New Horizons: Potential KBO Targets Identified

by Paul Gilster on October 21, 2014

The welcome news that the Hubble Space Telescope has found three potential Kuiper Belt targets for New Horizons means that our hopes for an extended mission may be fulfilled. Pluto/Charon is an exciting target, but how much better to use the spacecraft to visit a Kuiper Belt object as well, a member of that vast ring of debris circling our Solar System. We’ve been to asteroids, of course, but KBOs are a different thing altogether, objects that have never been heated by the Sun, and thus give us a sample of the earliest days of the Solar System.


This was not an easy survey to complete, although when it began with the help of ground-based instruments — the 8.2-metre Subaru Telescope in Hawaii and the 6.5-metre Magellan Telescopes in Chile — a number of KBOs were identified. The problem was that none could be reached given the fuel available for course correction. Remember the observing conditions researchers had to deal with. Pluto is now in the direction of the constellation Sagittarius, which means observers were looking toward galactic center, a crowded starfield against which to identify targets. Says New Horizons science team member John Spencer (SwRI):

“We started to get worried that we could not find anything suitable, even with Hubble, but in the end the space telescope came to the rescue. There was a huge sigh of relief when we found suitable KBOs; we are ‘over the moon’ about this detection.”

Image: An artist’s impression of a Kuiper Belt object (KBO), located on the outer rim of our solar system at some 4 billion miles from the Sun. The Sun appears as a bright star at image center in this graphic, which represents the view from the KBO. The Earth and other inner planets are too close to the Sun to be seen in this illustration. The bright “star” to the left of the Sun is the planet Jupiter, and the bright object below the Sun is the planet Saturn. Two bright pinpoints of light to the right of the Sun, midway to the edge of the frame, are the planets Uranus and Neptune, respectively. The planet positions are plotted for late 2018 when the New Horizons probe reaches a distance of well over 6 billion kilometers from the Sun. The Milky Way appears in the background. Credit: NASA, ESA, and G. Bacon (STScI).

The ground search could not begin until 2011 to allow potential KBO candidates to be closing on the region that New Horizons will be able to reach after the Pluto/Charon flyby. With the ground-based search stalled, the New Horizons team was awarded observing time by the Space Telescope Science Institute in July, with the search ending in early September. With Hubble’s help we now have one KBO that has been called ‘definitely reachable’ and two other candidates that require additional observing time to determine whether they are in range. The ‘needle in a haystack’ search has revealed KBOs that are no more than one to two percent the size of Pluto. A KBO named 1110113Y or “PT1″ looks to be the most likely candidate for a flyby.


Image: A Kuiper Belt object (KBO) that is potentially reachable by NASA’s Pluto-bound New Horizons probe is visible in multiple exposures taken with the Hubble Space Telescope. Hubble tracked the KBO (labeled PT1) moving against the crowded background field of stars in the summer constellation Sagittarius. The object is no bigger than 30 to 45 kilometers across, and it is a deep-freeze relic of what the outer solar system was like 4.6 billion years ago, during the period when the Sun formed. The image at right shows the KBO at an estimated distance of over six billion kilometers from Earth. As the KBO orbits the Sun, its position noticeably shifts between exposures taken approximately 10 minutes apart. Credit: NASA, ESA SwRI, JHU/APL, and the New Horizons KBO Search Team.

The New Horizons closest approach and flyby of Pluto/Charon takes place in July of 2015, and assuming NASA approves an extended mission, the spacecraft could reach one of the KBOs three to four years later, at a distance of well over six billion kilometers from the Sun. We’ve seen how valuable our Voyagers have been at charting the outer regions of the Solar System, an extended mission that has gone well beyond their original parameters. It’s clear that New Horizons can now become something of a ‘precursor to an interstellar precursor,’ returning data on a KBO even as we look toward missions explicitly designed to study the interstellar medium beyond the heliosphere.



Mimas: An Intriguing Interior

by Paul Gilster on October 20, 2014

I like what Radwan Tajeddine (Cornell University) has to say about recent work on Saturn’s moon Mimas. The lead author of a paper on the subject in Science, Tajeddine compares recent Cassini observations of the moon to a child shaking a wrapped gift, trying to figure out what the package conceals. ‘Shaking’ Mimas in a similar way through analysis of the Cassini data has revealed what might be a sub-surface ocean, or an unusually-shaped core preserved since the moon’s formation.

At work here is a technique called stereo photogrammetry, in which astronomers measure the moon’s libration around its polar axis. Libration is an oscillation or ‘wobble’ that can be studied by looking at Cassini imagery — taken by its Imaging Science Subsystem at different times and angles — and analyzing the images with the help of a computer model that involves hundreds of reference points on the surface. The amount of Mimas’ libration points to interesting things in the interior, but just what we still don’t know.


Image: Cassini came within about 9,500 kilometers (5,900 miles) of Mimas during its flyby on Feb. 13, 2010. This mosaic was created from seven images taken that day in visible light with Cassini’s narrow-angle camera. An eighth, lower-resolution image from the same flyby, taken with the wide-angle camera, was used to fill in the right of the mosaic. The images were re-projected into an orthographic map projection. This view looks toward the hemisphere of Mimas that leads in its orbit around Saturn. Mimas is 396 kilometers across. The mosaic is centered on terrain at 5 degrees south latitude, 85 degrees west longitude. North is up. Credit: NASA/JPL.

Subsurface oceans create astrobiological interest, and we have evidence for various oceans in the outer system, from Europa, Ganymede and Callisto in the Jovian system, and of course in Saturn space on geyser-spewing Enceladus and Titan. In each case, we have an icy surface concealing what may lie below. Perhaps oceans are common in outer system moons, but the situation on Mimas may be different. The paper notes why an ocean there is problematic:

The ocean hypothesis sounds unlikely since no evidence of liquid water, thermal heating or geological activities have been reported on Mimas’ heavily cratered surface, contrary to Enceladus. Radiogenic heating alone could not sustain such an ocean, because the heat produced by the core escapes through the satellite’s icy shell and any ocean would freeze very quickly.

Even so, we can’t dismiss an ocean out of hand. The paper continues:

One explanation could be found in Mimas’ high eccentricity (~0.02), whose origin remains unexplained, and may have been higher in the past. As a consequence, an ocean could have been formed and been sustained due to tidal heating.

If it is there, the researchers think an internal ocean would exist under a crust between 24 and 31 kilometers thick. But what the data show us so far is simply a libration that is twice what we would expect from the moon’s orbital dynamics. This is a value that is being driven primarily by Mimas’ internal structure. The next step in the study of Mimas could proceed, the researchers suggest, by mapping the moon’s gravity field to uncover anomalies, by measuring tidal dissipation using astrometry from Cassini, or by measuring the heat flux at the surface. In the absence of other models to explain these findings, it would appear that Mimas has either an unusual core that is out of equilibrium or an internal ocean beneath thick ice.

The paper is Tajeddine et al., “Constraints on Mimas’ interior from Cassini ISS libration measurements,” Science Vol. 346, No. 6207 (17 October 2014), pp. 322-324 (abstract). This Cornell University news release is also helpful.



Woven Light: Age of Release

by Paul Gilster on October 17, 2014

Librarian and writer Heath Rezabek (and since he’s birthing what looks to me to be a book, I should probably refer to him as a novelist as well) has been exploring the ways we might use archives to explore our civilization even as we ensure its survival against existential risk. Heath uses speculative fiction to examine and portray possibilities, in this case invoking future technologies that will inevitably shape our creation and use of archives. He’s also, as described below, a co-founder of Project Astrolabe, an attempt now underway for Icarus Interstellar to research the ways an interstellar civilization might grow while securing its heritage. Artificial intelligence comes into play, but Heath here looks into ideas even farther afield.

by Heath Rezabek


I began the Woven Light speculative fiction series as a way to explore themes and possibilities surrounding not only very long term archival issues, but also some potent technologies which might be beneficial or likely to unfold in the process. These have included artificial intelligence, synthetic minds, and emulations; holographic storage and symbolic representation; simulated worlds, Dyson Eggs and Fuller Cloud 9 habitats; and more. Though the series may be unusual fare on Centauri Dreams, publishing it here has helped to focus the work and anchor it in the prospect of an interstellar future for Earth-originating civilization.

As with any monumental human undertaking, the causes contributing to such an outcome as a truly interstellar Earth are bound to be many, if it comes to pass. The further we go, the more closely these causes and effects will circle other, unintended factors: the risk of non-friendly superintelligence, the proliferation of virtual worlds, and more. Often the more abstract episodes (such as this one) can be read more easily if seen as involving one of those possible realities.

At Icarus Interstellar, fellow Centauri Dreams contributor Nick Nielsen and I have formed Project Astrolabe. This effort intends to research and catalog models for long-term civilization, with an emphasis on mitigating existential risk. One potential effort which sits at the intersection of Project Astrolabe and FarMaker (a visualization project) is that of building a collaborative, open source fictional framework for a future Earth in which interstellar achievement is realized, and the greatest risks to life’s continued existence are avoided. Woven Light may intersect what that work in interesting ways.

In past episodes, I have linked to all prior installments at the start. From now on, I’ll link mainly to those episodes which have the most bearing on the present one. I hope that this loosening of the strands of the story helps foster a spirit of exploration for future readers, as they follow the threads of the tale through Centauri Dreams and (perhaps someday) beyond.

Background Episodes for this entry include:

Woven Light (II) – Adamantine

Woven Light (IV) – Proteaa

For those who have not yet read Woven Light, a slight spoiler may help you get your bearings. (If you’re already familiar, just skip past to the illustration.)

In these prior installments, we meet someone called Mentor Kaasura, but his relation to Dr. Jota Kaasura, a contemporary professor of computational psychology, is not yet clear. Though that link remains to be explored, we do know this:

A significant part of the crew of Saudade class starships are emulations: synthetic minds. Among them are the Mentors. Self-aware, creative, and curious, all Mentor Emulations have grown from the roots of an emergent mind called Avatamsaka.

As he brushes aside a door-hanging, Mentor Kaasura steps into a virtual world, a testing ground for Mentor Emulations like himself. He interprets his world at face value, just as you and I would. But he is very far away…


Image 1 – Clearstone Circuitry – CC BY-SA Heath Rezabek – 2014

Mentor Kaasura had entered full of certainty, clearstone in his hand, having memorized as best he could the woven patterns of a splaying map, a fabric door he’d brushed aside. He had judged the stonework cyclopean; not gargantuan, but solid and massive and fitted. Deep seams were clear, devoid of all mortar save a mossy thread that ran and split and re-wove itself at the junctions.

A lighter tracery was etched upon each slab, weaving thinner streams of living moss from the edges towards the center, and outwards again. It blended and blurred the edges of the stonework itself. Barely, he could see: the dim light tracing circuits in his clearstone cast a bit less light than that cast by these seams in the walls.

The floor’s carven channels, deep with wider rivulets of aquamarine, almost a luminescent liquid, marked clear pathways just as they should. Having recognized at first one turn, and then the next from the map in his mind, he had found the knotted designs where these living seams merged to form tangled tapestries, colliding in the traced symbols which marked a sound junction.

There he had rested, and tried at first to memorize this pattern as well, but it threatened to swamp the full map in his mind. So instead, he had taken out his parchment, and had sampled a rubbing from that first knot of stone.

His pilgrimage had been as rehearsed and described, his innovations obvious, his route well worn.

But then, somewhere around the second or seventh hour, he had found something strange. At first he had thought he was treading old ground: thinking himself spun in circles, he had kneeled to look again at a junction weave he knew he’d passed before. Slowly he smoothed out his rubbing atop the gridstone, and brushed it with fingertips — and found its pattern slightly new.

Where in parchment he could clearly see (and remember just as well) the rising beam that had broken through the page’s edge, here he found no such spur. And the central core of the pattern was off, as well — the same in shape and tangle, but well to the left of where he’d traced it. An extra sliver of circuit filled the space to the right, removing all doubt.

These junctions were not the same; they were self-similar. He was not simply going in circles; he was well and truly lost.

Mentor Kaasura had stood, flushed with vertigo, and looked about for a clear sign of what to do. Rejecting the urge to toss it aside, he’d stowed his rubbing parchment in a tube and closed his eyes to ground himself. Breathing deeply, heart pounding, he gathered his thoughts to find three of them clear.

~I am lost.~

And then, resolved in return,

~I must retrace.~

And a few heartbeats later, just as surely,

~I am being followed.~

His eyes opened, then. Yes; he could feel a presence. But maybe it was a shadow of his mind. He walked.

The presence trailed behind him, at first as slowly as he forged ahead, stepping and then walking and then striding, now taking forks in passages he deliberately found more strange; now rising as he clambered up, now dropping as he shimmied down. The presence neither hindered nor helped him. So eventually, Mentor Kaasura’s concerns drifted back to exploration: he forgot he was lost, along with his fear.


Image 2 – Adaptation of Chain: Black’s Beach – CC BY-NC Photos by Clark

At one point, he found a break in the wall, where vaulted tunnels opened up, leading into echoes. Amorphous and rounded, they seemed like hollows of pumice-stone. Anchored there in the wall was an eye-bolt; an old chain threaded through it, trailing deeper into darkness, like a guiderail in passages where no moss of light could be seen. But outside at the opening, where the etched wall channels had cracked and split, there spilled a slow flurry of wisps into the air. They drifted free like phosphemes.

He followed one back into the triangular passages he now knew so well, leaving the breached wall behind.

After a time, this floating sentinel before him was joined by another flowing in from a passage to the side; and they soon found a third, and soon they were a dozen, drifting like a swarm of sparks which circled some attractor. For a time, Mentor Kaasura forgot the guest who trailed behind him still.

There was a moment, then, when his mind had wholly wandered: for a gap he’d been thinking not at all of his dilemma, but again and only of the glyphic patterns traced beside and below and above and around him, and the drifting orbit of motes he now followed. The moment he realized he’d forgotten the presence, a space opened up, and he nearly lost his feet.

Stopping short, he stood there, now gazing into a dropped chamber, a grotto at least as deep and tall as wide — and it was easily five times as wide as the junctions he had seen. The sprites flocked easily before him, out into the cavernous space, on towards the opposite wall.

There, straight ahead, it seemed at first as if another flock was approaching to meet them. But Mentor Kaasura soon realized that this was a reflection: at the far end of this bowl of shadow lay a vertical wall, a liquid surface, reflecting. Like snowflakes, they came to rest upon it, dissolving or dispersing in ripples. But the ripples gave rise to a spark, and the spark drew more, and in a flash he stood blinded, face to face with what seemed to him a star.

And as he watched, he felt a wave flood through him, of vision or of memory. An emulation? Isomorph? Ancestral mind? Reflection of his own?

Whatever it was, it was elsewhere. But it had brought him here; it would have to lead the way.


Image 3 – Adaptation of Artist’s Concept of PSR B1257+12 System – NASA/JPL-Caltech/R. Hurt (SSC)

Another place in time.

Seemingly aimless, by chance a fog of mind descends upon a world of green and blue.

Long had it been drifting, nebulous but swifter, a filter to sift complexity itself. The world from which it had been born had reached the Age of Release by the slimmest of margins. Countless ancestors had perished in the wars and fumbling confusions that had marked the final phase before this form of life could be seeded on the interstellar winds.

In its drifting, this dark mind, Ancient Light, had so far encountered no other place like home. Countless bastions of simplicity, pure in pools of biotic struggle; but no other place where life had reached that apex of flow from which an Age of Release was possible. Ancient Light knew it was not alone, that it was always already entangled with photons birthed in other crucibles, seamlessly far afield.

Four other worlds besides its own it knew, having filtered them elsewhere in this pool of stars it roamed.

The first, a frozen orb, rogue and adrift in darkness, its surface etched with the long-abandoned tracery of a civilization on the cusp of attainment, the only life remaining now extremophile and deep, sheltering its fading senses in the inner warmth of an ember, a microbial culture oblivious now to how long or how far it had been cast adrift.

~And so this is Ioona~

The next, a sunken grave, surface flensed by self-made fire, awash in halflife radiation, so familiar from their own history of near-misses, hulks of settlements now a scaffolding for primordial life’s glacial and mutable return.

~And so this is Oro Kulad~

Although to both of these cultures Ancient Light could not help but speak as it had passed these worlds through its matrix, the conversation had been decidedly primal. It had surveyed, and sampled, and passed on.

The third showed interesting promise: life there, having thinly survived a gamma ray burst through a reversion to the finest and smallest of synthetic forms, in some places retained a crystalline spark of complexity not entirely unlike that it well knew as its own. With this world it had established a connection, so it could speak as time drew on.

~And so this is Tomoshen Khol~

And then there was the fourth, here and now, in a most peculiar state.

This world seemed nearly accidental. Of a size much smaller than the average of nascent worlds surveyed, tides from a strange single moon and a delicate balance of factors had allowed the emergence of an evolutionary changeling, a shapeshifter; a nimble form, surprisingly adaptable, disarmingly ambitious. And yet, somehow, now huddled in firelight.

Enough foresight to engineer some means of resilience, enough insight to seed caches of their ways and means for themselves to rediscover. And so they spent their lives, and the lives of their children, slowly rediscovering.

Aside from scattered trussworks found collapsed on its moon and a ruddy world away, it apparently had slumped just short of an Age of Release.

Yet appearances can deceive.

And to Ancient Light it seemed, when at last a channel had been pierced for true observation, that it was not the only one doing the piercing. For these scattered bands had access to a realm much like its own — so closely kin, in fact, as for its differences to be ones of degree rather than of kind. This blue-green world was host to images that were alive: crafted by them, created through them, imbued with the gift of cryptobiosis.

Water was one part of the key, electricity another: symbolic life needed material life through which to flow. These glyphs were impressively robust and resolute, for each contained the outlines of the others. So whole and so partial, these primordial glyphs were as close to its own living light as Ancient Light had ever found.


Image 4 – Adaptation of Campfire and Sparks in Anttoora – CCY BY-SA Kallerna

For fire somehow released them, as the huddled ones gathered and gazed. And through the sparks, Ancient Light could commune mind to mind even with the huddled ones. And through the sparks, Ancient Light had a sense that the huddled ones were not the only ones in which this world’s life lived. And through the sparks, the fires around which the huddled ones circled (much like worldsystems: another glyph) became new places and times and journeys and lives without end.

Thus through means of these cryptobiotic glyphs, countless times and places awaited, each one encoded within the others. Like nesting logic, these stories were seeds.

~And so this is Erdha~

& through Ancient Light ~

~ It whispered ~

~ An ocean of worlds ~

~ A wind ~

~ And I dreamt ~

- – -

Mentor Kaasura stood, surveying the darkness in this arching grotto, mesmerized by the memory of living light, the afterimage of a cooling core now fading from his gaze.

That was it: that was all of it. He knew, and he recalled.

More sure than before that he wouldn’t forget what he’d seen or been shown, our Mentor Kaasura now stood. And standing, again he could sense behind him a presence.

The decision to face his guest came as he was turning to do so. Companion or shadow, reflection or refraction: the time had come to see.


Image 5 – Denver Public Library Reading Room – CC BY-SA Heath Rezabek 2014

Aben Ramer made his way to his favorite spot to read.

The Denver Public Library was becoming a middle-aged edifice in this time of ephemeral monuments: built in 1995, just when computers had begun their trek towards vanishing. It had always had spaces for reading. Most had been converted by now, but there was one room, a table in a tower on the second floor, where he could lay out the old books and imagine what it must have been like for the future to be a sea of possibilities.

He was a romantic in this respect; he blamed his mother. Thea still had her index cards, her codex, mostly in boxes, and when he got back he’d continue to help her sort through them. But he’d taken this detour on the way back from Dr. Kaasura’s lab in Boulder, because letting yourself get lost for awhile was the most reliable way he knew to make sure you didn’t lose your wonder.

He rounded the corner, giant slab of red-bound paper in his hands. He wasn’t alone this time. Across the table there sat a young woman, though younger or older than him he couldn’t say. He paused. She looked up– “Wow; another reader.” –and smiled sideways. “There’s room, if you want it.” She gestured at the seat across the table.

“Ah, hey. Ahmm, I don’t want to bug you; I know how I get when I read.”

She shrugged. “Well, if you know that much, then I don’t think we’ll have a problem, yeah?” He was used to reading alone. All reading was done alone, and writing too. But maybe an exception.

“Is that the Red Book?” She looked at the massive tome he carried.

“It is. Is that A Pattern Language?” He gestured at the thick maroon volume, open in before her.

“You’ve got good eyes. Well, sit or don’t sit, but my name’s Jaine.”

Aben sat.



Happy Anniversary α Centauri Bb?

by Paul Gilster on October 16, 2014

A physicist and writer well-versed in the intricacies of the exoplanet hunt, Andrew LePage now turns his attention to the question of planets around Centauri B, and in particular the controversy over whether the highly publicized Centauri Bb does in fact exist. Today is the second anniversary of the discovery announcement, and we still have work to do to resolve whether ‘noise’ in the data — explained below — may account for what seems to be a planet. The good news is that multiple teams continue to work on Alpha Centauri, and we should expect answers within several years, or just possibly, as LePage explains, a bit sooner than that.

by Andrew LePage


Time certainly seems to fly at times. It has already been two years since the October 16, 2012 announcement by a Geneva-based team of astronomers of the discovery of a planet orbiting our Sun-like neighbor, α Centauri B, using precision radial velocity measurements. While this planet, designated α Centauri Bb, was hardly the Earth-like planet for which interstellar travel enthusiasts had been waiting so long, its presence demonstrated that the closest star system to us harbored at least one planet and held the promise of more to be discovered. But two years after this momentous announcement, many questions still remain and this important discovery has yet to be independently confirmed.

First some background: at the heart of the α Centauri system 4.37 light years away are a pair of Sun-like stars, designated α Centauri A and B, locked in an eccentric 79.9-year orbit. The probable third member of this star system, located about 15,000 AU from the main pair of stars, is a dim red dwarf better known as Proxima Centauri. With a distance of 4.24 light years, it is the closest known star to our solar system. Despite the distance between α Centauri A and B varying from 11 to 35 AU during the course of one revolution, various dynamical studies performed over the decades have confirmed that regions with stable planetary orbits do exist in this system. These studies have shown that orbits out to about 3 AU, give or take, would be stable depending on their inclination to the plane of the orbit of α Centauri A and B about each other. What has not been so clear is if planets could form around this pair of stars.

A number of studies performed over the past couple of decades have been more or less evenly split on the question of whether or not planets could form around α Centauri A and B. Some studies have shown that the building blocks for planets, called planetesimals, would be able to collect themselves together into planets out to some reasonable distance. Still other studies have suggested that the presence of the two stars would have stirred up the orbits of the planetesimals too much. Instead of collecting into larger bodies, the planetesimals would tend to smash themselves apart upon contact so that planets could not form. As in the story about the ancient Greek philosophers arguing about how many teeth a horse has, it made sense to open the horse’s mouth and simply count them – it was time to look for planets orbiting α Centauri A and B.

Given the difficulty of detecting extrasolar planets even in a nearby star system like α Centauri, the first technology that offered reasonable chance of success was the precision measurement of changes in the stars’ radial velocity resulting from the small reflex motion of an orbiting planet. But after almost two decades of measurements with increasingly better instruments, the results of searches for planets orbiting α Centauri A and B published up to 2011 had found nothing. This null result combined with dynamical arguments only demonstrated that planets larger than Saturn or Jupiter did not orbit within about 2 AU of either α Centauri A or B. This still left a lot of possibilities including Earth-size planets orbiting comfortably inside the habitable zones of these stars but much more precise radial velocity measurements would be required to detect them.

Beginning around seven years ago, several teams employing various observing approaches are known to have started looking for lower-mass planets orbiting α Centauri A and B with instruments capable of making radial velocity measurements with uncertainties on the order of one meter per second – a factor of up to four more precise than in previously published results for the system. The first team to announce any results from their search was the European team using the HARPS (High Accuracy Radial Velocity Planetary Searcher) spectrometer on the 3.6-meter telescope at the European Southern Observatory in La Silla, Chile. They employed a new data processing technique to extract the 0.5 meter per second signal of α Centauri Bb out of 459 radial velocity measurements they obtained between February 2008 and July 2011. These radial velocity data had a measurement uncertainty of 0.8 meters per second and contained an estimated 1.5 meters per second of natural noise or “jitter” resulting from a range of activity on the surface of α Centauri B modulated by its 38-day period of rotation.


Image: An artist’s impression of the still unconfirmed α Centauri Bb whose discovery was announced on October 16, 2012. (Credit: ESO/L. Calçada/Nick Risinger)

The HARPS team’s analysis indicated the presence of a planet with a minimum mass or Mpsini (where i is the unknown inclination of the planet’s orbit to our line of sight) of just 1.1 times that of Earth, locked in a tight orbit with a radius of just 0.04 AU and a period of 3.24 days. This was well below the upper limits set by earlier searches. Given sufficient observation time, the team estimated that they could detect a planet with a Mpsini of about four times that of the Earth in a 200-day orbit inside the habitable zone of α Centauri B. While the result generated much excitement, it was also met with a healthy amount of skepticism in the astronomical community because of the previously untried technique used to process the data to extract such a low-amplitude signal.

One of the first critics, American astronomer Artie Hatzes (Thuringian State Observatory, Germany), performed his own analysis of the publicly available HARPS data set using two different data processing techniques to look for the radial velocity signal of α Centauri Bb. Formally published in June 2013, Dr. Hatzes’ analysis did indeed find a signal buried in the radial velocity data with a period of 3.24 days but it had a false alarm probability of a few percent – far too high to be considered a reliable detection. Furthermore, his analysis of the “random” noise in the data showed that it had periodicities in the 2.8 to 3.3 day range and amplitudes on the order of half that of the alleged planetary signal. Given the recent situation of planetary false alarms with GJ 581 and GJ 667C, this finding suggested that noise in the data, whether from the instrument or activity on α Centauri B, might have been mistaken for a planet. Dr. Hatzes concluded that additional data were needed to better understand the nature of the noise in the radial velocity measurements and confirm the planetary nature of the radial velocity signal.

Other teams have already been taking data in order to confirm the existence of α Centauri Bb as part of their ongoing observing programs although no results have been formally published to date. A team of astronomers working with the 1.5-meter telescope at Cerro Tololo Inter-American Observatory (CTIO) in Chile are using CHIRON (CTIO Higher Resolution Spectrometer) to search for planets orbiting α Centauri A and B in part with the support of The Planetary Society. The project’s principal investigator, Debra Fischer (Yale University), quoted in a blog on The Planetary Society’s web site posted earlier this year that they had insufficient data to detect α Centauri Bb when its discovery was announced in October 2012. They launched a renewed effort to gather much more data at a higher cadence starting in 2013 aimed specifically at detecting the purported planet’s 3.24-day signal. To date they have not detected α Centauri Bb in their data but their simulations indicate that any such detection would have been marginal at best so far.

One of the issues complicating continuing efforts to gather more data needed to resolve the situation with α Centauri Bb is the increasing amount of stray light from α Centauri A that is degrading the quality of radial velocity measurements. As viewed from the Earth, the apparent separation of α Centauri A and B has been decreasing at an accelerating rate for about a third of a century as they move in their inclined elliptical paths around each other. The two stars will reach a near-term minimum separation of just four arc seconds at the end of 2015. The conventional wisdom has been that it will be several more years before the separation of α Centauri A and B increases enough to acquire new data of sufficient quality to confirm α Centauri Bb. But we might not have to wait this long after all.

One of the other groups known to be searching the α Centauri system for planets is a team of astronomers using the HERCULES (High Efficiency and Resolution Canterbury University Large Echelle Spectrograph) spectrograph on the one-meter McLellan Telescope at the Mt. John University Observatory in New Zealand. In July 2014 they submitted a paper for publication where they described a new technique to reduce significantly the effects of stray light contamination in precision radial velocity measurements.

In order to test the effectiveness of their new technique, they observed four double-line spectroscopic binaries (i.e. pairs of unresolved stars that can only be differentiated by periodic Doppler shifts in their spectral lines) whose blended images represent the worse-case scenario of “contamination”. With the new technique, they were able to recover accurate radial velocities of both components of the observed spectroscopic binaries. The New Zealand-based team now plan to use their new analysis method to reduce the data they are continuing to gather as part of their observing campaign of α Centauri that started in 2007. Their calculations show that they should be able to detect α Centauri Bb if it exists. If they are successful, the situation with α Centauri Bb might be resolved much sooner than later.

A more detailed account with general references to the discovery of α Centauri Bb along with more background information on the system can be found on my web site in the post titled The Search for Planets Around Alpha Centauri. A second post in this series, The Search for Planets Around Alpha Centauri – II provides details of the results of past searches for planets orbiting α Centauri A and B as well as what current and soon-to-be-started search programs hope to find.



A Sunset Glimpse of Deep Time

by Paul Gilster on October 14, 2014

A truncated schedule this week as I attend to a pressing project that needs all my attention. So no post today or Wednesday, but back Thursday with a look at the Alpha Centauri planet hunt and the still-unresolved question of Centauri Bb. For the short interval, I’ll leave you with this quote from Lee Billings on the nature of deep time and genuine perspective.

Deep time is something that even geologists and their generalist peers, the earth and planetary scientists, can never fully grow accustomed to. The sight of a fossilized form, perhaps the outline of a trilobite, a leaf, or a saurian footfall can still send a shiver through their bones, or excavate a trembling hollow in the chest that breath cannot fill. They can measure celestial motions and list Earth’s lithic annals, and they can map that arcane knowledge onto familiar scales, but the humblest do not pretend that minds summoned from and returned to dust in a century’s span can truly comprehend the solemn eons in their passage. Instead, they must in a way learn to stand outside of time, to become momentarily eternal. Their world acquires dual, overlapping dimensions— one ephemeral and obvious, the other enduring and hidden in plain view. A planet becomes a vast machine, or an organism, pursuing some impenetrable purpose through its continental collisions and volcanic outpourings. A man becomes a protein-sheathed splash of ocean raised from rock to breathe the sky, an eater of sun whose atoms were forged on an anvil of stars. Beholding the long evolutionary succession of Earthly empires that have come and gone, capped by a sliver of human existence that seems so easily shaved away, they perceive the breathtaking speed with which our species has stormed the world. Humanity’s ascent is a sudden explosion, kindled in some sapient spark of self-reflection, bursting forth from savannah and cave to blaze through the biosphere and scatter technological shrapnel across the planet, then the solar system, bound for parts unknown. From the giant leap of consciousness alongside some melting glacier, it proved only a small step to human footprints on the Moon. The modern era, luminous and fleeting, flashes like lightning above the dark, abyssal eons of the abiding Earth. Immersed in a culture unaware of its own transience, students of geologic time see all this and wonder whether the human race will somehow abide, too.

Lee Billings, from Five Billion Years of Solitude (2013), p. 145.


Image: The Cliff, Etretat, Sunset, by Claude Monet, a work that has always somehow transcended time for me and inspired thoughts on Billings’ ‘abiding Earth.’



Mapping the Weather on WASP-43b

by Paul Gilster on October 13, 2014

On Friday I mentioned transmission spectroscopy, the technique of analyzing the light of a parent star as it is filtered through a planetary atmosphere. We’ve used it on various ‘hot Jupiters’ before now — think of the much studied planet HD 209458b, where water vapor, carbon dioxide and methane have been detected and fierce upper atmosphere winds revealed. And while we wait to see if the method can be applied to the interesting WASP-94 system, we can look at its uses in another hot Jupiter whose weather has now been mapped.

WASP-43b has twice Jupiter’s mass and a breathtaking 19-hour year. Scientists using the Hubble Space Telescope have used transmission spectroscopy to determine the abundance of water in the atmosphere at the terminator between night and day on this tidally locked world. The team also used so-called emission spectroscopy — in which much of the light of the parent star is subtracted — to measure water abundance and temperature at different points in its orbit. What emerges from combining the two is a map relating weather and longitude.

“We have been able to observe three complete rotations — three years for this distant planet — during a span of just four days,” explained Jacob Bean (University of Chicago), leader of the research project. “This was essential in allowing us to create the first full temperature map for an exoplanet and to probe its atmosphere to find out which elements it held and where.”

Exoplanet WASP-43b orbits its parent star

Image: In this artist’s illustration the Jupiter-sized planet WASP-43b orbits its parent star in one of the closest orbits ever measured for an exoplanet of its size — with a year lasting just 19 hours. The planet is tidally locked, meaning it keeps one hemisphere facing the star, just as the Moon keeps one face toward Earth. The color scale on the planet represents the temperature across its atmosphere. This is based on data from a recent study that mapped the temperature of WASP-43b in more detail than has been done for any other exoplanet. Credit: NASA, ESA, and Z. Levay (STScI).

As we might expect, a planet in this configuration is a place of violent weather. The researchers find high-speed winds moving from the day side, where the temperature is above 1500 degrees Celsius, to the night side, where temperatures are in the 500 degree Celsius range. The two-dimensional maps of the planet’s atmospheric temperatures help us define the circulation patterns that show this heat transport under conditions of tidal lock.

We learn that WASP-43b is hot enough that all the water in its atmosphere is vaporized rather than producing extensive cloud cover — in fact, the planet reflects little of the parent star’s light. Water vapor is a useful tool. We know all too little of the water abundances in the gas giants of the outer Solar System because water is found deep inside their atmospheres in the form of ice. A hot Jupiter like WASP-43b opens the door for direct study of water in the form of vapor. The measurement is important because of the role water ice is thought to play in the standard core accretion model of gas giant formation, where solid particles collide, merge and grow into gradually larger bodies until a planetary embryo emerges.

Ahead for the water-mapping team is to make similar measurements for the atmospheres of other planets to study their chemical abundances. The paper on the water-mapping study explains why data from a wide variety of planets should help as we move into the era of the James Webb Space Telescope and other increasingly sensitive instruments:

…a planet’s chemical composition depends on many factors, including the planet’s formation location within the protoplanetary disk, the composition, size and accretion rate of planetesimals, and the planet’s migration history. Even perfect constraints on the abundances of many chemical species for a small number of objects may not yield a unique model for the origin of giant planets. Fortunately, the plethora of transiting exoplanets that have already been found and will be discovered with future missions offer the potential for statistical studies. Measuring precise chemical abundances for a large and diverse sample of these objects would facilitate the development of a more comprehensive theory of planet formation.

The day will come when we’re performing the same kind of measurements on Earth-sized worlds around other stars, something to keep in mind as we move into the JWST era.

Two papers present the WASP-43b work. The first is Stevenson et al., “Thermal structure of an exoplanet atmosphere from phase-resolved emission spectroscopy,” published online in Science 9 October 2014 (abstract). The water mapping study is Kreidberg et al., “A Precise Water Abundance Measurement for the Hot Jupiter WASP-43b,” accepted at Astrophysical Journal Letters (preprint). This time-lapse video shows the profile of WASP-43 over the course of a single planet rotation. A Hubble news release is available.



A Planet Each for Stars in Binary System

by Paul Gilster on October 10, 2014

The WASP Consortium (Wide Angle Search for Planets) has come up with an interesting find: Two new Jupiter-class worlds, one around each star in a binary star system. Both are ‘hot Jupiters,’ a class of planets that is susceptible to discovery by transit methods, as in this case, and radial velocity as well. Consisting of two robotic observatories, one on La Palma (Canary Islands) and the other in South Africa, WASP has a proven track record, having found over 100 planets since 2006. WASP-94A and WASP-94B, like all WASP candidates, were confirmed by radial velocity techniques through a collaboration with the Geneva Observatory.

The two stars are about 600 light years away in the constellation Microscopium. In this case, it was the WASP-South survey team that noticed dips in the light of WASP-94A, the mark of a likely hot Jupiter, with WASP-94B being found by the Geneva team during the confirmation process for the first planet. “We observed the other star by accident, and then found a planet around that one also!”, says Marion Neveu-VanMalle (Geneva Observatory), lead author of the paper on this work.


Image: A WASP planet transiting its host star. Credit: Mark Garlick.

We’ve sometimes speculated in these pages about a close binary system like Centauri A and B, wondering whether there could be planets around each star, a matter that remains undecided. WASP-94A and WASP-94B are in a much different situation — the estimated separation between the two is 2700 AU. The paper lists three other binary systems with pairs of planets. Like WASP-94, HD20782/HD20781 is a wide binary, with HD20782 hosting a Jupiter-mass planet and HD20781 two Neptune-class worlds.

We also have Kepler-132, an interesting system hosting three ‘super-Earths,’ with an angular separation too small to allow us to tell which of the two stars the planets are transiting, although researchers believe that the two planets with the shortest periods cannot be orbiting the same star. Finally, there is XO-2, another wide binary in which one star hosts a transiting hot Jupiter, while the other hosts two gas giants, one Jupiter-class, the other the mass of Saturn.

We may learn something interesting about the formation of hot Jupiters from the WASP-94 system. Planets like these should form far enough from their primary to allow ices to freeze out of the protoplanetary disk, while being later forced, presumably through interactions with another star or planet, into the inner system. The paper comments: “The discovery of a hot Jupiter around each star suggests that the same formation process took place and that similar favorable conditions boosted the migration of the planets.” Interactions between the two stars are problematic given the large separation but this may help us stretch our theories:

Even though at this stage nothing can be proven, there are recent dynamical theories relevant to this system. Moeckel & Veras (2012) described interactions in which a planet orbiting one component of a stellar binary can ‘jump’ to the other star. If the two giant planets were formed around the same star, planet-planet scattering could have occurred. This would have pushed one of the planets near the host star and ejected the second one, which could then have been captured by the stellar companion. As we do not know the eccentricity of the stellar system, we can also consider the ‘flipping machanism’ described by Li et al. (2014), in which a coplanar system leads to very high eccentricities for the planet.

We may also find the WASP-94 system valuable on other grounds. Like most of the WASP planets, WASP-94A and WASP-94B orbit stars that are relatively bright — most of the Kepler stars, by contrast, are faint. This Keele University news release quotes the university’s Coel Hellier speculating on the possibility of atmospheric studies through transmission spectroscopy, where the atmosphere of the transiting world can be analyzed as it moves onto and off the stellar disk during a transit. I can scarcely imagine what John Herschel, who first observed this stellar system back in 1834, would have made of possibilities like these.

The paper is Neveu-VanMalle et al., “WASP-94 A and B planets: hot-Jupiter cousins in a twin-star system,” in press at Astronomy & Astrophysics (preprint).



The Emergence of Solitary Stars

by Paul Gilster on October 9, 2014

Looking at the latest work from Carnegie’s Alan Boss reminds me once again of the crucial role computers play in astrophysical calculations. We’re so used to the process that we’ve come to take it for granted, but imagine where we’d be without the ability to model complex gravitational systems. To understand planet formation, we can simulate a protoplanetary disk around a young star and let a billion years pass in front of our eyes. And as our models improve, we can set the process in motion with ever greater fidelity.

Read Caleb Scharf’s The Copernicus Complex ( Farrar, Straus and Giroux, 2014) to see how much we’ve learned by ever more precise modeling. Back in the late 1980s, Jacques Laskar (Bureau des Longitudes, Paris), Gerald Sussman and Jack Wisdom (the latter two at MIT) developed mathematical approaches that could track changes to orbital motions to understand our solar system’s past. Their work and the wave of innovation that followed helped us understand exponential divergence over million-year time periods, a crucial factor, as Scharf shows, in how unpredictable planetary motions can be:

Newton’s physics and its application by scientists like Laplace had appeared to be describing a clockwork universe, a reality based on laws that could always lead you from point A to point B, through space and time. And although the concepts of chaos and nonlinearity were well-known by the time these numerical computer experiments were carried out on planetary motions, this was the first real confirmation that our solar system was neither clockwork nor predictable.

In other words, when dealing with astronomical time-frames, we begin to find outcomes that could not have been predicted as we run our simulations. We’re observing chaos at play in complex gravitational systems, where tiny interactions can ultimately change the trajectories of entire planets. Scharf’s discussion of these matters celebrates the computer’s ability to model these phenomena and observe different results, but it’s also a humbling reminder of our limitations in thinking that with enough information we can always predict the outcome.

Learning How Stars Form

What Alan Boss is modeling is the formation of stars, using three-dimensional models of the collapse of magnetic molecular cloud cores. His simulations depict the formation of stars as clusters of newly formed protostars come apart. What they show is that younger star and protostar populations have a higher frequency of multiple-star systems than older ones. In other words, many single-star systems like our own start out as multi-star systems, with stars being ultimately ejected to achieve stability. You can see the modeling at work below.


Image: The distribution of density in the central plane of a three-dimensional model of a molecular cloud core from which stars are born. The model computes the cloud’s evolution over the free-fall timescale, which is how long it would take an object to collapse under its own gravity without any opposing forces interfering. The free-fall time is a common metric for measuring the timescale of astrophysical processes. In a) the free-fall time is 0.0, meaning this is the initial configuration of the cloud, and moving on the model shows the cloud core in various stages of collapse: b) a free-fall time of 1.40 or 66,080 years; c) a free-fall time of 1.51 or 71,272 years; and d) a free-fall time of 1.68 or 79,296 years. Collapse takes somewhat longer than a free-fall time in this model because of the presence of magnetic fields, which slow the collapse process, but are not strong enough to prevent the cloud from fragmenting into a multiple protostar system (d). For context, the region shown in a) and b) is about 0.21 light years (or 2.0 x 1017 centimeters) across, while the region shown in c) and d) is about 0.02 light years (or 2.0 x 1016 cm) across. Credit: Alan Boss.

As the molecular cloud that will form a star collapses, how it fragments depends, Boss shows, on the initial strength of the magnetic field. If the magnetic field is strong enough, single protostars emerge, but below this level, the cloud begins to fragment into multiple protostars. From the paper:

The calculations produce clumps with masses in the range of ~0.01 to 0.5 M, clumps which will continue to accrete mass and interact gravitationally with each other. It can be expected that the multiple systems will undergo dramatic subsequent orbital evolution, through a combination of mergers and ejections following close encounters, resulting ultimately in a small cluster of stable hierarchical multiple protostars, binary systems, and single protostars. Such evolution appears to be necessary in order [to] produce the binary and multiple star statistics that hold for the solar-type stars in the solar neighborhood…

Those statistics are striking. Roughly two-thirds of the stars within 81 light years of the Earth are either binary or part of multi-star systems. And because what we see today as single stars can also be the result of ejection from a multi-star system, the formation of binary and multi-star systems seems to be commonplace. I’m interested in these findings because if we are to understand our own place in the cosmos, we’re beginning to see that we have to account for why single-star systems do not seem to be the default in the Milky Way.

The paper is Boss and Keiser, “Collapse and Fragmentation of Magnetic Molecular Cloud Cores with the Enzo AMR MHD Code. II. Prolate and Oblate Cores,” in press at The Astrophysical Journal (preprint).



Interstellar Flight: Risks and Assumptions

by Paul Gilster on October 8, 2014

The interstellar mission that Dana Andrews describes in his recent paper — discussed here over the past two posts — intrigues me because I’m often asked what the first possible interstellar mission might be. Sure, we can launch a flyby Voyager-class probe to Alpha Centauri if we’re willing to tolerate seventy-five thousand years in cruise, but what would we accept by way of acceptable cruise times? The lifetime of a human being? Multiple generations? And if we had to launch as soon as possible, what would the mission parameters be?

The mission that Andrews conceives grows out of questions like these. I can say upfront that this isn’t a mission I would want to fly on. For one thing, it’s a generation ship, so entire lives will be spent in cramped quarters, and the prospect of being overtaken by a later, faster ship is always there. But that’s not the point. 18th Century voyagers with a yen for the unknown could have waited for the age of steamships, but how could they have anticipated it? In any case, waiting would have cost them the journey that was in front of them. I think there will always be pioneers in search of experience unique to them, the first to step onboard as long as a viable mission presents itself.

Yesterday we looked at various propulsion strategies for Andrews’ starship, including a personal favorite, the Sailbeam design of Jordin Kare, which uses tiny micro-sails driven by laser as a stream of beamed energy that can be ionized when it arrives at the ship, providing thrust to a magsail. Dana Andrews knows a lot about magsails — working with Robert Zubrin in 1988, he showed that Robert Bussard’s interstellar ramjets would produce more drag than thrust, and the idea of turning a magnetic scoop into a magnetic sail began to grow. We’re seeing that it can be used both for acceleration and deceleration upon arrival.


Image: Interstellar generation ship configured for braking. Credit: Dana Andrews.

Several decades before the Andrews/Zubrin paper, Robert Forward had been taking note of laser developments at Hughes Research Laboratories in Malibu, CA. He already knew about solar sails, which had appeared in the work of Konstantin Tsiolkovsky and Fridrikh Tsander in the 1920s and which had been the subject of a technical paper by Richard Garwin in 1958. As a science fiction writer, Forward was surely aware as well of Carl Wiley’s “Clipper Ships of Space” article, which appeared under the byline Russell Saunders in Astounding Science Fiction. Why not, Forward mused, boost a solar sail with a laser?

I can see why Andrews included Forward’s laser lightsail ideas in the current paper, but the magsail seems like a far more likely candidate for the near-term mission that he describes. Even working with a minimal Forward configuration, we still have to solve problems of deployment and infrastructure that are huge, including, in Andrews’ calculations, a beam aperture fully 20 kilometers in diameter. He goes on to describe a lightsail mission with acceleration of 0.05 gees that reaches 2 percent of c in 155 days at a distance of 267 AU. “The minimum cost system is to invest in really good stationary optics,” he adds, “thereby allowing less power and smaller sails, but then beam jitter begins to dominate.”

Summing up the various propulsion methods discussed, Andrews comments:

We quickly examined four different near-term interstellar propulsion concepts. Each has its issues… The laser-powered ion thruster needs aggressive weights for the design to close, but has no obvious showstoppers. The Neutral Particle Beam concept appears workable at planetary distances, but requires very high acceleration and power levels to maintain divergence angles of one microradian or more. Projecting a beam of neutralized particles presents the problem of re-ionizing a dispersed cloud of particles, which is a definite showstopper. The Sailbeam propulsion has potential, but needs tests of the acceleration capability and is still power hungry (~4000 TW of electrical power for the example presented here). Even at 4000 TW it needs pointing accuracy better than a nanoradian to finish the acceleration. The laser-lightsail actually came off as relatively low risk at 800 TW of electrical power, but that is very dependent on the availability of a 20 km diameter diffraction-limited steering optic, and a one-gram/m2 lightsail (both risk factor 4+).


Image: Total energy usage comparison. Credit: Dana Andrews.

What makes predictions about spaceflight so tricky is that we can’t anticipate the emergence of disruptive technologies. The risk factors that Andrews develops as he looks at the progress of interstellar flight are, by his admission, estimates and ‘guesstimates,’ which is about the best we can do, and he characterizes near-term technologies as less than risk factor 4.


Image: Relative risk between candidate interstellar technologies.

We can all find things we might take exception to here and there in this list. You can see, for example, that Andrews characterizes a breakeven fusion reactor at risk factor 4, with a 40 year development time. Fusion has wreaked havoc with our predictions since the 1950s, and I think it’s optimistic to hope for working fusion power-plants even within this timeframe, though I know fusion-minded people who think we’re much closer. Fusion for starship propulsion he ranks at a risk factor of 7, needing 100 years to develop. Notice, too, that for the purposes of this mission, freezing or suspended animation are ruled out as being at risk factor 9, which would place their development 400 years out. A disruptive advance could negate this.

I find it useful to lay out our assumptions in such direct form. The biggest question I have regards fully closed-cycle biological ECLSS (Environmental Control and Life Support Systems). At a risk factor of 2.5 and 25 years of development, we could deploy these technologies on a generation ship, but will they be tested and ready by late in this century, when the starship would presumably be launched? My own guesstimate would lean toward a higher risk factor and 50 years for development, a tight but perhaps possible fit.

Other things the ship will need: Protection against galactic cosmic radiation (GCR), which Andrews proposes may be resolved by using magnetic fields to deflect charged particles away from the crew areas. He gives the topic a fuller discussion in a 2004 paper (see citation below). Dust in the interstellar medium poses a challenge because at 2 percent of c, impacting particles become plasma and can cause erosion to the spacecraft. Andrews notes this will need to be addressed in any starship design but doesn’t elaborate.

The conclusions [Andrews adds] are that near-term interstellar colonization flights are not completely science fiction, but there has to be a powerful requirement to generate the funding necessary to work many of the problems identified. The alternative is to wait a hundred years or so for low specific power fusion, or much longer for warp drive. We’ll see.

The paper is Andrews, “Defining a Near-Term Interstellar Colony Ship,” presented at the IAC’s Toronto meeting and now being submitted to Acta Astronautica. The 2004 paper is Andrews, “Things To Do While Coasting Through Interstellar Space,” AIAA-2004-3706, 40th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Fort Lauderdale, Florida, July 11-14, 2004.



Pondering Interstellar Propulsion Strategies

by Paul Gilster on October 7, 2014


Back in 1950, George Pal produced Destination Moon, a movie that was based (extremely loosely) on Robert Heinlein’s Rocketship Galileo. Under the direction of Irving Pichel, the film explained the basics of a journey to the Moon — using among other things an animated science lesson — to a world becoming intrigued with space travel. I’ve wondered in the past whether we might one day have an interstellar equivalent of this film, a look at ways of mounting a star mission in keeping with the laws of classical mechanics. Call it Destination Alpha Centauri or some such and let’s see what we get.

Christopher Nolan’s film Interstellar doesn’t seem to be that movie, at least based on everything I’ve seen about it so far. I did think the early trailers were interesting, evoking the human urge to overcome enormous obstacles and buzzing with a kind of Apollo-era triumphalism. The most recent trailer looks like starflight is again reduced to magic, although maybe there will be some attempt to explain how what seems to be wormhole transit works. While we wait to find out, I still wonder about what kind of mission we would mount if we really did have a world-ending scenario on our hands that demanded an interstellar trip soon.

[Addendum]: I’ve been reminded that Kip Thorne has a hand in the science of this film, which does make me more interested in seeing it.]

Dana Andrews, as we saw yesterday, has been exploring a near-term interstellar colony ship that could fit into this scenario. Today I want to return to the paper he presented at the recent International Astronautical Congress. Andrews defines near-term this way:

Near-term means we can’t use warp drive, fusion engines, or multi-hundred terawatt lasers (which could destroy civilization on Earth in the wrong hands). Affordable means we’re going to do design and cost trades and select the lowest risk option with reasonable costs…. The goal was to design the system with state of the art technologies, assuming there will be twenty to thirty years of Research and Development before we start construction.

I speculated yesterday that given the need for a robust space-based infrastructure, it was hard to see the methods Andrews studies being turned to the construction of an actual starship before at least 2070, but it’s probably best to leave chronologies out of the picture since every year further out makes prediction that much more likely to be wrong.

The ‘straw man point of departure’ design in Andrews’ paper invoked beamed lasers mounted on an asteroid for risk stabilization and momentum control, with a 500 meter Fresnel lens directing the beam to the spacecraft’s laser reflectors, where it is focused on solar panels to power up ion thrusters using hydrogen propellant. A 32-day boost period gets the mission underway and a magsail is used to slow from 2 percent of c upon arrival. Andrews is assuming a laser output of around 100 TW to make this system work.

Enter the Sailbeam

But the point-of-departure method is only one possibility Andrews addresses. He also looks at Jordin Kare’s Sailbeam concept, in which a large laser array (250 5.8 TW lasers) accelerates, one after another, a vast number of 20 centimeter spinning microsails. The acceleration is a blinding 30 million gees over a period of 244 milliseconds, with the ‘beam’ being kept tight during the brief acceleration by using ‘fast acting off axis lasers,’ and additional lasers downrange. Only for the first 977 kilometers are the sails actually under the beam.


Image: The Sailbeam propulsion schematic. Credit: Dana Andrews.

Notice the requirement for keeping the beam tight, an issue that has bedeviled concepts from laser-pushed lightsails to particle beam acceleration. After all, beam spread means you can’t deliver the bulk of the required power to the vessel that needs its energies. Assuming a tight microsail beam, however, the huge boost delivered at the beginning is sufficient, according to Kare’s numbers, to produce serious thrust for the starship. The beam reaches the ship, which vaporizes and ionizes the incoming sails by its own laser system, creating a plasma pulse that drives a magsail. Numerous issues arise with this method, as Andrews describes:

250 micro sails per second amounts to 3.2 kg/sec, and that requires 150 to 300 MW of laser power to ionize the microsails. Assuming 50% efficient lasers we need about a gigawatt of thermal power. Assuming risk factor three, that’s about 1000 mT of powerplant mass, which is actually less than the laser-powered ion propulsion system.

Could we beam the needed power to the ship? Perhaps, but there are other problems:

To maintain 0.01 gee with 250 microsails/second as the spacecraft approaches 2% c, we need to accelerate each microsail for 0.18 seconds. Since we assume each microsail launcher needs about 0.5 seconds to place, align, and spin up its microsail, we don’t need to increase the number of laser launchers to maintain the 0.01 gee. The problem is the long (730 day) acceleration time, which results in end of boost at 1238 AU. This requires very small sailbeam divergence (< Nano radian).


Image: Microsail plasma interaction magsail dipole field. Credit: Dana Andrews.

With regard to the divergence problem, you’ll remember our recent discussions about neutral particle beams, which occurred in the context of Alan Mole’s paper in JBIS on a small interstellar probe. Mole, in turn, had drawn on earlier work by Andrews in 1994. In that paper, Andrews described a 2000 kilogram probe driven by a neutral particle beam sent to a magsail (the same magsail would be used for deceleration in the destination system). Writing in these pages, James Benford later found this propulsion method inapplicable for interstellar uses because of unavoidable spread of the beam and a corresponding loss of efficiency, although he considered it quite interesting as a driver for fast interplanetary travel.

In the current paper, Andrews revisits particle beam propulsion as a third alternative to the ion thrusters in his point-of-departure design. He cites Benford’s critique in the footnotes and goes on to add that the divergence of the beam will limit the useful range of operation. That implies high accelerations at the beginning of the flight, when the spacecraft can exploit maximum power from the beam as it meets the required divergence angle requirements. To catch up on our Centauri Dreams discussions of these issues, start with The Probe and the Particle Beam and Sails Driven by Diverging Neutral Particle Beams, both of which launched discussions and follow-up articles that can be found in the archives.

But there is a final possibility, the laser-driven lightsail as envisioned by Robert Forward back in the 1960s and refined by him in numerous papers in the years following. Which of these best fits our need to launch a near-term mission aboard a crewed starship? More on the laser lightsail and Andrews’ thoughts on energy requirements when I wrap up the paper tomorrow.

The paper is Andrews, “Defining a Near-Term Interstellar Colony Ship,” presented at IAC 2014 (Toronto) and in preparation for submission to Acta Astronautica. The earlier Andrews paper is “Cost considerations for interstellar missions,” Acta Astronautica 34, pp. 357-365,1994. Alan Mole’s paper is “One Kilogram Interstellar Colony Mission,” Journal of the British Interplanetary Society Vol. 66, No. 12, 381-387 (available in its original issue through JBIS).