by Paul Gilster | Feb 5, 2014 | Exoplanetary Science |
When I was growing up, Alpha Centauri was utterly dismissed as a possible location for planets. A binary system couldn’t possibly produce them, I read, and it was assumed that planets could only be found around single stars like our own Sun. How times have changed. Now we know of plenty of multiple star systems with planets — the number is over ten percent of all known exoplanets — and while many of these are widely spaced, we’ve nonetheless found a few in tight circumstances indeed. HD196885 (Gamma Cephei) is an example, where the separation between the two stars is on the order of 20 AU, much like Centauri A and B.
How planets form around such stars is an interesting issue because, as a new paper considering dust in the Alpha Centauri system explains, the standard core-accretion model runs into problems with environments as perturbed as these. We can see the results in existing observations: Radial velocity methods detect no planets more massive than 2.5 Jupiter masses inside 4 AU of Centauri A or B, and large regions seem, from theoretical models, to be a real challenge to planet formation, with an outer limit for the process perhaps in the 2 AU region.
Image: Trajectory of Alpha Centauri B relative to A (fixed to the coordinate origin) as seen from the Earth (inclined ellipse) and face-on (horizontal ellipse). The orbit parameters are taken from Pourbaix et al. (2002). Credit: SiriusB/Wikimedia Commons.
What’s interesting about a new paper called “How dusty is α Centauri?” is that it gets into issues raised by the large number of planet-bearing stars known to show far-infrared emission from cool circumstellar dust. The authors, led by Joachim Wiegert (Onsala Space Observatory, Sweden), observed the Centauri stars to look for dust emissions there, presumably the results of collisions between objects up to planetesimal size. The team sought to understand the properties of the dust and to gauge the minimum temperature (Tmin) of the stellar chromospheres, a layer in the stars’ atmospheres that plays a role in how we determine the emission from cool dust in such systems.
The close-in Centauri stars, then, are templates that may help us find undetected dust disks around other stars. The team notes that it is::
…primarily concerned with the possible effects Tmin might have on the estimation of very low emission levels from exo-Edgeworth-Kuiper belt dust. The intensity of the stellar model photosphere beyond 20 to 40 µm is commonly estimated from the extrapolation of the spectral energy distribution (SED) into the Rayleigh-Jeans (RJ) regime at the effective temperature Teff. There is a potential risk that this procedure will overestimate actual local stellar emissions, which may be suppressed at the lower radiation temperatures. In those cases, where the SEDs are seemingly well fit by the RJ-extrapolations, the differences may in fact be due to emission from cold circumstellar dust (exo-Edgeworth-Kuiper belts) and, here, we wish to quantify the magnitude of such an effect.
As to Alpha Centauri itself, a slight infrared excess could be interpreted as dust emissions around both Centauri A and B. Disk modeling then produces the possibility of two circumstellar disks, the one around Centauri A being no larger than 2.78 ± 1.48 AU, while around Centauri B the limits are 2.52 ± 1.60 AU (the planet tentatively identified as Centauri Bb in 2012 is small enough and close enough to the star that it is inside the range of the authors’ disk models, and is thus ignored in their modelling). And while a circumbinary dust disk is possible at distances greater than 75 or so AU from the barycenter of the stars, no such disk was detected by these observations, and the authors note that other studies have made this scenario unlikely.
And this is interesting: “These size limits are reminiscent of the inner solar system, i.e., this opens the possibility for an asteroid belt-analogue for each star which forms dust discs through the grinding of asteroids and comets.” The results, displaying the possible disks and their snow lines, are shown in the figure below:
Image: Face-on circumstellar test-particle discs after ~ 103 periods shown with the stars close to periapsis. α Cen A is colour-coded blue (the left star and its orbit) and α Cen B is colour-coded red (the right star and its orbit). The green circles represent acrit around the stars and the magenta circles show estimates of their respective snow lines. Credit: Wiegert et al., taken from the paper.
We don’t yet know whether the excess at 24 µm the researchers found is a detection of warm dust or not — the paper describes the data as “marginal excesses” for both Centauri A and B, adding that “If due to circumstellar emission from dust discs, fractional luminosity and dust mass levels would be some 10 to 100 times those of the solar zodiacal cloud.” As we push deeper into the study of the nearest stars, we’ll use what we learn about dust emissions and their relation to stellar temperatures to study cool circumstellar disk possibilities around more distant targets.
The paper is Wiegert et al., “How dusty is α Centauri?” accepted at Astronomy & Astrophysics and available as a preprint. Thanks to Andy Tribick for the pointer to this one.
by Paul Gilster | Aug 12, 2013 | Culture and Society |
As we approach Starship Congress in Dallas, the Institute for Interstellar Studies has announced the creation of the Alpha Centauri Prize Awards, the first of which will be the ‘Progenitor Award,’ to be bestowed at this year’s Starship Congress on August 18. The winner will receive a certificate and $500 cash award donated by Icarus Interstellar, the organization behind the Dallas meetings. The winner is to be chosen from among those presenting at the Starship Congress.
The Dallas gathering that convenes this Thursday will be the third major interstellar conference so far this year, following conclaves in Huntsville (Tennessee Valley Interstellar Workshop), San Diego (Starship Century) and preceding September’s 100 Year Starship Symposium. In addition, a conference on The Philosophy of the Starship was held by I4IS in London in May. In that suddenly quickened climate for interstellar studies the judges for the Alpha Centauri Prize Progenitor Award are being asked to make their selections based on originality, direct relevance to interstellar flight, and the potential of the work to be viable both technologically and economically in this century.
Addendum: Kelvin Long writes to tell me that another Starship Century conference will take place in London this October, along with a conference on Project Icarus. More on these when I have further information.
The Progenitor Award is the first of what is to become a series of awards in deep space design. Subsequent awards are, according to an email from I4IS executive director Kelvin Long, to be launched over the next two years. The intent is to provide incentives for design work of relevance to interstellar flight. A recent post on the Institute for Interstellar Studies website offers this:
We will set technical standards for physicists, engineers, biologists and scientists to reach for, harnessing the skills of old, and building the skills of new. We will foster and encourage pathways to new design concepts which solve old problems, and generate insights into new ones…
As explained in the I4IS post, the larger motivation is to adopt the lessons learned in the Ansari X-Prize competition to spur innovation and create technical developments in interstellar research. Here the future competitions become ambitious indeed, encompassing design studies by international teams of 6-10 designers, with each team submitting a final report to a judging panel after a development time of one year, and recurrent competitions taking place every two to three years. The plan is to drive design work in a host of projected technologies:
The Alpha Centauri Prize would be an international competition that has the function of incentivizing research, contributing technical knowledge, developing designer capability whilst inspiring the public towards the vision of interstellar flight. It is one of the best ways to advance the prospects for interstellar travel, and to have separate design studies, which could be derived, iterated and improved. Over time, the concept would be worked upon by future generations and ultimately lead to a direct design blue print for an interstellar probe after several decades of running. Like the BIS/Icarus Interstellar Project Icarus and the soon to be announced I4IS Project Dragonfly, it is the hope that other teams around the world would be assembled to work on specific proposals investigated historically such as NERVA, Starwisp, Vista, Longshot, AIMStar, Orion or one of the many others.
Long and Icarus Interstellar’s Richard Obousy created Project Icarus in 2009 as both a continuation and redefinition of the 1970s era Project Daedalus. The competition foreseen in the Alpha Centauri Prize, unlike Icarus, does not focus on a single propulsion system but considers all options from solar sails to antimatter, eschewing redesigns of historical work to create what the site describes as ‘new and innovative design concepts.’ I4IS envisions competitions taking place every two to three years to increase the technological readiness of different propulsion schemes, with eventual cash prizes in the $10,000 to $100,000 range:
After running the competition for two decades we may find that what may emerge is not a single choice for going to the stars in the coming centuries, but instead a realization that it is a combination of approaches with highly optimized engineering designs that will be the way to go. This may suggest hybrid propulsion schemes and could for example be along the lines of a fusion-based drive with anti-proton catalyzed reactions but using a nuclear electric engine for supplementary power and perhaps a solar sail and MagSail for solar system escape or upon arrival. From the two decades of research will develop reliable engineering studies, practical progress of the technology and several clear front runner designs to focus initially divergent research options towards the proper investment into the clear front runner designs by a process of gradual down select.
Competitions have proven their worth in aviation and aerospace (think Lindbergh and Burt Rutan), but in those cases we were dealing with existing or near-term technology and building hardware. What I4IS intends with the Alpha Centauri Prize is to turn the same principles to work at design studies that will surely out-run present-day engineering. It’s an idea that worked with the volunteer teams that have designed Daedalus and are now designing Project Icarus. With government funding all but non-existent on most of these concepts, it’s heartening to think that philanthropic alternatives can be found to push studies across the spectrum of propulsion options. A torrent of research papers would be a welcome outcome of such competitions.
by Paul Gilster | Apr 25, 2013 | Deep Sky Astronomy & Telescopes |
Apropos of yesterday’s article on the discovery of Proxima Centauri, it’s worth noting that Murray Leinster’s story “Proxima Centauri,” which ran in Astounding Stories in March of 1935, was published just seven years after H. A. Alden’s parallax findings demonstrated beyond all doubt that Proxima was the closest star to the Sun, vindicating both Robert Innes and J. G. E. G. Voûte. Leinster’s mile-wide starship makes the first interstellar crossing only to encounter a race of intelligent plants, the first science fiction story I know of to tackle the voyage to this star.
The work surrounding Proxima Centauri was intensive, but another fast-moving star called Gamma Draconis in Draco, now known to be about 154 light years from Earth thanks to the precision measurements of the Hipparcos astrometry satellite, might have superseded it. About 70 percent more massive than the Sun, Gamma Draconis has an optical companion that may be an M-dwarf at about 1000 AU from the parent. Its bid for history came from the work of an astronomer named James Bradley, who tried without success to measure its parallax. Bradley was working in the early 18th Century on the problem and found no apparent motion.
Stellar parallax turned out to be too small an effect for Bradley’s instruments to measure. Most Centauri Dreams readers will be familiar with the notion of observing the same object from first one, then the other side of the Earth’s orbit, looking to determine from the angles thus presented the distance to the object. It’s no wonder that such measurements were beyond the efforts of early astronomers and the apparent lack of parallax served as an argument against heliocentrism. A lack of parallax implied a far greater distance to the stars than was then thought possible, and what seemed to be an unreasonable void between the planets and the stars.
It would fall to the German astronomer Friedrich Wilhelm Bessel to make the first successful measurement of stellar parallax, using a device called a heliometer, which was originally designed to measure the variation of the Sun’s diameter at different times of the year. As so often happens in these matters, Bessel was working on 61 Cygni at the same time that another astronomer — his friend Thomas Henderson — was trying to come up with a parallax reading for Alpha Centauri. Henderson had been tipped off by an observer on St. Helena who was charting star positions for the British East India Company that Alpha Centauri had a large proper motion.
Henderson was at that time observing at the Cape of Good Hope, using what turned out to be slightly defective equipment that may have contributed to his delays in getting the Alpha Centauri parallax into circulation. In any event, Bessel’s heliometer method proved superior to Henderson’s mural circle and Dollond transit (see this Astronomical Society of Southern Africa page for more on these instruments), and Bessel’s findings on 61 Cygni were accepted by the Royal Astronomical Society in London in 1842, while Henderson’s own figures were questioned.
Bessel thus goes down as the first to demonstrate stellar parallax. Henderson went on to tighten up his own readings on Alpha Centauri, using measurements taken by his successor at the Royal Observatory at the Cape of Good Hope, but it took several decades for the modern value of the parallax to be established. But both astronomers were on to the essential fact that parallax was coming within the capabilities of the instruments of their time, and by the end of the 19th Century, about 60 stellar parallaxes had been obtained. The parallax of Proxima Centauri, for the record, is now known to be 0.7687 ± 0.0003 arcsec, the largest of any star yet found.
Image: A portrait of the German mathematician Friedrich Wilhelm Bessel by the Danish portrait painter Christian Albrecht Jensen. Credit: Wikimedia Commons.
While the Hipparcos satellite was able to extend the parallax method dramatically, it falls to the upcoming Gaia mission to measure parallax angles down to an accuracy of 10 microarcseconds, meaning we should be able to firm up distances to stars tens of thousands of light years from the Earth. Indeed, working with stars down to magnitude 20 (400,000 times fainter than can be seen with the naked eye), Gaia will be able to measure the distance of stars as far away as the galactic center to an accuracy of 20 percent. The Gaia mission’s planners aim to develop a catalog encompassing fully one billion stars, producing a three-dimensional star map that will not only contain newly discovered extrasolar planets but brown dwarfs and thousands of other objects useful in understanding the evolution of the Milky Way.
One can only imagine what the earliest reckoners of stellar distance would have made of all this. Archimedes followed the heliocentric astronomer Aristarchus in calculating that the distance to the stars, compared to the Sun, was proportionally as far away as the ratio of the radius of the Earth was to the distance to the Sun (thanks to Adam Crowl for this reference). Using the figures he was working with, that works out to a stellar distance of 100 million Earth radii, a figure then all but inconceivable. If we translated into our modern values for these parameters, the stars Aristarchus was charting would be 6.378 x 1011 (637,800,000,000) kilometers away. The actual distance to Alpha Centauri is now known to be roughly 40 trillion (4 x 1013) kilometers.
by Paul Gilster | Feb 15, 2013 | Culture and Society |
If the title of this piece conjures up exotic images, that’s all to the good. In fact, I’m surprised that “Alpha Centauri Sunrise” hasn’t been the title of a science fiction story somewhere along the line, but a quick check shows no such reference. Thus when Robert Kennedy (The Ultimax Group) created a drink called the Alpha Centauri Sunrise at our recent conclave in Huntsville, he was breaking new ground. And maybe images of a double sunrise also came to mind, the view from an as yet undiscovered world where Centauri A is a bright flare in the morning sky while a still closer Centauri B begins to nudge up over the hills, flooding the scene with orange light.
And what happened to Proxima Centauri? It would not be a factor in a scene like that, its light so dim that it would not stand out from other stars in a completely dark sky. Only its proper motion would alert local astronomers to how close it was (roughly 15000 AU). But let’s drink to Proxima anyway. I promised the recipe for the Alpha Centauri Sunrise two weeks ago and it’s time to deliver, as a number of readers have reminded me. Here we have it, from the pen (or keyboard) of Robert Kennedy himself:
The Alpha Centauri Sunrise
Best served in a martini glass or a champagne flute to accentuate the color gradient.
Ingredients:
1 jigger Tennessee moonshine;
2 jiggers Red Bull (different cognate but Centauri sorta suggests a bull, plus one of the stars is reddish);
4-6 oz. orange juice;
½ tsp. grenadine (to make the red-to-yellow color gradient);
three little red berries (to represent the three suns of the triple star system: α Centauri A, α Centauri B, and Proxima Centauri.
If, after all these puns, your customer still doesn’t crack a smile, then instead of three little red berries, give him a big raspberry (literally or figuratively).
With an Alpha Centauri Sunrise in hand, you might want to recall some of the great science fiction venues where drinks like this might be served. Callahan’s Place is the creation of Spider Robinson (it’s immortalized in Callahan’s Crosstime Saloon), a place where time travelers make the occasional appearance and aliens from a variety of worlds might wander in at any time. Robinson devotees will recognize ‘Callahan’s Law’: “Shared pain is lessened, shared joy, increased – thus do we refute entropy.” And I would say that refuting entropy is a task worth accomplishing.
Callahan’s, of course, had forerunners, among them Gavagan’s Bar, which was the work of Fletcher Pratt and L. Sprague de Camp, depicted in a series of tall tales (most of them, I believe, written for John Campbell’s Unknown) and collected into a 1953 book. The one that comes most readily to my mind, though, is Arthur C. Clarke’s Tales from the White Hart, a 1957 collection that brought science fiction and pub culture to a triumphant peak. These stories are still lively today, and recall a time when the members of the British Interplanetary Society and science fiction fans met regularly at such venues.
Image: Alpha Centauri Sunrise creator Robert Kennedy with the finished product.
Of course there are wonderful movie and TV bars in science fiction, from the Mos Eisley Cantina on Tatooine (Star Wars) to Star Trek‘s Ten Forward, which is where I would have spent my time on the Enterprise whenever possible. But Britain’s pub culture gave birth not only to Clarke’s Harry Purvis, the raconteur who spun his tales, but also to the British Interplanetary Society’s later work on Project Daedalus, the fusion starship design. Much of their work took place in a pub called the Mason’s Arms, where propulsion concepts and target stars offered just as magical a look at reality as anything in the White Hart. Thus it’s written SF and the White Hart I come back to when thinking of starships and bars.
Harry Purvis could hardly have had better companions than I had in Huntsville. Looking toward next week, I’ll be tapping the ideas of one of these, Ken Roy (The Ultimax Group), whose thoughts on colonizing outer system and deep space objects dovetail beautifully into my own thinking on gradual expansion into the Oort Cloud. Ken is a frequent contributor with colleagues Robert Kennedy and David Fields in venues like JBIS and Acta Astronautica. We’ll soon be looking at an unusual take on terraforming and how it might transform human expansion.
Image: Robert Kennedy’s co-author Ken Roy (left) and ‘neighbor/fellow habitué of the Friday Night Dinner Club’ John Preston, with Alpha Centauri Sunrises in hand.
by Paul Gilster | Jan 23, 2013 | Communications and Navigation |
Back when I was working on my Centauri Dreams book, JPL’s James Lesh told me that the right way to do communications from Alpha Centauri was to use a laser. The problem is simple enough: Radio signals fall off in intensity with the square of their distance, so that a spacecraft twice as far from Earth as another sends back a signal with four times less the strength. Translate that into deep space terms and you’ve got a problem. Voyager puts out a 23-watt signal that has now spread to over one thousand times the diameter of the Earth. And we’re talking about a signal 20 billion times less powerful than the power to run a digital wristwatch.
Now imagine being in Alpha Centauri space and radiating back a radio signal that is 81,000,000 times weaker than what Voyager 2 sent back from Neptune. But lasers can help in a major way. Dispersion of the signal is negligible compared to radio, and optical signals can carry more information. Lesh is not a propulsion man so he leaves the problem of getting to Alpha Centauri to others. But his point was that if you could get a laser installation about the size of the Hubble Space Telescope into Centauri space, you could send back a useful datastream to Earth.
The probe would do that using a 20-watt laser system that would lock onto the Sun as its reference point and beam its signals to a 10-meter telescope in Earth orbit (placed there to avoid absorption effects in the atmosphere). It’s still a tough catch, because you’d have to use optical filters to remove the bright light of the Alpha Centauri system while retaining the laser signal.
But while the propulsion conundrum continues to bedevil us, progress on the laser front is heartening, as witness this news release from Goddard Space Flight Center. Scientists working with the Lunar Reconnaissance Orbiter have successfully beamed an image of the Mona Lisa to the spacecraft, sending the image embedded in laser pulses that normally track the spacecraft. It’s a matter of simultaneous laser communication and tracking, says David Smith (MIT), principal investigator on the LRA’s Lunar Orbiter Laser Altimeter instrument:
“This is the first time anyone has achieved one-way laser communication at planetary distances. In the near future, this type of simple laser communication might serve as a backup for the radio communication that satellites use. In the more distant future, it may allow communication at higher data rates than present radio links can provide.”
The Lunar Reconnaissance Orbiter, I was surprised to find, is the only satellite in orbit around a body other than Earth that is being tracked by laser, making it the ideal tool for demonstrating at least one-way laser communications. The work involved breaking the Mona Lisa into a 152 x 200 pixel array, with each pixel converted into a shade of gray represented by a number between 0 and 4095. According to the news release: “Each pixel was transmitted by a laser pulse, with the pulse being fired in one of 4,096 possible time slots during a brief time window allotted for laser tracking. The complete image was transmitted at a data rate of about 300 bits per second.”
Image: NASA Goddard scientists transmitted an image of the Mona Lisa from Earth to the Lunar Reconnaissance Orbiter at the moon by piggybacking on laser pulses that routinely track the spacecraft. Credit: NASA Goddard Space Flight Center
The image was then returned to Earth using the spacecraft’s radio telemetry system. We’ll soon see where this leads, for NASA’s Lunar Atmosphere and Dust Environment Explorer mission will include further laser communications demonstrations. The robotic mission is scheduled for launch this year, and will in turn be followed by the Laser Communications Relay Demonstration (LCRD), scheduled for a 2017 launch aboard a Loral commercial satellite. LCRD will be NASA’s first long-duration optical communications mission, one that the agency considers part of the roadmap for construction of a space communications system based on lasers.
If we can make this work, data rates ten to one hundred times higher than available through traditional radio frequency systems can emerge using the same mass and power. Or you can go the other route (especially given payload constraints for deep space missions) and get the same data rate using much less mass and power. The LCRD demonstrator will help us see what’s ahead.
In any case, it’s clear that something has to give when we think about leaving the Solar System. Claudio Maccone has gone to work on bit error rate, that essential measure of signal quality that takes the erroneous bits received divided by the total number of bits transmitted. Suppose you tried to monitor a probe in Alpha Centauri space using one of the Deep Space Network’s 70-meter dishes. Maccone assumes a 12-meter inflatable antenna aboard the spacecraft, a link frequency in the Ka band (32 GHz), a bit rate of 32 kbps, and forty watts of transmitting power.
The result: A 50 percent probability of errors. We discussed all this in these pages a couple of years back in The Gravitational Lens and Communications, so I won’t rehash the whole thing other than to say that using the same parameters but working with a FOCAL probe using the Sun’s gravitational lens at 550 AU and beyond, Maccone shows that forty watts of transmitting power produce entirely acceptable bit error rates. Here again you have to have a probe in place before this kind of data return can begin, but getting a FOCAL probe into position could pay off in lowering the mass of the communications package aboard the interstellar probe.
Whether using radio frequencies or lasers, communicating with a probe around another star presents us with huge challenges. James Lesh’s paper on laser communications around Alpha Centauri is Lesh, C. J. Ruggier, and R. J. Cesarone, “Space Communications Technologies for Interstellar Missions,” Journal of the British Interplanetary Society 49 (1996): 7-14. Claudio Maccone’s paper is “Interstellar radio links enhanced by exploiting the Sun as a Gravitational Lens,” Acta Astronautica Vol. 68, Issues 1-2 (2011), pp. 76-84.
