Paul Gilster's Blog, page 196

Electric Sail Concept Moves Forward

Just how we follow up on the investigations of New Horizons remains an open question. But we need to be thinking about how we can push past the outer planets to continue our study of the heliopause and the larger interstellar environment in which the Sun moves. I notice that Bruce Wiegmann, writing a precis of a mission concept called the Heliopause Electrostatic Rapid Transit System (HERTS) has drawn inspiration from the Heliophysics Decadal Survey, which cites the need for in situ measurements of the outer heliosphere and beyond.

It’s good to see a bit more momentum building for continuing the grand voyages of exploration exemplified by the Pioneers, the Voyagers and New Horizons. I often cite the Innovative Interstellar Explorer concept developed at Johns Hopkins (APL), which targets nearby interstellar space at a distance of over 200 AU, but whether we’re talking about IIE or Claudio Maccone’s FOCAL mission or any other design aimed at exiting the Solar System, the key problem is propulsion. Weigmann’s team at Marshall Space Flight Center has been awarded a Phase I grant from NASA’s Innovative Advanced Concepts office to work on a dramatic solution.

The Heliopause Electrostatic Rapid Transit System involves a sail and thus propellant-less propulsion, but it’s not the conventional solar sail that uses the momentum provided by solar photons. The nomenclature is confusing, because the electric sail HERTS is designed around would interact with the solar ‘wind,’ which is not made up of photons at all but a stream of charged particles that flows constantly though erratically from the Sun at high velocity. A spacecraft riding the solar wind could, by some calculations, move between five and ten times faster than our best outer-system result so far, the 17.1 km/sec Voyager 1.

Wiegmann explains the principle at play in the precis:

The basic principle on which the HERTS operates is the exchange of momentum between an array of long electrically biased wires and the solar wind protons, which flow radially away from the sun at speeds ranging from 300 to 700 km/s. A high-voltage, positive bias on the wires, which are oriented normal to the solar wind flow, deflects the streaming protons, resulting in a reaction force on the wires—also directed radially away from the sun. Over periods of months, this small force can accelerate the spacecraft to enormous speeds—on the order of 100-150 km/s (~ 20 to 30 AU/year). The proposed HERTS can provide the unique ability to explore the Heliopause and the extreme outer solar system on timescales of less than a decade.

If you’re an old Centauri Dreams hand, you’ll recognize the HERTS sail as the offspring of Pekka Janhunen (Finnish Meteorological Institute), whose concept involves long tethers (perhaps reaching 20 kilometers in length) extended from the spacecraft, each maintaining a steady electric potential with the help of a solar-powered electron gun aboard the vehicle. As many as a hundred tethers — these are thinner than a human hair — could be deployed to achieve maximum effect. While the solar wind is far weaker than solar photon pressure, an electric sail with tethers in place is still efficient, according to Janhunen’s calculations, and can create an effective solar wind sail area of several square kilometers.

Image: A full-scale electric sail consists of a number (50-100) of long (e.g., 20 km), thin (e.g., 25 microns) conducting tethers (wires). The spacecraft contains a solar-powered electron gun (typical power a few hundred watts) which is used to keep the spacecraft and the wires in a high (typically 20 kV) positive potential. The electric field of the wires extends a few tens of metres into the surrounding solar wind plasma. Therefore the solar wind ions “see” the wires as rather thick, about 100 m wide obstacles. A technical concept exists for deploying (opening) the wires in a relatively simple way and guiding or “flying” the resulting spacecraft electrically. Credit: Artwork by Alexandre Szames. Caption via Pekka Janhunen/Kumpula Space Centre.

MSFC’s Advanced Concepts Office has been studying the feasibility of the Janhunen sail during the past year, finding that the electric sail is able to reach velocities three to four times greater than any realistic current technology including solar (photon) sails and solar electric propulsion systems. Because we are dealing with a stream of particles flowing outward from the Sun (and because the electric sail can, like a solar sail, be ‘tacked’ for maneuvering), we are looking at a fast interplanetary propulsion system that avoids the deployment issues faced by large solar sails using photon momentum for their push. Deploying reels of tethers is, by comparison, straightforward.

Both photon-pushed sails and those riding the solar wind are limited by distance from the Sun, but the electric sail may have applications in future interstellar missions nonetheless. If we accelerate a (non-electric) sail by the use of a laser or microwave beam up to a small percentage of the speed of light, we could slow it down upon arrival by using the solar wind from the destination star, interacting with a tether system deployed as the spacecraft enters the new system. Having decelerated, the spacecraft could then use electric sail technology for exploration. Janhunen has explored the concept for electric sails (though not yet in detail), but an idea like this was also broached by Robert Zubrin and Dana Andrews for magnetic sail deceleration in 1990.

A key paper on electric sails is Janhunen and Sandroos, “Simulation study of solar wind push on a charged wire: solar wind electric sail propulsion,” Annales Geophysicae 25, (2007), pp. 755-767. For background, see Electric Solar Wind Sail Spacecraft Propulsion, which provides diagrams, a FAQ and various links to published papers.

NIAC: An Orbiting Rainbow

Remember Robert Forward’s beamed sail concepts designed for travel to another star? Forward was the master of thinking big, addressing questions of physics which, once solved, left it up to the engineers to actually build the enormous infrastructure needed. Thus his crewed mission to Epsilon Eridani, which would demand not only a large power station in the inner system but a huge Fresnel lens out between the orbits of Saturn and Uranus. A 75,000 TW laser system was involved, a ‘staged’ sail for deceleration at the destination, and as for that lens, it would mass 560,000 tons and be a structure at least a third the diameter of the Moon.

In addition to being a highly regarded physicist, Forward was also a science fiction writer who detailed his beamed sail concepts in Rocheworld (Baen, 1990), which grew out of a previous version in Analog. I always thought of the Epsilon Eridani mission as his greatest attempt to confound human engineering, but later came to think that vast structures like his outer system lens might be possible. Rather than legions of space-suited workers building the thing, perhaps nanotechnology could come to the rescue, so that one technology builds another. It’s another reason to include the possibility of vast structures in our SETI thinking.

Creating Apertures in Space

All of this is inspired by looking at the recent announcement from the NASA Innovative Advanced Concepts (NIAC) program, which has named twelve projects for Phase 1 awards and five for Phase 2. The latter receive up to $500,000 each over a two-year period, often growing out of ideas previously broached in a Phase 1 paper and refining the work explored therein. The Jet Propulsion Laboratory is leading one of the Phase II projects, one called ‘Orbiting Rainbows,’ that inevitably calls Forward to mind because it involves clouds of dust-like matter being shaped into the primary element for an ultra-large space aperture. In other words, a kind of lens.

Here’s a snippet from JPL’s Marco Quadrelli describing the principle:

Our objective is to investigate the conditions to manipulate and maintain the shape of an orbiting cloud of dust-like matter so that it can function as an ultra-lightweight surface with useful and adaptable electromagnetic characteristics, for instance, in the optical, RF, or microwave bands. Inspired by the light scattering and focusing properties of distributed optical assemblies in Nature, such as rainbows and aerosols, and by recent laboratory successes in optical trapping and manipulation, we propose a unique combination of space optics and autonomous robotic system technology, to enable a new vision of space system architecture with applications to ultra-lightweight space optics and, ultimately, in-situ space system fabrication.

Quadrelli points out that the cost of any optical system is always driven by the size of the primary aperture, which is why Forward’s vast lens seems so out of reach. Have a look at the image below for what Quadrelli proposes. Here we’re seeing a cloud of what he refers to as ‘dust-like objects’ that can be optically manipulated. The particles are shaped by light pressure into a surface that can be tuned to act coherently in specific frequencies. The idea seems to have grown out of recent work in the physics of optically manipulating small particles in the laboratory — think ‘optical tweezers’ of the kind that have made it possible to work at the nanotech scale.

Image: Creation of lenses out of clouds of tiny objects. Credit: Marco Quadrelli/JPL.

The Orbiting Rainbows study is all about the feasibility of making a single aperture out of a cloud of particles, but the implications are intriguing and Quadrelli names them in his short description. Multiple ‘aerosol lenses’ could be combined to create powerful tools for exoplanet research, all the while teaching us much about remote manipulation of clouds of matter in space. The goal is a completely reconfigurable, fault-tolerant lensing system of huge size and low cost. This would complement the next generation of extremely large telescopes on Earth and bring an entirely new approach to the operation of telescopes in space. Quadrelli’s description continues:

A cloud of highly reflective particles of micron size acting coherently in a specific electromagnetic band, just like an aerosol in suspension in the atmosphere, would reflect the Sun’s light much like a rainbow. The only difference with an atmospheric or industrial aerosol is the absence of the supporting fluid medium. This new concept is based on recent understandings in the physics of optical manipulation of small particles in the laboratory and the engineering of distributed ensembles of spacecraft swarms to shape an orbiting cloud of micron-sized objects.

I dwell on this JPL concept because we’ve recently been talking about the potential of tiny spacecraft operating in swarms, and considered them even in terms of propulsion, moving from Clifford Singer’s pellet ideas to Gerald Nordley’s intelligent ‘snowflake’ designs (see, for example, ‘Smart Pellets’ and Interstellar Propulsion, and the sequence of articles around it). These pellets would constitute a propellant stream for a departing spacecraft, but we’ve also seen Mason Peck’s ideas about satellites the size of a microchip that can be manipulated through interactions with the magnetic fields of planets, and perhaps accelerated to interstellar velocities (see Sprites: A Chip-Sized Spacecraft Solution).

So miniaturization, swarm operations and propulsion through natural interactions (Peck’s Sprites take advantage of the Lorentz force that affects charged particles moving through a magnetic field) all factor into evolving thinking about deep space. Nanotechnology enables new operations at both ends of the scale spectrum, perhaps constructing the vast structures of the science fictional imagination (Dyson spheres come to mind) and potentially enabling tiny spacecraft whose low mass makes getting them up to speed a much easier matter than heavy rockets.

Back to Quadrelli. His reflective particles aren’t ‘intelligent,’ but the principle of shaping them into apertures using autonomous robotic technology shares in some of the same premises. We’ll need to see, of course, just how this shaping works, how it manipulates what kinds of dust-like matter, and just how adaptive the resultant clouds are to changes in configuration, but that is what a Phase II study is all about. The point is, when we need something huge, like Clifford Singer’s 105 kilometer particle accelerator, nature can sometimes offer a better solution, like the acceleration of Mason Peck’s Sprites within Jupiter’s powerful magnetic fields.

Finding ways to assemble huge lenses is a project cut out of the same cloth. And at the Phase I level at NIAC, several studies cry out for later analysis here, including Webster Cash’s Aragoscope, which the University of Colorado scientist hopes will ‘shatter the cost barrier for large, diffraction-limited optics,’ producing the possibility of telescopes with a thousand times the resolution of the Hubble instrument. Also impressive is the Heliopause Electrostatic Rapid Transit System from Marshall Space Flight Center (Janhunen’s electric sails!) and Justin Atchison’s work at Johns Hopkins on Swarm Flyby Gravimetry, which again attracts me by its use of miniaturization and swarm technologies. More on these Phase I NIAC studies next week

Rosetta: Arrival at a Comet

How do you close on a comet? Very carefully, as the Rosetta spacecraft has periodically reminded us ever since late January, when it was awakened from hibernation and its various instruments reactivated in preparation for operations at comet 67P/Churyumov–Gerasimenko. The spacecraft carried out ten orbital correction maneuvers between May and early August as its velocity with respect to the comet was reduced from 775 meters per second down to 1 m/s, which is about as fast as I was moving moments ago on my just completed morning walk.

What a mission this is. When I wrote about the January de-hibernation procedures (see Waking Up Rosetta), I focused on two things of particular interest to the interstellar-minded. Rosetta’s Philae lander will attempt a landing on the comet this November even as the primary spacecraft, now orbiting 67P/Churyumov–Gerasimenko, continues its operations. We’re going to see the landscape of a comet as if we were standing on it, giving Hollywood special effects people legions of new ideas and scientists a chance to sample an ancient piece of the Solar System.

You’ll want to bookmark the Rosetta Blog to keep up. But keep in mind the other piece of the puzzle for future space operations. Rosetta will be looking closely at the interactions between the solar wind — that stream of charged particles constantly flowing from the Sun — and cometary gases. We’ll learn a great deal about the composition of the particles in the solar wind and probably get new insights into solar storms.

Remember that this ‘solar wind’ isn’t what drives the typical solar sail, which gets its kick from the momentum imparted by solar photons. But there are other kinds of sail. The Finnish researcher Pekka Janhunen has discussed electric sail possibilities, craft that might use the charged particles of the solar wind instead of photons to reach speeds of 100 kilometers per second (by contrast, Voyager 1 is moving at about 17 km/s). Rosetta results may help us understand how feasible this concept is.

Image: Comet 67P/Churyumov-Gerasimenko by Rosetta’s OSIRIS narrow-angle camera on 3 August from a distance of 285 km. The image resolution is 5.3 metres/pixel. Credit & Copyright: ESA / Rosetta / MPS for OSIRIS Team MPS / UPD / LAM /IAA / SSO / INTA / UPM / DASP / IDA.

That image is a stunner, no? Now that Rosetta has rendezvoused with 67P/Churyumov-Gerasimenko, we can think back not only to the orbital correction maneuvers but the three gravity assist flybys of the Earth and one assist at Mars, allowing a trajectory that produced data about asteroids Steins and Lutetia along the way. It’s been a long haul since 2004 and you can see why Jean-Jacques Dordain, the European Space Agency’s director general, is delighted:

“After ten years, five months and four days travelling towards our destination, looping around the Sun five times and clocking up 6.4 billion kilometres, we are delighted to announce finally ‘we are here.’ Europe’s Rosetta is now the first spacecraft in history to rendezvous with a comet, a major highlight in exploring our origins. Discoveries can start.”

Getting the hang of operations around the comet is going to be a fascinating process to watch. Right now the spacecraft is approximately 100 kilometers from the comet’s surface, and over the course of the coming six weeks, while close-up studies from its instrument suite proceed, it will nudge closer, down to 50 kilometers, and eventually closer still depending on comet activity. Remember that images from the OSIRIS camera showed a dramatic variation in activity between late April and early June as the comet’s gas and dust envelope — its ‘coma’ — brightened and then dimmed within the course of six weeks. These are quirky, lively objects, and we now proceed to teach ourselves the art of flying a spacecraft near them for extended periods.

This ESA news release tells us that the plan is to identify five landing sites by late August, with the primary site being chosen in mid-September. The landing is currently planned for November 11, after which we’ll have both lander and orbiter in operation at the comet until its closest solar approach in August of 2015. Comets are ancient pieces of the Solar System that may well have delivered the bulk of Earth’s oceans. Now we’ll see up close what happens to a comet as it approaches the Sun. Congratulations and Champagne are due all around for the planners, designers, builders and controllers of this extraordinary mission. Onward to the surface.

Keeping a Planet Alive

I’ve made no secret of my interest in red dwarf stars as possible hosts of life-bearing planets, and this is partially because these long-lived stars excite visions of civilizations that could have a stable environment for many billions of years. I admit it, the interest is science fictional, growing out of my imagination working on the possibility of life under the light of a class of stars that out-live all others. What might emerge in such settings, in places where tidal lock could keep the planet’s star fixed at one point in the sky and all shadows would be permanent?

Some of this interest grows out of an early reading of Olaf Stapledon’s 1937 novel Star Maker, in which the author describes life in the form of intelligent plants that live on such a tidally locked world. For that matter, Larry Niven developed an alien race called the Chirpsithra, natives of a red dwarf who have a yen for good drink and socializing with other species (you can sample Niven’s lively tales of these creatures in The Draco Tavern, a 2006 title from Tor). I tend to imagine red dwarf planet dwellers as something more like philosophers and sages than intelligent carrots or Niven’s incredibly tall barflies.

But no matter. A new paper from Christa Van Laerhoven and Rory Barnes (University of Washington) and Richard Greenberg (University of Arizona) has me absorbed in matters such as how close an Earth-class planet would need to be to stay habitable around a red dwarf. There’s no one answer because of the range of stellar temperatures between different types of red dwarf, but Laerhoven and company are looking at a star with a mass of 0.1 solar masses and a luminosity 1.15 x 10-3 times that of the Sun. Here it turns out that to receive the same incident flux as the Earth, the planet would need to orbit at 0.034 AU.

Now Mercury is about 0.38 AU from the Sun, which gives us a feel for how much cooler such a star must be. We can also note that because of their long lifetimes, many red dwarfs are much older than our Solar System, on the order of twice as old in some cases, and because a transiting Earth-class planet around such a star should be detectable (the transit depth would be huge), it’s possible that the first Earth-like habitable planet we find will be billions of years older than our own. Thus my visions of ancient races of philosophers under a darkened sky.

But maybe not. The Van Laerhoven paper makes the case that planets like these are going to be cooling internally as they age, enough so to cause problems. Plate tectonics are driven by heat, and we’re learning their necessary function in the carbon cycle that allows the planet to avoid greenhouse overheating. Here’s the issue (internal citations omitted):

On an Earth-like body, long before reaching twice Earth’s age, plate tectonics would probably have turned off as the planet cooled, primarily because solidification of the core would terminate the release of latent heat that drives mantle convection. While plate tectonics may not be essential for life on all habitable planets, an equivalent tectonic process to drive geochemical exchange between the interior and the atmosphere is a likely requirement. The necessary amount of internal heat for such activity is uncertain (even the mechanisms that govern the onset and demise of terrestrial plate tectonics are still poorly understood and controversial), but it seems likely that a planet ~10 Gyr would have cooled too much…

So my race of philosophers and poets may have a much shorter time to thrive than the ten trillion years its dim star will live. What we need is an additional heat source, and the possibility in play in this paper is tidal heating, which the paper argues calls for either non-synchronous rotation or an eccentric orbit. Even these are a problem because tidal effects gradually synchronize the rotation and circularize the orbit, but we need them to help us on geological timescales.

The solution may be another planet in the same system, an outer companion that can keep the inner planet’s orbit from circularizing and thus maintain the tidal stresses that heat the planet. The computer models the researchers used allow this effect to keep the inner world habitable for billions of years even when other internal sources of heat have long perished. The paper argues that this effect, while studied here only in terms of two-planet systems, can also come into play in systems with a larger number of planets. From the paper:

…a reasonable fraction of terrestrial-scale planets in the HZ of very old, low-mass stars may be able to sustain life, even though without a satisfactory companion they would have cooled off by now. The requirements on the outer planet are not extremely stringent. For example, one could well imagine a Neptune-size outer planet a few times farther out than the rocky planet with an orbital eccentricity ~0.01-0.02. Not only would such an outer planet yield an appropriate amount of tidal heating to allow life, but the heating would be at a steady rate for at least tens of Gyr.

Image: For certain ancient planets orbiting smaller, older stars, the gravitational influence of an outer companion planet might generate enough energy through tidal heating to keep the closer-in world habitable even when its own internal fires burn out. But what would such a planet look like on its surface? Here, UW astronomer Rory Barnes provides a speculative illustration of a planet in the habitable zone of a red dwarf. “The star would appear about 10 times larger in the sky than our sun, and the crescent is not a moon but a nearby Saturn-sized planet that maintains the tidal heating,” Barnes notes. “The sky is mostly dark because cool stars don’t emit much blue light, so the atmosphere doesn’t scatter it.” Credit: Rory Barnes / University of Washington.

It could be, then, that a planet in this configuration — a terrestrial world like the Earth orbiting a 0.1 solar mass star with an outer companion — could experience enough tidal heating to make it the longest lived surface habitat in the galaxy. Is such a world, as the authors speculate, a possible home for humanity in the remote future, when our own Earth becomes uninhabitable? For that matter, given that such worlds seem made to order for ancient civilizations, shouldn’t we consider them as good SETI candidates? The paper recommends that any search for habitable Earth-scale planets should include a search for outer system companions.

The paper is Van Laerhoven et al., “Tides, planetary companions, and habitability: Habitability in the habitable zone of low-mass stars,” Monthly Notices of the Royal Astronomical Society, published online 12 May, 2014 (abstract / preprint).

What We Want to Hear

“A man hears what he wants to hear and disregards the rest.”

So sang Simon & Garfunkel in their 1968 ballad “The Boxer.” Human nature seems to drive us to look for what we most want to happen. It’s a tendency, though, that people who write about science have to avoid because it can lead to seriously mistaken conclusions. In science itself there is a robust system of peer review to evaluate ideas. It’s not perfect but it’s a serious attempt to filter out our preconceptions. As with the flap about ‘faster than light’ neutrinos at CERN, we want as many qualified eyes as possible on the problem.

Journalists come in all stripes, but of late there has been a disheartening tendency to prove Paul Simon’s axiom. Not long ago we went through a spate of news stories to the effect that NASA was investigating warp drive. True enough — the Eagleworks team at Johnson Space Center, under the direction of Harold “Sonny” White, has been looking at warp drive possibilities for some time, though it could hardly be said to be a well-funded priority of the space agency. The budget for the Eagleworks effort has been small, and Eagleworks is only a small part of Dr. White’s job description, which focuses mostly on his acknowledged expertise in ion thrusters and related technologies.

But many of the recent stories went well beyond the facts, implying that warp drive is a major project at NASA. Numerous sites featured images of what the purported ship would look like, and the implication was that NASA had already produced designs for the vessel, meaning that breakthroughs that would allow faster than light propulsion were in the works. Anyone involved with the breakthrough propulsion community can tell you that this is not the case despite the exultant nature of some of the Internet postings. Dr. White himself has always criticized media hype and has done everything he can to distance himself from it.

Science proceeds through careful experimentation and theorizing. We also need to see well-developed analysis of any experimental apparatus that is producing anomalous results, to see if we can verify what’s going on. If the apparatus has a flaw, those operating it may not realize that effects apparently being generated by their theory are actually artifacts of the equipment being used. Such a result may be developing with regard to the White/Juday Interferometer, the key tool involved in the JSC studies of warp drive physics.

It’s not making any headlines, but a new study from Jeff Lee and Gerald Cleaver (both affiliated with the Early Universe Cosmology & Strings Group, Baylor University) has appeared, bearing a title that makes the paper’s case: “The Inability of the White-Juday Warp Field Interferometer to Spectrally Resolve Spacetime Distortions.” You can find it here. The tool in question is the one being used at Eagleworks to study possible space-time distortions of the sort that might lead one day to a warp drive. About it, the paper has this to say:

The White-Juday Warp Field Interferometer has been demonstrated to be incapable of resolving the minute distortions of spacetime created by both 106 V·m-1 electric fields and a 1 kg mass.

And this:

Variations in temperature were shown to produce potentially detectable changes in the refractive index of air, which could result in occasional spurious interference fringes. Although a more rigorous model, which considers a time-changing index of refraction gradient along the interferometer arm, would result in a smaller lateral beam deviation, the purpose for which the WJWFI is intended has been shown to be unachievable.

And this:

…were any signals to appear in the White-Juday Warp Field Interferometer, they would most often be attributable to either electronic noise or the classical electrodynamics interaction between the ionized air between the plates and the electromagnetic radiation of the laser.

Note that last point: Noise within the experimental equipment may be what is being observed.

What to make of this? Two things. First, we are trying to learn whether a particular experimental setup can do what its builders hope. Examining the apparatus is key to science, and it’s something that both the experiments and those reviewing the work take as a solemn responsibility. If the White-Juday Warp Field Interferometer doesn’t work as originally expected, this now gives the experimenters the opportunity to use this knowledge to add to their database, and possibly use it in refining future experimental efforts in this area.

Secondly, this entirely natural development of studying the apparatus and working out the implications doesn’t fare well when journalists jump to conclusions. It is entirely normal for ideas to be advanced in the give and take of conferences and scientific papers as researchers proceed with the dogged task of finding the truth. Journalism likes a good story, however, and the temptation to take tentative conclusions and make them sound permanent is irresistible. Thus we get headlines like The Washington Post’s This is the amazing design for NASA’s Star Trek-style space ship, the IXS Enterprise.

Sonny White, who is the kindest of men, is a friend, and every time I’ve talked to him about these matters he has pointed out to me how much he deplores the hype that accompanies work in these areas. Sonny would like there to be a way to get to a warp drive and so would I, and he may well want to rebut the paper above with a new analysis of his own. So the work proceeds, but it should always do so with the understanding that ideas can be blown far out of proportion in the era of a global Internet and a willingness to go for the big story rather than the considered truth. The truth here is that we are in a process of learning what works and what does not.

Enter the Quantum Vacuum Thruster

So we need to calm down. Over the past few days there has been a flare-up about so-called quantum vacuum thrusters, following a story in Wired that made several bold statements, such as the title: NASA Validates ‘Impossible’ Space Drive. It is true that Eagleworks tested a quantum vacuum thruster device, a ‘propellant-less microwave thruster’ which was developed by Guido Fetta. The work on what Fetta calls the ‘Cannae Drive’ was presented in late July at the 50th Joint Propulsion Conference in Cleveland. Independent of this effort, British scientist Roger Shawyer has been working on a similar thruster for years, one recently tested by a team in China.

I always appreciate it when people send me interesting links, and a number of readers passed the Wired story along. I can certainly understand their interest! For the propellantless thruster seems to violate the principle of conservation of momentum, a very big thing if true, and it’s also true that a drive that could do these things could lead to entirely new designs in propulsion. There is no sense, however, in which NASA could be said to have ‘validated’ this device.

Gizmodo popped up with a headline of its own, making the bald statement: NASA: New “impossible” engine works, could change space travel forever. The article also tells us: “the fact is that the quantum vacuum plasma thruster works and scientists can’t explain why.”

But does it work? To know, we would need to study the experimental apparatus carefully to make sure there were no effects happening within it that could replicate the minute perceived signal. In other words, we may be looking at equipment noise. My sources, which I consider highly reliable, tell me that a review of the equipment used in the JSC quantum vacuum thruster tests has been completed but because it has not yet been released, I cannot make a comment on it beyond saying that it will likewise upgrade our understanding of the kind of experiment that was run, and how valid the results might be.

I would love to see the emergence of a genuine ‘impulse’ engine of the sort that the media have written about and would rejoice in its implications. But we are part way into a complicated story that has reached no conclusion. Fortunately, several media stories have also appeared that have begun to take a more probing look at these matters, such as Don’t buy stock in impossible space drives just yet from Ars Technica and an essay in Popular Science quoting Michael Baine, chief of engineering at Intuitive Machines:

“Whenever you get results that have extraordinary implications, you have to be cautious and somewhat skeptical that they can be repeated before you can accept them as a new theory,” Baine says. “Really, it’s got to come down to peer review and getting that done before you can get any kind of acceptance that something exotic is going on here.”

The Chinese team in Xian claims results that back the quantum vacuum thruster idea. Let’s put their analysis under the same level of scrutiny. We have no choice in this, because finding a hole in conservation of momentum would be a result so unexpected that we can expect any laboratory producing such results to undergo examination about its methodology. We can also expect papers undergoing peer review that defend the findings. All of that would jibe with scientific method aimed at ferreting out the truth. But getting ahead of ourselves when we’re only part way into the story can only lead to confusion. As I said above, other shoes are about to drop on the quantum vacuum thruster story, and when they do, we’ll look at them with equal interest.

I love “The Boxer.” And when I think about the media reaction to many advanced propulsion stories, its lyrics keep coming to mind. Here’s the complete first verse:

I am just a poor boy.

Though my story’s seldom told,

I have squandered my resistance

For a pocketful of mumbles,

Such are promises

All lies and jest

Still, a man hears what he wants to hear

And disregards the rest.

I’m a writer and journalist, not a scientist. But the researchers I talk to are taken aback by the wave of hype that has accompanied many recent advanced propulsion stories. Let’s hope a bit of caution seeps in, for scientific breakthroughs do not come easily. If we are on the edge of one, which I seriously doubt, the matter will resolve itself because more and more data will be accumulated, subjected to review, and put through rigorous testing. What we want to hear is not what’s important. The universe parcels out its answers according to what is true.

Dining with Dirac

Ever wonder what it would have been like to sit in on a great occasion? I used to think about this in relation to a dinner party the painter Benjamin Haydon threw in 1817 at his London studio. At the ‘immortal dinner’ were, among others, William Wordsworth, John Keats and Charles Lamb, leading literary figures of their day. Fortunately, gatherings like these aren’t relegated to the 19th Century. In a piece that ran originally in The New York Review of Science Fiction, Gregory Benford describes an equally extraordinary evening with some of the greatest minds of our time: Martin Rees, Stephen Hawking, and Paul Dirac. A physicist and award-winning science fiction writer, Benford relates the particulars of a Cambridge sabbatical as scientists at the top of their form meet for an evening of bonhomie, whimsy and reflection.

by Gregory Benford

In science one tries to tell people, in such a way as to be understood by everyone, something that no one ever knew before. But in poetry, it’s the exact opposite.

–Paul Dirac

The invitation was on heavy bond in a delicious oyster color. I opened the Trinity College envelope noting it bore no stamp, apparently placed in my Institute of Astronomy mailbox by hand. Flowing script invited my wife Joan and me to evening meal with Professor Martin Rees.

Very good; the full High Table college show, then. In 1976 I was on sabbatical as a visiting fellow in Cambridge, England. I went there to study pulsars where they’d been discovered, but quickly became more interested in the luminous jet just seen in radio frequency maps of M87, the nearest active galaxy.

Martin Rees was then the Plumian Professor of physics and the director of the Institute of Astronomy, appointed just after the departure of Fred Hoyle. He had agreed to host my sabbatical, a stay that began my astrophysics career; I’ve spent the decades hence mostly on pulsars and galactic jets. In Cambridge I learned much more than I anticipated.

Precisely on time Joan and I walked through the Great Gate, the main entrance to the college, leading to the yawning Great Court. In the centre of the court stood an ornate fountain, traditionally fed by a pipe from Conduit Head in west Cambridge, not the unreliable Cam River nearby. A solemn porter in a black bowler hat welcomed us, remarking gruffly on the chilly air, and nodding at the invitation as I presented it. “Ah, the Rees room.”

Image: Cosmologist and astrophysicist Martin Rees, whose books and lectures explore not only astronomy but the bounds of science, philosophy and humanity’s place in the cosmos.

Trinity College undergraduates passed in gowns of dark blue. A statue of the college founder, Henry VIII, greeted us from a shadowy niche above the doorway. Martin Rees stood beside it, a slight man with a hawk nose and incisive gaze, bowing to Joan with a broad smile. I imagined we’d eat at the high table, as I had before for lunch, but instead Martin took us into a private dining room. I walked in with Joan and saw at the table two men and their wives: Paul Adrien Maurice Dirac and Stephen Hawking. Martin had said nothing to alert us.

Newton, Nehru and Maxwell were alumni of Trinity, and Dirac stood in such company; soon, so would Hawking and Rees.

The dining room was small, with room for six at the table. Soft lighting cast glows on the dark wood walls amid the scene of 700 years of academic elitism. The leadened plates stamped with the famed Trinity monogram framed a small salad. The flatware was heavy, dark silver and tall stemmed glasses ranked to the side. The servers wore formal tuxedo styled clothes and professionally disinterested faces. The headwaiter handled all dishes with white gloves and led the two solemn under-waiters.

I said very little through the salad, letting Joan carry our side. She entertained them with stories to adapting to English home appliances, her tinkling laughter softening the atmosphere. I reflected. Dirac had won a Nobel in 1933 for the first relativistic theory of particles, the Dirac equation. “The great papers of the other quantum pioneers were more ragged, less perfectly formed than Dirac’s,” my friend Freeman Dyson had said to me when I was in graduate school. Freeman had taken Dirac’s Cambridge quantum mechanics course as a precocious 19-year-old. Of Dirac’s discoveries, Freeman said, “His papers were like exquisitely carved marble statues falling out of the sky, one after another. He seemed to be able to conjure laws of nature from pure thought.”

This is an evening to keep your mouth shut, I thought, sitting at the centuries-old table and sipping a light Chardonnay (French, of course) served with the salad. Next, a tasty soup arrived, attended in strict silence by the stiff waiters. I noted that the French red wine was older than I was, a 1938 from the Fellows’ Cellar. A Haut Medoc, it was deep and rich with a surprising plum aftertaste.

Famously, Dirac’s wife Manci spoke little, and he even less. His colleagues in Cambridge jokingly defined a conversational unit of a dirac — one word per hour. Rees had related to me already how a while back all the physicists in Cambridge had newborn daughters, and someone said, “There must be something in the air.” Without pause Dirac, who had two daughters, said, “Or in the water.”

Dirac was a slight man and autistic, widely known as hard to draw out. He said this concentration proved crucial to his success as a theoretical physicist, for he could remain focused on a problem for a long time. He also could order information about mathematics and physics in a systematic way, employing his visual imagination and determination. (Decades later, I saw medical practice focus on this supposed disorder, “fixing” it with drugs and therapy. How many geniuses have we lost this way?)

I asked him how he concentrated solely on his research. “I don’t talk,” he said with admirable brevity and a smile. He also said he only stopped work on Sunday, when he took long strolls alone. He had struggled to find the Dirac equation for months, getting nowhere, then took his usual Sunday walk—and the entire solution came to him when he was crossing a small bridge. He hurried to a nearby pub, asked for lunch and wrote the equation on the back of the menu so he would not forget. He seldom looked directly at anyone, but this time he stared me in the eye. “There it was, out of nowhere.”

Image: Nobelist Paul Dirac, a founder of quantum mechanics and quantum electrodynamics, who predicted the existence of antimatter.

“Do you still have the menu?” I asked, eyes wide. When I said it would be a charming historical momento, he dismissively waved his hand. He had used it to start a fire in his chilly college rooms.

The Navy bean soup done, talk moved on. Some mention of English politics arose, at a time when Maggie Thatcher was moving to the fore, Martin squelched with, “I’m entirely infra-red,” which meant something like Trotsky. He had no wife then. Hawking’s wife rolled her eyes at this statement, saying nothing.

As the waiters smoothly placed plates of veal ala brochard before us, Hawking changed the tone of the conversation with his halting words. He wanted to talk about science fiction. Martin had told him I wrote the stuff. I’d had the impression that at Cambridge science fiction was something serious scientists never would do, and seldom discuss — especially at a table where Newton changed the world over bowls of steaming lentil soup—and I said so. Hawking gave a slanted grin. “Fred Hoyle has left us, but he is not forgotten.”

Hawking talked in slurred tones about what we now call his “chronology protection conjecture”. Why does nature apparently abhor a time machine? He said, “It seems that there is a Chronology Protection Agency which prevents the appearance of closed timelike curves and so makes the universe safe for historians.”

Martin pointed out that there was strong experimental evidence in favor of the conjecture — from the fact that we have not been invaded by hordes of tourists from the future. All this discussion Hawking eventually included in a book in the 2000s, along with his fears that our TV broadcasts, would bring ravening aliens to our door. He thought about such speculations in the 1970s, but apparently kept them largely to himself during his climb to fame.

Image: Theoretical physicist Stephen Hawking, famous among many things for his work on black holes and general relativity. Like Dirac, he held Cambridge’s Lucasian professorship of mathematics.

Dirac spoke about the walks he took around Cambridge, relating favorite routes in great detail, but otherwise had no small talk. Slowly Hawking turned the conversation around to what books we read, asking each of us. He then announced that since he was thirteen he had never bothered with the assignments in Literature classes, preferring science fiction. Dirac remarked, “In science one tries to tell people, in such a way as to be understood by everyone, something that no one ever knew before. But in poetry, and I suppose in fiction, it’s the exact opposite.”

To my surprise Rees assented. “But science fiction leads to science,” he said. Dirac was silent and looked puzzled.

Stephen spent a long while relating memories of sf short stories he’d read. Like many fans, Hawking could recall ideas but not authors or titles. He was a big Robert Sheckley fan, I deduced, from what his remembered plots. Rees said he thought science fiction was like a literary dialect. It had its own vernacular and insider terms, its unusual pronunciation patterns and rhythms. A native sf “speaker” uses the argot of an audience, one that knows what Delany later called the sf reader protocols – signals of broader meaning. A good example is, “The door dilated,” implying a changed world. Nods all round, though Dirac said he had read little sf beyond Wells and Brave New World. “Perhaps I should.”

We all agreed that aliens in fiction serve as a distorting mirror to show what humankind is not. Hawking spoke with jerky gestures, fighting the erosions from his Amyotrophic Lateral Sclerosis, which I knew as Lou Gehrig’s disease. His speech was slurred, brief and almost unintelligible, his conciseness a skill that later worked well in A Brief History of Time. Hawking’s fame was rising on his striking research ideas–that empty space wasn’t empty after all, and black holes aren’t black.

His wife, with her tight, focused look, scoffed at ideas like aliens, likening them to imaginary beings. Stephen retorted tartly that so were angels. A sudden silence around the table. I sipped the wine, which was excellent and still blossoming with rich new tones. This incident prefigured the issue of her Baptist faith versus his firm atheism, which eventually split them up.

I recalled this evening lately, looking over notes I made that very evening. My wife Joan died of cancer in 2002. In 2005 Rees was elevated to a life peerage, sitting as a crossbencher in the House of Lords as Baron Rees of Ludlow, a seat in the County of Shropshire. By then Astronomer Royal, he told the British Interplanetary Society, ‘”It is better to read first-rate science fiction than second-rate science; it’s no more likely to be wrong and is far more stimulating than second-rate science. And I think it’s good to read the great classics of science fiction.”

After a five course meal we had the finishing treat: an English, less sweet, version of crème brûlée, known as “Trinity burnt cream.”

Now Martin is master of Trinity College and the best known astronomer in the world. Recently, in Our Final Hour, he predicted that one of the two following outcomes is inevitable for humanity:

Extinction from runaway effects of new technology (nanotechnology, robotics) or else from uncontrolled scientific research; terrorist or fundamentalist violence; or destruction of the biosphere;

or else

Our expansion into space, survival through colonization. He now advocates free markets and believes that the wealthy will push back the frontiers of space.

Not infra-red any longer.

I never saw Dirac again, but have kept up with Hawking and Rees through the decades, visiting Cambridge often. They both use science fiction in their popular writing, whereas in the 1970s that was not the sort of thing you mentioned at High Table. Our world has changed, partly because of those men.

What distinguished them the most, I think, was their quiet verve, their wish to grapple with life. They were eager to deal with whatever came at them. Dirac probed our fundamental understanding of the world in his monk-like solitude. Hawking persevered against his crippling disease to become a major cosmologist. Rees cannily wove his way into great power, urging the Institute for Astronomy to the forefront of the field, becoming Astronomer Royal, and a major figure bringing science to the public as well.

The evening left a deep impression on me. On the walk home, I remarked to my wife that I would probably never have a better evening–at least, with my clothes on. She took that as a challenge and made the evening even more so.

From my time there I gathered background that eventually appeared in my 1980 novel Timescape, which explores how scientists confront the unknown. Cambridge is steeped in tradition, but its scientific culture is radical. I hope it remains so.

Copyright 2013 by Gregory Benford

101 Geysers on Enceladus (and What They Imply)

I’ve mentioned before the irony that we may discover signs of robust extraterrestrial life sooner around a distant exoplanet than right here in our own Solar System. The scenario isn’t terribly implausible: Perhaps we come up empty on Mars, or find ourselves bogged down with ambiguous results. As our rovers dig, we still have Europa, Enceladus and other outer system possibilities, but probably face a wait of decades before we could build and fly the missions needed to identify life.

Meanwhile, the exoplanet hunt continues. While we’ve had many a setback — the Space Interferometry Mission will always stand out in this regard, not to mention the inability to follow through with Terrestrial Planet Finder, Darwin and other high-end concepts — it’s just possible that within the next few decades, a space-based observatory will detect a solid biosignature from an exoplanet’s atmosphere. Even the James Webb Space Telescope should be able to detect the transmission spectrum of an Earth-class planet transiting a dim red dwarf star. Future instruments will be able to take atmospheric characterization down to an Earth 2.0 around a Sun-like star.

Then again, maybe the outer Solar System will prove so enticing that we do decide to make it a priority. We could be seeing this happen right now. Every new piece of evidence from Cassini helps to build the case that Enceladus is an attractive proposition for the life search, the latest news being that the Saturn orbiter has identified 101 distinct geysers erupting on the moon’s surface. We first detected geysers of ice particles and water vapor at Enceladus’ south pole almost a decade ago. Now we have a map of geysers erupting from the so-called ‘tiger stripe’ fractures coincident with surface hot spots.

Image: This two-image mosaic is one of the highest resolution views acquired by Cassini during its imaging survey of the geyser basin capping the southern hemisphere of Saturn’s moon Enceladus. It clearly shows the curvilinear arrangement of geysers, erupting from the fractures. From left to right, the fractures are Alexandria, Cairo, Baghdad, and Damascus. As a result of this survey, 101 geysers were discovered: 100 have been located on one of the tiger stripes , and the three-dimensional configurations of 98 of these geysers have also been determined. The source location of the remaining geyser could not be definitively established. These results, together with those of other Cassini instruments, now strongly suggest that the geysers have their origins in the sea known to exist beneath the ice underlying the south polar terrain. Credit: NASA/JPL-Caltech/SSI.

The reason this is so exciting is that the hot spots that Cassini’s heat-sensing instruments found in the south polar region are only a few tens of meters across. That means they’re too small to be produced by the kind of frictional heating that would be caused by the repeated flexing of Enceladus due to tidal effects from Saturn. Frictional heating could have accounted for the geyser phenomena by turning surface ice into vapor and liquid, but it now appears that we’re dealing with water from the ocean below being exposed by opening and closing of the fractures.

Carolyn Porco (Space Science Institute) is leader of the Cassini imaging team, and lead author of a new paper on the Cassini findings:

“Once we had these results in hand we knew right away that heat was not causing the geysers but vice versa. It also told us the geysers are not a near-surface phenomenon but have much deeper roots.”

The source of the material forming the geysers of Enceladus is thus found to be the sea that exists under the ice shell, a sea that Cassini’s gravity data on the moon has confirmed. This news release from CICLOPS (Cassini Imaging Central Laboratory for Operations) has more, including the results of a second paper in which the authors report that the brightness of the combined geyser plume as viewed by Cassini changes periodically during the moon’s orbit of Saturn. In most respects, the brightness variations track the expected tidal venting cycle.

But not entirely. What would be expected from the opening and closing of the fractures does not predict when the plume begins to brighten, a finding that could implicate the spin rate of Enceladus. Francis Nimmo (UC-Santa Cruz) is lead author on the second paper:

“It’s an interesting puzzle. Possibilities for the mismatch include, among other effects, a delay in the response of the ice shell, which would suggest tides are heating the bulk of the ice at the south pole, or subtle changes in the spin rate of Enceladus.”

That last remark points to the possibility that the liquid water under the Enceladan ice may be global, even if deeper under the south pole region. So we have yet another reason for fascination with a moon whose salty sea, known to contain organic compounds, is spouting geysers and, possibly, reaching the surface on occasion as a liquid. We have a potentially habitable environment under the ice that periodically offers up samples to nearby spacecraft.

Enceladus is too good a target to resist, and it’s worth remembering mission concepts like Life Investigation for Enceladus (LIFE), developed by Peter Tsou. LIFE could launch in the early 2020s, reaching Saturn in 2030 with the help of gravity assists along the way, capturing material from the Enceladus geysers with an aerogel collector like the one NASA used in its Stardust comet mission. With a final gravity assist at Titan, LIFE would then bring its samples back to Earth in 2036.

I’m remembering, too, NASA astrobiologist Chris McKay’s exhortation that the venting of water and organics into space is ‘an open invitation to go there.’ The German Aerospace Center (DLR) has likewise been exploring Enceladus mission concepts, envisioning a lander that would drill through the ice. Enceladus Explorer would use an ice drill probe to melt its way into a water-bearing crevasse to look for microorganisms, on the theory that any life in the plumes would have been destroyed by sudden exposure to space. Thus the need to probe the ocean itself.

So the ideas for sampling Enceladus for life are out there and they’ll doubtless increase as Cassini continues to demonstrate how potent an astrobiological target this moon is. Which concept should we choose, and for that matter, which should we choose between Enceladus and Europa in terms of life-seeking mission destinations for spacecraft that can be flown in the near future? Both have legitimate claims on our attention, and the possibility of plumes on Europa itself (see Water Vapor Detected Above Europa) may change the equation. Will these enticing moons motivate us to reach them before a near-term space telescope finds the first biosignatures around an exoplanet?

The papers are Porco et al., “How the Geysers, Tidal Stresses, and Thermal Emission Across the South Polar Terrain of Enceladus are Related,” The Astronomical Journal Vol. 148, No. 3 (2014), 45 (abstract) and Nimmo et al., “Tidally Modulated Eruptions on Enceladus: Cassini ISS Observations and Models,” The Astronomical Journal Vol. 148, No. 3 (2014) 46 (abstract). On the LIFE mission, see Tsou et al., “LIFE: Life Investigation For Enceladus: A Sample Return Mission Concept in Search for Evidence of Life,” Astrobiology Vol. 2, No. 8 (September 12, 2012). Abstract available.

HK Tauri: Misaligned Protoplanetary Disks

When I was a boy in ninth grade, I asked our science teacher whether the nearest star was likely to have planets. He loved the question because it gave him the chance to explain to the class that Alpha Centauri was a binary star (we left poor Proxima out of the discussion), and that as a binary, it couldn’t possibly have planets because their orbits would be too disrupted by gravitational effects to survive. That sounded reasonable to me, and I began putting my hopes on places like Tau Ceti and Epsilon Eridani, single stars with no disruptive companion.

Since then we’ve begun finding binary stars with planets and are learning about the diversity of exoplanetary systems, putting Alpha Centauri back into the game. A good thing, too, given the fact that binary stars are common, and keeping them in the planet hunt allows that many more chances to find an Earth 2.0, not to mention all the other interesting kinds of planets including ‘super-Earths’ that we’re locating. But the fact that binary systems can have planets doesn’t mean we can ignore the powerful effects two stars in the same system can have on the objects orbiting them.

Take the case of the interesting HK Tauri system, located some 450 light years from Earth in the constellation Taurus. Here we’re dealing with a young system, the two stars being between one and four million years old, the age range in which planet formation is believed to occur. Their separation is about 58 billion kilometers, which works out to 386 AU. Given the youth of the system, it’s not surprising to find protoplanetary disks here, one of them (HK Tauri B) edge-on and observable in visible or near-infrared wavelengths. The orientation of the disk helps to block the light of the central star, making observations at these wavelengths possible.

The other disk, around HK Tauri A, is best observed in millimeter-wavelength light because we do not see it edge-on and visible light observations are overpowered by the star’s light. Now we have new results from the Atacama Large Millimeter/submillimeter Array (ALMA), which draw on observations of the planet-forming disks in this system. A team led by Eric Jensen (Swarthmore College), able to measure the rotation of the HK Tauri A disk for the first time, has discovered that the two disks are mutually misaligned by at least 60 degrees.

Image: This artist’s impression shows a striking pair of wildly misaligned planet-forming gas discs around both the young stars in the binary system HK Tauri. ALMA observations of this system have provided the clearest picture ever of protoplanetary discs in a double star. The new result demonstrates one possible way to explain why so many exoplanets — unlike the planets in the Solar System — came to have strange, eccentric or inclined orbits. Credit: R. Hurt (NASA/JPL-Caltech/IPAC).

What we’re seeing is that when stellar orbits and protoplanetary disks are not in the same plane, the planets under formation are likely to end up in eccentric, tilted orbits, with the gravitational effects of one star perturbing the disk of the other. Is the disk arrangement we find here a unique case or a common process around binary stars? A good deal of work lies ahead before we can answer that question, and not all oddball exoplanet orbits can be explained by this mechanism. But in at least this case, disk misalignment is a powerful indicator. Says Jensen:

“Our results show that the necessary conditions exist to modify planetary orbits and that these conditions are present at the time of planet formation, apparently due to the formation process of a binary star system. We can’t rule other theories out, but we can certainly rule in that a second star will do the job.”

Image: This picture shows the key velocity data taken with ALMA that helped the astronomers determine that the discs in HK Tauri were misaligned. The red areas represent material moving away from Earth and the blue indicates material moving toward us. Credit: NASA/JPL-Caltech/R. Hurt (IPAC).

The paper on this work notes that the team’s findings are consistent with recent simulations of binary formation that predict such misalignments, especially in systems with separation greater than 100 AU, as we find here. Moreover, we may not always be aware of the companion responsible for a perturbed disk. From the paper:

While it remains to be seen how the protoplanetary disks in a statistical sample of young binary systems are oriented, it is suggestive that in the handful of systems where this measurement has been made, the misalignments are large. If this is a common outcome of the binary formation process, and especially if it extends to lower-mass binary companions (which may easily go undetected) as well, then perturbations by distant companions may account for many of the orbital properties that make the current sample of extrasolar planets so unlike our own solar system.

The paper is Jensen and Akeson, “Misaligned Protoplanetary Disks in a Young Binary System,” published online in Nature 31 July 2014 (abstract).

‘Hot Jupiters’: Drier Than Expected

Be aware of Open Source, a radio show on Boston’s WBUR that last week did a show about exoplanets and the possibility of extraterrestrial life. Earth 2.0 is available online, featuring David Latham (Harvard-Smithsonian Center for Astrophysics), Dimitar Sasselov (Harvard University), Jason Wright (Penn State) and Sarah Rugheimer (a PhD student at Harvard studying exoplanet atmospheres). The discussion ranges through the Kepler mission to the Fermi question and recent studies of exoplanet atmospheres, the latter particularly appropriate to today’s post.

For I want to talk today about ‘Hot Jupiters’ and their atmospheres, and what we can learn about planet formation by studying their composition. Hot Jupiters were a surprise when first discovered, but models of planetary migration seemed to explain them. We would expect a gas giant to form at or beyond the ‘snow line,’ where volatiles like water would form ice grains. As we saw in our discussion of Kepler-421b (see Transiting World at the Snow Line), planetary embryos that become gas giants should coalesce in this low temperature regime, with the resulting worlds richer in ice and water than the drier inner Solar System, which relies on volatile delivery by impacting comets or other objects with a formation history in the outer system.

Planetary migration is a way of getting those ‘hot Jupiters’ where they have been observed to be. We assume gravitational interactions with other young worlds that drive some gas giants into the inner system, taking a planet that has formed in the cold regions beyond the snow line into close proximity to the parent star. It would be reasonable to assume high water content in these worlds, but new work led by Nikku Madhusudhan (University of Cambridge, UK) comes up with a surprisingly different result.

Madhusudhan and team used near-infrared spectra of hot Jupiters observed by the Hubble Space Telescope, whose position in space allows accurate measurement of water in an exoplanetary atmosphere because it is far above contaminating water in the Earth’s own atmosphere. The method is transmission spectroscopy, in which some of the star’s light passes through the atmosphere of a planet in transit across its face as seen from Earth. The spectrum that results tells us much about the molecules in the atmosphere, but the researchers are finding only a small fraction of the water predicted by standard planet formation models.

Madhusudhan calls the result ‘astonishing,’ and adds:

“It basically opens a whole can of worms in planet formation. We expected all these planets to have lots of water in them. We have to revisit planet formation and migration models of giant planets, especially ‘hot Jupiters’, and investigate how they’re formed.”

Image: This graph compares observations with modeled infrared spectra of three hot-Jupiter-class exoplanets that were spectroscopically observed with the Hubble Space Telescope. The red curve in each case is the best-fit model spectrum for the detection of water vapor absorption in the planetary atmosphere. The blue circles and error bars show the processed and analyzed data from Hubble’s spectroscopic observations. Credit: NASA, ESA, N. Madhusudhan (University of Cambridge), and A. Feild and G. Bacon (STScI).

The planets in question are HD 189733b, HD 209458b, and WASP-12b, with temperatures ranging from 800 to 2200 degrees Celsius. The water measurement of HD 209458b is the highest-precision measurement of any chemical compound in an exoplanet, and while it does find water, the low abundance creates problems for core accretion scenarios of planet formation beyond the snow line. Is our Solar System unusual in its high water content?

One thing to remember is that exoplanets are, in certain respects, easier for us to measure than some of the worlds in our own system. We know little about the constituents of the planetesimals that formed our own gas giants. The paper explains this seeming paradox while also pointing to an upcoming space mission that can help (internal references omitted for brevity):

Atmospheric elemental abundances of solar-system giant planets have led to important constraints on the origin of the solar system. The observed super-solar enrichments of C, S, N, and inert gases, support the formation of Jupiter by core accretion. However, the oxygen abundance of Jupiter is yet unknown. The upper atmosphere of Jupiter (P < 1 bar) has T < 200 K, causing water to condense and to be confined to the deepest layers (> 10 bar), requiring dedicated probes to measure it. The upcoming Juno mission to Jupiter aims to measure its O abundance, which is important to estimate the amount of water ice that was available in the planetesimals forming Jupiter and the rest of the solar system.

So Juno should be able to give us a better read on Jupiter’s oxygen, thus helping us better understand the kind of planetesimals that formed in our system’s earliest days. As to the measurements of exoplanets vs. planets closer to home:

The O/H and C/O ratios are easier to measure for hot giant exoplanets than they are for solar-system giant planets. The vast majority of extrasolar gas giants known have equilibrium temperatures of ~1000-3000 K, thus hosting gaseous H2O in their atmospheres accessible to spectroscopic observations.

HD 189733b, HD 209458b, and WASP-12b are good choices because they range widely in temperature, with HD 189733b being one of the ‘coolest’ hot Jupiters known, and Wasp 12b one of the hottest. On the matter of equilibrium temperature (Teq), I’m drawing on Sara Seager’s book Exoplanet Atmospheres: Physical Processes (Princeton, 2010), which explains that equilibrium temperature is the temperature attained by an isothermal planet after it has attained complete equilibrium with the radiation from the star it orbits. The Madhusudhan paper adds that these hot Jupiters have the best spectroscopic precision of all the hot Jupiters that have been observed using the transmission spectroscopy technique.

So we have high-quality results that have the researchers looking at various scenarios to explain low water abundances. The paper adds that the Galileo probe reported a low H20 abundance in Jupiter that was explained by saying the probe moved through an unusually dry region. But at least one alternative explanation came in a 2004 paper suggesting that Jupiter may have formed by planetesimals dominated by tar rather than water ice. The Madhusudhan results reawaken such questions and cause us to look anew at formation and migration models for all giant planets.

The paper is Madhusudhan et al., “H2O abundances in the atmospheres of three hot Jupiters,” The Astrophysical Journal Letters Vol. 791, No. 1 (2014) L9 (abstract / preprint). On carbonaceous matter in the formation of Jupiter, see Lodders, “Jupiter Formed with More Tar than Ice,” The Astrophysical Journal Vol. 611, No. 1 (2004), 587 (abstract).

Tight Measurement of Exoplanet Radius

Both the Kepler and Spitzer space telescopes had a role to play in recent work on the planet Kepler-93b, whose size is now known to an uncertainty of a mere 120 kilometers on either side of the planet. What we have here is the most precise measurement of an exoplanet radius yet, a helpful result in the continuing study of ‘super-Earths,’ a kind of world for which we have no analogue in our own Solar System. A third instrument also comes into play, for studies of the planet’s density derived from Keck Observatory data on its mass (about 3.8 times Earth’s mass) and the known radius indicate this is likely an world made of iron and rock.

And that is absolutely the only similarity between Kepler-93b and Earth, for at 0.053 AU, six times closer than Mercury to the Sun, the planet’s surface temperature is estimated to be in the range of 760 degrees Celsius. The planet is 1.481 times the width of Earth. The accuracy of the measurement is the story here, a result so precise that, in the words of Sarah Ballard (University of Washington), lead author of the paper on these findings, “it’s literally like being able to measure the height of a six-foot tall person to within three quarters of an inch — if that person were standing on Jupiter.”

Image: Using data from NASA’s Kepler and Spitzer Space Telescopes, scientists have made the most precise measurement ever of the size of a world outside our solar system, as illustrated in this artist’s conception. The diameter of the exoplanet, dubbed Kepler-93b, is now known with an uncertainty of just one percent. Credit: NASA/JPL-Caltech.

Just how the measurement was made is a story in itself. The Spitzer instrument provided data for seven transits of Kepler-93b between 2010 and 2011, three of them studied with a new observational technique called ‘peak up’ that halved the uncertainty of Spitzer’s own radius measurements. Kepler-93 thus served as a test subject for the new technique, which was developed in 2011 and allows tighter control over how light affects individual pixels in the observatory’s infrared camera. The paper examines all seven light curves in detail.

Meanwhile, we have the Kepler data, which provided light curves as well as the dimming of the star caused by seismic waves in motion in the interior. Now we’re in the realm of asteroseismology, which is a powerful way to probe the makeup of individual stars. Asteroseismic measurements over a long observational baseline can provide useful information about the density of the star (with a precision of 1 percent) as well as its age (within 10%). Such measurements require a long observational baseline at high cadence — cadence refers to the time between observations of the same target — as well has high photometric precision.

When we have both an asteroseismic density measurement of the exoplanet host star as well as a transit light curve, we can improve the precision of our radius measurements. Sara Seager (MIT) and colleagues examined host star densities in relation to planetary orbits and the radius of the star as early as 2003, and later work by a team led by Philip Nutzman (Harvard-Smithsonian CfA) used asteroseismology along with transit light curves to constrain the radius of HD 17156b, highlighting a method that has been found to be relevant to a wide number of recent studies.

From the paper:

The Kepler mission’s long baselines and unprecedented photometric precision make asteroseismic studies of exoplanet hosts possible on large scales… Kepler-93 is a rare example of a sub-solar mass main-sequence dwarf that is bright enough to yield high-quality data for asteroseismology. Intrinsically faint, cool dwarfs show weaker-amplitude oscillations than their more luminous cousins. These targets are scientifically valuable not only as exoplanet hosts, but also as test beds for stellar interior physics in the sub-solar mass regime.

The combination of the Kepler data and Spitzer’s new technique was powerful, and adds luster to the already rich history of Spitzer’s Infrared Array Camera (IRAC) in exoplanetary science. The instrument has been helpful in mapping planetary weather and characterizing super-Earth atmospheres, and has been a major tool in ruling out exoplanet false-positives, because an actual planet will present the same transit depth no matter the wavelength at which it is observed. After losing its coolant in 2009, the telescope, now dubbed ‘warm Spitzer,’ continues to provide key readings that are now enhanced with the development of the ‘peak up’ process.

Kepler-93 is a star of approximately 90 percent of the Sun’s mass and radius, located some 300 light years from Earth. With the Spitzer data corroborating the find and the use of asteroseismology to constrain the result, we wind up with an error bar that is just one percent of the radius of Kepler-93b. A planet thought to be 18,800 kilometers in diameter might be bigger or smaller than that by about 240 kilometers, but no more, an outstanding result for exoplanetary science and a confirmation of the power of asteroseismology in determining stellar radii.

The paper is Ballard et al., “Kepler-93b: A Terrestrial World Measured to within 120 km, and a Test Case for a New Spitzer Observing Mode,” The Astrophysical Journal Vol. 790, No. 1 (2014), 12 (abstract / preprint). A JPL news release is also available.