A Microlensing Opportunity for Centauri A

by Paul Gilster on October 25, 2016

First light for the European Extremely Large Telescope (E-ELT) is scheduled for 2024, a useful fact given that a few years later, we may be able to use the instrument in a gravitational lensing opportunity involving Alpha Centauri. Specifically, Centauri A is expected to align with the star 2MASS 14392160-6049528, thought to be a red giant or supergiant and far more distant than Alpha Centauri. This will create an event that not just the E-ELT but other instruments, like the GRAVITY instrument on the Very Large Telescope Interferometer (VLTI), will be able to study — GRAVITY is capable of extremely high accuracy astrometry.

A team of French astronomers led by Pierre Kervella (CNRS/Universidad de Chile) is behind this new study, which involved fine-tuning our knowledge of the trajectories of Centauri A and B. Remember that we see gravitational lensing when a massive object like a star distorts the spacetime around it, so that light from the more distant object must follow a curved path to reach us. The amount of mass in the closer star affects the extent of this deflection, and when one or more planets orbit the star, they become theoretically detectable.


Image: The predicted trajectory of Alpha Centauri A and B. Credit: ESO.

The Centauri A event is to occur in 2028, by which time we may well have knowledge of other planets around one or more of the primary Alpha Centauri stars. But a particularly useful aspect of microlensing is that it is not reliant on proximity to the star. Unlike transit studies or radial velocity analysis, which can produce evidence for a close-in planet quickly but require lengthy study for planets further out in the system, microlensing allows us to spot planets in wider orbits, even if the observation in which we find them is transitory and does not repeat.

So this lensing event is another potential window into this intriguing system, around which at present we know only of the world orbiting Proxima Centauri. Recall that the latter has also been studied in two microlensing events, one in October of 2014, the second in February of 2016. We found Proxima b through radial velocity methods (although the star was also the subject of transit studies), but microlensing is useful even when it fails to turn up planets, since it gives us a way of refining our estimates of a given star’s mass.

Alpha Centauri gives us serious opportunities for microlensing because the star field behind it is densely populated, thanks to the system’s location near the plane of the galaxy as seen from Earth. The Kervella paper studies conjunctions with background stars that will occur in coming decades, based on observations of the field surrounding Alpha Centauri that Kervella and co-author Fréderic Thévenin (Observatoire de la Côte d’Azur) began over a decade ago.


Image: An enlargement of the conjunction that will occurs in 2028, with the Einstein ring of Alpha Cen A represented in cyan color. Credit: Pierre Kervella.

The conjunction of Centauri A with 2MASS 14392160-6049528 is the most favorable of the conjunctions involving Alpha Centauri over the next three decades. From the paper, which references the background star as S5; i.e., one of the background stars numbered in the survey:

During the approaches, astrometric measurements will reveal the relativistic deflection of the background star light with a high signal to noise ratio. For the conjunction with S5, we may be able to directly observe the gravitational splitting of the distant source image using the E-ELT, VLTI/GRAVITY or ALMA. The astrometric monitoring of the relative positions of α Cen and the S stars may reveal the presence of planets through secondary gravitational lensing. In addition, the light from the background star will possibly be subject to photometric variations induced by transits of low mass objects present in the α Cen system.

Potential results include, in other words, everything from high and low mass planets to asteroids and comets. The authors believe the conjunctions in coming decades will give us highly accurate information about the Alpha Centauri stars’ proper motion, orbital parameters and parallax values. And note this: “This accuracy will be valuable for high-precision modeling of the two stars of α Cen, and for the preparation of the recently announced Breakthrough Starshot initiative to send ultra-fast light-driven nanocrafts to α Centauri.”

The Kervella paper is “Close stellar conjunctions of α Centauri A and B until 2050,” Astronomy & Astrophysics 594 (2016), A107 (abstract). On microlensing and Proxima Centauri, see Sahu et al., “Microlensing Events by Proxima Centauri in 2014 and 2016: Opportunities for Mass Determination and Possible Planet Detection,” Astrophysical Journal Volume 782, Issue 2 (2014). Abstract available.



Red Dwarfs: Oldest Known Circumstellar Disk

by Paul Gilster on October 24, 2016

Determining the age of a star is not easy, but one way of proceeding with at least some degree of confidence is to identify the star as a member of a stellar association. Here we’re talking about a loose cluster of stars of a common origin. Over time, the stars have begun to separate, but they still move together through space. It was the Armenian astronomer Viktor Ambartsumian, the founder of the Byurakan Observatory, who discovered the nature of these associations and demonstrated that they were composed of relatively young groups of stars.

Stellar associations, or young moving groups (YMGs), provide an outstanding place to study the evolution of protoplanetary disks around young stars, for all associated stars have a similar age. Indeed, their galactic motion can be traced back to their place of origin. Another benefit: Exoplanets in such infant systems are often still hot, well within the capabilities of our near-infrared direct imaging techniques. Many direct imaging and disk evolution surveys in recent years have focused on the members of young moving groups.

Without Ambartsumian’s discovery of these groups, we would be hard pressed to come up with the age of the interesting red dwarf tagged AWI0005x3s. We’ve just learned, through a team led by Steven Silverberg (University of Oklahoma), working with a group of citizen scientists using the Disk Detective site, that this star has a warm circumstellar disk, an interesting find in its own right because we haven’t found many disks around red dwarfs.

But AWI0005x3s is found in the Carina association, on the order of 200 light years away in the Carina nebula, and appears to be moving with it. That pegs the young red dwarf as highly unusual, as Silverberg explains:

“Most disks of this kind fade away in less than 30 million years. This particular red dwarf is a candidate member of the Carina stellar association, which would make it around 45 million years old [like the rest of the stars in that group]. It’s the oldest red dwarf system with a disk we’ve seen in one of these associations.”


Image: Artist’s concept of the newly discovered disk. Credit: Jonathan Holden.

Looking through the paper, I learned that the intriguing AWI0005x3s disk would be the oldest ever observed around an M-dwarf, assuming the star can be confirmed as a member of Carina (the authors argue that the star has a probability of over 90 percent of being part of the association). Astronomers have found a disk frequency of about 6 percent around M-dwarfs less than 40 million years old, dropping to 1.3 percent around older members of this stellar class.

So where does AWI0005x3s fit in? The paper contrasts what we see in M-dwarfs with other types of star:

…debris disks are detected around 32 ± 5% of young A stars with Spitzer/MIPS (Su et al. 2006), and around 1−6% of old (∼ 670 Myr) Sun-like (F5-K9) stars with Spitzer/MIPS (Urban et al. 2012). Survival models predict that M dwarf debris disks occur at a similar frequency as disks around Sun-like stars, and that the dearth of detections to date is either due to systems having blackbody-like dust close to their central star, or due to systems having a smaller amount of dust distributed over a larger orbital separation (Heng & Malik 2013).

But other possibilities are still in play, including accelerated disk dissipation through interactions with a young stellar wind. Its age places AWI0005x3s in a potentially useful place in relation to other M-dwarfs, as the paper makes clear:

Our new M dwarf debris disk would bridge the gap between YMG and field M dwarf disks. Given their common spectral type (both M5.5V), this system could be a young analog for the Proxima Centauri system (Anglada-Escudé et al. 2016), as well.

The authors believe that AWI0005x3s is a potential target for study via adaptive optics on large telescopes and should be within range for high-contrast imaging, which could allow us to resolve the structure of the disk and potentially identify exoplanet candidates.

As for Disk Detective, it’s well worth a look. In fact, some 30,000 people have gotten involved in viewing short videos from surveys like the Wide-field Infrared Survey Explorer mission (WISE) and Two-Micron All Sky Survey (2MASS) projects, with some two million classifications of celestial objects now achieved. Eight of the citizen scientists involved are listed as co-authors on the AWI0005x3s paper, which is now available online.

The paper is Silverberg et al., “A New M Dwarf Debris Disk Candidate in a Young Moving Group Discovered with Disk Detective,” Astrophysical Journal Letters Vol. 830, No. 2 (14 October 2016). Abstract / preprint.



Witnessing Titan’s ever-changing seasons has been a major payoff of the Cassini mission, whose end is now close enough (September, 2017) to cause us to reflect on its accomplishments. We now see winter settling in firmly in the southern hemisphere, along with a strong vortex now developing over the south pole. When Cassini arrived in 2004, we saw much the same thing, only in the northern hemisphere. Athena Coustenis (Observatoire de Paris) is presenting results on Titan’s climate at the ongoing joint meeting of the American Astronomical Society’s Division for Planetary Sciences and 11th European Planetary Science Congress.

“Cassini’s long mission and frequent visits to Titan have allowed us to observe the pattern of seasonal changes on Titan, in exquisite detail, for the first time,” says Dr. Coustenis. “We arrived at the northern mid-winter and have now had the opportunity to monitor Titan’s atmospheric response through two full seasons. Since the equinox, where both hemispheres received equal heating from the Sun, we have seen rapid changes.”

The overall cycle of heat circulation on Titan is clearly defined. Warm gases rise at the summer pole as cold gases subside at its winter equivalent. The equinox occurred on Titan in 2009, and since then Cassini has observed a reversal of the system. A strong, revolving pattern of circulation, or vortex, has developed in the stratosphere over the south pole, one that is enriched in trace gases that are otherwise rarely found in Titan’s atmosphere. Cassini also revealed an atmospheric hot spot developing at high altitudes within months of the equinox, while its counterpart in the northern hemisphere had greatly diminished two years later.


Image: Slipping into shadow, the south polar vortex at Saturn’s moon Titan still stands out against the orange and blue haze layers that are characteristic of Titan’s atmosphere. Images like this, from NASA’s Cassini spacecraft, lead scientists to conclude that the polar vortex clouds form at a much higher altitude — where sunlight can still reach — than the lower-altitude surrounding haze. This view looks towards the trailing hemisphere of Titan (5,150 kilometers across). North on Titan is up and rotated 17 degrees to the left. Images taken using red, green and blue spectral filters were combined to create this natural-color view. The image was taken with the Cassini spacecraft narrow-angle camera on July 30, 2013. Credit: NASA/JPL-Caltech/Space Science Institute.

Within the polar vortex over the south pole, trace gases are accumulating as sunlight diminishes. Here again the parallel is direct. We now see, according to this Europlanet news release, the appearance of complex hydrocarbons and nitriles like methylacetylene and benzene, which were before observed only at high northern latitudes. Coustenis again:

“We’ve had the chance to witness the onset of winter from the beginning and are approaching the peak time for these gas-production processes in the southern hemisphere. We are now looking for new molecules in the atmosphere above Titan’s south polar region that have been predicted by our computer models. Making these detections will help us understand the photochemistry going on.”

While the onset of winter led to a swift temperature drop of 40 degrees Celsius in the stratosphere over the southern pole, the warming effects in the northern hemisphere as the seasons change have been much more gradual, with a 6-degree rise since 2014. In these northerly regions, Cassini has found trace gases that persist into the summer. Although these should eventually disappear, Coustenis says an area of depleted molecular gas and aerosols has emerged across the entire northern hemisphere at an altitude of 400-500 kilometers.

High altitudes on Titan are, in other words, complicated, and while we’re developing a consistent picture thanks to Cassini’s twelve years of observations, these complex effects bear further study. Remember that although we’re entering Cassini’s last year, we have a two-part endgame to go through that involves a final close flyby of Titan to reshape the spacecraft’s orbit. In its new trajectory, Cassini will make 22 passes through the gap between the rings and the planet.

The so-called Grand Finale begins in April of 2017 and takes us to a first dive through the ring/planet gap on April 27. It should be quite a ride, with the closest observations ever made of Saturn, including mapping the planet’s magnetic and gravity fields at high precision, along with samples of particles in the main rings and gases from Saturn’s outer atmosphere. In addition, we should get spectacular views of the rings when, in November of this year, Cassini begins a series of 20 passes just beyond the outer edge of the main rings. Cassini has not gotten this close to the rings since its arrival at Saturn in 2004; we’ll see the ring structure at high resolution. The spacecraft’s final dive into Saturn is planned for September 15, 2017.

“While it will be sad to say goodbye, Cassini’s final act is like getting a whole new mission in its own right,” said Linda Spilker, Cassini project scientist at NASA’s Jet Propulsion Laboratory, Pasadena, California. “The scientific value of the F ring and Grand Finale orbits is so compelling that you could imagine an entire mission to Saturn designed around what we’re about to do.”



New Work on Planet Nine

by Paul Gilster on October 20, 2016

Considering how long we’ve been thinking about a massive planet in the outer Solar System — and I’m going all the way back to Percival Lowell’s Planet X here — the idea that we might find the hypothetical Planet Nine in just three years or so is a bit startling. But Caltech’s Mike Brown and colleague Konstantin Batygin, who predicted the existence of the planet last January based on its effects on Kuiper Belt objects, are continuing to search the putative planet’s likely orbital path, hoping for a hit within the next few years, a welcome discovery if it happens.

The duo are working with graduate student Elizabeth Bailey, lead author of a new study being discussed at the American Astronomical Society’s Division for Planetary Sciences meeting in Pasadena, which is occurring in conjunction with the European Planetary Science Congress. The new paper is all about angles and alignments, focusing on the fact that the relatively flat orbital plane of the planets is tilted about six degrees with respect to the Sun. That’s an oddity, and Planet Nine, hypothesized to be about ten times the mass of the Earth and in an orbit averaging 20 times Neptune’s distance from the Sun, just may be the cause.

The calculations on display in the new paper depict a planet some 30 degrees out of alignment with the orbital plane of the other planets. That can help to explain orbital observations of Kuiper Belt objects, but also the unusual system-wide tilt, which stands out because of the assumed formation of the planets through the collapse of a spinning cloud into a disk and, eventually, a collection of planets orbiting the Sun. We would expect the angular momentum of the planets to maintain a rough alignment with the Sun along the orbital plane.

Unless, of course, something is disrupting the system. Throw in the angular momentum of Planet Nine, based on its assumed mass and distance from the Sun, and profound effects on the system’s spin become evident, creating a long-term wobble that shows up in the system’s tilt. As Bailey puts it, “Because Planet Nine is so massive and has an orbit tilted compared to the other planets, the Solar System has no choice but to slowly twist out of alignment.”

And this from the paper:

… a solar obliquity of order several degrees is an expected observable effect of Planet Nine. Moreover, for a range of masses and orbits of Planet Nine that are broadly consistent with those predicted by Batygin & Brown (2016); Brown & Batygin (2016), Planet Nine is capable of reproducing the observed solar obliquity of 6 degrees, from a nearly coplanar configuration. The existence of Planet Nine therefore provides a tangible explanation for the spin orbit misalignment of the solar system.


Image: This artistic rendering shows the distant view from Planet Nine back towards the sun. The planet is thought to be gaseous, similar to Uranus and Neptune. Hypothetical lightning lights up the night side. Credit: Caltech/R. Hurt (IPAC).

The six-degree tilt we see between planetary disk and Sun thus fits into the team’s calculations regarding Planet Nine’s size and distance from the central star. And if this does indeed turn out to be the explanation, speculation will then center on how Planet Nine came to be so far out of line with the other planets. We know that gravitational interactions in young planetary systems can sometimes result in disruption, causing some planets to be thrown out of their systems, and others to be moved into distant orbits. Such gravitational byplay may well be the reason for Planet Nine’s unusual position. Now we just need to discover the planet.

I also want to mention that Renu Malhotra (University of Arizona) and team have continued their analysis of a possible Planet Nine, likewise presenting their results at the AAS/EPSC meeting in Pasadena. Through analysis of what they call ‘extreme Kuiper Belt Objects’ —on eccentric orbits with aphelia hundreds of AU out — the team finds a clustering of orbital parameters that may point to the existence of a planet of 10 Earth masses with an aphelion of more than 660 AU. Two orbital planes seem possible, one at 18 degrees offset from the mean plane, the other inclined at 48 degrees.

Dr. Malhotra confirmed in an email this morning that her own constraints on the current position of this possible planet line up with Mike Brown and team at Caltech. But her team continues to point out that we have no detection at this point, and much to learn about the orbits of the Kuiper Belt objects under study. From her paper:

…we note that the long orbital timescales in this region of the outer solar system may allow formally unstable orbits to persist for very long times, possibly even to the age of the solar system, depending on the planet mass; if so, this would weaken the argument for a resonant planet orbit. In future work it would be useful to investigate scattering efficiency as a function of the planet mass, as well as dynamical lifetimes of non-resonant planet-crossing orbits in this region of the outer system. Nevertheless, the possibility that resonant orbital relations could be a useful aid to prediction and discovery of additional high mass planets in the distant solar system makes a stimulating case for renewed study of aspects of solar system dynamics, such as resonant dynamics in the high eccentricity regime, which have hitherto garnered insufficient attention.

The Bailey, Batygin & Brown paper is “Solar Obliquity Induced by Planet Nine,” accepted for publication in the Astrophysical Journal (preprint). The Malhotra paper is Malhotra, Volk & Wang, “Coralling a Distant Planet with Extreme Resonant Kuiper Belt Objects,” Astrophysical Journal Letters Vol. 824, No. 2 (20 June 2016). Abstract / preprint.



New Horizons: Looking Further Out

by Paul Gilster on October 19, 2016

We’re getting close on New Horizons data, all of which should be downlinked as of this weekend. Although that’s a welcome marker, it hardly means the end of news from the doughty spacecraft. For one thing, we have years of analysis ahead of us as we bring the abundant data from the spacecraft’s instrument packages into focus. For another, we’re still in business out there in the Kuiper Belt, heading for that interesting object 2014 MU69.

Who knows what will turn up at the latter, given our propensity to be surprised at every turn in interplanetary exploration, from Triton’s volcanic plains to fabulously fractured Miranda. And, of course, Pluto and Charon themselves, which turned out to be so interesting that Alan Stern, principal investigator for New Horizons, is already talking about future missions there.

But back to 2014 MU69, which has continued to be the subject of Hubble observations even as New Horizons homes in on the object. As this news release from the mission points out, MU69 is the smallest KBO ever to have its color measured, a reddish hue that confirms its identity as part of the ‘cold classical’ region of the Kuiper Belt. These are objects with low orbital eccentricity and inclination that are not in orbital resonance with Neptune. Reddish-brown tholins formed by solar radiation interacting with simple organic compounds are common here.

“The reddish color tells us the type of Kuiper Belt object 2014 MU69 is,” says Amanda Zangari, a New Horizons post-doctoral researcher from Southwest Research Institute. “The data confirms that on New Year’s Day 2019, New Horizons will be looking at one of the ancient building blocks of the planets.”


Image: 2014 MU69 travels diagonally across a dense field of stars and noise in the background. Credit: NASA, ESA, SwRI, JHU/APL, and the New Horizons KBO Search Team.

New Horizons has now covered a third of its distance from Pluto to MU69, with the target approximately a billion kilometers away. The analysis of New Horizons data, meanwhile, is turning up interesting things on Charon, where we find landslides, a feature that has not yet been spotted on Pluto’s surface, although we’ve found them on worlds as diverse as Mars and Iapetus. The Charon landslides are the farthest ever observed from the Sun.


Image: Scientists from NASA’s New Horizons mission have spotted signs of long run-out landslides on Pluto’s largest moon, Charon. This image of Charon’s informally named Serenity Chasma was taken by New Horizons’ Long Range Reconnaissance Imager (LORRI) on July 14, 2015, from a distance of 78,717 kilometers. Arrows mark indications of landslide activity. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

Likewise, we learn that while Pluto’s atmosphere is hazy but largely cloud-free, a handful of possible clouds have turned up in New Horizons imagery. That would point to an atmosphere still more complex than expected. And the variations in surface brightness on Pluto itself are telling. Some of these regions, particularly in Pluto’s now famous heart-shaped region, are among the most reflective in the Solar System. This has implications for what may be occurring on another deep space object, says Bonnie Buratti (JPL), a co-investigator on the New Horizons science team:

“That brightness indicates surface activity. Because we see a pattern of high surface reflectivity equating to activity, we can infer that the dwarf planet Eris, which is known to be highly reflective, is also likely to be active.”


Image: Pluto’s present, hazy atmosphere is almost entirely free of clouds, though scientists from NASA’s New Horizons mission have identified some cloud candidates after examining images taken by the New Horizons Long Range Reconnaissance Imager and Multispectral Visible Imaging Camera, during the spacecraft’s July 2015 flight through the Pluto system. All are low-lying, isolated small features-no broad cloud decks or fields – and while none of the features can be confirmed with stereo imaging, scientists say they are suggestive of possible, rare condensation clouds. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

We’re a long way from through with New Horizons, which should make its flyby of 2014 MU69 on January 1, 2019 after the series of four course changes that adjusted its trajectory. We may have other outer system news to discuss as the joint meeting of the American Astronomical Society Division for Planetary Sciences and European Planetary Science Congress in Pasadena continues this week. But for now, I particularly like Alan Stern’s words:

“We’re excited about the exploration ahead for New Horizons, and also about what we are still discovering from Pluto flyby data. Now, with our spacecraft transmitting the last of its data from last summer’s flight through the Pluto system, we know that the next great exploration of Pluto will require another mission to be sent there.”



Antimatter Sail: Focus on Storage

by Paul Gilster on October 18, 2016

An antimatter sail, as described yesterday in the work of Gerald Jackson and Steve Howe, is an exciting idea particularly because it relies on only small amounts of antimatter, tapping its energies to create fission in a uranium-enriched sail. Thus the uranium is the fuel and the antimatter, as Jackson says, is the ‘spark plug.’ We reduce the needed amount of antimatter and define what the new Kickstarter campaign calls “…the first proposed antimatter-based propulsion system that is within the near-term ability of the human race to produce.”

The antimatter sail produces fission by allowing antimatter, stored probably as antihydrogen, to drift across to the sail, and as we saw yesterday, the potential for velocities up to 5 percent of lightspeed mean that such a sail could be deployed on interstellar missions. Proxima Centauri naturally emerges as a target, but Jackson and Howe’s work is not a result of recent interest in that star and its one known planet. The 2002 study in which they describe the antimatter sail was originally created for a probe to 250 AU (a Kuiper Belt and heliopause mission), drawing on work on a 10 kg instrument payload that was done at the Jet Propulsion Laboratory.


Image: Antiproton striking the depleted uranium coating on a carbon sail. Credit: Gerald Jackson/Hbar Technologies.

The just launched Kickstarter campaign is to re-think that earlier work in the context of an unmanned mission to a nearby solar system. Among the specific campaign goals is to create a detailed design for long-lived antimatter storage, a key issue in any such concept. After all, we have to store the antimatter in such a way that it does not annihilate with the normal matter surrounding it. We’ve known that this can be done for some time. In fact, it was back in 1984 that physicist Hans Dehmelt demonstrated how to hold a single positron in a cylinder using electric and magnetic fields. Dehmelt gave his device the name ‘Penning trap,’ a nod to Frans Penning, a Dutch physicist who saw that a magnetic field could steer electrons into tight orbits.

Dehmelt would go on to be awarded the Nobel Prize in Physics in 1989 for co-developing the Penning trap technique with Wolfgang Pauli (each shared one-half of the prize). By adjusting the voltage and strength of the magnetic field, the Penning trap would become a viable way to store tiny amounts of antimatter.

But as antimatter storage has become feasible, problems grow as we begin storing more and more of the stuff. Try to contain large amounts of positrons or antiprotons and their like charges repel. Thus large quantities of antimatter — and compared to current levels of production, we need large quantities indeed — experience repulsive forces between them that quickly become stronger than the magnetic container can handle. Soon the magnetic ‘bottle’ begins to leak and the antiparticles are destroyed.

In his book Antimatter (Oxford University Press, 2010), Frank Close notes that even a millionth of the amount of antimatter needed for a Mars trip would create tons of electric force on the walls of the tank. That’s a daunting thought, and this is for a nearby target, although Close isn’t thinking in terms of the antimatter sail concept, which minimizes the amount of antimatter needed. Even so, high-capacity storage of antimatter has to be addressed.

What Jackson and Howe have been looking at for their antimatter sail involves storing the antimatter in the form of antihydrogen (a positron orbiting an antiproton). Here we’re at the heart of the original work the duo did for NASA’s Institute for Advanced Concepts, from which the antimatter sail took root. A key goal of the new Kickstarter campaign is to produce a design report describing how to build an antihydrogen storage bottle that can be used aboard a spacecraft. Their extensive experience with the issues makes Jackson and Howe an ideal team to push spacecraft antimatter storage forward.

I’m looking back at the original NIAC report Steve Howe prepared for NASA, which envisions storing antihydrogen in the form of frozen pellets rather than in traps, the idea being to use integrated circuit chips of the kind we have become familiar with through today’s microprocessors. We would deploy the same kind of etching technology to create a series of tunnels on each chip, with wells at periodic intervals where the antihydrogen pellets would be held. Changing the voltage allows pellets to move down these tunnels from well to well.

Remember the concept here: The antimatter, as it emerges from the storage device (held some 12 meters behind the sail) is then accelerated so that it drifts out into contact with the sail. But Jackson and Howe’s thinking is clearly evolving on this matter. The plan discussed on the Kickstarter site is to create an antihydrogen storage bottle based on methods Robert Millikan and Harvey Fletcher used in the early 20th Century to measure the charge of the electron. As this involved observing charged oil droplets between two metal electrodes, its uses may supercede the kind of chip technology originally envisioned. And further research, Jackson says, may actually involve antilithium rather than antihydrogen as the optimum form of antimatter.

A key requirement, of course, is portability — this is a storage method that has to be applicable to spacecraft. At Hbar Technologies’ headquarters, Jackson and Howe have the first portable storage bottle for such uses, one able to store positrons or antiprotons at liquid helium temperatures in a hard vacuum. Jackson’s experience with storage also extends to the design and construction of particle accelerator storage rings. Forcing storage technology forward is the need to house amounts of antimatter too large to store as charged elementary particles.


Image: Original portable antimatter storage bottle built by Penn State University and JPL.

I’ve focused on storage, but it’s clear that a lot of things have to go right to get to an interstellar antimatter sail. Key parameters for the Kickstarter effort are to drill down further on antimatter storage issues while at the same time describing renewed antiproton production possibilities at Fermi National Accelerator Laboratory. The effort will recast the original study by way of firming up or adjusting earlier parameters in sail physics and engineering. The thrust-test apparatus developed through the NIAC grant will be upgraded and test facilities identified; here I would imagine experiments involving uranium-laden foils and antiproton interactions. Jackson and Howe also want to design an antihydrogen experiment to be funded in a later campaign.

To me, the possibility of a renewed and improved antiproton process at Fermilab is quite interesting, as we’ve seen what minute amounts our technologies are currently able to generate. In addition to an investigation like this, I would imagine Jackson and Howe will want to look at James Bickford’s ideas on natural antimatter production within the Solar System (see, for example, Antimatter Acquisition: Harvesting in Space, or search the archives here for more).

Bickford argues that space harvesting of antimatter is five orders of magnitude more cost effective than producing antimatter on Earth, an idea Jackson and Howe may want to contest if the highly restricted antimatter yields at Fermilab can be adjusted and improved. For more, see the Kickstarter page for this effort and keep an eye on Jackson and Howe’s Antimatter Drive site. The page there is not yet populated, but the intent is to archive all previous work on the antimatter sail, including a great deal of continuing studies on these storage concepts.



Antimatter and the Sail

by Paul Gilster on October 17, 2016

An antimatter probe to a nearby star? The idea holds enormous appeal, given the colossal energies obtained when normal matter annihilates in contact with its antimatter equivalent. But as we’ve seen through the years on Centauri Dreams, such energies are all but impossible to engineer. Antimatter production is infinitesimal, the by-product of accelerators designed with a much different agenda. Moreover, antimatter storage is hellishly difficult, so that maintaining large quantities in a stable condition requires multiple breakthroughs.

All of which is why I became interested in the work Gerald Jackson and Steve Howe were doing at Hbar Technologies. Howe, in fact, became a key source when I put together the original book from which this site grew. This was back in 2002-2003, and I was captivated with the idea of what could be called an ‘antimatter sail.’ The idea, now part of a new Kickstarter campaign being launched by Jackson and Howe, is to work with mere milligrams of antimatter, allowing antiprotons to be released from the spacecraft into a uranium-enriched, five-meter sail.

Reacting with the uranium, the antimatter produces fission fragments that create what could be considered a nuclear-stimulated ablation blowing off the carbon-fiber sail. As to the reaction itself, Jackson and Howe would use a sheet of depleted uranium U-238 with a carbon coating on its back side. Here’s how the result is described in the Kickstarter material now online:

When antiprotons… drift onto the front surface, their negative electrical charge allows them to act like an orbiting electron, but with different quantum numbers that allow the antiprotons to cascade down into the ground orbital state. At this point it annihilates with a proton or neutron in the nucleus. This annihilation event causes the depleted uranium nucleus to fission with a probability approaching 100%, most of the time yielding two back-to-back fission daughters.

Now we get into a serious kick for the spacecraft:

A fission daughter travelling away from the sail at a kinetic energy of 1 MeV/amu has a speed of approximately 13,800 km/sec, or 4.6% of the speed of light. The other fission daughter is absorbed by the sail, depositing its momentum into the sail and causing the sail (and the rest of the ship) to accelerate.

The concept relies, as Jackson said in a recent email, on using antimatter as a spark plug rather than as a fuel, converting the energy from proton-antiproton annihilations into propulsion.


Image: The original antimatter probe concept. Credit: Gerald Jackson/Hbar Technologies.

The current work grows out of a 2002 grant from NASA’s Institute for Advanced Concepts but the plan is to develop the idea far beyond the Kuiper Belt mission Jackson and Howe initially envisioned. Going interstellar would take not milligrams but tens of grams of antimatter, far beyond today’s infinitesimal production levels. In fact, while the Fermi National Accelerator laboratory has been able to produce no more than 2 nanograms of antimatter per year, even that is high compared to CERN’s output (the only current source), which is 100 times smaller.

Even so, interest in antimatter remains high because of its specific energy — two orders of magnitude larger than fusion and ten orders of magnitude larger than chemical reactions — making further research highly desirable. If the fission reaction the antimatter produces within the sail is viable, we will be able to demonstrate a way to harness those energies, with implications for deep space exploration and the possibility of interstellar journeys.

The original NIAC work led to a sail 5-meters in diameter, with a 15-micron thick carbon layer and a uranium coating 293 microns thick. Interestingly, the study showed that the sail had sufficient area to remove any need for active cooling of the surface. Indeed, the steady-state temperature of the sail would be 570𝆩 Celsius, below the melting point of uranium.


Image: A cloud of anti-hydrogen drifts towards the uranium-infused sail. CREDIT: Hbar Technologies, LLC/Elizabeth Lagana.

The work was based around a 10 kg instrument payload to be delivered to 250 AU within 10 years. Turning to interstellar possibilities, Breakthrough Starshot has been talking about reaching 20 percent of lightspeed with a beamed laser array pushing small sails. Jackson and Howe now seek roughly 5 percent of c, making for a mission of less than a century to reach Proxima Centauri, where we already know an interesting planet awaits.

But here’s a significant difference: Unlike Breakthrough Starshot’s flyby assumptions, the antimatter sail mission concept is built around decelerating and attaining orbit around the target star. In the absence of magsail braking against Proxima’s stellar wind, this would presumably also involve antimatter, braking with the same methods to allow for long-term scientific investigation, thus avoiding the observational challenges of a probe pushing past a small and probably tidally-locked planet at 20 percent of lightspeed.

Here’s how Jackson describes deceleration in his recent email:

Our project considers deceleration and orbit about the destination star a mission requirement. There are serious implications for spacecraft velocity when the requirement of deceleration at the destination is imposed. Either drag or some other mechanism needs to be invoked at the destination, or enough extra fuel must be accelerated in order to accomplish a comparable deceleration. Because the rocket equation equates probe velocity with mass utilization, a staged spacecraft architecture is envisioned wherein a more massive booster accelerates the spacecraft and a smaller second stage decelerates into the destination solar system.

The discovery of Proxima b, that interesting planet evidently in the habitable zone around the nearest star, continues to energize the interstellar community. The Kickstarter campaign, just underway and with a goal of $200,000, hopes to upgrade earlier antimatter sail ideas into the interstellar realm. Tomorrow I want to say a few more things about the antimatter sail and the issues the Kickstarter campaign will address as it expands the original work.



Cosmology: Shelter from the Storm

by Paul Gilster on October 14, 2016

I had thought while the power was out this past week that I would like to write about cosmology when it came back. That’s because there’s nothing like a prolonged power outage to adjust your perspective. The big picture beckons. In my case, it was thinking about how trivial being out of power was compared to those who had lost so much more in the wake of the recent hurricane.

So thinking about the cosmos became my shelter from the storm. I appreciated the emails from so many of you, but aside from a major chunk of tree that landed on the roof, we did just fine. In fact, it was deeply moving to see people from the neighborhood — some I knew, some I only recognized — turn up to get up on the roof and move that tree. I’m always reminded to do more for the people around me when I see something like this, and apprehensive that my resolution to do so all too often gets put aside as normal life returns.

The Universe We Can See

Reading by candlelight really is wonderful, and I ask myself why I don’t do it more often. There is something about that soft, flickering light on a well-printed page. And I found reading about cosmology by candlelight was especially pleasing, a way of connecting to a past way of life that had its own conceptions about the cosmos. One thing I read was my notes from work just received when the power went, a paper on galactic structure I fortunately printed just in time.

The conclusion of this one is straightforward: There are at least 10 times as many galaxies in the observable universe as previously thought. Christopher Conselice (University of Nottingham, UK) led an international team that produced this result. I was reading about it in a room full of candles and thinking about room after room of candles stretching out into infinity.

That homely thought weds the prosaic and the vast, sort of the way Douglas Adams did in his famous line on the size of the universe in The Hitchhiker’s Guide to the Galaxy:

“Space is big. You just won’t believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space.”

I’m sure you’re familiar with the line, but it still raises a chuckle. And this team’s findings cause yet another shift in perspective. Thus Conselice:

“It boggles the mind that over 90% of the galaxies in the Universe have yet to be studied. Who knows what interesting properties we will find when we observe these galaxies with the next generation of telescopes.”


Image: The GOODS South Field (Great Observatories Origins Deep Survey). GOODS draws on observations from the Spitzer, Hubble and Chandra observatories as well as the European Space Agency’s Herschel and XMM-Newton, along with ground-based facilities, to survey the distant universe across the electromagnetic spectrum. Credit: GOODS/Conselice et al.

Before this, we relied on such measures as the Hubble Deep Field images from the 1990s, which helped us arrive at an estimate of between 100 and 200 billion galaxies in the observable universe. Conselice and team studied the matter using Hubble imagery as well as other published data to create a 3D view, estimating the number of galaxies at various times in the history of the cosmos. Their mathematical models pointed to the result, that we have not been able to see most of the galaxies that are out there in our cosmological horizon.

Moreover, the work offers an interesting perspective on galaxies throughout time, one showing how their numbers have changed as the universe evolved. The paper explains that the total number density of galaxies in the universe declines with time:

[The] total number density… declines by a factor of 10 within the first 2 Gyr of the universe’s history, and a further reduction at later times. This decline may further level off between redshifts of z = 1 and z = 2. The star formation rate during this time is also very high for all galaxies, which should in principle bring galaxies which were below our stellar mass limit into our sample at later times. This would naturally increase the number of galaxies over time, but we see the opposite. This is likely due to merging and/or accretion of galaxies when they fall into clusters which are later destroyed through tidal effects, as no other method can reduce the number of galaxies above a given mass threshold.

So what is known as the ‘top-down formation of structure’ in the universe is supported by this work. Most of the galaxies in the first few billion years of the universe were similar in mass to the satellite galaxies we see surrounding the Milky Way. Galactic mergers reduce the total number. And although Conselice and colleagues show us that there are so many galaxies that almost every point in the sky contains part of one, most of these galaxies are invisible not just to the human eye but to our best telescopes.

If that rings a bell, it’s because we’re homing in on Olbers’ paradox, the idea that the darkness of the night sky is in contradiction to the assumption of an infinite universe filled with stars (based on the work of the German astronomer Heinrich Wilhelm Matthias Olbers, 1758-1840). Why is the night sky as dark as it is? I won’t get into the details on dust and gas absorption of light that the paper offers, but will simply quote the finding from the Conselice paper:

It would… appear that the solution to the strict interpretation of Olbers’ paradox, as an optical light detection problem, is a combination of nearly all possible solutions – redshifting effects, the finite age and size of the universe, and through absorption.

The paper is Conselice et al., “The Evolution of Galaxy Number Density at z < 8 and its Implications,” to be published in the Astrophysical Journal. Preprint here.



Working in the Dark

by Paul Gilster on October 11, 2016

Hurricane Matthew’s effects continue to be felt in the form of flooding, power outages and downed trees. I’m now told not to expect power for 4-6 days. The situation obviously impacts my ability to post here. I’ll try to keep up with comment moderation when possible. Will get things back to normal whenever the lights come back on.


Spiral Density Waves: Clue to Planet Formation?

by Paul Gilster on October 7, 2016

Have a look at the spiral of pinwheeling dust that can be seen around the young star Elias 2-27. We’re looking at gravitational perturbations in a protoplanetary disk that, as this National Radio Astronomy Observatory news release says, mimic the vast arms we expect in a spiral galaxy. But here we’re looking at a process with implications for planet formation, one that draws on data from the Atacama Large Millimeter/submillimeter Array (ALMA). This is the first time a spiral density wave has been detected in a protoplanetary disk’s planet formation areas.


Image: ALMA peered into the Ophiuchus star-forming region to study the protoplanetary disk around the young star Elias 2-27. Astronomers discovered a striking spiral pattern in the disk. This feature is the product of density waves – gravitational perturbations in the disk. Credit: L. Pérez (MPIfR), B. Saxton (NRAO/AUI/NSF), ALMA (ESO/NAOJ/NRAO), NASA/JPL Caltech/WISE Team.

Some 450 light years from Earth in the Ophiuchus star-forming region, Elias 2-27 is about half the mass of the Sun, though its protoplanetary disk is massive. Although the young star (about a million years old, according to current estimates) is shrouded by the molecular cloud from which it grew, ALMA was able to peer into the mid-plane of the disk to identify the spiral density waves. The spiral arms extend as much as 10 billion kilometers away from the host star.

All this catches the eye because while we can account for star formation from the collapse of gas and dust under the influence of gravity, we need a mechanism to keep enough material from falling into the protostar to ensure that it doesn’t spin up enough to shred itself. The protostellar disk projects angular momentum outward, and is where we can expect planets to form. But standard core accretion models have problems explaining the formation of planets 20 to 30 AU out, where the disk may not be dense enough to allow the process to be efficient.

Gravitational instabilities in the outer disk, however, can produce the kind of dense spiral arms we see here, with new material being pushed out into regions far from the star and collapsing under its own gravity to begin planet formation. Andrea Isella (Rice University) explains:

“We don’t completely understand how planets form, but we suspect there are two ways: Either small particles stick together until they form something like the Earth or Mars, or accreting gas forms a planet like Saturn or Jupiter. But this process works only very close to the star, within a few astronomical units (roughly the distance from the Sun to the Earth), because that’s where all the material is, and it has to have enough density… If a disk is massive enough to be gravitationally unstable, a spiral will form naturally.”

Thus the spiral arms of Elias 2-27 may be the manifestation of an instability that gives birth to a particular kind of exoplanet. Near to the star, the ALMA observations found a flattened dust disk that extends out beyond 30 AU, followed by a narrow band of sharply diminished dust that may indicate a planet in formation. The spiral arms extend outward from the edge of this gap in the disk. Lead author Laura Pérez (Max Planck Institute for Radio Astronomy) notes that an upcoming program will use ALMA data to home in on similar protoplanetary disks as we try to find out whether Elias 2-27’s spiral density waves actually do reveal planet(s) in formation.

The paper is Pérez et al., “Spiral density waves in a young protoplanetary disk,” Science Vol. 353, Issue 6307 (30 September 2016), pp. 1519-1521 (abstract). A Rice University news release is also available.