.

  • James M. Essig March 11, 2008, 4:57

    Hi Folks;

    Great commentary!

    One can imagine extracting hydrogen or hydrogen and/or helium from any of the gas giant planets within our solar system as a fuel source to power fusion powered space craft.

    Perhaps large space ark like spacecraft with a dry wieght of 10 million to 100 million metric tons could scoup up 100 million to one billion metric tons of fusion fuel respectively on their journeys out of the solar system to other star systems through out the Milky Way and even the local super-cluster and beyond if no faster way of space travel is developed other than effiecient fusion rockets with Isp in the 2.5 million to 3 million range.

    One can imagine that after one billion such launches, we would start to deplete Earth’s biosphere of hydrogen if we used Earth based hydrogen for fuel. The math for this assumption is Earth Hydrogen Supply = Ehs = (10 EXP 8)(10 EXP 9) = 10 EXP 17. The mass of the Earth’s oceans is roughly 10 EXP 18 metric tons. However the mass of Jupiter is about 317 times that of Earth and the Mass of Saturn is about 95 times that of Earth while the mass of Neptune is about 17 times that of Earth and the mass of Uranus is about 14 times that of Earth. The combined mass of these four planets is about 450 times the mass of the Earth or a total of roughly 10 EXP 24 metric tons. That is enough fusion fuel to launch 10 EXP 15 space ark missions involving 100 million metric ton dry mass space craft loaded with a billion metric tons of fuel. This works out to 100 thousand launches per year over a 10 billion year period or 100 million luanches per year over a 10 million year period. By that time we ought to have learned how to collect cometary hydrogen and helium from the Oort clould which has an estimated mass on the order of one solar mass.

    The Oort cloud has an extimated cometary mass of about one solar mass or 10 EXP 27 metric tons. This is enough fuel for 10 EXP 18 space ark missions involving 100 million metric ton dry wieght space craft with a loaded fueled wieght of 1 billion metric tons. That works out to a million launches per year for a trillion years, a billion launches per year for a billion years, or a trillion launches per year for a million years.

    If fusion runway propulsion is assummed, then the number of possible flights increases and the gamma factor obtainable by the space craft increases dramatically.

    Now if some method is developed to to break hydrogen and/or helium down into energy directly without antimatter annihilation, then the number of possible flights for a terminal velocity of 0.15 C goes by about 3 orders of magnitude. The number of space craft flights remains roughly the same for complete mass energy conversion rocket powered schemes and terminal gamma factors of about 10 for highly efficient rockets.

    Now if we learn how to use hydrogen and any direct mass to energy conversion technologies more effeciently such as for various antigravity or other gravatic propulsion schemes wherein a slight gravity imbalance could be used to boot-strap a much more intense gravatic or antigravatic spacetime warp, then the possibility of the number of space flight goes up by many orders of magnitude, perhaps even wherein the velocity of the craft can reach several to many orders of magnitude greater than C. Note that I once read an article in which it was reported by a group of physicists that such an space warp drive type of mechanism might utilize an initial slight space warp of some exotic kind wherein upon being set up, the space warp would quicky boot strap itself up into a very intense bubble like warp in space on which the craft would metaphorically ride. It was suggested that such a craft could travel as fast as the entire span of the Milky Way Galaxy in 2 seconds Earth time although the scientists conceded that they had no idea how to set up this exotic form of space warp.

    Now the figures for spacetime travel over the next million to one trillion years involved hydrogen available from our solar system and the Oort cloud. But this supply is only about 10 EXP – 23 of the total supply of hydrogen within our observable universe and perhaps only 1/(infinity) of the hydrogen supply within our Big Bang Universe assumming that our universe is of infinite spatial extent as some big bang models suggest. What’s more, Our big bang might be only one of many, perhaps an ensemble if not infinite number of such big bangs with laws simmilar to, or perhaps identical to our own. And so I am encouraged that the fuel of the stars or hydrogen is a universal cosmic energy source that can power mankinds star faring civilization as we develope it for time periods of cosmic duration into the future. Let us start by encouraging and supporting research into ordinary fusion rocket schemes
    that can reach, say, a terminal 0.1 C to 0.2 C so that we can start our cosmic journey out into the great black ocean of space and time. Let the fun begin!



  • george scaglione March 11, 2008, 8:24

    jim,funny thing this topic should come up! literally every time i see something about space it hits me right between the eyes that the universe is a HUGE source of power!!!! if only we could tap into these things we would have sufficient energy to do almost anything we wished from pushing our spacecraft ever faster to building space stations across the solar system,etc. etc. the possibilities would only be limited by imagination ! suggest we all try to develop these ideas as much as we possibly can! so,jim and everybody else…with respect,your friend george

  • Edg Duveyoung March 11, 2008, 11:14

    Where could space exploration be right now if money wasn’t an issue? let’s say that since 1960 money has been flowing.

    Would we have humans walking on Mars, a permanent Moon base, a Bussard’s engine that was providing free energy to the entire world, rovers on other planets, a complete survey of all possible Earth impactors, massive SETI efforts, a 400 inch Hubble, 550 A.U. telescopes, light sails, 2001 Space Odyssey accomplishments, commercially viable hydrogen production? I’d love the big brains here to speculate about this as an exercise in obtaining clarity about what political decisions are “costing us” in scientific advancements.

    NASA and congressional funding priorities are mostly political instead of scientific, it seems.

    Just as coal and oil technologies enjoy a lot of funding for basic research while so many green-technologies — that are already showing commercial oomph — are languishing because of low/no political support, it seems fair to criticize the lack of funding for promising propulsion technologies that also impact the oil and coal industries and could possibly provide almost free energy to the entire world.

    I know I’m indulging in a major paranoid dynamic of my personality here, but whenever BIG MONEY is at stake I have to at least examine the politicization of research budgets.

    Thanks for all the education about plutonium etc. I should have done an hour of googling the topics before posting here about it.

    The concept that elitists are making decisions for us is too broad, conceptually, to be jammed into this thread; they belong on another thread, or, Web site. Sorry for the thread highjacking — erp! ;-)


  • James M. Essig March 11, 2008, 15:39

    Hi George and Edg;

    Thanks for the comments.


    Good comments. Space is huge with inexhuastable energy reserves. My thinking is that we need to put much money into effecient fusion rockets and improved interstellar ramjet concepts since these are self contained systems that do not require beamed energy or mass to reach mildy to moderately relativistic velocities. There are, as we know, numerous types of fusion rocket designs that have been proposed and they should be persued vigorously. This will at least get us to our stellar nieghboors while we develope more capable systems to go even further afield.


    I share your above mentioned thinking. Our elitist leaders seem to be real short sighted in terms of long term prospects for our civilization.

    I remember as a third and fourth grader watching the Apollo missions return to Earth’s atmosphere. Every class room in my school had a TV to which each class was glued to, in a certain sense of awe, as the reporter mentioned the distance from or relative velocity of the space craft as it approached Earth’s atmosphere. At one point I remember faintly, I am not sure of the exact details, of the ticking off of the space crafts distance in 100 mile increments. The class seemed to understand that the space craft was traveling about 100 miles every 13 seconds.

    I remember my next door neighboors across the street inviting everyone in the nearby houses in our working class neighborhood over for a nighttime party to watch one of the Apollo space craft land on the Moon. It seemed at that instant, everyone in the country was united in pride and awe regardless of whether they had family fighting in Vietnam, were Vietnam War protestors, were in support of womens rights or not or indifferent, were conservative, liberal, democrat, republican, white, African American or what have you. We had all forgotten our differences and stood a little taller.

    We need such a program again, perhaps this time on a Global scale to unite humanity as never before. Going to Mars I think would be a great start, and then further out from there. There is too much to explore out there not to do so. On Earth, we are keenly aware of the limits of growth in our human population. With the whole universe to reach out into and colonize in part, there is virtually an unlimited gift of life that we can give to our would be descendents that otherwise would not be born into existence. Such providing the casual means for more human beings to come into existence is perhaps one of the ultimate acts of charity that we can manifest others as a species not to mention the wonderment, awe, and pride we can instill in future generations as we prepare the groundwork for these later generations to set boots on planets around nearby stars.

    The ramifications and the possibilities for humanity are enourmous. We have the ability to wage war at a horrific and selfish level and end civilization and perhaps life on our planet all in a lazy afternoon or we can plan for the survival of our species essentially forever. Our leaders just don’t seem how much of a cross-roads we are at. We need real responsible leadership of the people, derived from the people, for the people. This seems to have been forgotten.



  • george scaglione March 12, 2008, 8:25

    jim,edg…yes we need to develop better systems of propulsion very badly! the amount of energy inherent in space only invites us! i also remember that feeling of ,wow,100 miles every 13 seconds!! we sure need another major space project to bring us all together and yes mars could be it! thank you very much and i hope to hear from you both again george

  • Edg Duveyoung March 12, 2008, 12:22

    I was in Italy during the second moon landing. It wasn’t even the FIRST landing, but oh boy did it get the world’s attention.

    I was attending an educational seminar that took itself very seriously, but there was I — and about 500 others too — stopping everything and rigging up a TV set to watch in this auditorium where a very important lecture was supposed to be being given, but all stopped for the landing.

    Space exploration can capture the imagination of the whole world and give us all at least one thing we can agree about.

    I’m still rooting for a planet to be discovered in the Centauri system and having the masses clamor for “science to get us there now!”

    I for one wouldn’t want the masses to know just how far away the closest star is or how impossibly huge the challenges would be to invent the MILLION NEW THINGS that would have to be invented to get there.

    I remember thinking “Geeze, why is President Kennedy saying we should reach the Moon by the end of the decade? Why not put a little more money into it and do it next year?”

    Sigh…..I was one of the masses, see?


  • george scaglione March 12, 2008, 16:25

    edg yes i understand how you felt.i probably did too at that stage of the game!but yes you are very right.will the average guy realize as we do the distances(!!!!) involved in interstellar flight! as a matter of fact in that same vein i just had a look at a book which just came out by dr michio kaku.it seems to divide all the things we’d like to see in physics into three parts…what will not happen as soon as we want…what will take longer…and what may possibly never happen! excellent work guess everybody here should buy a copy. respectfully your friend george

  • James Fincannon March 14, 2008, 11:01

    While there is a fuel availability issue for radioistope power systems (which is especially critical for outer planetary travel), a non-mass optimum solution to this is available.

    Solar photovoltaic power systems are feasible even at the low outer planetary solar intensities. Triple junction solar cells have been shown to provide power at the low temperature, low intensity conditions beyond Jupiter. Missions have been examined for Saturn for instance. While they are ungainly large, the mass is not prohibitive (using Ultraflex or Concentrator/SLASR type solar arrays). Even operation to Pluto is possible IF the operating power levels are reduced/duty cycled.

    Although radioisotopes have been the historical mass optimum approach, when there are such limited supplies of fuel that they do not allow missions, then something must be done.

  • ljk April 21, 2008, 9:49

    Review: To the End of the Solar System

    In the early years of the Space Age, NASA pursued efforts to develop
    nuclear-powered rockets that held the promise of opening up the solar
    system. Jeff Foust reviews a book that examined the technology and
    policy issues associated with that ultimately failed effort.


  • ljk May 9, 2009, 8:35

    May 8, 2009

    NASA is Running Out of Plutonium

    Written by Ian O’Neill

    Decommissioning nuclear weapons is a good thing. But when our boldest space missions depend on surplus nuclear isotopes derived from weapons built at the height of the Cold War, there is an obvious problem.

    If we’re not manufacturing any more nuclear bombs, and we are slowly decommissioning the ones we do have, where will NASA’s supply of plutonium-238 come from? Unfortunately, the answer isn’t easy to arrive at; to start producing this isotope, we need to restart plutonium production.

    And buying plutonium-238 from Russia isn’t an option, NASA has already been doing that and they’re running out too…

    This situation has the potential of being a serious limiting factor for the future of spaceflight beyond the orbit of Mars.

    Exploration of the inner-Solar System should be OK, as the strength of sunlight is substantial, easily powering our robotic orbiters, probes and rovers. However, missions further afield will be struggling to collect the meagre sunlight with their solar arrays. Historic missions such as Pioneer, Voyager, Galileo, Cassini and New Horizons would not be possible without the plutonium-238 pellets.

    So the options are stark: Either manufacture more plutonium or find a whole new way of powering our spacecraft without radioisotope thermal generators (RTGs). The first option is bound to cause some serious political fallout (after all, when there are long-standing policies in place to restrict the production of plutonium, NASA may not get a fair hearing for its more peaceful applications) and the second option doesn’t exist yet.

    Full article here:


  • ljk October 8, 2009, 23:45

    Plutonium Shortage Could Stall Space Exploration

    by Nell Greenfieldboyce

    September 28, 2009

    Los Alamos National LaboratoryA pellet of plutonium-238 dioxide glows red because of heat generated by the radioactive decay of the material.

    NASA is running out of the special kind of plutonium needed to power deep space probes, worrying planetary scientists who say the U. S. urgently needs to restart production of plutonium-238.

    But it’s unclear whether Congress will provide the $30 million that the administration requested earlier this year for the Department of Energy to get a new program going.

    Nuclear weapons use plutonium-239, but NASA depends on something quite different: plutonium-238. A marshmallow-sized pellet of plutonium-238, encased in metal, gives off a lot of heat.

    “If you dim the lights a little bit, it glows a little red, because it’s very hot,” says Stephen Johnson, director of space nuclear systems and technologies at the Idaho National Laboratory.

    All that heat can be converted into electricity. “And this electricity is very, very useful, when you’re in a remote or a hostile environment,” says Johnson, “such as when you’re in space and when you’re too far away from sun to use solar power.”

    Full article here:


  • ljk October 30, 2010, 0:11

    AAS email: Plutonium 238 Production: An Ongoing Issue for Washington


    “It is not often that a $30M issue in an appropriations bill gets a lot of attention. In fact, it is not uncommon to hear Senate staff state that they have $100M round off errors, but recent language in Congressional legislation appropriating funds of this magnitude can have a direct, negative impact on the long-term success of US planetary science and, potentially, creative approaches to power generation for future astrophysics missions, earth observation missions and other research activities in space.”

  • ljk September 24, 2011, 21:32



    Robert Park – September 16, 2011

    In a move that could end deep space exploration, congressional budget cutters left off the $15 million needed to restart the production of Pu- 238. Pu-238 is not bomb stuff; it’s a non-fissile isotope used to supply heat for thermoelectric generators (RTGs). With no moving parts, RTGs are one of the simplest and most reliable technologies ever devised. They generate electrical power to operate long-duration space missions too distant from the Sun for solar panels. Let’s see, that will cut a $300 billion budget by 0.0005%. Well, it’s a start.