Remembering an Astronautical Pioneer

by Claudio Maccone

Physicist Les Shepherd, whose funeral is today, left friends throughout the astronautical community. Claudio Maccone, who worked with Shepherd on many occasions, was quick to offer his recollections of this remarkable man whose standards of excellence and unflagging support helped many young scientists as they embarked on careers in space science.

A young guy (44 years old, i.e. “young” by IAA standards) joins the IAA Interstellar Space Exploration Committee (ISEC) headed by Les Shepherd and Giovanni Vulpetti: that happened at the World Space Congress in Washington, D.C., USA, also known as the 43rd IAC, August 28 – September 5, 1992).

I was then working at Alenia Spazio SpA in Torino (Turin), Italy, and I had this secret love for future interstellar space missions (“secret” since at my space company nobody was interested, of course). So, I consulted with my good old friend and “teacher” (he is senior than I) Giovanni Vulpetti, who was in a similar position at Telespazio in Rome as I was at Alenia Spazio in Turin (the two space companies were then “rival” in Italy, trying to get the same funds from both ESA and ASI, and sometimes from NASA also), which added “risk” to our mutual conversations. Giovanni said: “If you come to Washington D.C. at your expenses (Alenia would never have covered a mission for me to deal with interstellar exploration) then I will introduce you to Dr. Leslie Shepherd, a physicist of the highest distinction and Chair of the IAA Interstellar Exploration Committee (ISEC). At that time I had just run in Turin the first conference ever about the FOCAL space mission to 550 AU, held on June 18th, 1992, at the Politecnico di Torino (Figure 1), and so I decided to try.

I flew to Washington and met there with Les Shepherd for the first time. He was an aristocrat of physics, you know, but with a typical British sense of humor. After Giovanni introduced us to each other, at a point I felt proud enough to tell that I had done my Ph.D. at the Department of Mathematics of the University of London King’s College.

Dr. Leslie Shepherd replied “I’ll forgive you. I did my Ph.D. at University College in Gower Street!”, and of course I had to shut up, having completely forgotten the century-old rivalry between the two most renowned Colleges (so they say) on the University of London. Later I must have thought something like this: “Gee…, I just met him and immediately spoiled my reputation with this British Aristocrat of Science”. But that wasn’t the case, due to Les and Giovanni’s open-mindedness. Indeed, by the time of the 1997 IAF Congress in Torino (my town) I had already raised to the position of Secretary of ISEC, with Les Shepherd as Chair and Giovanni Vulpetti as Co-Chair. Unfortunately, ISEC was finally disbanded by the IAA in the restructuring of the IAA that took place around the year 2000, and so ISEC had to… be reinvented in other forms…

Distribution of ETCs at Various Rates of Occurrence

Average Interval Between ETC's Appearance (Yrs)Total Number of ETCsAv. Distance Between ETCs (L.Y.)No. of Stars Per ETC*
*Idealized Galaxy
Av. No. of ETCs within 1000 Yrs. of our AgeAv. Dist. (L.Y.) to Nearest ETC within 1000 Yrs. of our Age
1 billion

100 million

10 million

1 million










4 million

40 million

400 million

4 billion









8.25 billion

825 million

82.5 million

8.25 million
























Figure 1: First conference ever about the FOCAL space mission to 550 AU, held on June 18th, 1992, at the Politecnico di Torino (Turin, Italy).

The relativistic rocket equation discovered by Jakob Ackeret: something Les and I had in common…

When I was a physics student in Turin (1967-72) I had to pass an exam called “Meccanica Superiore” (Higher Mechanics). That was of course classical mechanics (as opposed to quantum mechanics) and the textbook was the one on which generations of physicists have learned the subject: Classical Mechanics by Herbert Goldstein (1950). On page 213 of that book, Exercise 10, was my favorite: the relativistic rocket equation discovered by the Swiss aeronautical engineer Jakob Ackeret (1898 – 1981) and published by him in German in Helvetica Physica Acta in April 1946. Les Shepherd once told me that he also had admired that equation from the very first time he saw it, since it plainly pointed out that Einstein’s (special) relativity was not just something for particle physicists only: it could equally well be applied to relativistic interstellar flight! Actually, Les told me that he was the one who had Ackeret’s 1946 paper translated from the German into English and published in the Journal of the British Interplanetary Society, Vol. 6 (1947), pages 116-123. Thank you, Les!

Anecdote #1: how a British aristocrat could be tough enough to reject a paper he did not like

Over the years after 1992 Les, Giovanni and I also became involved in selecting the papers admitted for presentation at the ISEC Session of the IAF Congress (as the current IAC were then called) to be held on that year. I remember (it must have been at the Paris Spring Meeting in 1994) that we were uncertain about admitting or rejecting a paper for the next IAF in Jerusalem (October 9–14, 1994) (at that time my position at Alenia had somewhat improved, and so I could finally attend the IAF in Jerusalem at the company expenses: wow !). Well, I must confess that I always tried to admit papers even if I did not like them: this was due to my “prejudice” that, sometimes, young people do not have the funds to register and travel to big conferences, and they get those funds from their companies only if their paper is accepted.

So, Les and I were politely arguing about one such case, and, using the “British polite persuading techniques” that I had learned at King’s College London during my Ph.D. I thought that I could convince Les to accept that paper. Well, I was wrong. “You gave me a whole bunch of good reasons” – Les said (I still remember his words) – “why this paper must be rejected” and he was able to reverse all my arguments one by one, until the paper was finally indeed rejected. Gee… that was the generation of Defenders of the British Empire that won World War Two, you know…

Anecdote #2: how a British aristocrat could be friendly enough to “teach” Fred Astaire’s words to an Italian newcomer…

Before you go on, please click here and listen to this song. Well, this is the famous Fred Astaire – in Follow The Fleet (1936) – “We Saw The Sea” is the song. Les Shepherd was then 18 years old and so it is quite natural that he came to learn by heart all the words in the song. But it was not so natural that he could teach them many years later to an Italian newcomer learning English like myself. You see, to understand the situation better, you must remember that the Internet has not been available all that long. So, once in the late 1990’s Les and I were talking at some coffee break and I think I told Les that, in order to improve my colloquial English, I liked to watch original Anglo-American movies in TV, since the songs there could not be translated into Italian and so were the authentic songs. I added that I particularly enjoyed the above song that you just heard a little ago. Well, I had hardly finished talking when Les immediately taught me the words. He knew them all by heart, unbelievable to me! What a good Friend he was!

Anecdote #3: how a British aristocrat and his Wife could be kind enough to forgive lower-class people about their lack of culture…

Finally here is a story about Les Shepherd, his wife and my mother. Well, I am not ashamed to confess that I come from very low-class people: my father was a Pirelli worker, my mother was a dress maker, and they did not speak any foreign language. But I was their only child, they loved me, and always supported my hunger for knowledge, until I got to King’s College London for a Ph.D. in mathematics.

But let me go back to Les and his wife. Once they had to call me on the phone from England, for some reason. I was not home, I was working at Alenia, and my mother answered the phone. She heard a lady speaking English, that she could not understand at all. At a certain point, however, my mother heard this Italian word “Pastore… Pastore… Pastore”, and yet that lady on the phone kept talking English again… When I came back home after work, my mother told about that strange call, and that was a mystery for me too.

Until I met with the Shepherds several months later and Mrs. Shepherd said to me: “You know, we tried to call you on the phone… [Shepherd is Pastore in Italian], but your mother did not understand and she put the phone down.


Les Shepherd, RIP

There are so many things to say about Les Shepherd, who died on Saturday, February 18, that I scarcely know where to begin. Born in 1918, Leslie Robert Shepherd was a key player in the creation of the International Astronautical Federation (IAF), becoming its third president in 1957 — this was at the 8th Congress in Barcelona just a week after the launch of Sputnik — and in 1962 he would be called upon to serve as its president for a second time. A specialist in nuclear fission who became deeply involved in nuclear reactor technology, Shepherd was one of the founding members of the International Academy of Astronautics (IAA), and served as chairman of the Interstellar Space Exploration Committee, which met for the first time at the 1984 IAF Congress in Lausanne, Switzerland.

The IAF Congress in Stockholm the following year was the scene of the first ISEC symposium on interstellar flight, one whose papers were subsequently collected in one of the famous ‘red cover’ issues of the Journal of the British Interplanetary Society. But let’s go back a bit. Those ‘red cover’ issues might never have occurred were it not for the labors of Shepherd, who was an early member and, in 1954, the successor to Arthur C. Clarke as chairman of the organization. It was in 1952 that Shepherd’s paper “Interstellar Flight” first appeared in JBIS, a wide-ranging look at the potential for deep space journeys and their enabling technologies. He served again as BIS chairman (later president) between 1957 and 1960 and from 1965 to 1967.

I first learned about Shepherd’s career through conversations with Giancarlo Genta at the Aosta interstellar conference several years ago, a fitting place given his long association and friendship with Italian interstellar luminaries Giovanni Vulpetti, Claudio Maccone and Genta himself. It was only later that I learned that Shepherd was the organizer and first chairman of the Aosta sessions, which continue today. When I heard of Shepherd’s death, I wrote to both Vulpetti and Maccone for their thoughts, because from everything I could determine, the “Interstellar Flight” paper was one of the earliest scientific studies on how we might reach the stars. I considered it a driver for future investigations and suspected that it had a powerful influence on the next generation of scientists.

Dr. Vulpetti was able to confirm the importance of the work, which looked at nuclear fission and fusion as well as ion propulsion and went on to ponder the possibilities of antimatter. The latter is significant given how little antimatter propulsion had been studied at the time. Vulpetti goes on to say:

It may be interesting to consider the state of particle physics in which the paper of 1952 was written… For understanding some key aspects, we have to remember (a) that Dr. Shepherd was born in 1918, (b) the positron was discovered in 1932 by C. D. Anderson (USA), and (c) the antiproton was found in 1955 by E. Segré (Italy). Thus, [Shepherd] was aware of the prediction of antiproton existence made by P.A.M. Dirac in the late 1920s and 1930, but – when he wrote the paper – there existed no experimental evidence about the antiproton. Nevertheless, Dr. Shepherd realized that the matter-antimatter annihilation might have the capability to give a spaceship a high enough speed to reach nearby stars. In other words, the concept of interstellar flight (by/for human beings) may go out from pure fantasy and (slowly) come into Science, simply because the Laws of Physics would, in principle, allow it! This fundamental concept of Astronautics was accepted by investigators in the subsequent three decades, and extended/generalized just before the end of the 2nd millennium.

That the 1952 paper was ground-breaking should not minimize the contribution Shepherd made in other papers, including his 1949 and later collaborations with rocket engineer Val Cleaver on the uses of atomic energy in rocket technologies that not only examined nuclear-thermal propulsion but looked as well at the kind of nuclear-electric schemes we are now seeing actively used in operations like the Dawn mission to Vesta and Ceres. But it may be that his thoughts on antimatter in “Interstellar Flight” were his most provocative at the time — he published that paper a year before Eugen Sänger’s famous paper on photon rockets. Vulpetti adds that his work revealed the “important relationship between the mass of an annihilation-based rocket spaceship and its payload mass. This was confirmed in the 1970s, and generalized in the 1980s.”

Reading through the pages of “Interstellar Flight” is a fascinating exercise. Shepherd must confront not only the immense distances involved but the fact that at the time, we had no knowledge of any planetary systems other than our own. Yet even in this very early era of astronautics he is thinking through the implications of future technologies, and I suspect that a few science fiction stories may also have crossed his path as he pondered the likelihood of ‘generation ships’ that could take thousands of people on such journeys. From the paper:

It is obvious that a vehicle carrying a colony of men to a new system should be a veritable Noah’s Ark. Many other creatures besides man might be needed to colonize the other world. Similarly, a wide range of flora would need to be carried. A very careful control of population would be required, particularly in view of the large number of generations involved. This would apply alike to humankind and all creatures transported. Life would go on in the vehicle in a closed cycle, it would be a completely self-contained world. For this and many other obvious reasons the vehicle would assume huge proportions: it would, in fact, be a very small planetoid, weighing perhaps a million tons excluding the dead weight of propellants and fuel. Even this would be pitifully small, but clever design might make it a sufficiently varied world to make living bearable.

So wide-ranging is the “Interstellar Flight” paper that it also takes in relativistic flight (here there is no option, he believes, but antimatter for propulsion) and time dilation as experienced by the crew, and goes from there to an examination of the interstellar medium and the problems it could present to such a fast-moving vehicle. Shepherd saw early on that collisions with dust particles and interstellar gas had to be considered if a vehicle were moving at a substantial percentage of the speed of light, working out that at velocities of 200,000 kilometers per second or more, the oncoming flux would be about 1011 times as intense as that found at the top of the Earth’s atmosphere. He saw that a considerable mass of material would have to shield the living quarters of any spacecraft moving at these velocities. Project Daedalus would, in the 1970s, re-examine the problem and consider various mechanisms for shielding its unmanned probe.

Leslie Shepherd had many collaborators and, as Dr. Vulpetti told me, encouraged wide studies in propulsion systems for deep space exploration (Vulpetti himself was one of Shepherd’s collaborators — I give the citation below). I thank Giovanni Vulpetti and Claudio Maccone for their thoughts on Shepherd. Thanks also to Kelvin Long and Robert Parkinson of the BIS for helpful background information. But we’re not done: Dr. Maccone was kind enough to send along some personal recollections of Shepherd and his work that I want to run tomorrow — I had originally intended to publish them today but they give such a good sense of the man that I want to run them in their entirety as a tribute to a great figure in astronautics whose loss is deeply felt.

Les Shepherd’s ground-breaking paper on interstellar propulsion is “Interstellar Flight,” JBIS, Vol. 11, 149-167, July 1952. His 1994 paper with Giovanni Vulpetti is “Operation of Low-Thrust Nuclear-Powered Propulsion Systems from Deep Gravitational Energy Wells,” IAA-94-A.4.1.654, IAF Congress, Jerusalem, October 1994.


Project Icarus: Contemplating Starship Design

Andreas Tziolas, current leader of Project Icarus, gave a lengthy interview recently to The Atlantic’s Ross Andersen, who writes about starship design in Project Icarus: Laying the Plans for Interstellar Travel. Icarus encounters continuing controversy over its name, despite the fact that the Icarus team has gone to some lengths to explain the choice. Tziolas notes the nod to Project Daedalus leader Alan Bond, who once referred to “the sons of Daedalus, perhaps an Icarus, that will have to come through and make this a much more feasible design.”

I like that sense of continuity — after all, Icarus is the follow-on to the British Interplanetary Society’s Project Daedalus of the 1970s, the first serious attempt to engineer a starship. I also appreciate the Icarus’ team’s imaginative re-casting of the Icarus myth, which imagines a chastened Icarus washed up on a desert island planning to forge wings out of new materials so he can make the attempt again. But what I always fall back on is this quote from Sir Arthur Eddington which, since I haven’t run it for two years, seems ready for a repeat appearance:

In ancient days two aviators procured to themselves wings. Daedalus flew safely through the middle air and was duly honoured on his landing. Icarus soared upwards to the sun till the wax melted which bound his wings and his flight ended in fiasco. The classical authorities tell us, of course, that he was only “doing a stunt”; but I prefer to think of him as the man who brought to light a serious constructional defect in the flying-machines of his day. So, too, in science. Cautious Daedalus will apply his theories where he feels confident they will safely go; but by his excess of caution their hidden weaknesses remain undiscovered. Icarus will strain his theories to the breaking-point till the weak joints gape. For the mere adventure? Perhaps partly, this is human nature. But if he is destined not yet to reach the sun and solve finally the riddle of its construction, we may at least hope to learn from his journey some hints to build a better machine.

I love the bit about straining theories to the breaking point, and also ‘hints to build a better machine.’ Anyway, those unfamiliar with the Icarus project can use the search engine here to find a surfeit of prior articles, or check the Icarus Interstellar site for still more. You’ll also get the basics from the Andersen interview, which goes into numerous issues, not least of which is propulsion. Tziolas notes that Project Icarus has focused on fusion, although ‘the flavor of fusion is still up for debate.’ Seen as an extension of the Daedalus design, fusion is a natural choice here, because what Icarus is attempting to do is to re-examine what Daedalus did in light of more modern developments. But the He3 demanded by Daedalus is a problem because it would involve a vast operation to harvest the He3 from a gas giant’s atmosphere.

Image: Icarus project leader Andreas Tziolas. Credit: Icarus Interstellar.

As the team studies fusion alternatives, other options persist. Beamed propulsion strikes me as a solid contender if you’re in the business of starship design in a world where sustained fusion has yet to be demonstrated in the laboratory, much less in the tremendously demanding environment of a spacecraft. We’ve already had solar sails deployed, the Japanese IKAROS being the pathfinder, and laboratory work has likewise demonstrated that beamed propulsion via microwave or laser can drive a sail. But beam divergence is a problem, which is why Robert Forward envisioned giant lenses in the outer system demanding a robust space manufacturing capability. So the dismaying truth is that at present, both the fusion and beamed sail options look to be not only beyond our engineering, but well beyond the wildest dreams of our budgets.

Where we are clearly making the most progress in interstellar terms is in the choice of a destination. Obviously, we lack a current target, but within the next two decades it is well within our capabilities to launch the kind of ‘planet-finder’ spacecraft that can not only home in on an Earth analogue around another star but also study its atmosphere. It would be all to the good if we found that blue and green world we’re hoping for orbiting a nearby system like Centauri B, but we’re going to be learning very soon (depending on what gets budgeted for and when) which nearby stars have planets that might be suitable targets. Icarus is not just looking for any old rock — the goal is to design a craft that could reach a world that could be habitable for humans.

Image: One vision of the Icarus craft, by the superb space artist Adrian Mann.

Why that criterion? Survival of the species is a serious interstellar motivation. Tziolas asks whether, if humanity becomes capable of going to the stars and chooses not to, it wouldn’t deserve a Darwin Award, the kind of achievement that marks its recipient as doomed. But motivations cover a wide range. I’ve written about the human urge to explore on many occasions, and Tziolas talks about pushing back technological boundaries as another prime driver. Hard problems, in other words, drive us to push the envelope in terms of solutions:

In order to achieve interstellar flight, you would have to develop very clean and renewable energy technologies, because for the crew, the ecosystem that you launch with is the ecosystem you’re going to have for at least a hundred years. With our current projections, we can’t get this kind of journey under a hundred years. So in developing the technologies that enable interstellar flight, you could serendipitously develop the technologies that could help clean up the earth, and power it with cheap energy. If you look toward the year 2100, and assume that the 100 Year Starship Study has been prolific, and that Project Icarus has been prolific, at a minimum we’d have break-even fusion, which would give us abundant clean energy for millennia. No more fossil fuels.

And let’s not forget ongoing miniaturization, also a prime player in any starship technology:

We’d also have developed nanotechnology to the point where any type of technology that you have right now, anything technology-based, will be able to function the same way it does now, but it won’t have any kind of footprint, it will only be a square centimeter in size. Some people have characterized that as “nano-magic,” because everything around you will appear magical. You wouldn’t be able to see the structures doing it, but there would be light coming out of the walls, screens that are suspended that you can move around any surface, sensors everywhere — everything would be extremely efficient.

This is a lively and informative interview, one that circles back to the need to drive a shift in cultural attitudes as a necessary part of any long-haul effort. Some of this is simply practical — the creative souls who volunteered their engineering and scientific skills on Daedalus are retiring and some have already passed away. A new design requires a new generation of interstellar engineers, one recruited from that subset of the population that continues to take the long view of history, acknowledging that without the early and incremental steps, a great result cannot occur.


FTL Neutrinos: Closing In on a Solution

The news that the faster-than-light neutrino results announced to such widespread interest by the OPERA collaboration have now been explained has been spreading irresistibly around the Internet. But the brief piece in ScienceInsider that broke the news was stretching a point with a lead reading “Error Undoes Faster-Then-Light Neutrino Results.” For when you read the story, you see that a fiber optic cable connection is a possible culprit, though as yet an unconfirmed one.

Sean Carroll (Caltech) blogged on Cosmic Variance that while he wanted to pass the news along, he was reserving judgment until a better-sourced statement came to hand. I’ve thought since the beginning that a systematic error would explain the ‘FTL neutrino’ story, but I still was waiting for something with more meat on it than the ScienceInsider news. It came later in the day with an official CERN news release, and this certainly bears quoting:

The OPERA collaboration has informed its funding agencies and host laboratories that it has identified two possible effects that could have an influence on its neutrino timing measurement. These both require further tests with a short pulsed beam.

So we have not just one but two possibilities here, both with ramifications for the neutrino timing measurements and both needing further testing. And let’s go on with the news release:

If confirmed, one would increase the size of the measured effect, the other would diminish it. The first possible effect concerns an oscillator used to provide the time stamps for GPS synchronizations. It could have led to an overestimate of the neutrino’s time of flight. The second concerns the optical fibre connector that brings the external GPS signal to the OPERA master clock, which may not have been functioning correctly when the measurements were taken. If this is the case, it could have led to an underestimate of the time of flight of the neutrinos. The potential extent of these two effects is being studied by the OPERA collaboration. New measurements with short pulsed beams are scheduled for May.

Image: Detectors of the OPERA (Oscillation Project with Emulsion-tRacking Apparatus) experiment at the Italian Gran Sasso underground laboratory. Credit: CERN/AFP/Getty Images.

We may well be closing on an explanation for a result many scientists had found inconceivable. Here’s a BBC story on the possibility of trouble with the oscillator and/or an issue with the optical fiber connection. We learn here that a new measurement of the neutrino velocity will be taken in 2012, taking advantage of international facilities ranging from CERN and the Gran Sasso laboratory in Italy to Fermilab and the Japanese T2K. The story quotes Alfons Weber (Oxford University), who is working on the Minos effort to study the neutrino measurements at Fermilab:

“I can say that Minos will quite definitely go ahead… We’ve already installed most of the equipment we need to make an accurate measurement. Even if Opera now publish that ‘yes, everything is fine’, we still want to make sure that we come up with a consistent, independent measurement, and I assume that the other experiments will go forward with this as well.”

So this is where we are: An anomalous and extremely controversial result is being subjected to a variety of tests to find out what caused it. If I were a betting man, I would put a great deal of money on the proposition that the FTL results will eventually be traced down to something as mundane as the optical fiber connector that is now the subject of so much attention. But we’ll know that when it happens, and this is the way science is supposed to work. OPERA conducted numerous measurements over a three year period before announcing the FTL result. Let’s now give the further work time to sort out what really happened so we can put this issue to rest.


M-Dwarfs: A New and Wider Habitable Zone

I want to work a new paper on red dwarf habitability in here because it fits in so well with yesterday’s discussion of the super-Earth GJ1214b. The latter orbits an M-dwarf in Ophiuchus that yields a hefty 1.4 percent transit depth, meaning scientists have a strong lightcurve to work with as they examine this potential ‘waterworld.’ In transit terms, red dwarfs, much smaller and cooler than the Sun, are compelling exoplanet hosts because any habitable worlds around them would orbit close to their star, making transits frequent.

When I first wrote about red dwarfs and habitability in my Centauri Dreams book, it was in connection with the possibilities around Proxima Centauri, but of course we can extend the discussion to M-dwarfs anywhere, this being the most common type of star in the galaxy (leaving brown dwarfs out of the equation until we have a better idea of their prevalence). Manoj Joshi and Robert Haberle had published a paper in 1997 that described their simulations for tidally locked planets orbiting red dwarf stars, findings that held open the possibility of atmospheric circulation moderating temperatures on the planet’s dark side. There seemed at least some possibility for extraterrestrial life on such a world, although the prospect remains controversial.

Image: X-ray observations of Proxima Centauri, the nearest star to the Sun, have shown that its surface is in a state of turmoil. Flares, or explosive outbursts occur almost continually. This behavior can be traced to the star’s low mass, about a tenth that of the Sun. In the cores of low mass stars, nuclear fusion reactions that convert hydrogen to helium proceed very slowly, and create a turbulent, convective motion throughout their interiors. This motion stores up magnetic energy which is often released explosively in the star’s upper atmosphere where it produces flares in X-rays and other forms of light. X-rays from Proxima Centauri are consistent with a point-like source. The extended X-ray glow is an instrumental effect. The nature of the two dots above the image is unknown – they could be background sources. Credit: NASA/CXC/SAO.

A lot of work has been done on M-dwarfs and habitability in the years since, and we also have the problem of this class of stars emitting flares of X-ray or ultraviolet radiation, making the prospects for life still uncertain. It would be helpful, then, if we could find a way to back a planet off from its host star while still allowing it to be habitable. The flare problem would be partially mitigated, and tidal lock might not be a factor. Joshi, now studying planetary atmospheric models at the University of East Anglia, has recently published a new paper with Haberle (University of Reading) arguing that the habitable zone around M-dwarfs may actually extend as much as 30 percent further out from the parent star than had been previously thought.

At issue is the reflectivity of ice and snow. M-dwarfs emit a much greater fraction of their radiation at wavelengths longer than 1 ?m than the Sun does, a part of the spectrum where the reflectivity (albedo) of snow and ice is smaller than at visible light wavelengths. The upshot is that more of the long-wave radiation emitted by these stars will be absorbed by the planetary surface instead of being reflected from it, thus lowering the average albedo and keeping the planet warmer. Joshi and Haberle modeled the reflectivity of ice and snow on simulated planets around Gliese 436 and GJ 1214, finding both the snow and ice albedos to be significantly lower given these constraints.

The finding has no bearing on the inner edge of the habitable zone, as the paper notes:

The effect considered here should not move the inner edge of the habitable zone, usually considered as the locus of orbits where loss rates of water become significant to dry a planet on geological timescales (Kasting et al 1993), away from the parent M-star. This is because when a planet is at the inner edge of the habitable zone, surface temperatures should be high enough to ensure that ice cover is small. For a tidally locked planet, this implies that ice is confined to the dark side that perpetually faces away from the parent star; such ice receives no stellar radiation, which renders albedo effects unimportant.

The dependence of ice and snow albedo on wavelength is small for wavelengths shorter than 1 ?m, which is where the Sun emits most of its energy, so we see little effect on our own climate. But the longer wavelengths emitted by red dwarfs could keep snow and ice-covered worlds warmer than we once thought. How various atmospheric models would affect the absorption of the star’s light is something that will need more detailed work, say the authors, but they consider their extension of the outer edge of the habitable zone to be a robust conclusion.

The paper is Joshi and Haberle, “Suppression of the water ice and snow albedo feedback on planets orbiting red dwarf stars and the subsequent widening of the habitable zone,” accepted by Astrobiology (preprint).