It’s fascinating to watch the development of online preprint services from curiosity (which is what the arXiv site was when Paul Ginsparg developed it in 1991) to today’s e-print options, hosted at Cornell and with mirrors not just at the original Los Alamos National Laboratory site but all around the world. Then, too, the arXiv is changing in character, becoming an early forum for discussion and debate, as witness Ian Crawford’s comments on Jean Schneider’s Astrobiology paper. We looked at Crawford’s criticisms of Schneider yesterday. Today we examine Schneider’s response, likewise a preprint, and published online in a fast-paced digital dialogue.
Schneider (Paris Observatory) focuses here on nuclear fusion and antimatter by way of making the case that interstellar flight will be a long and incredibly difficult undertaking. A bit of context: Schneider’s real point in the original Astrobiology piece wasn’t to offer a critique of interstellar flight ideas, but to call attention to the gap that will occur after we have made the first detection of biosignatures on exoplanets. We’ll have evidence of life when that occurs, but it may be centuries, in Schneider’s view, before we know what that life looks like, because unlike relatively nearby places like Mars, we’ll find it a monumental undertaking to send a probe to an exoplanet.
Antimatter and Its Dilemmas
Crawford, of course, questioned whether interstellar flight was as difficult to imagine as Schneider believed, and the two remain on separate paths. Schneider’s concerns about antimatter are solid: He’s looking at what it would take to produce the antimatter needed to power up a 100-ton spacecraft, and finds that the vehicle would require 1027 erg at 0.1c, the velocity Crawford uses as a base. And the problem here is daunting, for the total energy needed to produce the requisite antimatter is 200 terawatts over ten years of continuous production. Today’s total instantaneous energy production on Earth is about 20 terawatts.
But the problem gets trickier still. Schneider doesn’t go into antimatter storage, but listen to what Frank Close says about the issue in his book Antimatter (Oxford University Press, 2009):
The propaganda for antimatter power is that to take a manned spaceship to Mars, the three tonnes of chemical propellant could be reduced to less than a hundred of a gram of antimatter, the mass of a grain of rice….However, the promoters of such hopes say less about the weight of the technology that will be required for containing the antimatter. Large numbers of antiprotons or positrons imply a large concentration of electric charge, which has to be contained. To store even one millionth of the amount needed for a Mars trip would require tons of electric force pushing on the walls of the fuel tank…
And so on. These are major issues. We’ve been able to store about a million anti-protons at once thus far, which seems like a big number, but Close points out that it’s a billion billion times smaller than what you would need for a gram. Be aware, too, that since the discovery of the anti-proton in 1955, the total of our anti-proton production is less than a millionth of a gram. None of this is to rule out future advances in antimatter production or collection (we’ve looked at James Bickford’s ideas on antimatter harvesting from space before in these pages). But you can see why Schneider is a skeptic about antimatter as rocket fuel, at least as of now.
Fusion: A Promise Deferred
The fusion argument divides between those who see the bright promise vs. those who see the frustrating history of the idea. Schneider notes how conceptually simple fusion is, but the simple fact is that seventy years after the invention of the basic concept of deuterium/tritium fusion, we still can’t make it work in any stable production facility. He continues:
The ITER fusion facility is not expected to achieve production energy at a demonstration level before 2030, that is, almost a century after the nuclear fusion concept was invented. The author correctly mentions the developments in miniaturization. As an example, he cites the National Ignition Facility (a similar, less advanced project called « Mega Joule Laser » exists in Europe). But this facility, with all its control and cooling systems, is presently quite a non-miniaturized building. In spite of the fact that presently it will only provide impulsive (non continuous) fusion energy, presently at a slow rate of one impulse per hour, one can imagine that in the future these impulses can be accumulated to provide a sufficient acceleration to the spacecraft. But it requires an initial energy of a few mega joules per 1 nanosecond impulse, and in the spacecraft this energy must come from somewhere.
Schneider is open to ideas, and notes how presumptuous it is to predict what will happen after a century or more, calling for the debate over these issues to go on. In certain respects, I don’t find his views as different from Crawford’s as they might at first appear. While he correctly cites the interstellar dust problem and the danger of high-speed, debilitating collisions with particles, both authors are aware of how much we have to learn about the interstellar medium. As we start accumulating the data we’ll need, we have to take risk evaluation into account.
Calculating the Odds
How? Current missions may launch with somewhere between 1 and 0.1 percent chance of failure, but we already know that given an unforeseen breakthrough, interstellar missions will be incredibly costly, at least in the early going. Reducing the risk is thus mandatory (Schneider would like to see it go to something like one in several thousand), and doing that increases the cost. I don’t imagine Crawford would argue with this, though the two disagree on timelines, with Crawford more optimistic about the near-term, and Schneider arguing that centuries will likely pass before we can speak about a true interstellar probe. Referring to the Technology Readiness Level classification, a key part of risk evaluation, he has this to say:
For interstellar travel, we are at best at level 1 (or even 0.5), while a « Flight proven » mission will realistically require first a precursor mission to secure the technological concept, including shielding mechanisms, at say 500 to 1000 Astronomical Units. As a comparison, I can take the nulling interferometry concept for the infrared detection of exo-Earths. It was invented in the late 70s (Bracewell 1978) and is still not foreseen for a launch by ESA and NASA before 2030, that is, 50 years after the invention of the concept for a mission at least 100 times easier and cheaper than interstellar travel.
The Millennial View
Those with a near-term bent will see a profound gulf between these two views, but I think those of us who are looking long-term will find the debate converging on common themes. Neither of these men rules out interstellar flight on scientific grounds, and both are aware of the huge difficulties that must be overcome for it to occur. If you are an optimist over a given technology, you might believe, for example, that fusion will succeed in revolutionizing space propulsion and thus pave the way for an early mission. But if you’re content with the idea that interstellar flight is going to occur, the question of just which century it occurs in carries a bit less weight.
I don’t, then, find this statement off-putting:
To deserve an interstellar travel mission, an exoplanet will require very solid clues of biosignatures (to quote Willy Benz, « Extraordinary claims require exceptional proofs »). I hope that current radial velocity monitorings will discover the existence of habitable planets around Alpha Cen A or B, and that in the coming decades these planets will reveal solid biosignatures. But what if the nearest planet with credible biosignatures lies at 10 pc? Even at a speed of 0.1c, the travel will last 400 years.
True enough, and if biosignatures do turn out to be this rare, we’ll have to re-evaluate our options (and our motives) if we don’t develop the technologies to make the journey faster than 0.1c. But the issue isn’t whether we can cross ten parsecs. It’s whether interstellar flight as an idea has any merit, and it remains to be seen what mission concepts might one day develop to much closer stars. We’re in that necessary period of evaluation that is, as we speak, opening up new solar systems on a regular basis, and what we find in them will doubtless affect the motivations for a mission.
I’m with Crawford in noting the large number of propulsion options being investigated in the literature, but like Schneider, I wouldn’t want to single out a particular one as the most likely to succeed (although I admit to a continuing interest in beamed lightsail concepts). It’s too early for that, and the excitement of this era in our investigations is precisely that we’re at the entry level, scrambling for ideas, weighing technologies, speculating about future breakthroughs like nanotech that could revolutionize the game. Let the debate continue indeed, and let’s hope we can keep it at the lively level these two scientists have established through their dialogue.
The paper is Schneider, “Reply to ‘A Comment on ‘”The Far Future of Exoplanet Direct Characterization” – the Case for Interstellar Space Probes,'” accepted at Astrobiology (preprint).