Falling Space Reactors: Assessing the Risk

A new NASA report examines various scenarios in which nuclear reactors that are used to power spacecraft could accidentally reenter the Earth’s atmosphere.

“There are a number of types of reentry events that can potentially occur with missions containing fission reactors. Each type of reentry event can produce a variety of possible adverse environments for the fission reactor,” the report said.

The postulated scenarios include accidental reentry upon launch, reentry from orbit, and reentry during Earth flyby.

“There are three potential outcomes for a fission reactor in a reentry scenario,” the report explains. “First, the fission reactor can burn up in the atmosphere due to the aerothermal loads imparted to it during reentry. Second, it can survive the reentry and impact the Earth’s surface with or without additional spacecraft components. Finally, it can break apart during reentry, but its various components survive reentry and impact the Earth’s surface (a scattered reentry).”

See Fission Reactor Inadvertent Reentry: A Report to the Nuclear Power & Propulsion Technical Discipline Team, by Allen Camp et al, NASA/CR−2019-220397, August 2019.

A conference on “Nuclear Energy in Space: Nonproliferation Risks and Solutions” will be held in Washington DC on October 17 that will focus on the anticipated use of highly enriched uranium in space nuclear reactors, and the feasibility of using low enriched uranium instead. The conference is sponsored by the Nuclear Proliferation Prevention Project (NPPP) at the University of Texas at Austin.

Several previous technical analyses have concluded that use of low enriched uranium in space reactors is in fact feasible, but that it would probably require a reactor of significantly larger mass.

See “White Paper – Use of LEU for a Space Reactor,” August 2017 and “Consideration of Low Enriched Uranium Space Reactors” by David Lee Black, July 2018.

Garwin on Serendipity and Solar Sailing

In July, the Planetary Society’s Lightsail 2 spacecraft demonstrated the viability of “solar sailing,” becoming “the first spacecraft in Earth orbit propelled solely by sunlight.”

But the practicality of solar sailing was first described six decades earlier by physicist Richard L. Garwin.

“It is difficult to exaggerate the importance of solar radiation pressure for the propulsion of satellites or space ships within the solar system,” he wrote in the Journal of the American Rocket Society in March 1958, when he was 30 years old. “Although the acceleration is numerically small, the velocity changes in reasonable times by significant amounts.”

This week, Garwin reflected on this and other episodes in his lifetime of problem solving and technical innovation. He spoke to post-doctoral researchers from the Harvard Physics Department. See Serendipities from Long Ago by Richard L. Garwin, keynote address, September 11, 2019.

How did he come up with solar sailing?

“As physicists do, I had been thinking about how things worked or could work and learned about radiation pressure, as did everybody in high school,” he said.

Not everyone grasped the concept immediately, Garwin noted.

“I recall that when the Chief Scientist of the U.S. Air Force was asked about this proposal at a press conference, he explained that even if it would work, it could only be used for going outward beyond Earth orbit around the Sun and not for going inward, because radiation pressure was radially outward from the Sun.”

“What he missed, of course, was that the fact that the sail was in Earth orbit or, for that matter solar orbit, meant that a reflective sail could be angled so as to provide a force perpendicular to the sail, that would have a component either along the velocity vector or in the opposite direction, so that the orbital velocity component could be increased or reduced; thus, the SS could either gain or lose energy and so spiral in or out from the Sun, or in Earth orbit.”

A New Category of Presidential Directives

President Trump created an entire new category of presidential directives to present his guidance for the U.S. space program. The new Space Policy Directive 1 was signed on December 11 and published in the Federal Register today.

“President Donald Trump is sending astronauts back to the Moon,” enthused NASA public affairs in a news release.

But the directive itself does no such thing. Instead, it makes modest editorial adjustments to the 2010 National Space Policy that was issued by President Obama and adopted in Presidential Decision Directive 4.

Obama’s policy had stated:

“Set far-reaching exploration milestones. By 2025, begin crewed missions beyond the moon, including sending humans to an asteroid. By the mid-2030s, send humans to orbit Mars and return them safely to Earth;”

Trump’s new SPD-1 orders the deletion and replacement of that one paragraph with the following text:

“Lead an innovative and sustainable program of exploration with commercial and international partners to enable human expansion across the solar system and to bring back to Earth new knowledge and opportunities. Beginning with missions beyond low-Earth orbit, the United States will lead the return of humans to the Moon for long-term exploration and utilization, followed by human missions to Mars and other destinations;”

And that’s it. At a White House signing ceremony on December 11, President Trump said grandly that “This directive will ensure America’s space program once again leads and inspires all of humanity.”

But it’s hard to see how that could be so. The Trump directive does not (and cannot) allocate any new resources to support a return to the Moon, and it does not modify existing authorities or current legislative proposals.

Interestingly, it also does not modify the many other provisions of Obama’s 14-page space policy, including requirements “to enhance U.S. global climate change research” and “climate monitoring.” Unless and until they are modified or revoked, those provisions remain in effect.

Pluto Flyby Mission Powered by Plutonium

News of the Earth these days is such that one welcomes news from elsewhere, especially when it concerns a prospect as spectacular as the impending flyby of Pluto by the NASA spacecraft New Horizons that will take place on July 14.

In reality, of course, New Horizons also represents news from Earth, having been built by humans and launched from Cape Canaveral in January 2006. Moreover, the New Horizons probe is not simply a technological artifact; it is the result of a political process and a policy debate. At issue were not only the parameters of the mission — its scope, timing, budget, and so on — but also the fact it uses a nuclear power source fueled, appropriately if controversially, by plutonium.

The plutonium-238 isotope used by New Horizons is an exceptionally hazardous material that is dangerous to produce, manufacture into suitable form, handle, transport and launch. The hazards are sufficient, in the eyes of some, to preclude its use altogether.

NASA and Department of Energy engineers did not dismiss public concerns about the safety of plutonium-fueled power sources, but they argued that the risks could be mitigated to an acceptably low level by proper design.

“Safety was the principal design driver for the [plutonium-fueled General-Purpose Heat Source used aboard New Horizons],” according to a 2006 retrospective account of its development. “The main safety objective was to keep the fuel contained or immobilized to prevent inhalation or ingestion by humans.” See “Mission of Daring: The General-Purpose Heath Source Radioisotope Thermoelectric Generator” by Gary L. Bennett, et al.

In effect, the design of the plutonium power source was predicated on the assumption that a launch accident or other mishap would occur, and that any resulting health and safety impact had to be minimized. Simulations were performed to validate the design, but fortunately no real-world test of the safety of the device under extreme conditions ever came to pass.

The GPHS plutonium power source has been used successfully on some of the boldest and most productive missions of space exploration ever undertaken, including Galileo, Ulysses, Cassini, and New Horizons.

For the most part, these missions were conducted with commendable openness, especially in earlier years. When one young critic raised questions about the use of plutonium power sources and the hazards of high-velocity Earth flybys in the Galileo mission prior to its 1989 launch, project manager John Casani of NASA Jet Propulsion Laboratory forthrightly invited him to come inspect the spacecraft in its clean room at JPL and to discuss the alternatives.

“Pluto is going to change us,” wrote analyst Dwayne Day last month, anticipating the possible consequences of the New Horizons mission for science, art, culture, politics, and space policy. See “Deep in space, corner of No and Where,” The Space Review, June 15, 2015.

NASA Releases Space Nuclear Power Study

NASA has released a long-awaited Nuclear Power Assessment Study that examines the prospects for the use of nuclear power in civilian space missions over the next 20 years.

The Study concludes that there is a continuing demand for radioisotope power systems, which have been used in deep space exploration for decades, but that there is no imminent requirement for a new fission reactor program.

The 177-page Study, prepared for NASA by Johns Hopkins University Applied Physics Laboratory, had been completed several months ago but was withheld from public release due to unspecified “security concerns,” according to Space News. Those concerns may have involved the discussion of the proposed use of highly enriched uranium as fuel for a space reactor, or the handling of plutonium-238 for radioisotope power sources.

Nuclear power can be enabling for a variety of space missions because it offers high power density in compact, rugged form. Radioisotope power sources (in which the natural heat of decay is converted into electricity) have contributed to some of the U.S. space program’s greatest achievements, including the Voyager I and II probes to the outer solar system and beyond. But development of nuclear reactor technology for use in space has been dogged by a repeated series of false starts in which anticipated mission requirements failed to materialize.

“The United States has spent billions of dollars on space reactor programs, which have resulted in only one flight of an FPS [fission power source],” the new NASA report noted. That was the 1965 launch of the SNAP 10-A reactor on the SNAPSHOT mission. It had an electrical failure after a month’s operation and “it remains in a 1300-km altitude, ‘nuclear-safe’ orbit, although debris-shedding events of some level may have occurred,” the report said.

The development and use of space nuclear power raises potential environmental safety and public health issues. As a result, the NASA report said, “it may be prudent to build in more time in the development schedule for the first launch of a new space reactor. Public interest would likely be large, and it is possible that opposition could be substantial.”

In any case, specific presidential approval is required for the launch of a nuclear power source into space, pursuant to Presidential Directive 25 of 1977.

“For any U.S. space mission involving the use of RPS [radioisotope power sources], radioisotope heating units, nuclear reactors, or a major nuclear source, launch approval must be obtained from the Office of the President,” the report noted.

Wanted: Astronomer with Top Secret Clearance

NASA’s orbiting James Webb Space Telescope will be “the premier observatory of the next decade, serving thousands of astronomers worldwide, and studying every phase in the history of our Universe, ranging from the first luminous glows after the Big Bang, to the formation of solar systems capable of supporting life on planets like Earth, to the evolution of our own Solar System.”

So why does its Director need to have a Top Secret/SCI security clearance, as specified in the job description posted last month on USA Jobs?

Clearly, the secrets of the universe do not lend themselves to, or require, national security classification controls, let alone non-disclosure agreements or polygraph testing.

But in practice, the civilian space program intersects the national security space program at multiple points, and former CIA analyst Allen Thomson suggested that the future Webb Director might need a Top Secret intelligence clearance in order to engage with the National Reconnaissance Office on space technology and operations, for example.

The Webb Space Telescope “will complement and extend the discoveries of the Hubble Space Telescope, with longer wavelength coverage and greatly improved sensitivity,” according to NASA. “The longer wavelengths enable the Webb telescope to look much closer to the beginning of time and to hunt for the unobserved formation of the first galaxies, as well as to look inside dust clouds where stars and planetary systems are forming today.”

The Webb Telescope has a projected launch date in 2018.