Category Archives: Space Applications

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


The Mini-Mag Orion Space Propulsion System

By Stan Tackett

ANST logoIn my previous article on the history of nuclear pulse propulsion, I outlined three research programs in nuclear propulsion systems for space travel.  The first of these, Project Orion, was investigated in the 1950s and 1960s as a very serious and practical option for space travel.  Its only limiting factor was the signing of the International Test Ban Treaty in 1963 that barred the detonation of nuclear weapons in space.

Fast forward to 2003.  Andrews Space & Technology (AS&T) introduced an innovative propulsion system that could significantly shorten round trips from Earth to Mars (from two years to only six months) and enable our spaceships to reach Jupiter within a year of space travel. The system is called the Miniature Magnetic Orion (Mini-Mag Orion for short), and is an optimization of the 1958 Orion interplanetary propulsion concept.  The system has the potential to dramatically affect interplanetary space travel.

The original Orion project was headed by Ted Taylor from General Atomics, who together with the famous physicist Freeman Dyson suggested ejecting nuclear explosives behind a spacecraft in order to propel it forward. The Mini-Mag system uses a magnetic field to trigger an explosion of compressed material in the form of small pellets weighing several grams. This explosion, although significantly weaker than a nuclear explosion, creates plasma that is directed through a magnetic nozzle to generate vehicle thrust. The proposed technology enables the production of thrust at high efficiency, allowing drastic reduction of interplanetary travel time. According to calculations performed by AS&T, this type of propulsion system could produce the same thrust as the Space Shuttle Main Engine, with 50 times more efficiency.

The Mini Mag Orion concept

The Mini Mag Orion concept

Due to the magnetic compression thrust technology, spacecraft could be smaller and lighter. The spacecraft itself would only need to carry a relatively small amount of fissionable material as fuel, and would be able to reach speeds of approximately 10% of the speed of light. Dr. Dana Andrews, AS&T Chief Technology Officer and Mini-Mag Orion inventor, and Roger Lenard from the Sandia National Laboratories, have published a paper describing their research into the Mini-Mag Orion (MMO) concept in the Acta Astronautica – Journal of the International Academy of Astronautics.

In the framework of their research into the subject, the scientists conducted an experiment that tested the process of compressing a simulated fissile material in a magnetic field. From a 2003 press release issued by Andrews Space, Inc.:

The experiment validated the physical process behind the MMO concept, substantiating MMO’s potential of enabling shorter interplanetary trip time for near-term space travel,” said AS&T Principal Investigator Ralph Ewig. “We are still far from constructing an actual vehicle, but the present research will chart the course for human missions to other planets in the near future. The Mini-Mag Orion system shows significant promise, and the successful completion of our experiment demonstrated the physics and validated our approach for a near-term, in-space, advanced propulsion system,” said Dr. Andrews.

In their Acta Astronautica paper, Dr. Andrews (Andrews Space, Inc.) and Dr. Lenard (Sandia National Laboratories) describe these technologies and their own recent studies of the Mini-Mag Orion concept, reducing the size of the vehicle drastically by using magnetic compression technology.  The two scientists have studied this process using Sandia National Laboratories’ Z-Pinch Machine, the world’s largest operational pulse power device.

The Z-Pinch Machine

Sandia Lab’s Z Machine

The interstellar version of Mini-Mag Orion couples highly efficient pulsed nuclear propulsion with beamed propulsion; that is, a pellet stream of fissionable particles beamed toward the spacecraft that continuously fuels the departing ship.  A Mini-Mag Orion vehicle could attain ten percent of light speed using the combination, according to Andrews and Lenard.  Deceleration of the vehicle at its destination would be accomplished via a magnetic sail, a large superconducting ring which uses intercepted charged particles to slow the spacecraft down.

Perhaps the most important aspect of the system is that it is another demonstration that the formidable distances of interstellar space can be conquered, using technologies which we already understand and could conceivably build within this century.


tackett a 100x128Stan 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.

Nuclear Pulse Propulsion: Gateway to the Stars

By Stan Tackett

ANST logoIn this first of a series of articles on nuclear propulsion for space travel, allow me to enlighten each of you about the fascinating history of this technology. This post will cover three early projects, with posts to follow that will explore other technologies along with an assessment of future prospects.

The great astronomer Carl Sagan once said that one cannot travel fast into space without traveling fast into the future. He was, of course, referring to the time dilation effect made manifest by speeds very close to the speed of light, 300,000 km/s. Sagan was also a strong proponent of nuclear power for use in space propulsion systems, in particular nuclear pulse propulsion. He outlined three of these in his award-winning series Cosmos: Project Orion, Project Deadalus, and the Bussard Ramjet.

Nuclear pulse propulsion is a theoretical method of spacecraft propulsion that uses nuclear explosions for thrust. It was first developed as Project Orion by the Defense Advanced Research Projects Agency (DARPA), an agency of the U.S. Department of Defense, after a suggestion by Stanislaw Ulam in 1947. Newer designs using inertial confinement fusion have been the baseline for most post-Orion designs, including Project Daedalus and Project Longshot.

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Project Orion starship

Project Orion

Project Orion was the first serious attempt to design a nuclear pulse rocket. The design effort was carried out at General Atomics in the late 1950s and early 1960s. The idea of Orion was to react small directional nuclear explosives against a large steel pusher plate attached to the spacecraft with shock absorbers. Efficient directional explosives maximized the momentum transfer, leading to specific impulses in the range of 6,000 seconds, or about 12 times that of the Space Shuttle Main Engine. With refinements, a theoretical maximum of 100,000 seconds (1 MN·s/kg) might be possible. Thrusts were in the millions of tons, allowing spacecraft larger than eight million tons to be built with 1958 materials.

The reference design was to be constructed of steel using submarine-style construction, with a crew of more than 200 and a vehicle takeoff weight of several thousand tons. This low-tech single-stage reference design would reach Mars and back in four weeks from the Earth’s surface (compared to ≈50 weeks for NASA’s current chemically powered reference mission). The same craft could visit Saturn’s moons in a seven-month mission (compared to chemically powered missions of about nine years).

A number of engineering problems were found, and solved, over the course of the project. Many of these related to crew shielding and pusher-plate lifetime. The system appeared to be entirely workable, and was under serious development in the United States, when the project was shut down in 1965. The primary reason given was that the Partial Test Ban Treaty made it illegal to detonate nuclear explosions in space (before the treaty, the United States and the Soviet Union had already detonated at least nine nuclear bombs, including thermonuclear bombs, in space; i.e., at altitudes over 100 km).

There were also ethical issues that would be associated with launching such a vehicle from within the earth’s magnetosphere. Calculations showed that the fallout from a takeoff could be projected to lead to the premature death of between 1 and 10 people. Thus, this project would be entirely feasible if the ship were launched from outside the magnetosphere—the only remaining difficulty being, of course, transporting everything to the launch point.

One useful mission for this technology near-term would be to deflect an asteroid that could collide with the earth, which was depicted dramatically in the 1998 film Deep Impact (in which the author appeared as an extra). The extremely high performance would permit even a late launch to succeed, and the vehicle could effectively transfer a large amount of kinetic energy to the asteroid by simple impact—in the event of an imminent asteroid impact, the theorized effect of health effects from fallout would probably not be considered prohibitive. An automated, one-way mission would eliminate the most problematic issue of the design: the shock absorbers.

Orion is an example of an interstellar space drive that could theoretically be constructed—with available technology.

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Project Daedalus

Project Daedalus

Project Daedalus was a study conducted between 1973 and 1978 by the British Interplanetary Society (BIS). The objective was to design a plausible interstellar spacecraft that could reach Alpha Centauri in roughly 45 years, resulting in a velocity of about 10 percent of the speed of light. A dozen scientists and engineers  worked on the project. At the time, fusion research appeared to be making great strides, and in particular, inertial confinement fusion (ICF) appeared to be adaptable as a rocket engine.

ICF uses small pellets of fusion fuel, typically lithium deuteride (6Li2H), with a small deuterium/tritium trigger at the center. The pellets are thrown into a reaction chamber where they are hit on all sides by lasers or another form of beamed energy. The heat generated by the beams explosively compresses the pellet, to the point where fusion takes place. The result is a hot plasma, and a very small “explosion” (compared to using a fission “bomb” to compress and heat the fusion fuel, as in a thermonuclear bomb).

For Daedalus, this process was run within a large electromagnet that formed the rocket engine. After the reaction, which was ignited by electron beams, the magnet funneled the hot gas to the rear for thrust. Some of the energy was diverted to run the ship’s systems and engine. In order to make the system safe and energy efficient, Daedalus was to be powered by Helium-3 fuel that would be mined from the atmosphere of Jupiter.

Currently, designing an ICF system efficient enough for a Daedalus design remains considerably beyond our technical capabilities. Some designs, however, are on the drawing board awaiting confirmation.

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Bussard ramjet

Bussard Ramjet

The noted physicist Robert W. Bussard in 1960 proposed a fascinating method of space propulsion capable of advanced interstellar spaceflight. This variant of a fusion rocket uses enormous electromagnetic fields as a “scoop” to collect and compress hydrogen from interstellar space. High speeds force the reactive mass into a progressively constricted magnetic field, compressing it until thermonuclear fusion occurs. The magnetic field then directs the energy as rocket exhaust, thereby accelerating the vessel.

In deep space, there is only one hydrogen atom for every 10 cubic centimeters of space. This means that the frontal scoop would need to be hundreds of kilometers across to scoop enough hydrogen atoms to funnel through to the reactor. Also, at ship speeds close to the speed of light, these atoms are traveling at the same relativistic speeds; the resulting cosmic rays would effectively fry the ship’s passengers. To counter this, Bussard proposed ionizing these atoms at a safe distance using a laser beam, and using a powerful magnetic field to funnel the ionized atoms into the ship, bypassing the ship’s hull.

There is an amazing characteristic of such a ship, assuming the highly advanced engineering and construction can someday be accomplished and the proposed fusion drive can be brought into existence. Let’s assume a constant acceleration of 1g during the first half of the ship’s journey, whereupon the ship decelerates to its destination at the same 1g for the comfort of all aboard. The resulting velocity of the ship for most of the journey would be very close to the speed of light. This would mean that the relativistic effects of time dilation come into play for the passengers.

For such a hypothetical voyage, Barnard’s Star—six light-years away—could be reached in a little under eight years, ship time. For longer voyages, even the center of our Milky Way galaxy could be reached in just 21 years. As Sagan said,  traveling fast into space means traveling fast into the future—because those left behind on earth during such a hypothetical journey would perceive things very much differently. For them, millions of years would have passed.  Relativistic travels make distant interstellar space travel feasible—but only for those on board the voyage.

In subsequent installments in this series, the history, science, and technology of these and other space nuclear propulsion projects will be explored in depth. Stay tuned.


tackett a 100x128Stan 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.

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.

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.


Mars Rover Curiosity featured in ReActions

From the American Nuclear Society to teachers interested in the nuclear sciences

The September/October edition of ReActions, an American Nuclear Society information resource newsletter for teachers, is available online. This issue features NASA’s fascinating, nuclear-powered rover Curiosity, which is busy making discoveries as it explores the Gale Crater on the surface of Mars. For example, Curiosity again made news and history just a few weeks ago when it discovered an ancient stream bed, from a time when water flowed across the surface of the red planet.

ReActions features some of the many ways that nuclear science and technology is important in everyday life, and includes one of the many classroom group research activities available through ANS. Teachers will want to be sure to use the many ANS information resources available for their students, and the many online information resources in the issue.

This issue of ReActions also takes an in-depth look at the hows and whys of the use of radioisotope power systems in historic space missions, such as the Curiosity rover, with guest contributor Wes Deason of the ANS Aerospace Nuclear Science and Technology Professional Division.

ReActions is a great resource for K-12 science teachers to keep up-to-date on developments in nuclear science education. The newsletter highlights hands-on activities that teachers can use in the classroom, as well as ANS online educational resources and materials.

ReActions also has information on the upcoming ANS teacher workshops in San Diego (November 10—register now, early registration (save $35) ends this Thursday) and Phoenix (December 6–8 and February 24). See the ReActions issue online for more details.


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.

NASA’s Roadmap to the Nuclear Thermal Rocket

By Wes Deason

It is certainly exciting times for NASA and the space nuclear community, as physical testing of nuclear thermal rockets (NTRs) and associated components has begun at NASA and the Department of Energy laboratories across the country. Nuclear thermal propulsion, as discussed in a previous article, is just one form of nuclear propulsion with extensive research behind it, and the only form with an extensive testing background. Near-term efforts by NASA will focus on preparation for ground and flight tests of a scalable Nuclear Thermal Rocket around 2020. However, the larger purpose of the recently restarted testing track is to develop an engine for manned travel to an asteroid, and eventually to our neighboring planet, Mars.

This effort to develop a modern NTR suitable for human travel is part of the Exploration Technology Development and Demonstration program, as described in the NASA 2011 budget under the Advanced In-Space Propulsion component. To begin research, NASA will focus on accomplishing five tasks that will need to be completed before ground technology demonstration can begin.

The roadmap

The first of these tasks is conceptual. NASA must gain an understanding of the current standing of NTR technology and determine, in addition to the currently planned work, what needs to be done to advance technology to an acceptable level. This work will guide any physical testing and set the emphasis on which testing should be prioritized to maximize return on research. NASA must gain a full understanding of how a ground testable NTR can scale to one that is flight testable and applicable to manned missions.

Second, NASA will be pursuing a detailed assessment of the in-core materials used in a modern nuclear thermal rocket (see Houts et al NETS 2012). Building on the extensive research done in the 1970s, they will develop new nuclear fuel fabrication systems, and complete extensive testing to select a final form. Currently, the two competing fuels are the NERVA graphite fuel, developed for NTRs in the 1970s, and a more recently developed fuel called tungsten CERMET. In their final form, both are seeded with uranium, allowing for power production when in a reactor configuration. These fuel types are designed to withstand temperatures greater than 2500° Kelvin, as well as all the stresses inherent to launch. For comparison, titanium, a metal with a very high melting temperature, melts at around 2000° Kelvin. To understand the behavior of these fuel types under many conditions, a testing device called the NTR Element Environment Simulator has been constructed at NASA’s Marshall Space Flight Center, which can provide non-nuclear test results crucial to fuel development (see Houts et al NETS 2012).

Nuclear Thermal Rocket Element Environmental Simulator

The third task is a conceptual analysis of the nuclear and thermal fluid aspects of a ground testable NTR. This analysis will generate models for design and eventual construction of a NTR, and enable cost savings down the road as most of the conceptual issues will be discovered and corrected before a physical prototype is constructed. Much work has previously been done in this area for a modern nuclear thermal rocket, and the breadth of codes and expertise available is large.

The fourth task for NASA will be the development of their NTR test facility. The method that will be explored is the Subsurface Active Filtration of Exhaust (SAFE) option, which utilizes boreholes drilled at the Nevada Test Site. With the SAFE option, NTR exhaust will be fired into the boreholes, as opposed to open air testing of NTRs, as was done for testing in the 1970s. Because of the low run time of these reactors, the inventory of radioactive fission products is very low; however, the use of the SAFE option will allow greater control of any fission products that are produced and lost in the coolant flow. To begin development of the SAFE testing facility, NASA will conduct hot gas injection tests into the boreholes, which will allow for understanding of the effectiveness of the alluvium (porous rock) to holdup and filter radioactive materials.

The final task that NASA will need to complete will be a conceptual plan for NTR development after ground and flight tests are completed. The purpose of this plan will be to sustain the technology level of the NTR so that the system will remain usable for years to come. By building on the knowledge and experience gained in the previously mentioned tests, the NTR will need to be applied to a mission to remain a viable technology option. This will mean regular use in missions beyond low earth orbit such as moon, asteroid, or Mars missions.

Experimental research in space nuclear technology, such as this current work by NASA, has been very weak for quite some time, and it is good to see it being pursued again (this holds true for any advanced propulsion system, not just nuclear thermal propulsion). In the end, the hope remains that nuclear technology will provide the key for unlocking the solar system for humankind.

Much of the information in this post came from a talk by Dr. Stanley Borowski at the NETS 2012 conference.  Dr. Borowski is a researcher at NASA Glenn Research Center in Ohio.



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 the American Nuclear Society.

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

By Wesley Deason

Part III: Nuclear Thermal Propulsion

Today’s post is the final installment of a series concerning space nuclear propulsion (Part I) (Part II). Previous posts discussed nuclear reactor safety and nuclear electric propulsion. Today I will focus on the other extensively researched nuclear space propulsion method: nuclear thermal propulsion.

Nuclear thermal propulsion

Nuclear thermal propulsion (NTP) involves the direct heating and expulsion of a propellant using nuclear power. To accomplish this, nuclear thermal rockets (NTRs) normally consist of three components: a propellant tank, a nuclear power generator, and a nozzle. As in nuclear electric rocket systems, the component that sets various NTRs apart is the type of nuclear generator used.

Most systems that have been designed and tested have used a nuclear reactor to provide heat, while some others have examined the concept of radioisotope power. In the end, the determining factor for which nuclear power generator type should be used is the purpose for which the system was designed. If a nuclear thermal rocket is intended to power a mission to Mars or beyond, a nuclear reactor is a necessity as a power source.


The concept of the nuclear thermal rocket was first developed in the 1950s as a solution for safe and reliable travel to Mars. The research program subsequently developed in the late 1950s and 1960s was unprecedented for space nuclear technology. Through the program, many NTRs were designed, built, and tested. The test site for these systems was Jackass Flats, a location adjacent to what is now the Nevada National Security Site, which lies about 65 miles northwest of Las Vegas.

Famous tests in the program included PHOEBUS 2A, the most powerful nuclear reactor ever to be operated, and NRX-A2, a reactor that was purposefully placed under a very fast power transient to prove its safety. Later NTRs were designed with a specific application in mind, as they were considered for the eventual final stage for the famous Saturn V rocket. Unfortunately, funding for the NTRs, and even the Saturn V rocket, eventually vanished due to a change in the nation’s priorities after the Apollo lunar landings. Despite this change, the program is today considered a technical success, as the tests showed that a system could be safely built and operated.

Some Reactors tested in Rover Program -- Space Nuclear Power by Angelo and Buden


But why choose nuclear thermal rockets—and nuclear propulsion in general—over chemical propulsion technology, which has been used for carrying payload from earth to space for over 50 years? The answer lies in the tremendous energy density present in nuclear power, and its inherent flexibility in application. NTRs are able to heat any propellant that is pushed through its core, unlike chemical rockets that must rely on the combustion of propellant for energy transfer. Because of this feature, NTRs can heat and expel the most efficient propellant possible, which is hydrogen gas, allowing for a large reduction in the overall mass that must be carried from earth’s surface to orbit.

In addition, all nuclear propulsion methods are inherently capable of providing long-term electricity production. Bimodal NTRs (BNTRs) can accomplish this by coupling a dynamic power conversion system to the reactor system. These systems are designed to run an additional coolant through selected channels in the reactor core, spinning a turbine, and producing electricity. Unlike solar power, nuclear power can operate independent of its location and orientation in space, providing electricity for energy intensive life support systems and scientific equipment.

Humanity’s route to the solar system

Nuclear power offers an unmatched capability for producing the massive amounts of energy required to travel in and out of the gravity wells of our solar system. Whether nuclear power is applied as a means of heating a propellant, as in nuclear thermal propulsion—or as a generator of electricity, as in nuclear electric propulsion—nuclear power stands as humanity’s route to the solar system.



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 the American Nuclear Society.

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

Part II: Electric propulsion and fission power generation in space

(Part I, “Space nuclear reactor safety,” is here)

By Wesley Deason

Ever since man set foot on Earth’s moon, explorers have envisioned traveling out of Earth’s orbit and into space beyond. To do so, however, will require a propulsion device capable of traveling farther than any used before. These devices will be powered by nuclear energy. In this post, I will discuss nuclear electric propulsion, one of the two primary nuclear propulsion concepts considered by engineers for near-term space travel. Nuclear thermal propulsion, the other primary concept, will be explored in a later post.

Electric propulsion, also commonly referred to as “ion thrust propulsion,” uses electrical power to accelerate ions to very high speeds to provide thrust for a spacecraft. Nuclear electric propulsion is electric propulsion whose power source is fission reactor based, or radioisotope decay based. Electric propulsion is not a new technology, and is well understood. Currently, solar powered electric propulsion devices are used at a small scale to keep satellites in their correct orbit. For operation at a larger scale, however, where much higher thrust values will be needed, or operation at a distance from the sun where solar power is incapable of providing the necessary energy intensity, nuclear electric propulsion will be required.

Aside from the ion thruster used to provide thrust for a nuclear electric propelled spacecraft, its other defining characteristic is the type of nuclear electric generator needed to provide power. Nuclear electric generators used for power production in space can come in many shapes and sizes, depending on the power requirements and spacecraft dimensions. For small power production needs in missions requiring a low thrust, radioisotope power systems can be used. In these systems, the radioactive decay heat from a radioisotope is converted to electricity through the use of a heat-to-work conversion device, or a heat engine. The most common of these are Radioisotope Thermoelectric Generators (RTGs) and Advanced Stirling Radioisotope Generators (ASRGs). You can find out more about these systems by reading my previous post on plutonium-based radioisotope power systems.

SNAP 10-A, fission-based space power system launched in 1965

For larger thrust requirements, however, fission-based power systems become a necessity. Missions requiring such high thrust will be manned and/or carry a large onboard capacity for conducting science. These mission requirements are also often outside the capability of chemical (or even nuclear thermal) propulsion. Individually, the previously stated requirements are not difficult to meet. For example, earth-based power reactors generate enough power to light a large city, but the thought of launching them into space to produce power is absurd. Alternatively, fission power systems have been proposed that are about the size of a small car, which is a relatively small payload to put into low earth orbit. Unfortunately, these systems can produce only a fraction of the electric power that could be produced by that same small car. The ideal space nuclear electric generator would meet both requirements of size and power. To evaluate competitive designs for nuclear electric propulsion systems, engineers seek the smallest system mass possible for a given power production level.


To meet these system requirements, engineers must consider different technologies from those used in earth-based nuclear reactors. For example, the first and only fission-based space power system to be flown by the United States, the SNAP-10A spacecraft, used thermoelectrics, which is the same power conversion technology used by RTGs to produce electricity. Thermoelectrics, however, while dependable, are very inefficient, and excess heat produced by the reactor must be rejected away from the spacecraft. In space, this heat rejection can only be in the form of radiative energy. For those unfamiliar with methods of heat transfer, radiative heat transfer is how heat lamps heat food at a local fast food restaurant, how heat is lost from a vacuum sealed Thermos, and even how the earth is heated by the sun. This may seem unintuitive at first but if you think about it, there are no lakes or rivers of water in space to sweep away excess heat like earth-based power systems. In space nuclear power systems, large panels are heated to high temperatures in order to reject this excess heat. Thus the size, and accordingly the temperature, of these radiator panels drive the power system to be as efficient and high temperature as possible.

Many technologies have been discussed as being capable of achieving such power production goals, with some being invented primarily for this purpose. One of the simpler systems may use a combination of helium and xenon gas as coolant, which can spin a turbine to produce electricity. This was the system designed and proposed for use in the Jupiter Icy Moons Orbiter (JIMO), a space exploration program under serious consideration only a few years ago. More complicated systems propose boiling potassium to spin a turbine, although the zero gravity environment of space makes the task more difficult to accomplish. Lastly, some propose suspending the fuel in a gaseous form, allowing it to flow through a magnetohydrodynamic generator (MHD), which uses the ionized fuel particles to produce electricity. The best way to explain an MHD generator is to think of it as a reverse ion thruster, where charged particles induce a current to produce electricity.

Prometheus nuclear electric Deep Space Vehicle, incorporating JIMO Mission Module

Nuclear electric propulsion has great potential. Its ability to provide propulsion to anywhere in the solar system makes it a viable competitor when the human race decides to explore beyond the gravity well of earth. Like most nuclear technologies, research will continue and technological advancements will continue to be made in the meantime.



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 the American Nuclear Society.


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.