On the Nuclear Imperative

Sometimes our concerns about the human future are eerily like those of our ancestors. Giancarlo Genta (Politecnico di Torino) likes to quote an ancient Assyrian tablet on the matter, one said to have been rendered around 2800 BC:

Our Earth is degenerate in these latter days; bribery and corruption are common; children no longer obey their parents; every man wants to write a book and the end of the world is evidently approaching.


Marvelous as it is, the quote may be apocryphal, as Genta noted in his recent talk at the deep space conference in Aosta. In fact, I suspect it is, unless the bit about every man wanting to write a book was literally written ‘every man wants to keep his household accounts on a clay tablet’ or some such. But whatever the case, the quote echoes similar sentiments found throughout history, thoughts that evoke a golden age when things were just plain better than they are in the present and the future did not seem so dark.

Which is not to say we don’t have serious problems that could limit our civilization, or even end it. The Assyrians didn’t have to cope with nuclear weaponry, or biotechnology, or the possibility of nanotech developing in unforeseen directions. Genta dismisses none of these problems, but his talk at Aosta made the point that immediate perils are not the best way to promote space exploration. If, in other words, we cite commodity shortages or climatic changes as a rationale for getting humans off-planet and thus saving the species, we run the risk of losing credibility if our predictions are wrong.

Image: Giancarlo Genta’s Lonely Minds in the Universe (2007) examines our view of extraterrestrial life as it has evolved over time, with a focus on historical accuracy that illuminates the discussion and challenges many stereotypes.

Here’s a bit of Dr. Genta’s paper from the Aosta proceedings:

While it is certainly true that there are limits to the development of the human species on this planet, these limits seem to be still quite far from being reached. The proposers of the space option should thus promote it as a great opportunity to improve the standard of living of all human beings and not as a last resort attempt to avoid an impending ruin.

We can make a case for improving living standards through space exploration, but only if we take the necessary next steps. Today, our launch technologies are essentially half a century old, with only minor improvements along the way. In our attempt to bootstrap a spacefaring civilization, we need to be thinking long-term and improving our ways of getting out of Earth’s gravity well. On this score, Genta is a proponent of nuclear energy, believing it alone will allow our emergence as a true spacefaring species. Here he speaks from his perspective as a deeply practical mechanical engineer:

The use of nuclear energy for space propulsion in Earth orbit and beyond is just a matter of political will and only marginally of technology: sure, technological advances are required, but after more than 50 years of theoretical studies the ideas are clear and what are still needed are just details. Nuclear-thermal propulsion was demonstrated on the ground in the 1970s and could be used by now for deep-space propulsion. It is true that the performance of such systems can be improved well beyond those demonstrated up to now, but what we have could allow anyway a large improvement if compared with chemical propulsion.

But transitioning to next generation technologies — or catching up in terms of a developing but unused capability — is a demanding process. More on this:

What we really need is to have nuclear powered spacecraft for interplanetary missions, even if their performance were only marginally better than those of chemical propulsion: we need to gain experience in building and operating nuclear systems in space and to make people used to this technology. Performance of nuclear thermal propulsion will improve in due course, but if we wait to start until improved systems are available, everything will be delayed indefinitely.


Image: Giancarlo Genta discussing Moon-rover prototypes at the Mechatronics Laboratory of the Politecnico di Torino in Verrès, a lovely town in the Aosta Valley. Photo by Roman Kezerashvili (New York City College of Technology).

Anyone advocating nuclear propulsion in today’s climate of opinion is sure to have a fight on his hands, but Genta believes the time for this fight is propitious. We’re already seeing signs that in the power industry, nuclear options are making a comeback in terms of public acceptance — the phrase ‘nuclear renaissance’ is in the air in some quarters, indicating that we may be ready to move past the era of kneejerk rejection of the nuclear idea. Funding remains a problem, but we come back again to having to sell our future in space one mission at a time, a laborious task but an essential one.

The space option is a long-term perspective, which will naturally be implemented in due time. Perhaps it is hard to accept that progress toward space must be done step by step, but trying shortcuts may be dangerous. In a situation of scarce funds a hard competition between missions and technologies should be avoided. The efforts should be concentrated in areas that may prove to be enabling technologies, even if this may result in postponing some important scientific results.

There is no more important enabling technology than one that would get us to low-Earth orbit cheaply. Genta noted the space elevator concept in his talk but expressed concerns about the size of the investment needed to build it. In any case, a space elevator raises its own safety concerns. He sees nuclear technology as an achievable solution to the low-Earth orbit problem that should not be put off in hopes of a vastly more expensive future solution. Political will is a tricky thing to summon, but making a sustained, long-term case for space as a key player in our economic future may help overcome the obstacle.

The paper is Genta, “The Nuclear Renaissance: From the Power Industry to Space Exploration?” in Proceedings of the Sixth IAA Symposium on Realistic Near-Term Advanced Scientific Space Missions, Aosta Italy (2009), pp. 13-18. As with other papers from this conference, this one will appear at a future date in Acta Astronautica.


Cometary Catastrophe? Not So Fast…

Once again we’re asked to reconsider our views about the outer Solar System. In this case, the area in question is the Oort Cloud, which begins at roughly 1000 AU and continues, by some estimates, as far as three light years from the Sun. It’s a spherical cloud of comets, probably numbering in the billions of objects, most of which will never be observed because of their distance and faint signature.

Getting comets into the inner Solar System is necessary for closer observation, but it’s also risky for living beings. At least, that’s been the prevailing belief, given that comet collisions with Earth could theoretically produce extinction events. New research, however, has begun to challenge this view. Nathan Kaib (University of Washington) and doctoral adviser Thomas Quinn have developed computer models to study how comet clouds behave.


The simulations trace the evolution of comets over a 1.2 billion year period and allow the team to estimate the highest number of comets possible in the inner Oort Cloud. The team assumed the inner Oort Cloud as the source of long-period comets, those whose orbits take from 200 to tens of millions of years to make their way around the Sun (short-period comets originate in the Kuiper Belt — Halley’s comet is the most obvious example). 3200 long-period comets are known, with Hale-Bopp being the most recent.

Image: Comet Hale-Bopp photographed above above Val Parola Pass in Italy’s Dolomite Mountains. Credit: A. Dimai, (Col Druscie Obs.), AAC.

The outer Oort Cloud is also a source for long-period comets, but by working with the assumption that the inner Oort was the only source, Kaib and Quinn could estimate the highest possible number of comets in that inner Oort region. The results cause them to doubt the case for comets as causes of planetary extinctions.

In fact, says Quinn, no more than two or three comets would have struck the Earth during the most powerful comet ‘shower’ of the last 500 million years. Three impacts occurred nearly simultaneously about forty million years ago, resulting in a minor extinction event that may have been the result of a comet shower. That event, notes Kaib, was the most intense in the fossil record:

“That tells you that the most powerful comet showers caused minor extinctions and other showers should have been less severe, so comet showers are probably not likely causes of mass extinction events.”

The results show the power of Jupiter and Saturn’s gravitational fields, which can cause comets to be ejected into interstellar space or drawn in for collisions with the gas giants themselves. We saw evidence for the latter just last week with an evident cometary impact on Jupiter. Those comets that do make it through the gravitational screening of the gas giants do not, by this thinking, stand as the culprits for major mass extinctions.

If Kalb and Quinn are right, we can relax a bit about comets, while asteroids remain a potential threat to Earth. The team’s work also tells us something about how cometary orbits are disrupted. Long-period comets from the inner Oort, according to the simulations, can emerge in the inner system without the nudge of a neighboring star as it passes close by the Solar System. The inner Oort thus becomes a livelier place than previously imagined.

The paper is Kaib and Quinn, “Reassessing the Source of Long-Period Comets,” Science Express 29 July 2009 (abstract).

Related: Be aware of JPL’s new Asteroid Watch site, which also tracks comets, offering the latest research on near-Earth objects and news about NEO discoveries and flybys. A Twitter feed is also available, along with RSS options and a downloadable widget.


Enceladus: More Evidence of Liquid Water

I’m pushed for time this morning but do want to catch up with Cassini news, in particular the recent findings from Enceladus. The plumes of water vapor and ice particles erupting from the moon continue to capture the imagination. Cassini’s Ion and Neutral Mass Spectrometer was used during Enceladus flybys in July and October of 2008, with results just released in Nature. Out of all this we get this interesting find, as discussed by Hunter Waite (SwRI), who is lead scientist on the instrument involved:

“When Cassini flew through the plume erupting from Enceladus on October 8 of last year, our spectrometer was able to sniff out many complex chemicals, including organic ones, in the vapor and icy particles. One of the chemicals definitively identified was ammonia.”

William McKinnon (Washington University, St. Louis) calls ammonia “sort of a holy grail for icy volcanism,” noting that this is our first unambiguous detection of ammonia on an icy satellite of a giant planet. Finding it is intriguing because the presence of ammonia can keep water in liquid form at lower temperatures than we might expect.


Image: The Cassini spacecraft acquired this view of Enceladus just after the spacecraft passed within 25 kilometers (15 miles) of the surface on Oct. 9, 2008. Remarkably, only a handful of craters are visible in this view, indicating the relatively young age of this surface. Credit: NASA/JPL/Space Science Institute.

This JPL news release talks about ammonia as a natural antifreeze, and notes that with the right amount of ammonia, water can exist in a liquid state at temperatures as low as 176 Kelvin (-143 degrees Fahrenheit). Waite again:

“Given that temperatures in excess of 180 Kelvin (-136 degrees Fahrenheit) have been measured near the fractures on Enceladus where the jets emanate, we think we have an excellent argument for a liquid water interior.”

Cassini has already racked up five Enceladus flybys, with two more scheduled for November and another two in the spring of 2010. We know we have organics and a strong possibility of liquid water within the Saturnian moon. The amount of water involved is still unknown, but potentially habitable environments are still in play.

The paper is Waite et al., “Liquid water on Enceladus from observations of ammonia and 40Ar in the plume,” Nature 460 (23 July 2009), pp. 487-490 (abstract).


Notes & Queries 7/27/09

Tau Zero in the Press

Edinburgh-based journalist Ian Brown offers up an overview of interstellar issues in Scotland’s Sunday Herald. The core of the story is an interview Brown conducted with Tau Zero founder Marc Millis, who as Brown notes was formerly the manager of NASA’s Breakthrough Propulsion Physics project. The Tau Zero Foundation grows out of that work (though it retains no NASA connection), and it’s good to see us getting publicity in a popular science story that captures TZF’s imaginative spirit while avoiding sensationalism. Brown calls us “a grass-roots network of physicists, mathematicians, engineers and science journalists,” an accurate description.

Here’s a snippet quoting Millis on the nature of interstellar striving:

“How much we accomplish is, of course, tied to the resources we acquire. The focus will be on making incremental progress rather than big projects.”

As a physicist, he knows the sheer immensity of the challenge. Many scientists believe we will never reach the stars. With interstellar propulsion we’re not limited by our own technological abilities but also by fundamental laws of physics such as the speed of light. Does Millis really believe we can do it?

“Since the survival of humanity is at stake, I think it would be irresponsible not to try,” he says.

“Just by showing humanity how hard it is to escape our precarious existence on this one small planet, we hope to cause the ripple effect of making people behave more peacefully and environmentally responsibly. If these efforts do lead to practical interstellar flight, we will have ensured humanity’s survival.”

Right now we’re talking about the Moon and Mars, spurred on by the recent Apollo 11 anniversary, and by Buzz Aldrin’s commitment to push on to the Red Planet. But the discovery of an Earth-like world around another star will surely capture the public imagination, as it did briefly when Gliese 581 c was (evidently erroneously) thought to be habitable. A blue and green world around a Sun-like star will pump up that interest again, and to greater effect. How to harness the enthusiasm? Imagination coupled with scientific rigor is, Millis believes, the key to progress and a central tenet of Tau Zero.

Physics as Cultural Resource

The Internet is the auto-didact’s dream, a chance to bone up on subjects grown musty with disuse or to master entirely new fields. How energizing, then, to see that Microsoft’s Bill Gates is putting classic physics lectures by Richard Feynman onto the Net, using part of his abundant fortune to acquire the rights and develop interactive software to show off the material to good effect. From a New York Times story discussing the site:

When it is completed it will offer searchable transcripts from the lectures as well as commentary from well-known physicists. The site also offers individual viewers the ability to annotate and take notes. Currently, the first lecture has an “extras” features on the right side of the Web page which includes interactive animations and other components.


Feynman had a remarkable gift for making a subject entertaining while not giving up an inch on the rigor necessary to understand it. That’s no small feat, and a rarity on most campuses today. Being accessible doesn’t change the fact that the intellectual bar needs to be set high enough so as not to over-simplify. I’ve listened to a number of Feynman lectures (made available through Audible.com) on my evening walks and continue to learn from the man despite the sometimes murky audio. The question now is, what other great lecturers can we likewise bring to the public?

The Thousand Year Disk

Those of us concerned with data preservation note with alarm that all that good content we’ve locked up on CDs and DVDs has a vanishingly short lifetime. But a Utah firm called Millenniata is, according to the Long Now Blog, about to begin offering DVDs that can protect their data for a thousand years. Capable of being read by a standard DVD drive, the disks are made of proprietary materials the company claims will survive the millennium intact.

Long Now has the necessary follow-up point: It’s one thing to have an imperishable disk, but how do we know that, after a thousand years, the technology to read it will be readily available? Comments the site: “A Millennial DVD player would be a vast departure from the current crop of devices that barely last beyond their one-year warranties.” True, but the short-term is also significant for journalists, and I notice the Millenniata site (www.millenniata.com) has been down since I read this story.


Galactic Life in Context

Does complex life emerge at a gradual, uniform rate? If so, we can come up with one answer to the Fermi paradox: We have not detected signs of extraterrestrial life because the time needed for complex life to appear generally exceeds the life of a star on the main sequence. But the assumption that intelligence appears over time with a gradual inevitability — a key tenet of work by Brandon Carter, Frank Tipler and others in the 1980s, may not in fact be true. Solar system-wide events connect life with its stellar environment, while galaxy-wide events provide yet another context.

Punctuated Evolution Among the Stars

Milan ?irkovi? (Astronomical Observatory, Belgrade) and colleagues have much to say about this in a new paper in Astrobiology. It’s a rich treatment of our older assumptions and newer thinking about punctuated evolution, the idea that life actually evolves in spasms rather than smooth ascents. Species remain relatively stable for long periods but endure sudden changes that can create evolutionary innovation. The paper relies not only on studying the frequency of extinction events on Earth but also on the idea that a system of stars interacts strongly with its galactic environment.

Here the theories abound. Consider just one of the discontinuities that could affect life on a galaxy-wide level. Gamma-ray bursts (GRBs) caused by merging neutron stars or collapsing, supermassive stars could cause massive extinctions. In one calculation, the collapse of ? Carinae could deposit in Earth’s upper atmosphere energy equal to the simultaneous explosion of one kiloton nuclear bombs per square kilometer over the surface of the hemisphere facing the blast. Other calculations show galactic GRBs being fatal to eukaryotes up to a distance of 14,000 parsecs, creating a vast ‘zone of lethality.’

But such events change over time. From the paper:

Since the regulation mechanism exhibits secular evolution, with the rate of catastrophic events decreasing with time, at some point the astrobiological evolution of the Galaxy will experience a change of regime. When the rate of catastrophic events is high, there is a sort of quasi-equilibrium state between the natural tendency of life to spread, diversify, and complexify, and the rate of destruction and extinctions. When the rate becomes lower than some threshold value, intelligent and space-faring species could arise in the interval between the two extinctions and make themselves immune (presumably through space-faring technology) to further extinctions.

A Broader View of Catastrophe and Change

But GRBs are only one possible mechanism illustrating a star system’s dependence on the galaxy around it. ?irkovi? and team list several others:

  • Cometary impact-causing ‘galactic tides’
  • Neutrino irradiation
  • Clumpy cold dark matter
  • Climate changes induced by spiral arm crossings

All of these have been studied in recent astrophysical literature. And in focusing on them, this paper homes in on a crucial point. We’re beginning to realize that it’s unrealistic to restrict the study of astrobiology to a closed system involving a single star and its planets. The Galactic Habitable Zone (GHZ), first introduced by Guillermo Gonzalez and colleagues in 2001 and followed up by Charles Lineweaver, has come into its own. And as ?irkovi? notes:

The GHZ constitutes an annular ring several kpc wide, comprising the solar circle at a galactocentric distance of 8.5 kpc; and, although its definition does not mention intelligent beings, it should clearly be the main target of SETI studies. In both astrobiology and Earth sciences, a paradigm shift toward an interconnected, complex view of our planet has already been in place for quite some time in both empirical and theoretical work…

A Galactic Phase Transition

The paper goes on to speculate that the factors above could each play a role in creating an astrobiological phase transition within the galaxy that indicates we may, contrary to common assumption, be living in a galaxy aswarm with intelligence, but most of it at stages of evolution not terribly different from our own. This would be the result of what ?irkovi? calls ‘galactic neocatastrophism,’ a galactic punctuated evolution. And it leads to the conclusion that there is no reason to reject the existence of extraterrestrial intelligence in the Milky Way.

A rare Earth? Maybe only in time. It’s possible we’re just early for the dance.

The paper is ?irkovi? et al., “Galactic Punctuated Equilibrium: How to Undermine Carter’s Anthropic Argument in Astrobiology,” Astrobiology Vol. 9, No. 5 (2009), pp. 491-501 (abstract). For more on the GHZ, ?irkovi? points to the excellent review of recent thinking on the galactic habitable zone in Gonzalez, “Habitable Zones in the Universe,” Origins of Life and Evolution of Biospheres 35 (2005), pp. 555-606.