by Paul Gilster | Oct 24, 2011 | Exoplanetary Science |
An orange dwarf star a bit smaller than our Sun is giving us valuable clues about how water-covered planets like Earth may evolve. TW Hydrae is 176 light years away, so young (5 to 10 million years) that it is still in the early stages of forming a planetary system. Working with data from ESA’s Herschel space observatory, astronomers have found cold water vapor in the disc of dust and gas that surrounds the star. It’s a significant find, because while we’ve found warmer water vapor in proto-planetary discs closer to their star, we now see evidence for much larger amounts of water in the outer disc, where the material for icy comets is found. Current theory holds that water will be far scarcer in the inner solar nebula around a coalescing system, meaning extensive oceans would have to be delivered by impacting objects from the outer regions.
The Herschel data show the distinct signature of water vapor, probably produced when ultraviolet radiation from the central star warms ice-coated dust grains, causing some water molecules to break free of the ice to create the thin layer of gas found by Herschel’s Heterodyne Instrument for the Far-Infrared (HIFI). TW Hydrae’s disc extends to 196 AU, and the assumption is that as matter within the disc grows into planets, much of the outer dust and ice will coalesce to become comets. Cometary bombardment in the emerging solar system could then produce oceans on the inner worlds there, a process that this work indicates may be common. Says Caltech’s Geoff Blake, one of the team of researchers investigating TW Hydrae: “These results beautifully confirm the notion that the critical reservoir of ice in forming planetary systems lies well outside the formation zone of Earthlike planets.”
Image: This image shows an artist’s impression of the icy protoplanetary disc around the young star TW Hydrae (upper panel) and the spectrum of the disc as obtained using the HIFI spectrometer on ESA’s Herschel Space Observatory (lower panel). The graph in the lower panel shows the spectral signature of water vapour in the disc. Water molecules come in two “spin” forms, called ortho and para, in which the two spins of the hydrogen nuclei have different orientations. By comparing the relative amounts of ortho and para water, astronomers can determine the temperatures under which the water formed. Lower ratios indicate cooler temperatures, though in practice the analysis is much more complicated. The ratio of ortho to para water observed in TW Hydrae’s protoplanetary disc is low enough to point to the presence of cold water vapour. Credit: ESA/NASA/JPL-Caltech/M. Hogerheijde (Leiden Observatory).
Michiel Hogerheijde ( Leiden University) relates the process to our own Solar System:
“The detection of water sticking to dust grains throughout the disc would be similar to events in our own Solar System’s evolution, where over millions of years, similar dust grains then coalesced to form comets. These comets we believe became a contributing source of water for the planets.”
Simulations that folded the new Herschel data in with Spitzer observations as well as ground-based studies allowed the team to calculate the total amount of water in the TW Hydrae disc, an amount equal to several thousand Earth oceans. Studying such raw materials of planetary formation should help us understand how systems evolve, which is why upcoming Herschel studies of three more young stars with similar discs should be so interesting. The expectation is that more water vapor should turn up, supplying additional evidence for the kind of icy reservoir from which water-covered worlds can draw as infant solar systems emerge.
The paper is Hogerheijde et al., “Detection of the Water Reservoir in a Forming Planetary System,” Science Vol. 334 no. 6054, pp. 338-340 (21 October 2011). Abstract available.
Related: Earth’s Oceans: A Cometary Source After All?
by Paul Gilster | Oct 21, 2011 | Missions |
by K.F.Long, co-founder Project Icarus
Kelvin Long is well known to Centauri Dreams readers. The physicist and aerospace engineer is, in addition to being one of the most energetic voices in the service of interstellar propulsion studies, the co-founder of Project Icarus, the successor to the 1970s-era Project Daedalus starship design study. Here Kelvin looks at where the ongoing Icarus effort stands in terms of fusion, placing that propulsion option in the context of the broader questions raised by pushing a payload to the stars.
Back in December 2009 I wrote an article titled Project Icarus and the Motivation Behind Fusion Propulsion. This was an attempt to justify the initial design choice of the team as part of the engineering requirements for the study that is Project Icarus.
Despite this article and other discussions we have had, we have recently learned something from our experience at the 100 Year Starship Study Symposium: People still don’t understand the Daedalus connection and fusion choice behind Project Icarus. I shall attempt to explain it in order that the Centauri Dreams readers can follow the process that led to this apparently controversial decision.
Firstly, let us address what makes Project Icarus so unique – its Daedalus heritage. Why was the project chosen to be this particular way? There are four reasons for having made this decision:
- Because Project Daedalus was the only full systems integrated study ever performed in the interstellar community and no-one had gone back and revisited an old design in this way.
- To provide for a solid foundation to start with by putting the new design team in contact with the original Daedalus team and others in the community.
- To provide for a reliable technology maturity comparison given the nearly four decades of scientific progress, something else not attempted before.
- To give the team images to feed the media whilst we were developing our design and allow us to inspire the public, thereby building the momentum behind the project.
On the evidence for how the team has grown to date and the opportunities that have come our way, it is the view of this author that these decisions have been validated. We started from a very strong foundation, initially supported by the British Interplanetary Society and the Tau Zero Foundation. We made all of the right connections and gradually embedded ourselves within the interstellar community. In particular, we have gone out of our way to meet many of the giants of the field such as Geoffrey Landis, George Miley, Marc Millis, Terry Kammash, Eric Davis to name a few, and convince them that what we were doing had intellectual value. From discussions it appears they are largely in agreement.
Image: Kelvin Long (left) and Alan Bond, one of the original Daedalus designers, at the headquarters of the British Interplanetary Society. Credit: K. Long.
Training the Starship Designers
Now let’s look at another bit of thinking behind the project, the interstellar community itself. I have already mentioned some names above. Some of the other greats in the subject have sadly left this world having made an astonishing contribution to the literature of interstellar flight. This includes inspiring people like Carl Sagan, Robert Forward and Robert Bussard, to name just a few. There are many other greats still working away. Greg Matloff is one of them, working as a consultant to Project Icarus by mentoring the design team and passing on his wisdom and experience. He still publishes papers and lectures at the same pace as the rest of us. Similarly, many of the original Project Daedalus team are still around, including Alan Bond, Anthony Martin and Bob Parkinson. But they are either in retirement or busy with other projects. Alan Bond of course is the Managing Director of Reaction Engines Ltd and this keeps him busy enough. Despite this, he still manages to find time to keep an eye on developments in the field, even attending the BIS World Ship symposium back in September this year.
But to cut to the chase, how many young people are working on interstellar research? Where is the next generation working on interstellar research problems? Who will pick up the baton and stand on the shoulders of our interstellar research giants? Enter Project Icarus.
Project Icarus is at heart a training exercise. It is an exercise in designer capability at the extreme end of aerospace engineering. The choice of fusion propulsion is not relevant to this ‘educational program’. All of the people involved in Project Icarus are essentially in interstellar school and hope sometime, perhaps around the year 2014 – 2015, to graduate first in their class.
We all have our pet favourites for how to get to the stars. My personal favourite happens to be internal and external nuclear pulse propulsion although I am also very interested in antimatter concepts. Additionally, other than Project Icarus, I am also involved with two solar sail projects. Others within the Icarus team also have their favoured methods for reaching the stars, including microwave beaming concepts to faster-than-light drives. One of these methods or a combination of these methods, a propulsion hybrid, may someday be the actual method by which we attain the journey and reach those far off destinations. But guess what, it doesn’t matter what the option is if there isn’t anyone around capable of doing the work and advancing that option technically. So Project Icarus aims to train the design team so that when they complete the project they are ‘capable’ of doing the necessary calculations to assess all of these options and thereby advance them all incrementally.
Fusion and Future Design
The Project Terms of Reference (ToR) document stipulate that the propulsion system must be mainly fusion based propulsion, to maintain continuity with Project Daedalus and allow a claimed ‘redesign’, which would otherwise be difficult to justify. From internal discussions within the team, we have interpreted this to mean that energy generation through fusion reactions should be responsible for around ~80-90% of the thrust generation during the boost phase. This work is defined under the Primary Propulsion module, led by Richard Obousy, the former Project Leader. This leaves around ~10-20% of the thrust generation during the boost phase to be augmented with alternative propulsion technologies. This work is defined under the Secondary Propulsion module, led by Andreas Tziolas, the current Project Leader. Project Icarus officially started in September 2009. Since that time members of the team have done various calculations and trade studies pertaining to nuclear fusion, antimatter, nuclear thermal propulsion, plasma drives, solar sails, magSails, Medusa sails, microwave beams, Orion-type drives and even Vacuum Energy concepts. A ToR needed to be defined at the beginning to constrain the design problem, otherwise a five year study would turn into a ten year study.
When Project Icarus is finally over, it is my personal aspiration that members of the design team will go off and seed other design projects across the propulsion spectrum, because they will have the knowledge and the skills to do so. Essentially, Project Icarus is injecting energy into the subject of interstellar studies. It is hoped that this energy will act as a catalyst and spread across the entire subject, through inspiration, hard work, enthusiasm and compelling reasons for trying.
Image: Icarus being assembled in Earth orbit. Note the SKYLON spaceplane delivering components. Credit: Adrian Mann.
What you are witnessing in Project Icarus ladies and gentlemen is a classroom in action, played out on the World Wide Web. It is a pilot program for a Starflight Academy. If you want to train a bunch of people up quickly, the best way to do that is to throw them in the deep end with an engineering problem and say “go solve that”. This is precisely what the Project Icarus Study Group is attempting to do, where the specific exam problem is defined by our ToR, merely a mechanism for facilitating this goal. Perhaps someday a real Starflight Academy will exist, and teams will similarly be solving problems relating to many interstellar propulsion concepts. And it’s not just all about propulsion of course. A spacecraft design needs structure, materials, communications, reliability and a variety of other assessments to prove that it is a credible concept. We’re working on all those issues too and in the future will be taking more active steps to communicate some of that research to you the readers – something else we realized from the Orlando 100 Year Starship Study Symposium. People want to know some of our research findings and we need to start talking.
On a final note, I ask the readers not to see Project Icarus as an initiative that necessarily advocates fusion propulsion as the best way forward in reaching the stars. It happens to be the view of some members of the team, but others take a different view. Fusion propulsion and the Daedalus design, as studied within Project Icarus, is merely a vehicle upon which to train the eager young space cadets for the future. So that when we really do need that starship in a hurry, whatever the propulsion option of choice, we have a team ready and waiting to go and design it. Meanwhile, we will continue to wave our fists at the Sun and dare to “fly closer to another Star”, building on our forefathers who did that seminal Project Daedalus study in the 1970s. Like a son to a father, we hope to make them proud of our efforts and along the way find a way to build a better machine.
by Paul Gilster | Oct 20, 2011 | Deep Sky Astronomy & Telescopes |
Planetary migration can play a huge role in the evolution of a solar system, as witness the thinking that it was a migration of the gas giants Jupiter and Saturn that brought about the Late Heavy Bombardment, a time four billion years ago when impacts from space pock-marked the Moon and inner planets. The migration model, first proposed at the Observatoire de la Côte d’Azur and thus known as the Nice model, posits a long-lasting cometary bombardment caused when gravitational effects of the migration scattered icy bodies in the Kuiper Belt. Most of these would have been ejected from the system, but others would have been sent on planet-intersecting paths.
If the Nice model is correct, then we may have an explanation for at least part of the water that wound up on our blue and green planet today, with obvious consequences for the development of life. That makes events similar to the Late Heavy Bombardment of considerable interest when we can find them in other solar systems, and new work with Spitzer Space Telescope data says we’ve now done just that. The nearby bright star Eta Corvi is the site of a band of dust that shows up with the help of Spitzer’s infrared detectors. The star is approximately one billion years old, young enough to mimic what we believe was the situation around our own Sun in that era.
Image: This artist’s conception illustrates a storm of comets around a star near our own, called Eta Corvi. Evidence for this barrage comes from NASA’s Spitzer Space Telescope, whose infrared detectors picked up indications that one or more comets was recently torn to shreds after colliding with a rocky body. In this artist’s conception, one such giant comet is shown smashing into a rocky planet, flinging ice- and carbon-rich dust into space, while also smashing water and organics into the surface of the planet. A glowing red flash captures the moment of impact on the planet. Yellow-white Eta Corvi is shown to the left, with still more comets streaming toward it. Credit: NASA/JPL-Caltech.
What we see around Eta Corvi is telling. The researchers found evidence in the dust disc of water ice, organics and rock, all of which indicate a cometary source. From the paper:
We interpret this as demonstrating that the parent body for the ? Corvi warm circumstellar dust was a large object created early in the system’s history, that it formed outside the ice line of the ? Corvi system, and that it retained much of its icy volatiles and primitive material.
Moreover, the dust band in question is close enough to Eta Corvi that Earth-like planets could exist there, leaving the implication that we’re looking at the results of a collision between a planet and one or more comets. Is this what our own Solar System might have looked like during the Late Heavy Bombardment? It’s a distinct possibility. Lead author Carey Lisse (JHU/APL): “We believe we have direct evidence for an ongoing Late Heavy Bombardment in the nearby star system Eta Corvi, occurring about the same time as in our solar system.”
Two bands of dust surround Eta Corvi, the second being a much colder ring at about 150 AU in a region that bears obvious analogies to our Kuiper Belt and may be a reservoir for comets. It’s intriguing that the light signature emitted by the inner Eta Corvi dust band resembles the composition of the Almahata Sitta meteorite, fragments of which were recovered in the Sudan in 2008. The Kuiper Belt may well have been the origin of the Almahata Sitta object. Eta Corvi thus becomes a wonderful laboratory for the study of cometary bombardment because it contains both warm and cold dust reservoirs with strong spectral features, and its rarity may itself be telling us something, as the paper goes on to note (internal references omitted for brevity):
Large photometric studies of many debris disks and targeted spectroscopic studies of individual debris disks conducted with the Spitzer Space Telescope have constrained the typical locations of debris-producing collisions, the evolution of the emission from these collisions, and, in some cases, the chemical composition of and events identified by the colliding parent bodies. Most debris disks have dust temperatures colder than ~ 150—200 K, consistent with debris produced from icy colliding planetesimals… The frequency of warm (> 200 K) debris dust is low, < 5—10% regardless of age, indicating that planetesimal collisions in the terrestrial zone or asteroid belt-like regions are rare or have a small observational window, consistent with expectations from theory...
That small observational window would certainly fit current thinking about the Late Heavy Bombardment as a period of intense activity that moderated following planetary migration. In any case, debris disks around young stars are proving their worth, suggesting as they do a reservoir of colliding planetesimals. Our own more mature system, of course, has two belts — relatively warm dust and metal-rich bodies in the asteroid belt between 2 and 4 AU and the cold dust and icy planetesimals of the Kuiper Belt beyond 30 AU. By studying such discs around other stars, we are looking at system evolution in action and learning more about our system’s past.
The paper is Lisse, et al., “Spitzer Evidence for a Late Heavy Bombardment and the Formation of Urelites in ? Corvi at ~1 Gyr,” accepted by the Astrophysical Journal (preprint).
by Paul Gilster | Oct 19, 2011 | Culture and Society |
Nuclear rocket designs are hardly new. In fact, it was clear as early as the 1950s that conventional chemical rocketry was inefficient, and programs like Project Rover, set up to study the use of nuclear reactors to heat liquid hydrogen for propulsion, aimed at the kind of rockets that could get us beyond the Moon and on to Mars. The NERVA rocket technology (Nuclear Engine for Rocket Vehicle Application) that grew out of all this showed great promise but ran afoul of political and economic issues even as the last Apollo missions were canceled. Nor is the public wariness of nuclear methods likely to vanish soon, yet another hurdle for future ideas.
But making people aware of what has done and what could be done is good practice, as Kenneth Chang does by example in his recent piece on the 100 Year Starship Symposium, which bears the optimistic title Not Such a Stretch to Reach for the Stars. In interstellar terms, propulsion is the biggest problem of all. Chang’s article suggests a pathway through conventional rocketry and into nuclear-thermal designs, with reference along the way to using nuclear engines to generate the electrical fields that power up an ion engine. The goal on this pathway is fusion, though Chang admits no one has yet built an energy-producing fusion reactor.
The Daedalus concept was fusion-based, and the ongoing Icarus project that followed is now examining Daedalus to note the effect of thirty years of new technology. But Chang has also talked to James Benford, whose interest in laser and microwave beaming remains strong. Leave the propellant behind and you’ve maximized payload, in addition to working with known physics and apparently achievable engineering. And there continue to be startling new concepts like those of Joseph Breeden, who finds a more extreme way to create an engineless vehicle:
From his doctoral thesis, Dr. Breeden remembered that in a chaotic gravitational dance, stars are sometimes ejected at high speeds. The same effect, he believes, could propel starships.
First, find an asteroid in an elliptical orbit that passes close to the Sun. Second, put a starship in orbit around the asteroid. If the asteroid could be captured into a new orbit that clings close to the Sun, the starship would be flung on an interstellar trajectory, perhaps up to a tenth of the speed of light.
“The chaotic dynamics of those two allow all the energy of one to be transferred to the other,” said Dr. Breeden, who came toting copies of a paper describing the technique. “It’s a unique type of gravity assist.”
What I call the ‘joy of extreme possibility’ has animated interstellar studies since the days of Robert Forward. It works like this: We know the distances between the stars are so vast as to dwarf the imagination. Indeed, most people have no notion of them, seeing an interstellar mission as merely a next step once we have explored the outer system, a kind of juiced-up Voyager. The scientists and engineers who work on these matters, knowing better, realize how far beyond our current technologies these journeys really are. So they’re not afraid to speculate even at the absolute far end of the plausible (and often beyond that). Work your way through interstellar papers like these and you pick up an infectious, jazzy brainstorming. It’s the kind of mental riffing on an idea that a John Coltrane or a McCoy Tyner does with a musical theme.
And by the way, Chang is careful to get those distances across to readers. I’m always interested in homely comparisons because you can use them to boggle audience minds when speaking about interstellar flight. This is useful, because a boggled mind often becomes a curious one, and while you can never predict these things, occasionally interstellar studies gain a new adherent. Chang cites a Richard Obousy analogy: If the Earth were Orlando and Alpha Centauri were in Los Angeles, then the Voyager spacecraft would have traveled but a single mile.
Even after all these years, that one still boggles my own mind. Chang again:
Another way of looking at the challenge is that in 10,000 years, the speed of humans has jumped by a factor of about 10,000, from a stroll (2.6 m.p.h.) to the Apollo astronauts’ return from the Moon (26,000 m.p.h.). Reaching the nearest stars in reasonable time — decades, not centuries — would require a velocity jump of another factor of 10,000.
It’s good to see the 100 Year Starship Study steadily percolating in the news. Maybe one day these concepts will not seem as esoteric as they do today. I note as I write this, for example, that my word processor flags the word ‘starship’ as a spelling error. We need to set deeper roots into the culture than that. We can start by doing what conference organizer David Neyland told Chang he wants to do, to establish a bar high enough that people “will actually go start tackling some of these really hard problems.” Of course, the real bar is set by nature, and it’s the highest bar we as a species have faced in terms of travel times and distance. But the joy of extreme possibility only ignites the spirit when everything is on the table and the challenge is immense.
by Paul Gilster | Oct 18, 2011 | Exoplanetary Science |
I’ve put off writing about Wesley Traub’s paper on the frequency of planets in the habitable zone because I knew Adam Crowl had reservations about Traub’s method. We talked about this at the 100 Year Starship Symposium, which led to Adam’s agreeing to writing this piece for Centauri Dreams. How you define a habitable zone is, of course, a critical matter, especially when you’re dealing with a topic as compelling as extrasolar planets that can support life. Adam places Traub’s work in the context of earlier attempts at defining the habitable zone and finds HZ estimates different from Traub’s, though one is surprisingly similar to a much earlier study.
by Adam Crowl
The recent paper by Wesley Traub [reference below] has estimated the frequency of terrestrial (“Earth-like”) planets in the Habitable Zone (HZ) of their stars based on statistical analysis of the recent Kepler data release, but the frequency computed, of ~34(+/-14)% around FGK stars, is dubious due to the assumption of wide HZ limits. Before I discuss the specifics, let’s look at the modern history of the “Habitable Zone”.
The modern discussion really began with Stephen Dole’s “Habitable Planets for Man”, a RAND commissioned study from the early 1960s, eventually updated in 1970, and popularized with Isaac Asimov. Dole based his HZ limits on the criterion that a significant fraction of a terrestrial planet would experience a “hospitable climate”. He didn’t examine the effect of atmosphere, and derived the HZ limits of 0.725 – 1.25 AU, from just outside the orbit of Venus and a bit closer to the Sun than Mars at its closest. Applying statistical analysis to various features of the known planets, then extrapolating to other stars, Dole found that potentially 645 million Earth-like planets might exist in the Galaxy.
In the mid 1970s Michael Hart developed the first evolutionary models of the atmosphere of an Earth-like planet, finding Earth to be poised on a virtual knife-edge, tipping towards a Runaway Greenhouse if closer than 0.95 AU and Runaway Glaciation if further out from the Sun than 1.01 AU. When this criterion was applied to other stars, the frequency of Earth-like planets was less than 1 in a quarter million stars, or less than 400,000 Earths in a Galaxy of 100 billion stars.
Hart’s limits seemed overly sensitive to climate perturbations, and further work on the evolution of Earth’s atmosphere in the 1980s led to the paper “Habitable Zones Around Main Sequence Stars”, (Kasting, Whitmire & Reynolds, 1993) , which redefined the debate. What James Kasting and colleagues discovered was a powerful feedback loop between the levels of carbon dioxide in the atmosphere, geological weathering and the heat input from the Sun.
This creates a self-regulated surface temperature which can keep water in its liquid range out to a significant distance from the Sun. The chief uncertainty came from the complication of dry-ice clouds. Past 1.37 AU clouds of dry-ice begin forming and by 1.67 AU the cloud cover becomes total, negating the effectiveness of the carbon dioxide greenhouse effect. Some preliminary work on water clouds also suggests the inner radius of the habitable zone, just 0.95 AU, might be extended to closer to Venus.
Traub’s paper has somewhat more generous HZ limits. Traub examined three cases, with the ranges from 0.72-2.0 AU in the best case, a nominal HZ of 0.8-1.8 AU, and a “conservative” 0.95-1.67 AU. Using the observed planetary radii distribution and the orbital radii, Traub was able to compute the frequency of terrestrial planets in these HZ as 34(+/-14)%, with the extremes providing the error bar limits.
Here’s where just what is computed and why is important. The ranges used by Traub for the HZ apply to specifically liquid water compatible planets with extensive greenhouse gas atmospheres. Such worlds, with up to several bars of carbon dioxide for atmosphere, are only distantly “Earth-like”, much like Mars or Venus can be called Earth-like. The Earth we know, with an oxygen rich, carbon dioxide poor, atmosphere is somewhat more sensitive to climatic instability. If more conservative HZ ranges are used a quite different result is obtained.
The HZ, inside of the CO2 cloud limit found by Kasting, et.al., is the more restrictive 0.95-1.37 AU. This gives a frequency of just 13.3%. If we use the Continuously Habitable Zone (0.95-1.15 AU), also from Kasting, et.al., then the frequency drops to a mere 6.3%. Using Hart’s even more restricted range drops the frequency to less than 2%. Another caveat is that the planet frequency estimated is limited to stars in the mass-range 1.13-1.01 solar masses and is yet to be extended into the wider population of stars which make up ~80% of the Galaxy.
The HZ limits derived by Kasting et.al. assumed ocean-dominated terrestrial planets. The broader range of land dominated “desert planets” (Abe et.al., 2011), with water bodies limited to circum-polar lakes/ice-caps, increases the HZ range to 0.75-1.3 AU, and a corresponding frequency of 17.3%. Incidentally this range is equivalent to that derived by Dole’s (1964) ground-breaking study.
So, in conclusion, the high frequency of “Earth-like” planets derived by Traub, is tempered somewhat when a more precise Earth-like Habitable Zone range is used. Planets warm enough for liquid water thanks to multi-bar atmospheres of carbon dioxide, methane or hydrogen, while probably conducive to extremophiles, aren’t “Earth-like” as usually understood, and this caveat should be more widely appreciated when making such estimates.
The paper is Traub, “Terrestrial, Habitable-Zone Exoplanet Frequency from Kepler,” available online as a preprint. Other references:
Y.Abe, A.Abe-Ouchi, N.H.Sleep, and K.J.Zahnle. “Habitable Zone Limits for Dry Planets”, Astrobiology, Volume 11, Issue 5, pp. 443-460 (2011).
S.H. Dole, Habitable Planets for Man, Blaisdell, New York (1964).
M.H. Hart, “Habitable zones about main sequence stars”, Icarus, 37: 351-357 (1978).
J.F. Kasting, D.P. Whitmire and R.T. Reynolds. “Habitable Zones Around Main Sequence Stars” Icarus 101: 108-128 (1993).
