Catching Up with FOCAL

Michael Chorost has written a fine essay on Claudio Maccone’s FOCAL mission concept for The New Yorker blog. Centauri Dreams regulars will know Chorost from several previous posts here, particularly a discussion on SETI that I talked about in On Cosmic Isolation, where he analyzed the hunt for extraterrestrial civilizations in terms of problems of perception, with reference to his own thoughts on deafness, cochlear implants and neurotechnology. Mike is the author of the superb World Wide Mind (Free Press, 2011) that examines the interface between future humans and future machines.


I also like to remind readers of something Mike wrote on his own blog last year, which refers to a book I deeply admire and issues I’ll be writing about in future essays here. In particular, how do we deal with advanced alien civilizations if we run across them, and would the gap between us and them defeat our attempts at communication? Chorost takes a positive view:

I’d like to be optimistic. I’d like to think we’d be better off than preliterates puzzling over Wikipedia on an iPad. In his book The Beginning of Infinity, David Deutsch argues that humans crossed a crucial threshold with the scientific method. We now know that everything is explainable in principle, if we make the effort to understand it. Arthur C. Clarke famously said that any sufficiently advanced technology will seem like magic. This may be true, but we will not mistake it for magic. We have a postmodern openness to difference, a future-oriented culture, and well-established methodologies for studying the unknown. Our relative horizons are much larger than our ancient ancestors’ were.

More on all these issues in last year’s post SETI: Contact and Enigma. For today, though, some thoughts about the FOCAL mission, which envisions sending a probe out past 550 AU, the distance at which light is bent around the Sun by its gravity to form a gravitational lens. I say ‘beyond 550 AU’ because the focal line goes to infinity, and also because at 550 AU itself, we have severe coronal distortion effects to deal with, so a FOCAL mission, if the technology and the science check out to make it happen, would begin its observations further out and would keep traveling as it made them.

Lensing and Technology

We’ve seen recently –and Chorost writes about this — that there is to be an attempt to examine Proxima Centauri for planets because of an upcoming gravitational lensing event. The star will pass in front of a far more distant star in October of 2014, and an analysis of that event may flag the presence of small planets that have thus far eluded detection. Maccone, of course, has been arguing since the early 1990s that we can stop waiting for chance observational occurrences and start exploiting the gravitational focus with a directed mission. His book Deep Space Flight and Communications (Springer, 2010) is an analysis of mission possibilities.

My conversations with Claudio Maccone about FOCAL go back to one of Ed Belbruno’s New Trends in Astrodynamics and Applications conferences at Princeton in 2005, where Greg Matloff, his wife C and I had breakfast with Claudio and, over orange juice, coffee and a stack of enormous pancakes, talked about the problems and potential solutions for getting the mission to work. Since then the indefatigable Maccone has worked tirelessly on the issues and in his new book discusses, among other things, the kind of antenna deployment such a probe would use, and the prospects for using gravitational lensing not only for astronomical observations but communications.

Chorost had numerous exchanges with Maccone for this piece and does an admirable job at making the basics clear to a lay audience. He’s particularly interested in SETI possibilities, something out of which the initial FOCAL work grew through meetings in Italy in the 1990s that examined the kind of missions that could be run using solar sail technologies. From the essay:

Maccone wants to use the sun as a gravitational lens to make an extraordinarily sensitive radio telescope. He did not invent the idea, which he calls FOCAL, but he has studied it more deeply than anyone else. A radio telescope at a gravitational focal point of the sun would be incredibly sensitive. (Unlike an optical lens, a gravitational lens actually has many focal points that lie along a straight line, called a focal line; imagine a line running through an observer, the center of the lens, and the target.) For one particular frequency that has been proposed as a channel for interstellar communication, a telescope would amplify the signal by a factor of 1.3 quadrillion.

Could we actually build such a craft and, having sent it on its journey to a place roughly five times as far as Voyager 1 has reached to begin observations, untangle the information it sent us? We’re pushing theory and technology hard and have much to do with both before we can be sure it will work. For that matter, when Mike asked me how we could get Maccone’s payload into this kind of trajectory within the lifetime of a human researcher, I had to say that with present-day methods, we’d be limited to a solar sail (perhaps with a close solar pass to boost acceleration) or nuclear-electric technologies that might be used in conjunction with such a sail.


Image: Claudio Maccone (left), Jill Tarter and myself at the 100 Year Starship conference last year in Houston. What an evening that was, and yes, FOCAL was a major topic when Claudio and I had dinner the next evening. Thanks to Thomas Hair for snapping this.

But then what? Long-time readers will know of my admiration for A. E. van Vogt’s short stories (his novels, alas, are another matter, although I have a lingering fondness for The Weapon Shops of Isher). In particular, the story “Far Centaurus,” which ran in the January, 1944 issue of Astounding, sticks with me because its Centauri-bound crew finds the destination already settled by people who had left long after them on faster ships. So if a 50-year FOCAL mission is enabled by sail or hybrid technologies, can we guarantee that twenty years after launch, we won’t have something better — a fusion option, perhaps — that will pass the initial mission along the way?

It’s always a tradeoff, and the fact is that we could have waited before sending our Voyagers to the edge of the Solar System. If we had, we’d still be waiting, because it’s proving mighty hard to come up with that next big propulsion breakthrough due to funding limitations and perceived lack of public enthusiasm for major space projects. New Horizons is arcing toward Pluto/Charon and at one point was moving a bit faster than Voyager 1, but the Sun’s gravity has slowed it back down, leaving the intrepid Voyager 1 at 17.1 kilometers per second as our fastest moving object.

Or, I should say, ‘our fastest moving object that is departing the Solar System,’ since we’ve been able to get solar probes like Helios up to faster speeds. In any case, FOCAL is one of those mission concepts that, like Innovative Interstellar Explorer, demand innovative thinking and a serious gut-check in terms of what is possible and what we want to do. For me, the scope of the challenge makes it all the more fascinating to study such missions. Pushing hard at our limits awakens creative thought and suggests unexpected options in a future that is anything but foreordained.


Asteroids in our Future

NASA has released an Asteroid Initiative Request for Information on the issue of asteroid retrieval. It’s an interesting document both in its audience — the agency is making a point about soliciting comments not only from academics, scientists and engineers but the general public — but also because of the issues it explores. Being sought are ideas on how best to capture an asteroid, land an astronaut on one, and change its orbit, not necessarily in that order. The Los Angeles Times quotes NASA associate director Robert Lightfoot on the public component of NASA’s initiative:

“Too often, by the time we present a mission to the public, it has already been baked, and there’s not much we can change. This is your chance to present your ideas to us before the mission is baked.”

If you’re interested in contributing, move quickly, for the deadline for responses is July 18, with a workshop to follow in September.

The creation of a Solar System-wide infrastructure will necessarily precede any interstellar probes, if only because the methods we are studying to make such a probe happen all involve large construction projects in interplanetary space and resource retrieval from places as far away as the gas giants. But making the early infrastructure viable could well be the result of asteroid activities through companies like Planetary Resources and Deep Space Industries, or whoever manages to sustain an economic model for exploiting these interesting objects.

Asteroids are compelling targets for mining everything from gold, iron, nickel and platinum to water that can be extracted to support human settlements. But the case for developing our asteroid capabilities is also wrapped up in planetary defense, and it’s interesting to see this section of the NASA RFI:

Asteroid Deflection Demonstration: NASA is interested in concepts for deflecting the trajectory of an asteroid using the robotic Asteroid Redirection Vehicle (ARV) that would be effective against objects large enough to do significant damage at the Earth’s surface should they impact (i.e. > 100 meters in size). These demonstrations could include but [are] not limited to: a. Use of the ARV to demonstrate a slow push trajectory modification on a larger asteroid. b. Use of the ARV to demonstrate a “gravity tractor” technique on an asteroid. c. Use of ARV instrumentation for investigations useful to planetary defense (e.g. sub-surface penetrating imaging) d. Use of deployables from the ARV to demonstrate techniques useful to planetary defense (e.g. deployment of a stand alone transponder for continued tracking of the asteroid over a longer period of time).

10,000 NEOs and Counting

All of this is wrapped up inside the larger agency effort to capture and de-spin an asteroid and redirect it into translunar space, as described in the document. Just after the release of the NASA Request for Information on June 18, we learned that the 10,000th near-Earth object, asteroid 2013 MZ5, was detected by the Pan-STARRS-1 telescope in Hawaii. Near-Earth objects (NEOs) can approach the Earth’s orbital distance within 45 million kilometers. Known NEOs are as large as 40 kilometers (1036 Ganymed) or as small as a meter in diameter. Asteroid 2013 MZ5 turns out to be about 300 meters across and is not in an orbit that is considered hazardous.

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Image: Asteroid 2013 MZ5 as seen by the University of Hawaii’s PanSTARR-1 telescope. In this animated gif, the asteroid moves relative to a fixed background of stars. Asteroid 2013 MZ5 is in the right of the first image, towards the top, moving diagonally left/down. Credit: PS-1/UH.

I’ve been reading Don Yeomans’ book Near-Earth Objects: Finding Them Before They Find Us (Princeton University Press, 2012), an excellent overview of the field that I’ll be reviewing here in coming weeks. In this JPL news release Yeomans, manager of NASA’s Near-Earth Object Program Office at JPL, comments on the overall effort to track down NEOs:

“The first near-Earth object was discovered in 1898. Over the next hundred years, only about 500 had been found. But then, with the advent of NASA’s NEO Observations program in 1998, we’ve been racking them up ever since. And with new, more capable systems coming on line, we are learning even more about where the NEOs are currently in our solar system, and where they will be in the future.”

A glimpse of that future is provided by Lindley Johnson, who is part of NASA’s Near-Earth Object Observations Program. Johnson notes the significance of finding the 10,000th NEO but adds “…there are at least 10 times that many more to be found before we can be assured we will have found any and all that could impact and do significant harm to the citizens of Earth.” So we keep looking. NASA expects there are about 15,000 NEOs that are 140 meters in size and more than a million that reach 30 meters. The latter is a figure the agency cites as being the minimum size needed to cause ‘significant devastation’ in populated areas.

The news release has this to say about the NEOs we’ve already discovered:

Of the 10,000 discoveries, roughly 10 percent are larger than six-tenths of a mile (one kilometer) in size – roughly the size that could produce global consequences should one impact the Earth. However, the NASA NEOO program has found that none of these larger NEOs currently pose an impact threat and probably only a few dozen more of these large NEOs remain undiscovered.

The Near-Earth Object Observations Program is indeed, as Yeomans says, ‘racking them up.’ Working through the Catalina Sky Survey, the University of Hawaii’s Pan-STARRS survey and MIT’s LINEAR survey, NEOs are being discovered at a rate of about 1,000 per year. All observations flow to the Minor Planet Center in Cambridge MA in an effort that is clearly making progress on finding and cataloging objects. We now need to emphasize the effort to study the kind of deflection and trajectory-altering techniques NASA describes in the new RFI.


Philosophy, Intention and GJ 667C

The star Gliese 667C is as intriguing as it is because it underlines in triplicate the ‘habitability’ question, which surfaces every time a planet is discovered in a zone around its star where liquid water could exist on the surface. This is the classic definition of ‘habitable zone,’ meaning not so much a place where humans could live — we have no knowledge of other conditions on these worlds, knowing little more than their minimum mass — but a place where a basic condition for life as we know it is possible.

I’m much in favor of considering exotic environments for life, and these would include venues ranging from the upper clouds of Venus to the depths of the icy gas giant satellites in our own system. But when we read about ‘habitable zones’ in most scientific papers, we’re usually falling back on the liquid water criterion because it’s hard enough to search for any kind of life on a distant world, much less a kind that we don’t even know exists. Liquid water is a starting point, a baseline in the search for life-supporting worlds, but it hardly means the hunt for life stops there.

My own speculations about systems like GJ 667C become purely imaginative. I notice how many systems we’re finding that have ‘super-Earths,’ a category of planet that doesn’t even appear in our own Solar System. I also note that small M-dwarfs can produce clusters of planets that, in the case of GJ 667C, are close enough that three can be squeezed into the habitable zone. Imagine a civilization emerging on one such world, with other possible life-bearing planets so tantalizingly close — what a boost that would give to the proponents of the local space program!


Image: This picture shows the sky around multiple star Gliese 667. The bright star at the centre is Gliese 667 A and B, the two main components of the system, which cannot be separated in this image. Gliese 667C, the third component, is visible as a bright star, very close and just under A and B, still in the glare of these brighter stars. The very subtle wobbles of Gliese 667C, measured with high precision spectrographs including HARPS, revealed it is surrounded by a full planetary system, with up to seven planets. Credit: ESO/Digitized Sky Survey 2. Acknowledgement: Davide De Martin.

Searching for Life’s Equations

In Are We Alone, a fine essay in Aeon Magazine, astrobiologist Caleb Scharf (Columbia University) places the exoplanet hunt in a cosmic context. We may be getting used to them now, but the early proliferation of ‘hot Jupiters’ and now ‘super-Earths’ was not an expected outcome at the beginning of exoplanet detection — for that matter, the pulsar planets found around PSR B1257+12 in 1991 certainly raised eyebrows as orbiting the remnant of a once massive star. Scharf points out, too, that 70 Virginis b and 16 Cygni Bb, discovered not long after the detection of planets around 51 Pegasi in 1995, were moving on highly elliptical orbits.

These wonderful finds — and news like GJ 667C delivered yesterday — keep reminding us that the old idea that our Solar System is somehow representative of what we’ll find elsewhere is deeply flawed. Says Scharf:

Many exoplanets… follow orbits that are far more elliptical than any followed by the major planets around our Sun. More puzzling still, the most frequent type of configuration, the one that has earned the moniker of ‘the default mode of planetary formation’, is that of closely packed worlds, on orbits that take mere days or weeks to loop around their stars. These compressed versions of our own system seem, for now, to be far more normal than our own. But if we’re not normal, what are we? That’s a question we can’t answer yet, because our census of stars and planets is still woefully incomplete.

Back to the habitability question. With the prospect of tens of billions of stars in our galaxy harbouring planets of Earth-size and into the super-Earth range, we live in a universe that gives us the chance to find out whether life is rare or common. Being in a habitable zone does not mean a planet sustains life, and it will be the job of our next-generation instruments to help us study planetary atmospheres so we can unlock the numbers on just what the odds are. Scharf speaks of an ‘equation for abiogenesis’ that we can search for throughout a galaxy that accommodates us by giving us a vast number of targets. We can thus take a measure of our own significance:

If we lived in a cosmos with only a few planets, we could never deduce the true probability of abiogenesis with any precision, even if they all harboured life — as imagined by earlier advocates of plurality. We might never be lucky enough to find one of these worlds within examining range, and all would be lost among the stellar fields of the Milky Way. The existence of billions of planets gives us a chance to write the equation, a chance to pin down the relationship between habitability and actual habitation.

So the excitement of finds like the planets around GJ 667C isn’t that we have yet more evidence that clement places for life can exist in the cosmos. It’s that we have another target — at 22 light years a relatively close one — that we’ll eventually be able to use as we make the crucial call on life’s equations. In the coming century, a discovery in either direction, that life is a rarity or is as common as, well, planets in the habitable zone, will have profound implications for philosophy as well as practical action. It will guide our intentions as we develop the technologies to reach other stars


Gliese 667C: Three Habitable Zone Planets

Gliese 667C keeps getting more interesting. In the past we’ve looked at studies of this star in a triple system just 22 light years away, work that had identified three planets around the star. As one of these was in the habitable zone, this small red dwarf (about a third of the Sun’s mass) quickly engaged the interest of those thinking in terms of astrobiology. Now we get news that GJ 667C may actually host up to seven planets, with three evidently in the habitable zone.

I would say Mikko Tuomi (University of Hertfordshire, UK) is guilty of a bit of understatement. He’s quoted in this ESO news release thusly:

“We knew that the star had three planets from previous studies, so we wanted to see whether there were any more. By adding some new observations and revisiting existing data we were able to confirm these three and confidently reveal several more. Finding three low-mass planets in the star’s habitable zone is very exciting!”

Exciting indeed — we’ve never found three super-Earths within the same star’s habitable zone, in this case a realm closer to the parent star than the planet Mercury in our system. The work drew on data from the UVES spectrograph on ESO’s Very Large Telescope (Chile), as well as the Carnegie Planet Finder Spectrograph at the Magellan II site in Chile, the HIRES spectrograph on the 10-meter Keck instrument on Mauna Kea, and previous data from the HARPS (High Accuracy Radial velocity Planet Searcher) on the ESO 3.6-meter instrument in Chile.

What we wind up with after a thorough analysis of the radial velocity data for GJ 667C are five signals described by ESO as ‘very confident,’ with a sixth signal that is tentative and a seventh that is more tentative still. From the paper:

— – Up to seven periodic signals are detected in the Doppler measurements of GJ 667C data, being the last (seventh) signal very close to our detection threshold.
— The signi?cance of the signals is not affected by correlations with activity indices and we could not identify any strong wavelength dependence with any of them.
— The ?rst six signals are strongly present in subsamples of the data. Only the seventh signal is uncon?rmed using half of the data only. Our analysis indicates that any of the six stronger signals would had been robustly spotted with half the available data if each had been orbiting alone around the host star.

A Densely Packed Habitable Zone

The habitable zone here is found to lie between 0.095–0.126 AU and 0.241–0.251 AU. Two planets exist on the star-side of the habitable zone, three within it, and the last two further out in the system. The assumption here, echoed by ESO, is that all five of the inner planets including the three in the habitable zone are tidally locked, with one side in permanent sunlight and the other in darkness. The skies above one of the habitable zone planets could present an interesting view indeed, as the ESO artist’s impression below conveys.


I want to look more closely at the author’s conclusions on the three habitable zone planets, starting with planet c, which is closer to the inner edge of the HZ than the Earth is in our system. Global climate here would depend upon the properties of the atmosphere:

If the atmosphere is thin, then the heat absorbed at the sub-stellar point cannot be easily transported to the dark side or the poles. The surface temperature would be a strong function of the zenith angle of the host star GJ 667C. For thicker atmospheres, heat redistribution becomes more signi?cant. With a rotation period of ? 28 days, the planet is likely to have Hadley cells that extend to the poles (at least if Titan, with a similar rotation period, is a guide), and hence jet streams and deserts would be unlikely. The location of land masses is also important. Should land be concentrated near the sub-stellar point, then silicate weathering is more e?ective, and cools the planet by drawing down CO2 (Edson et al. 2012)….

The authors describe planet f as ‘a prime candidate for habitability’:

It likely absorbs less energy than the Earth, and hence habitability requires more greenhouse gases, like CO2 or CH4. Therefore a habitable version of this planet has to have a thicker atmosphere than the Earth, and we can assume a relatively uniform surface temperature. Another possibility is an “eyeball” world in which the planet is synchronously rotating and ice-covered except for open ocean at the sub-stellar point (Pierrehumbert 2011).

And finally, about planet e, which receives:

…only a third the radiation the Earth does, and lies close to the maximum greenhouse limit. We therefore expect a habitable version of this planet to have > 2 bars of CO2. The planet might not be tidally locked, and may have an obliquity that evolves signi?cantly due to perturbations from other planets. From this perspective planet e might be the most Earth-like, experiencing a day-night cycle and seasons.


Image: This diagram shows the system of planets around the star Gliese 667C. A record-breaking three planets in this system are super-Earths lying in the zone around the star where liquid water could exist, making them possible candidates for the presence of life. This is the first system found with a fully packed habitable zone. The relative approximate sizes of the planets and the parent star are shown to scale, but not their relative separations. Credit: ESO.

Nature of the Super-Earths

As to the composition of the super-Earths around GJ 667C, the authors note the ‘packed configuration’ of the system, with all planets inside 0.5 AU, and go on to say:

…the planets either formed at larger orbital distances and migrated in (e.g. Lin et al. 1996), or additional dust and ice ?owed inward during the protoplanetary disk phase and accumulated into the planets Hansen & Murray (2012, 2013). The large masses disfavor the ?rst scenario, and we therefore assume that the planets formed from material that condensed beyond the snow-line and are volatile rich. If not gaseous, these planets contain substantial water content, which is a primary requirement for life (and negates the dry-world HZ discussed above). In conclusion, these planets could be terrestrial-like with signi?cant water content and hence are potentially habitable.

Is GJ 667C the first among many M-dwarf systems containing several potentially habitable worlds each? The authors speculate that this is the case, and if that is so, then we may ultimately learn that life is more common on worlds around these small red dwarfs than around any other class of star. It’s a notion worth thinking about, given that M-dwarfs make up perhaps as much as 80 percent of the stellar population.

It’s worth mentioning that this study includes a reanalysis of earlier data that underlines the growing power of our archived observations to inform new work. Thus Guillem Anglada-Escudé (University of Göttingen), who worked with Tuomi on this project: “These new results highlight how valuable it can be to re-analyse data in this way and combine results from different teams on different telescopes.” The new tools of digital storage and analysis mean that we are gathering data at a clip far beyond what we can exhaustively analyze, meaning that surprises may await in numerous datasets when weighed against hints from new studies.

The paper is Anglada-Escudé et al., “A dynamically-packed planetary system around GJ 667C with three super-Earths in its habitable zone”, accepted for publication in Astronomy & Astrophysics.


Centaurs and their Implications


One of the themes I often use in my talks is the ‘filling out’ of our picture of the Solar System. In addition to the asteroid belt, we’ve added the icy bodies of the Kuiper Belt and the vast expanse of the Oort Cloud into what once seemed a relatively simple, nine-planet solar system. I could easily add to the ranks the population of so-called ‘Centaurs,’ small bodies that populate the space between the giant planets and show characteristics of both comets and asteroids.

10199 Chariklo is the largest Centaur yet discovered (260 kilometers in diameter), and Saturn’s moon Phoebe may be a captured Centaur, in which case images of it from the Cassini orbiter offer us our first detailed view of such an object. Both Chiron (discovered in 1977) and 60558 Echeclus show signs of a cometary coma; both are classified as asteroids and comets (as is 166P/NEAT). Although differences in definition exist, most agree that Centaurs orbit the Sun between Neptune and Jupiter and eventually cross the orbits of one or more of the gas giants.

Image: Saturn’s moon Phoebe, possibly a captured Centaur. Credit: NASA.

Now we have news from Universidad Complutense of Madrid (UCM) that Crantor, a large asteroid with a diameter of 70 kilometers orbits the Sun in exactly the same time period as Uranus. The asteroid’s orbit, says Carlos de la Fuente Marcos, one of the study’s authors, “…is controlled by the Sun and Uranus but is unstable due to disturbances from nearby Saturn.” Two other asteroids — 2010 EU65 and 2011 QF99 — are also associated with Uranus. The latter is in a stable Trojan orbit, moving 60 degrees in front of Uranus, while Crantor and 2010 EU65 show ‘horseshoe’ orbits that result periodically in close encounters with the planet.

About Crantor itself we have much to learn:

(83982) Crantor is remarkable in several respects: it is the first known minor body to be trapped in a 1:1 mean motion resonance with Uranus; it currently moves in a complex, horseshoelike orbit when viewed in a frame of reference co-rotating with Uranus; and it could be the “Rosetta Stone” for understanding why the overall number of Uranus co-orbitals appears to be significantly below that of Jupiter or Neptune. The object is placed and removed from its horseshoe orbit by the mechanism of the precession of the nodes. This precession is accelerated by the perturbative effects of Saturn. The chaotic nature of the orbit of this object constraints the degree of predictability of its dynamical evolution on timescales longer than a few 10 kyr.

The horseshoe shape of a ‘horseshoe orbit’ appears when we map the movement of the asteroid in relation to the Sun and, in this case, Uranus. While the asteroid always orbits the Sun in the same direction, it periodically catches up with the planet and falls behind again, tracing out the horseshoe outline in relation to Sun and planet. 3753 Cruithne was the first object confirmed to follow a horseshoe orbit, in this case near the Earth, while multiple objects in such orbits have been found around Jupiter, with others identified in association with Venus and Mars.


Image: A NASA illustration of a horseshoe orbit, in this case showing the orbit in relation to the Earth.

I don’t want to leave the topic of Centaurs behind without mentioning an interesting 2010 paper, although its emphasis is on Neptune rather than Uranus. Jonathan Horner (University of Durham) and Patryk Sofia Lykawka (Kinki University, Japan) make the case that a significant fraction, if not more, of the Centaur population comes from the planetary Trojan clouds, considered here as ‘stable reservoirs of objects moving in 1:1 mean-motion resonance with the giant planets…’ The researchers’ simulations show that there should be an ongoing movement of objects into the Centaur population. They go on to suggest that the Neptune Trojans could be the main source of new Centaurs. And I thought this was interesting (from their paper):

We suggest that further observational work is needed to constrain the contribution made by the Neptune Trojans to the ongoing flux of material to the inner Solar system, and believe that future studies of the habitability of exoplanetary systems should take care not to neglect the contribution of resonant objects (such as planetary Trojans) to the impact flux that could be experienced by potentially habitable worlds.

The de la Fuente Marcos paper is “Crantor, a short-lived horseshoe companion to Uranus,” Astronomy & Astrophysics 551: A114, March 2013 (abstract). The Horner/Lykawka paper is “Planetary Trojans – the main source of short period comets?,” International Journal of Astrobiology 9, 227-234 (2010). Preprint online.