Category Archives: NASA

Happy Landings Anniversary, Mars Curiosity Rover

A quick note of congratulations to NASA’s Mars Curiosity rover project team on the first anniversary of a daredevil landing on Mars on August 6, 2012.


The project has been a scientific and engineering triumph of the first order. Also, it is a prime example of the application of nuclear technology in scientific research—Curiosity is one of a long line of historic space missions powered by a Plutonium-238 radioisotope thermoelectric generator. See the September/October 2012 issue of ANS ReActions for a discussion of the Curiosity rover and its plutonium heartbeat.

As part of its birthday celebration, NASA compiled a time-lapse video of Curiosity’s first year of diggings, drillings, and travels on Mars. Now, the rover treks toward a rendezvous with Mount Sharp—possibly a site where the chemical ingredients needed for life have been best preserved.

Thanks to for that music soundtrack!

Other Curiosity-related stories on ANS Nuclear Cafe:
ANS Nuclear Matinee: Measuring Radiation on Mars
Converting heat into electricity without moving parts
Nuclear-powered Mars rover Curiosity lands safely
ANS Nuclear Matinee: Mars Rover Curiosity, A Nuclear Powered Mobile Laboratory
Plutonium in Space: Why and How?
Shannon Bragg-Sitton of INL discusses nuclear space applications


Friday Matinee – How Nuclear Power Saves 1.8 Million Lives

NASA scientist Dr. Pushker Kharecha and Dr. James Hansen (the leading climate scientist in the US) recently authored a study which conservatively estimates nuclear power has saved 1.8 million lives, which otherwise would have been lost due to fossil fuel pollution and associated causes, since 1971.

DNews has the story:

For more information, see Ashutosh Jogalekar’s blog at Scientific American Nuclear power may have saved 1.8 million lives otherwise lost to fossil fuels, may save up to 7 million more.

Thanks to DNews


The Cassini-Huygens Mission to Saturn

By Stan Tackett

Cassini-Huygens is a Flagship-class NASA-ESA-ASI robotic spacecraft sent to the Saturn system. It has studied the planet and its many natural satellites since its arrival there in 2004, as well as observing Jupiter and the Heliosphere, and testing the theory of relativity. Launched in 1997 after nearly two decades of gestation, it includes a Saturn orbiter Cassini and an atmospheric probe/lander Huygens that landed in 2005 on the moon Titan. Cassini is the fourth space probe to visit Saturn and the first to enter orbit, and its mission is ongoing as of 2013.  It is powered by a plutonium power source, and has facilitated many landmark scientific discoveries in its mission to the stars.

A Cassini RTG before installation

Because of Saturn’s distance from the Sun, solar arrays were not feasible as power sources for this space probe.  To generate enough power, such arrays would have been too large and too heavy. Instead, the Cassini orbiter is powered by three radioisotope thermoelectric generators (RTGs), which use heat from the natural decay of about 33 kg (73 lb) of plutonium-238 (in the form of plutonium dioxide) to generate direct current electricity via thermoelectrics. The RTGs on the Cassini mission have the same design as those used on the New HorizonsGalileo, and Ulysses space probes, and they were designed to have a very long operational lifetime. At the end of the nominal 11-year Cassini mission, they will still be able to produce 600 to 700 watts of electrical power. One of the spare RTGs for the Cassini mission was used to power the New Horizons mission to Pluto and the Kuiper belt.

A glowing-hot plutonium pellet that will become the power source for the probe’s RTG

To gain interplanetary momentum while in flight, the trajectory of the Cassini mission included several gravitational slingshot maneuvers: two fly-by passes of Venus, one more of the Earth, and then one of the planet Jupiter. The terrestrial fly-by maneuver was successful, with Cassini passing by 500 km (310 mi) above the Earth on August 18, 1999. Had there been a malfunction causing the Cassini space probe to collide with the Earth, NASA’s complete environmental impact study estimated that, in the worst case (with an acute angle of entry in which Cassini would gradually burn up), a significant fraction of the 33 kg of plutonium-238 inside the RTGs could have been dispersed into the Earth’s atmosphere. NASA estimated the odds against that happening at more than 1 million to one.

Selected events and discoveries for Cassini

Cassini made its closest approach to Jupiter on December 30, 2000, and made many scientific measurements. About 26,000 images of Jupiter were taken during the months-long flyby. Cassini produced the most detailed global color portrait of Jupiter yet (see image at right), in which the smallest visible features are approximately 60 km (37 mi) across. The New Horizons mission to Pluto captured more recent images of Jupiter, with a closest approach on February 28, 2007.

A major finding of the flyby, announced on March 6, 2003, was of Jupiter’s atmospheric circulation. Dark “belts” alternate with light “zones” in the atmosphere, and scientists had long considered the zones, with their pale clouds, to be areas of upwelling air, partly because many clouds on Earth form where air is rising. But analysis of Cassini imagery showed that individual storm cells of upwelling bright-white clouds, too small to see from Earth, pop up almost without exception in the dark belts.

Other atmospheric observations included a swirling dark oval of high atmospheric-haze, about the size of the Great Red Spot, near Jupiter’s north pole. Infrared imagery revealed aspects of circulation near the poles, with bands of globe-encircling winds, with adjacent bands moving in opposite directions.  The same announcement also discussed the nature of Jupiter’s rings. Light scattering by particles in the rings showed the particles were irregularly shaped (rather than spherical) and likely originate as ejecta from micrometeorite impacts on Jupiter’s moons, probably Metis and Adrastea.

Tests of General Relativity

On October 10, 2003, the Cassini science team announced the results of tests of Einstein’s Theory of General Relativity, which were done by using radio waves that were transmitted from the Cassini space probe. This remains the best measurement of post-Newtonian parameter γ; the result γ = 1 + (2.1 ± 2.3) × 10-5 agrees with the predictions of standard General Relativity.

The radio scientists measured a frequency shift in the radio waves to and from the spacecraft, while those signals traveled close to the Sun. According to the Theory of General Relativity, a massive object like the Sun causes space-time to curve, and a beam of radio waves (or light, or any form of electromagnetic radiation) that passes by the Sun has to travel farther because of this curvature.

The extra distance that the radio waves traveled from the Cassini craft, past the Sun, to the Earth delayed their arrival. The amount of this time delay provided a sensitive test of the calculated predictions of Einstein’s Relativity Theory.

Although some measurable deviations from the values that are calculated using the General Theory of Relativity are predicted by some unusual cosmological models, no deviations were found by this experiment. Previous tests using radio waves that were transmitted by the Viking and Voyager space probes were in agreement with the calculated values from General Relativity to within an accuracy of one part in one thousand. The more refined measurements from the Cassini space probe experiment improved this accuracy to about one part in 51,000, with the measured data firmly supporting Einstein’s General Theory of Relativity.

New moons of Saturn

Using images taken by Cassini, three new moons of Saturn were discovered in 2004. They are very small and were given the provisional names S/2004 S 1, S/2004 S 2, and S/2004 S 5, before being named Methone, Pallene, and Polydeuces at the beginning of 2005.

On May 1, 2005, a new moon was discovered by Cassini in the Keeler gap. It was given the designation S/2005 S 1, before being named Daphnis. The only other known moon inside Saturn’s ring system is the moon Pan.

A fifth new moon was discovered by Cassini on May 30, 2007, now known as Anthe.

A press release on February 3, 2009, showed a sixth new moon found by Cassini. The moon is approximately 1/3 of a mile in diameter within the G-ring of the ring system of Saturn, and is now named Aegaeon.

A press release on November 2, 2009, mentions the seventh new moon found by Cassini on July 26, 2009. It is presently labeled S/2009 S 1, and is approximately 300 m (984 ft.) in diameter in the B-ring system.

Phoebe flyby

On June 11, 2004, Cassini flew by the moon Phoebe. This was the first opportunity for closeup studies of this moon since the Voyager 2 flyby. It also was Cassini’s only possible flyby for Phoebe due to the mechanics of the available orbits around Saturn.

The first closeup images were received on June 12, 2004, and mission scientists immediately realized that the surface of Phoebe looks different from asteroids visited by spacecraft. Parts of the heavily cratered surfaces look very bright in those pictures, and it is currently believed that a large amount of water ice exists under its immediate surface.

Saturn rotation

In an announcement on June 28, 2004, Cassini program scientists described the measurement of the rotational period of Saturn. Since there are no fixed features on the surface that can be used to obtain this period, the repetition of radio emissions was used. These new data agree with the latest values measured from Earth, and constitute a puzzle to the scientists. It turns out that the radio rotational period has changed since it was first measured in 1980 by Voyager, and that it is now six minutes longer. This does not indicate a change in the overall spin of the planet, but is thought to be due to movement of the source of the radio emissions to a different latitude, at which the rotation rate is different.

Orbiting Saturn

On July 1, 2004, the Cassini spacecraft flew through the gap between Saturn’s F and G rings and achieved orbit, after a seven-year voyage. Cassini is the first spacecraft to ever orbit Saturn.

The Saturn Orbital Insertion maneuver performed by Cassini was complex, requiring the craft to orient its high-gain antenna away from Earth and along its flight path, to shield its instruments from particles in Saturn’s rings. Once the craft crossed the ring plane, it had to rotate again to point its engine along its flight path, and then the engine fired to decelerate the craft by 622 m/s (1391 mph) to allow Saturn to capture it. Cassini was captured by Saturn’s gravity at around 8:54 p.m. Pacific Daylight Time on June 30, 2004. During the maneuver, Cassini passed within 20,000 km (12,000 mi) of Saturn’s cloud tops.

Titan flybys

Cassini had its first distant flyby of Saturn’s largest moon, Titan, on July 2, 2004, only a day after orbit insertion, when it approached to within 339,000 km (211,000 mi) of Titan and provided the best look at Titan’s surface to date. Images taken through special filters (able to see through the moon’s global haze) showed south polar clouds thought to be composed of methane, and surface features with widely differing brightness. On October 27, 2004, the spacecraft executed the first of 45 planned close flybys of Titan, when it flew a mere 1,200 kilometers above the moon. Almost four gigabits of data were collected and transmitted to Earth, including the first radar images of the moon’s haze-enshrouded surface. It revealed the surface of Titan (at least the area covered by radar) to be relatively level, with topography reaching no more than about 50 meters in altitude. The flyby provided a remarkable increase in imaging resolution over previous coverage. Images with up to 100 times better resolution were taken and are typical of resolutions planned for future Titan flybys.

Huygens lands on Titan

Cassini released the Huygens probe on December 25, 2004, by means of a spring and spiral rails intended to rotate the probe for greater stability. Huygens entered the atmosphere of Titan on January 14, 2005, and after a two-and-a-half-hour descent landed on solid ground. Although Cassini successfully relayed 350 of the pictures that it received from Huygens of its descent and landing site, a software error failed to turn on one of the Cassini receivers and resulted in the loss of the other 350 pictures.

Enceladus flybys

Enceladus backdropped by Saturn’s ring shadows in 2007

During the first two close flybys of the moon Enceladus in 2005, Cassini discovered a “deflection” in its local magnetic field that is characteristic for the existence of a thin but significant atmosphere. Other measurements obtained at that time point to ionized water vapor as being the atmosphere’s main constituent. Cassini also observed water ice geysers erupting from the south pole of Enceladus, which gives more credibility to the idea that Enceladus is supplying the particles of Saturn’s E ring. Mission scientists hypothesize that there may be pockets of liquid water near the surface of the moon that fuel the eruptions, making Enceladus one of the few bodies in the Solar System known to contain liquid water.

On March 12, 2008, Cassini made a close flyby of Enceladus, getting within 50 km of the moon’s surface. The spacecraft passed through the plumes extending from its southern geysers, detecting water, carbon dioxide, and various hydrocarbons with its mass spectrometer, while also mapping surface features with its infrared spectrometer that were measured to be at much higher temperature than their surroundings. Cassini was unable to collect data with its cosmic dust analyzer due to an unknown software malfunction.

Radio occultations of Saturn’s rings

In May 2005, Cassini began a series of occultation experiments to measure the size-distribution of particles in Saturn’s rings, and measure the atmosphere of Saturn itself. For more than four months, Cassini completed orbits designed for this purpose. During these experiments, Cassini flew behind the ring plane of Saturn, as seen from Earth, and transmitted radio waves through the particles. The radio signals were received on Earth, where the frequency, phase, and power of the signal were analyzed to help determine the structure of the rings.

Spoke phenomenon verified

In images captured September 5, 2005, Cassini detected spokes in Saturn’s rings, previously seen only by the visual observer Stephen James O’Meara in 1977 and then confirmed by the Voyager space probes in the early 1980s.

Lakes of Titan

Radar images obtained on July 21, 2006, appear to show lakes of liquid hydrocarbon (such as methane and ethane) in Titan’s northern latitudes. This is the first discovery of currently-existing lakes anywhere besides Earth. The lakes range in size from one to 100 kilometers across.

Titan “sea” (left) compared at scale to Lake Superior (right)

On March 13, 2007, the Jet Propulsion Laboratory announced that it had found strong evidence of seas of methane and ethane in the northern hemisphere of Titan. At least one of these is larger than any of the Great Lakes in North America.


A Saturnine hurricane

In November 2006, scientists discovered a storm at the south pole of Saturn with a distinct eyewall. This is characteristic of Earth’s hurricanes and had never before been seen on another planet. Unlike a Terran hurricane, the storm appears to be stationary at the pole. The storm is 8,000 kilometers (5,000 mi) across, and 70 kilometres (43 mi) high, with winds blowing at 560 km/hr  (350 mph).

Great Storm of 2010 and its aftermath

Storm in the North 2011

On October 25, 2012, Cassini witnessed the aftermath of the massive Great White Spot storm that recurs roughly every 30 years on Saturn. Data from Cassini’s composite infrared spectrometer instrument indicated a powerful discharge from the storm that caused a temperature spike in the stratosphere of Saturn 150 °F (83 kelvins) above normal. Simultaneously, a huge increase in ethylene gas was detected by NASA researchers at Goddard Research Center in Greenbelt, Maryland. Ethylene is a colorless and odorless gas that is highly uncommon on Saturn and is produced both naturally and through man-made sources on Earth. The storm that produced this discharge was first observed by Cassini on December 5, 2010, in Saturn’s northern hemisphere. The storm is the first of its kind to be observed by a spacecraft in orbit around Saturn as well as the first to be observed at thermal infrared wavelengths, allowing scientists to observe the temperature of Saturn’s atmosphere and track phenomena that are invisible to the naked eye. The spike of ethylene gas that was produced by the storm reached levels that were 100 times more than those thought possible for Saturn. Scientists have also determined that the storm witnessed was the largest, hottest stratospheric vortex ever detected in our solar system, initially being larger than Jupiter’s Great Red Spot.

Mission extension

On April 15, 2008, Cassini received funding for a two-year extended mission. This consisted of 60 more orbits of Saturn, with 21 more close Titan flybys, seven of Enceladus, six of Mimas, eight of Tethys, and one targeted flyby each of Dione, Rhea, and Helene. The extended mission began on July 1, 2008, and was renamed the Cassini Equinox Mission as it coincided with Saturn’s equinox.

A proposal was submitted to NASA for a second mission extension, provisionally named the extended-extended mission or XXM. This was subsequently approved and renamed the Cassini Solstice Mission. It will see Cassini orbiting Saturn 155 more times, conducting 54 additional flybys of Titan, and 11 more of Enceladus. The chosen mission ending is a series of very close Saturn passes, passing inside the rings, then a plunge into the Saturn atmosphere around the 2017 northern summer solstice, to destroy the spacecraft.



Stan Tackett holds undergraduate degrees in mathematics and computer science, and is currently pursuing a Master’s degree in computer science with specializations in uses of artificial intelligence in the nuclear industry. His interests in nuclear engineering include nuclear propulsion for space travel, fusion, computational fluid dynamics and reactor physics. In his spare time he reads Piers Anthony as much as possible, and enjoys writing and editing crossover science fiction stories.

Friday Nuclear Matinee: Low Energy Nuclear Reactions

The ANS Nuclear Cafe today brings faithful viewers a short interview with Dr. Joseph M. Zawodny, senior research scientist at NASA Langley Research Center. Zawodny discusses research on “Low Energy Nuclear Reactions” at NASA Langley, and the incredible potential of this new form of nuclear power—IF theory is validated by experimental results.

See this basic explication of the science from Dennis Bushnell, chief scientist at NASA Langley Research Center: Low Energy Nuclear Reactions: The Realism and the Outlook (caution: labs blowing up, windows melting…)

The American Nuclear Society conducted an LENR panel session organized by Mr. Steven B. Krivit at the ANS 2012 Winter Meeting.

Ultra low momentum neutron catalyzed nuclear reactions
on metallic hydride surfaces seminal paper by Widom and Larsen

Discover Magazine Big Idea: Bring Back the “Cold Fusion” Dream

As Bushnell says, some seriously “strange” things are going on—possibly with the potential to change the world.

Thanks to NASA Langley, and tip of cap to Nuclear Energy Institute Facebook

ANS Nuclear and Emerging Technologies for Space (NETS 2013) Topical Meeting

The 2013 ANS Topical Meeting on Nuclear and Emerging Technologies for Space (NETS 2013) will be held February 25–28, 2013, at the Albuquerque Marriott in Albuquerque, New Mexico.

NETS serves as a major communications network and forum for professionals and students working in the area of space nuclear technology. The NETS meeting facilitates the exchange of information among research and management personnel from international government, industry, academia, and the national laboratory systems.

NETS 2013 will address topics ranging from overviews of current space programs to methods of meeting the challenges of future space endeavors, with a focus on nuclear technologies and applications.  See the NETS program page for meeting tracks and topics.

NETS 2013 is hosted by the Aerospace Nuclear Science and Technology Division (ANSTD) of the American Nuclear Society with co-sponsors Aerojet and the ANS Trinity Local Section.

Register Now

Hotel Reservations

See the Nuclear and Emerging Technologies for Space meeting page for much more information. We hope to see you in Albuquerque.


ANS Nuclear Cafe Matinee: DUFF Space Nuclear Reactor Prototype

A joint Department of Energy and NASA team has demonstrated a simple, robust fission reactor prototype [note: see Comments for more accurate and complete description] intended for development for future space exploration missions. The DUFF (Demonstration Using Flattop Fissions) experiment represents the first demonstration in the United State—since 1965—of a space nuclear reactor system to produce electricity.

The uranium–powered reactor is the first use of a “heat pipe” to cool a small  nuclear reactor (measuring one foot!) and power a Stirling engine. The following short video from Los Alamos National Laboratory explains the hows and whys:

See this article from Los Alamos on the details of the DUFF experiment recently successfully conducted.  Also, see this CNN article for an excellent description.

Many future space missions will only be feasible with the use of reliable and safe nuclear energy, and this proof-of-concept is a steppingstone toward that future.


ANS Nuclear Cafe Matinee: Radiation Belt Storm Probes

NASA’s Radiation Belt Storm Probes mission is scheduled for launch early on Thursday morning, August 30. How and why? An ANS Nuclear Cafe double feature matinee:

Quite a lot of fascinating “right stuff” goes into getting a scientific mission into orbit. A behind-the-scenes look at Radiation Belt Storm Probes launch preparation:

Once these dual satellites are in orbit, the mission will allow us to better understand fundamental radiation and particle acceleration processes throughout the universe. So, what are the Van Allen Radiation belts, and what will the Radiation Belt Storm Probes do there?


Nuclear and Emerging Technologies for Space 2013: Call for Papers

February 25–28, 2013 • Albuquerque Marriott, N.M.

Abstract Submissions Due: September 4, 2012

On February 25–28, 2013, the Aerospace Nuclear Science and Technology Division of the American Nuclear Society will hold the 2013 Nuclear and Emerging Technologies for Space (NETS 2013) topical meeting in Albuquerque, N.M. This conference represents the second stand-alone topical meeting in Albuquerque since the previous Space Technologies and Applications International Forum, and follows a successful meeting in 2012 held in conjunction with the 43rd Lunar Planetary Science Conference.

Topic areas

NASA is currently developing capabilities for robotic and crewed missions to the Moon, Mars, and beyond. Strategies that implement advanced power and propulsion technologies, as well as radiation protection, will be important in accomplishing these missions. NETS serves as a major communications network and forum for professionals and students working in the area of space nuclear technology. Every year NETS facilitates the exchange of information among research and management personnel from international government, industry, academia, and the national laboratory systems. To this end, the NETS 2013 meeting will address topics ranging from overviews of current programs to methods of meeting the challenges of future space endeavors.


Track 1: Current Space Architectures and Missions
Space Science and Exploration Missions
Industrial Programs
Defense Architectures
Spacecraft Concepts and Design
Lunar and Planetary Surface Concepts
Mission Analysis and Validation Missions
Space Policy and Procedures

Track 2: Present Enabling Capabilities
Plutonium-238 Production
Radioisotope Power Systems
Power Conversion Systems and Components
Supporting Technologies (including Heat Rejection and Power
Management & Distribution)
Space Radiation Environment and Protection
Impact on Human Operations 

Track 3: Near-Term Nuclear Technologies
Reactor and Shield Design
Reactor Simulation
Fuels Development
Materials and Radiation Testing
Alternative Radioisotopic Systems and Applications
Systems Integration
Tools and Modeling
Testing and Validation 

Track 4: Augmenting Nuclear Capabilities
Advanced Reactor Concepts
Advanced Fuels and Materials
Hybrid Nuclear Systems
Enhanced Computational Methods
Improved Radioisotopic Power System Design
Nuclear Enabled In-Situ Resource Utilization 

Track 5: Innovative and Advanced Technologies
Low Alpha Multi-Megawatt Power Systems
Fusion Systems
Non-Traditional Methods
Novel Mission Design


Home Page for NETS 2013

Meeting Chairs and Contact Information

ANS Nuclear Matinee: Measuring Radiation on Mars

Even before its successful landing earlier this week, NASA’s Mars Science Laboratory was already sending back important scientific data—about the radiation exposure that astronauts might face during a mission to the Red Planet.

Now, the Curiosity rover’s Radiation Assessment Detector is collecting information about the radiation environment on the surface of Mars. Cosmic rays and energetic particles from the sun can be very important factors for past or present life on Mars, and for future human exploration as well. Don Hassler, principal investigator for Curiosity‘s Radiation Assessment Detector, explains.



Nuclear-powered Mars rover Curiosity lands safely

An image sent by NASA’s Curiosity rover shortly after landing

The nuclear-powered roving robotic laboratory Curiosity touched down early on August 6, and is beaming back images while undergoing system checks. The Curiosity landing has generated worldwide interest, including interest in its plutonium power source.

A short internet news roundup highlighting Curiosity‘s use of nuclear technology for its source of power:

Steve Aplin at the Canadian Energy Issues blog, in “This educational moment brought to you by plutonium, and the end of the Cold War,” provides an excellent overview of Curiosity‘s radioisotope thermoelectric generator and its origins — in the end of the Cold War — and explains how Curiosity‘s successful landing is a triumph of cooperation between two former enemies.

Matt Wald at the New York Times Green BlogNuclear Pack Powers Rover on Mars” provides a succinct overview of the reasoning behind using a plutonium power source for Curiosity instead of solar.

At the Nuclear Energy Institute’s Nuclear Notes,A Nuclear-Powered Space Rover Lands on Mars, Brings New Hope for Space Exploration” covers some of the far-reaching implications Curiosity’s mission has for the nuclear energy field — especially in space exploration.

Dan Yurman at Idaho Samizdat inNASA Mars vehicle uses nuclear power source interviewed Stephen Johnson, director of Idaho National Laboratorys Space Nuclear Systems and Technology Division, about Curiosity shortly after launch.

From the mainstream press Los Angeles Times today, “Mars rover draws on nuclear power for trek around Red Planet” outlines the role of Curiosity‘s radioisotope thermal generator, developed by engineers at Hamilton Sundstrand Rocketdyne in partnership with the U.S. Department of Energy.

Readers are referred to ANS Nuclear Cafe’s recent “Mars Rover Curiosity, A Nuclear Powered Mobile Laboratory” containing a NASA video featuring Ashwin Vasavada, deputy project scientist for the Mars Science Laboratory, explaining Curiosity‘s Multi-Mission Radioisotope Thermoelectric Generator.

ANS contributor Wes Deason in “Plutonium in Space: Why and How?” delves into the advantages of using plutonium in radioisotope generators for space missions.

Shannon Bragg-Sitton of INL discusses nuclear space applications” and speaks at length about the Curiosity rover in this ANS Nuclear Cafe video shortly after the launch of the mission. Dr. Bragg-Sitton served as chair of the 2012 ANS Nuclear and Emerging Technologies for Space conference.

The Jet Propulsion Lab Mars Science Laboratory website provides Mars Science Laboratory mission background information and breaking news.


ANS Nuclear Matinee: Mars Rover Curiosity, A Nuclear Powered Mobile Laboratory

Early on Monday morning (1:31AM Eastern Daylight Time), after having traveled 352 million miles, NASA’s robotic rover Curiosity is scheduled to touch down inside the Gale Crater on the surface of Mars. Soon after, it will begin looking for clues about possible early forms of Martian life.

The Curiosity rover carries much more scientific equipment than previous Mars rovers. How to run so much heavy, power-intensive scientific research equipment for a mobile laboratory on another planet? Nuclear power!

Ashwin Vasavada, deputy project scientist for the Mars Science Laboratory, explains in this week’s video.

Space nuclear propulsion: Humanity’s route to the solar system

Part I:  Space nuclear reactor safety

by Wesley Deason

Though humans have successfully traveled from the earth to the moon, our exploration of the remainder of the solar system has been limited to robotic space probes which, once set in their trajectory, were not designed to return to earth. The data returned from these probes has been of tremendous importance for our understanding of the solar system and regions beyond, but human exploration beyond earth’s orbit remains to be achieved. There are a number of concepts currently under study that would allow us to break out of earth’s gravity well. The most studied and discussed are nuclear electric propulsion and nuclear thermal propulsion. Before I jump into an explanation of those concepts and their respective differences, however, I want to address their similarity: Both are powered by a nuclear reactor.

The primary principle that drives the immense energy production of a nuclear reactor is the process of nuclear fission, in common terms the “splitting of atoms.” This process, induced in uranium through the absorption of a neutron, releases very large amounts of energy when part of the mass of the original uranium atom is converted into energy as the atom splits apart (E=mc2). The fission process also releases additional neutrons that can be used to invoke fission in other uranium atoms. If enough uranium atoms are present, the chain of fissions can be maintained at a steady rate and this configuration of uranium is said to have reached “critical mass.” Extended over a long period of time, this process allows a nuclear reactor to produce large amounts of energy. Fission energy becomes particularly useful and indeed necessary when large amounts of energy are required while availability of fuels or other energy sources is low. With this amazing energy generation capability, however, questions about its safety can, and should, be asked.

Is it safe to launch nuclear reactors into space?

Space reactors must be able to endure specific circumstances that are unique to their transport to outer space. Most importantly, the reactor must remain “subcritical” until required by the mission to commence operation. One classic design requirement for space reactors is that the reactor remain subcritical after a water submersion (a launch accident scenario). Water around a submerged reactor behaves as a neutron moderator, a material which slows fast-moving neutrons. In order to meet this important design criterion, reactors will often contain a material that will absorb moderated or slowed neutrons before they can cause fission in the uranium fuel.

If there were a highly unlikely launch accident in which reactor fuel escaped containment, the environmental effects would remain minimal. Uranium, the fuel that drives modern reactors, is a naturally occurring radioactive element that has a half-life of around 700 million years (for the uranium-235 isotope). This means that it releases energy through radioactive decay at a very slow rate. Also, uranium is an alpha emitter. As discussed in my previous post on plutonium-238, alpha radiation is generally not harmful to humans, provided its emitters are not inhaled or ingested. The more highly radioactive constituents that comprise spent nuclear fuel would not be present before reactor operation commences in space.

Are nuclear reactors dependable and controllable for power generation in space?

To address the controllability and dependability of nuclear reactors, we must consider the methods and physical processes that allow a reactor to be controlled. The main concepts are the effects of negative temperature feedback and the active removal of neutrons through the use of neutron absorbing materials and leakage control.

Core arrangement – Space nuclear power by Angelo & Buden

Negative feedback within a nuclear reactor can come from two main effects, both of which are related to the slowing of the fission chain reaction due to a temperature rise. First is the commonly-known material property of thermal expansion. As a reactor core heats up, it will expand in size, causing the uranium fuel within to spread farther apart and absorb fewer neutrons for fission. Second, due to the neutron absorption properties of nuclei, when the temperature of uranium rises, it is more likely to absorb a neutron but not cause fission. From a safety and control perspective, negative temperature feedback can aid in preventing a reactor from producing too much power and overheating.

There are also methods to actively control a nuclear reactor by removing neutrons from the reactor. These include control rods, drums, shutters, and windows. Control rods and drums use boron, an element with a large neutron absorption ability, to remove neutrons from the reactor before they can cause fission of the uranium atoms. Control rods insert boron directly into the central region of the reactor to adjust power or shut it down. Control drums are a more popular alternative for compact space reactors; the drums contain an absorber section that is rotated towards or away from the reactor to adjust power. Shutters and windows are largely unique to space reactors as they take advantage of the vacuum of space. When these shutters or windows open, they allow neutrons to leak out of the system, thus slowing the chain reaction. These features, along with others specific to a selected reactor design, allow well-designed space reactors to maintain containment of radioactive materials in case of accident.

Kiwi A Prime nuclear thermal rocket built and tested in the 1960s

Nuclear reactors, due to their ability to produce large amounts of energy at any location, will be the required energy source for future human space travel outside of earth’s orbit. Future installments in this series will focus on how nuclear reactors are applied in the two most-studied nuclear space propulsion technologies:  nuclear electric and nuclear thermal propulsion.



Wes Deason is a graduate student in nuclear engineering at Oregon State University working on the safety analysis of vented fuel systems for gas-cooled fast breeder reactors. He is a former summer fellow for the Center for Space Nuclear Research and the current student liaison for the Aerospace Nuclear Science and Technology Division of ANS.

ANS’s Loewen visits local sections

Eric Loewen, president of the American Nuclear Society, kept up his rapid pace last week as he visited the ANS local section in Aiken, S.C., on February 15, and the one in Charlotte, N.C., on February 16. Loewen, as the featured speaker at the meetings of the two sections, presented his personal talk titled “Plutonium: Promise or Peril”.

During the morning on the 15th, Loewen toured the MOX Fuel Fabrication Facility on the Savannah River Site, in South Carolina. The facility,which is being built by the Department of Energy’s National Nuclear Security Administration, will convert surplus nuclear weapon-grade plutonium into reactor fuel for use in commercial nuclear power plants starting in 2016. Under a 2000 agreement, the United States and Russia will dispose of 68 metric tons of surplus plutonium, enough material for many thousands of nuclear weapons (see Shaw Areva MOX Services for more info).

Later on the 15th, Loewen was hosted by Stephen Sheetz of the Savannah River National Laboratory for a tour of the lab and other facilities on the Savannah River Site.

At the MOX Fuel Fabrication Facility: Zachary Kosslow (ANS), Amanda Bryson (Shaw Areva MOX Services), Eric Loewen (ANS-president), and Kevin Hall (NNSA).


NNSA-MOX Federal Project Director Clay Ramsey illustrates with ANS's Loewen how a fuel pellet boat will be used in the MOX fuel fabrication process.

The dinner meeting that featured Loewen on the 15th was attended by about 160 people. The dinner was hosted by Citizens for Nuclear Technology Awareness, in cooperation with ANS. “Dr. Loewen’s presentation was very well received by all in attendance,” said Amanda Bryson, chair of the Savannah River ANS local section. “The event brought together professionals at all stages of their careers from all over the Central Savannah River Area, representing many facets of the nuclear industry in the area. This was one of the best-attended events for ANS–Savannah River in the past year, and provided the opportunity for lively and thought-provoking interaction among our membership and the membership of Citizens for Nuclear Technology Awareness. It was a pleasure and a privilege to have Dr. Loewen visit.”

The next day, in Charlotte,  Loewen was interviewed on WFAE NPR Radio Charlotte. Click the “Listen” button at the 

WFAE webpage to tune in to the interview via the Comments page, or tune in to the interview directly

Dr. Clint Wolfe (Exec. Dir. CNTA), Dr. Loewen, Karen Bonavita (CNTA)

“Dr. Loewen had over 100 attentive local section members as an audience,” said Thomas Doering, chair of the Piedmont-Carolinas ANS local section, regarding Loewen’s talk in Charlotte on the 16th. “The Peidmont-Carolinas section historically has drawn nearly 100 local members for over two years; the greater Charlotte area is considered the energy capital of the nation. Dr. Loewen’s talk focused on the misconceptions of plutonium and how other energy sources suffered from a similar beginning.”

When asked about his trip, Loewen said, “I’m just so impressed with the vibrancy and vitality of these sections. They really are greater than the sum of their parts, and their parts are pretty great.”

Carolinas Section Officers James Bakke, Thomas Doering - chair, ANS President Loewen, Myron Koblansky, Andrew Sowder.

Plutonium in Space: Why and How?

By Wes Deason

The reasons for using plutonium in space missions are often unclear to those outside the mission planning community. Observers may see or hear only that the space mission is nuclear related, and that the power source uses plutonium.

Plutonium is a word that in some communities has very negative connotations. Plutonium was needed to create the atomic weapons of the Cold War, is highly regulated by proponents of nuclear nonproliferation, and is one of the causes of the environmental woes at the Hanford site in Washington State. On the other hand, it is also the element that has been used to safely power many space missions, including the Voyager, Galileo, Cassini, New Horizons, and the most recent Mars rover, Curiosity.

So, why is plutonium still used if it has issues associated with it?

The answer is that plutonium exists in multiple nuclear forms, or isotopes. Isotopes occur in elements naturally due to differing number of neutrons in the nucleus. While relatively unimportant on the chemical level, on the nuclear level isotopes of a single element can behave very differently. Plutonium-239, the isotope of plutonium with 94 protons and 145 neutrons, is a fissile isotope, meaning that after the absorption of a non-energetic neutron it has a possibility of splitting, or fissioning. Because of this capability, plutonium-239 can be used in nuclear reactors and weapons. Plutonium samples with a large fraction of the plutonium-239 isotope are referred to as weapons-grade plutonium.

However, devices that use plutonium to produce power use the plutonium-238 isotope, which has 94 protons and 144 neutrons. It is not fissile, and cannot be used in atomic bombs or nuclear reactors. Plutonium-238 is useful for radioisotope heat sources, and radioisotope power systems, because it decays radioactively, releasing a particularly useful form of radiation called alpha radiation.

Alpha radiation is simply energized and completely ionized helium atoms, which lose their energy in the form of heat when interacting with other matter. This energy loss mechanism is similar to how friction generates heat on a surface. Alpha radiation is generally not harmful to humans, provided its emitters are not inhaled or ingested; alpha particles can be stopped by the outermost layer of skin.

Pu-238 is safe and can produce heat, but why is it preferred over other power sources?

Radioisotope power systems are useful for space applications for two main reasons:

  • First, they are very versatile. Unlike solar power sources, radioisotope power systems do not rely on correct orientation toward the sun, nor do they depend on proximity to the sun.
  • Second, the power from plutonium-238 lasts a long time. The half-life of plutonium-238, or the amount of time it takes for the power produced by the isotope to decrease by half, is 87.7 years.

A power system fueled by plutonium-238 can last for a very long time. This is, of course, dependent upon the reliability of the heat-to-electricity conversion components. The most common power conversion method—a static system known as thermoelectric conversion—is very reliable and can last for decades.

Future radioisotope power systems will adopt a new method for power conversion called the Stirling cycle—a dynamic (moving) cycle—which will allow for higher efficiency and lower mass systems. The new generators will be termed Advanced Stirling Radioisotope Generators. For more information on radioisotope power systems, see this page maintained by the Department of Energy.

Where do we get plutonium-238? Can it be found naturally?

Unfortunately, plutonium-238 cannot be found naturally. This is because it is radioactive and will have almost completely decayed into a different element after a geologically short period of 1000 years. Thus, plutonium-238 must be produced using nuclear reactors.

During the Cold War, when weapons-grade plutonium production was at full scale, plutonium-238 was a byproduct that could be saved and used for space power production.  Since the 1990s, however, the United States has stopped production of weapons-grade plutonium, yet we continue to plan space missions that require the use of plutonium-238. NASA and the DOE have discussed plans to use national laboratory reactors to produce plutonium-238 for general purpose applications, but it is questionable if they will be able to supply a sufficient amount to meet national needs.

Another concept, proposed by the Center for Space Nuclear Research (CSNR), uses flexible TRIGA research reactors to produce a higher quantity of Pu-238 per year at lower cost. For more information on low cost plutonium-238 production, contact the CSNR.

Regardless of its source, Pu-238 remains an important tool for scientific research. Many space missions have been powered by plutonium-238, and future missions will continue to be enabled by it. Its long lasting heat generation—coupled with a dependable power conversion system—allows it to be used in many environments and configurations. The use of plutonium-238 can be expected to become even more important as space exploration pushes further outward to Mars, Jupiter, their moons, and beyond!

This article is the first of a monthly series of ANS Nuclear Cafe entries on nuclear space topics by the ANS Aerospace Nuclear Science and Technology Professional Division.



Wes Deason is a graduate student in nuclear engineering at Oregon State University working on the safety analysis of vented fuel systems for gas-cooled fast breeder reactors. He is a former summer fellow for the Center for Space Nuclear Research and the current student liaison for the Aerospace Nuclear Science and Technology Division of ANS.

Photo Time!

The Curiosity rover (Click to enlarge/Photo: NASA)

This summer should see the first use of a nuclear-powered land vehicle—on Mars! On November 26, NASA launched the Mars Science Laboratory (MSL), which includes a rover named Curiosity, from the Kennedy Space Center in Florida. The MSL/Curiosity package is by far the largest object ever intended to land on Mars and remain functional afterward. That is why Curiosity, in its operations on the Martian surface, will be powered by a multi-mission radioisotope thermoelectric generator fueled with plutonium-238. Curiosity is described as being the size of an automobile.
Read more about Curiosity and the Mars mission in the January 2012 issue of Nuclear News magazine, available in hard copy and electronically for American Nuclear Society members (must enter ANS user name and password in Member Center).