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Voyager: Pressure at the Edge of the System

One of these days we’ll have a spacecraft on a dedicated mission into the interstellar medium, carrying an instrument package explicitly designed to study what lies beyond the heliosphere. For now, of course, we rely on the Voyagers, both of which move through this realm, with Voyager 1 having exited the heliosphere in August of 2012 and Voyager 2, on a much different trajectory, making the crossing in late 2018. Data from both spacecraft are filling in our knowledge of the heliosheath, where the solar wind is roiled by the interstellar medium.

A new study of this transitional region has just appeared, led by Jamie Rankin (Princeton University), using comparative data from the time when Voyager 2 was still in the heliosheath and Voyager 1 had already moved into interstellar space. Leaving the heliosheath, the pressure of the Sun’s solar wind is affected by particles from other stars, and the magnetic influence of our star effectively ends. What the scientists found is that the combined pressure of plasma, magnetic fields, ions, electrons and cosmic rays is greater than expected at the boundary.

“In adding up the pieces known from previous studies, we found our new value is still larger than what’s been measured so far,” said Rankin. “It says that there are some other parts to the pressure that aren’t being considered right now that could contribute.”

Image: This is an illustration depicting the layers of the heliosphere. Credit: NASA/IBEX/Adler Planetarium.

Thus the Voyager data continue to be robust, giving us a look into a dynamic and turbulent region through which future missions will have to pass. The particular area that the study’s authors focused on is called a global merged interaction region, a wave of outrushing plasma produced by bursts of particles from the Sun in events like coronal mass ejections. Such an event is visible in Voyager 2 data from 2012, causing a decrease in the number of galactic cosmic rays, one that Voyager 1 would go on to detect four months later.

Traveling at nearly the speed of light, galactic cosmic rays are atomic nuclei from which all of the surrounding electrons have been stripped away. The difference between how this change in their numbers was detected by the two spacecraft is instructive. Still within the heliosheath at the time, Voyager 2 saw a decrease of galactic cosmic rays in all directions around the spacecraft, whereas at Voyager 1’s vantage beyond the heliosphere, only those galactic cosmic rays traveling perpendicular to the magnetic fields in the region decreased.

This intriguing asymmetry flags the crossing of the heliosheath, though the study’s authors are quick to point out that why this directional change in cosmic rays occurs remains unknown. They are able to calculate the larger than expected total pressure in the heliosheath, and discover that the speed of sound in the heliosheath is roughly 300 kilometers per second (remember that the speed of sound in any medium is simply the speed at which disturbances in pressure propagate, in this case the result of interactions in the solar wind).

Image: The Voyager spacecraft, one in the heliosheath and the other just beyond in interstellar space, took measurements as a solar event known as a global merged interaction region passed by each spacecraft four months apart. These measurements allowed scientists to calculate the total pressure in the heliosheath, as well as the speed of sound in the region. Credit: NASA’s Goddard Space Flight Center/Mary Pat Hrybyk-Keith.

“There was really unique timing for this event because we saw it right after Voyager 1 crossed into the local interstellar space,” Rankin said. “And while this is the first event that Voyager saw, there are more in the data that we can continue to look at to see how things in the heliosheath and interstellar space are changing over time.”

The paper is Rankin et al., “Heliosheath Properties Measured from a Voyager 2 to Voyager 1 Transient,” Astrophysical Journal Vol. 883, No. 1 (25 September 2019). Abstract.

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Enceladus: New Organic Compounds via Cassini Data

While I’m working on the project I discussed the other day, I’m trying to keep my hand in with the occasional article here, looking forward to when I can get back to a more regular schedule. Things are going to remain sporadic for a bit longer this month, and then again in mid-November, but I’ll do my best to follow events and report in when I can. I did want to take the opportunity to use an all too brief break to get to the Enceladus news, which has been receiving attention from the space media and, to an extent, the more general outlets.

We always track Enceladus news with interest given those remarkable geysers associated with its south pole, and now we return to the Cassini data pool, which should be producing robust research papers for many years. In this case, Nozair Khawaja (University of Berlin) and colleagues have tapped data from the spacecraft’s Cosmic Dust Analyzer (CDA) to study the ice grains Enceladus emits into Saturn’s E ring, finding nitrogen- and oxygen-bearing compounds. These are similar to compounds found on Earth that can produce amino acids. Says Khawaja:

“If the conditions are right, these molecules coming from the deep ocean of Enceladus could be on the same reaction pathway as we see here on Earth. We don’t yet know if amino acids are needed for life beyond Earth, but finding the molecules that form amino acids is an important piece of the puzzle.”

Image: This illustration shows how newly discovered organic compounds — the ingredients of amino acids — were detected by NASA’s Cassini spacecraft in the ice grains emitted from Saturn’s moon Enceladus. Powerful hydrothermal vents eject material from Enceladus’ core into the moon’s massive subsurface ocean. After mixing with the water, the material is released into space as water vapor and ice grains. Condensed onto the ice grains are nitrogen- and oxygen-bearing organic compounds. Credit: NASA/JPL-Caltech.

So let’s clarify the process. What the Cosmic Dust Analyzer is looking at appears to be organics that would have been dissolved in the ocean beneath Enceladus’ surface. These would have evaporated from the ocean and then condensed, freezing on ice grains inside fractures in the crust. Rising plumes would have accounted for these materials being blown into space.

We begin to get a window into what might be produced within the ocean, though the view is preliminary. In the excerpt below, note that the scientists classify various types of ice grains on Enceladus according to a taxonomy: Type 1 represents grains of almost pure water ice, Type 2 shows features consistent with grains containing significant amounts of organic material, and Type 3 is indicative of salt-rich water ice grains. The study homes in on Type 2:

It is highly likely that there are many more dissolved organic compounds in the Enceladean ocean than reported here… In this investigation of Type 2 grains, the initial constraints, in particular the choice of salt-poor spectra, favoured the identification of compounds with high vapour pressures. Despite the expected solubility of potential synthesized intermediate- or high-mass compounds, their low vapour pressures mean that they will not efficiently evaporate at the water surface and thus remain undetectable not only in the vapour, but also those Type 2 grains forming from it. Potential soluble biosignatures with higher masses might therefore be found in spectra from Type 3 grains, which are thought to form from oceanic spray (Postberg et al. 2009a, 2011). Finding and identifying such biosignatures will be the main goal of future work.

Image: With Enceladus nearly in front of the Sun from Cassini’s viewpoint, its icy jets become clearly visible against the background. The view here is roughly perpendicular to the direction of the linear “tiger stripe” fractures, or sulci, from which the jets emanate. The jets here provide the extra glow at the bottom of the moon. The general brightness of the sky around the moon is the diffuse glow of Saturn’s E ring, which is an end product of the jets’ material being spread into a torus, or doughnut shape, around Saturn. North on Enceladus (505 kilometers, or 314 miles across) is up and rotated 20 degrees to the left. Credit: NASA/JPL/Space Science Institute.

The researchers believe that similarities between the hydrothermal environment found on Enceladus and what we see on Earth prioritizes the exploration of the Saturnian moon for life. After all, we know of many places on our planet where life develops without sunlight, with the vents supplying the energy that fuels reactions leading to the production of amino acids. Despite the remarkable strides made by Cassini, its Cosmic Dust Analyzer was not, the authors say, designed for deep probing of this question. That makes high-resolution mass spectrometers a key component of any dedicated mission designed to explore the organic chemistry beneath the ice.

The paper is Khawaja et al., “Low-mass nitrogen-, oxygen-bearing, and aromatic compounds in Enceladean ice grains,” Monthly Notices of the Royal Astronomical Society Vol. 489, Issue 4 (November 2019), pp. 5231–5243 (full text).

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Alan Boss: The Gas Giants We Have Yet to Find

The news of a gas giant of half Jupiter’s mass around a small red dwarf, GJ 3512 b, continues to resonate. It goes to what has become a well enshrined controversy among those who follow planet formation models. While core accretion is widely accepted as a way of building planets, gravitational instability has remained an option. We are not talking about replacing one model with another, but rather saying that there may be various roads to planet formation among the gas giants. In any case, GJ 3512 b makes a strong case that we have much to learn.

When I think about gravitational instability, I go back to the work of Alan Boss (Carnegie Institution for Science), as he has long investigated the concept. I learned about it from his papers and his subsequent book The Crowded Universe (Basic Books, 2009). Here’s how Boss describes it there:

Proponents of the top-down mechanism… envision clumps of gas and dust forming directly out of the planet-forming disk as a result of the self-gravity of the disk gas. The clumps would result from the intersections of random waves sloshing around the disk, waves that look much like the arms in spiral galaxies such as the Milky Way. When two spiral arms pass through each other, they momentarily merge to form a wave with their combined heights, just as waves do on the surface of an ocean. Such a rogue wave might rapidly lead to the formation of a clump massive enough to be self-gravitating, and so hold itself together against the forces trying to pull it apart. Once such a self-gravitating clump forms, the dust grains within the clumps settle down to the center of the protoplanet and form a core…

What emerges, then, is a solid core with gaseous envelope. In other words, this ‘disk instability’ model produces a planet that has the same structure as one derived from core accretion. What makes GJ 3512 b so interesting is that its position around a star as small as its host is hard to explain without a truly massive disk, and if that disk were to form, it would be unstable, and the gravitational instability model could come into play. The creator of this model, by the way, is Alastair Cameron (Harvard University), who spawned the notion in 1972. Other key players were Gerard Kuiper and Soviet scientist Victor Safronov, though it was Boss who revived the idea in 1997 and began developing computer models showing how it could occur.

Image: Simulation of the disk of gas and dust surrounding a young star. Credit: Alan Boss.

So what does Boss think of GJ 3512 b? As you might guess, he’s energized by the result:

My new models show that disk instability can form dense clumps at distances similar to those of the Solar System’s giant planets. The exoplanet census is still very much underway, and this work suggests that there are many more gas giants out there waiting to be counted.”

The work he is referring to is a new paper in press at The Astrophysical Journal that suggests there is a likelihood that gas giants in Jupiter-like orbits may be plentiful, with the inherent biases built into our observational techniques making them hard to find. As for how many of these may be formed from disk instability, Boss is computing various protoplanetary disk models to continue the investigation. As he notes in the new paper:

These models are intended to be first steps toward creating a hybrid model for exoplanet population synthesis, where a combination of core accretion and disk instability works in tandem to try to reproduce the exoplanet demographics emerging from numerous large surveys using ground-based Doppler spectroscopy and gravitational microlensing or space-based transit photometry (e.g., Kepler, TESS).

Image: The black box encapsulating Jupiter denotes the approximate region of exoplanet discovery space where Alan Boss’ new models of gas giant planet formation suggest significant numbers of exoplanets remain to be found by direct imaging surveys of nearby stars. NASA’s WFIRST mission, slated for launch in 2025, will test the technology for a coronagraph (CGI) that would be capable of detecting these putative exoplanets. Top Right: This simulation of the disk of gas and dust surrounding a young star shows dense clumps forming in the material. According to the proposed disk instability method of planet formation, they will contract and coalesce into a baby gas giant planet. Credit: Alan Boss.

We’re slowly eking out planets in wider orbits that are tricky for radial velocity, where their signals are more difficult to tease out than large planets close to their star, and also for transit work, because the transits would occur over an orbital period of five years or more. Boss continues to champion the concept that one size may not fit all when it comes to planet formation, with GJ 3512 b a striking case in point. Given a sufficiently massive protoplanetary disk, giant planets form in his models within 20 AU. Another targeted investigation for WFIRST will come out of all this when the mission launches some time in the next decade.

The paper is Boss, “The Effect of the Approach to Gas Disk Gravitational Instability on the Rapid Formation of Gas Giant Planets,” in press at The Astrophysical Journal (preprint).

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An Unusual Gas Giant in a Red Dwarf System

The gas giant GJ 3512 b does not particularly stand out at first glance. About 30 light years from the Sun, it orbits its host star in 204 days, discovered by radial velocity methods by the CARMENES collaboration, which is all about finding planets around small stars. But look more deeply and you discover what makes this find provocative. GJ 3512 b turns out to be a gas giant with about half the mass of Jupiter, and small red dwarfs like this one aren’t supposed to host such worlds.

In fact, GJ 3512 b is at least an order of magnitude more massive than what we would expect from current theoretical models, making it an interesting test case for planet formation. Core accretion models assume the gradual agglomeration of material in a circumstellar disk, with small bodies banging into each other and growing over time until their gravity is sufficient to draw in an atmosphere from the surrounding gas. This gas giant defies the model, evidently having formed directly from the disk through gravitational collapse.

Image: Comparison of GJ 3512 to the Solar System and other nearby red-dwarf planetary systems. Planets around solar-mass stars can grow until they start accreting gas and become giant planets such as Jupiter, in a few millions of years. However, up to now astronomers suspected that, except for some rare exceptions like GJ 876, small stars such as Proxima, TRAPPIST-1, Teegardern’s star, and GJ 3512 were not able to form Jupiter mass planets. Credit: Guillem Anglada-Escude – IEEC/Science Wave, using SpaceEngine.org (Creative Commons Attribution 4.0 International; CC BY 4.0).

As for the host star, GJ 3512 has about 12 percent the mass of the Sun. The disks of gas and dust that surround such low mass stars are assumed to contain insufficient material to form planets like this. Consider: The Sun is 1050 times heavier than Jupiter, while the mass ratio between GJ 3512 and GJ 3512 b is 270. A much more massive debris disk would be needed to build this planet the conventional way, but if a disk of more than a tenth of the stellar mass is present, the star’s gravity cannot keep the disk stable. Gravitational collapse can then occur, as in star formation, but no disks this massive have been found around young red dwarf stars. Is this new exoplanet evidence that such disks can indeed form and be productive?

Things get even trickier when we consider other planets in the same system. At least one other planet is thought to exist, and the elliptical orbit of GJ 3512 b offers evidence for the gravitational effect of a possible third planet just as massive, one that may have been ejected. So now we have a small red star that would have needed to produce multiple massive planets, taking us well beyond current models. In a paper on this work from researchers at the Max Planck Institute for Astronomy, the University of Lund in Sweden and the University of Bern, the authors argue for gravitational disk collapse as the only viable method of formation.

“Until now, the only planets whose formation was compatible with disk instabilities were a handful of young, hot and very massive planets far away from their host stars,” says Hubert Klahr, who heads a working group on the theory of planet formation at the MPIA. “With GJ 3512 b, we now have an extraordinary candidate for a planet that could have emerged from the instability of a disk around a star with very little mass. This find prompts us to review our models.”

Image: Visualisation of the radial velocity (RV) measurement time-series and residuals obtained with CARMENES. Panel a illustrates how the RV of GJ 3512 (vertical axis) changes with time indicated in days since 8 December 2014, 12:00 p.m. UT (Universal Time, horizontal axis). HJD stands for Heliocentric Julian Day. Both the visual (blue symbols) and the infrared (red symbols) channels agree well. The black solid curve is the best orbital fit to the data. After subtracting the contribution of GJ 3512 b, panel b shows the residual, which indicates the presence of a long-term period hinting to a second planet. Panels c and d depict the residuals of the best overall orbital fit for the two CARMENES channels. Credit: Morales et al. (2019)/MPIA.

Considering pebble accretion vs. gravitational instability of the disk around this star, the scientists must look at a time early in formation when the disk was still massive relative to the star. The authors were unable to model an accretion process that would explain this system. But the competing model of gravitational instability results in a disk that is gravitationally unstable in a range of viscosities and surface densities at distances below 100 AU. From the paper:

The estimated masses of the fragments formed are less than that of Jupiter, consistent with the mass of GJ 3512 b. Except for unrealistically low values of a [disk viscosity], fragmentation of the disk occurs at radii of ≳10 au, so the planets must have migrated a substantial distance from their formation locations to their present positions. This is possible given the large mass of the disk with respect to the planet, and is often seen in numerical simulations of disk fragmentation. For realistic viscosity a > 0.01, disk fragmentation typically occurs at radii of a few tens of au, and the total disk mass within this radius is ~30 MJ. Disks cannot extend too far beyond this fragmentation radius, because the total disk mass would become extremely large (up to 1 M within 100 au). Thus, the planetary system around GJ 3512 favors the gravitational instability scenario as the formation channel for giant planets around very-low-mass stars.

If you’re looking for a comparison, consider TRAPPIST-1, a star with many of the characteristics of GJ 3512. Here we have seven planets with masses equal to or less than the mass of the Earth, and the ‘bottom up’ accretion model fits with observation. But GJ 3512 b all but forces us to look at models where the planet forms directly from gravitational collapse in the disk. We’d still like to know why GJ 3512 b hasn’t migrated closer to its star, an indication that the mysteries of this system may prove fodder for a great deal of future analysis.

The paper is Morales, et al. “A giant exoplanet orbiting a very low-mass star challenges planet formation models”, Science 27 September 2019 (abstract).

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Looking Back, and Ahead

Centauri Dreams was launched as a website in 2004 for a specific reason. I was wrapping up my book of the same name and wanted to build a simple database of news stories related to the angles on interstellar flight I had covered in the book. I intended the site to be used for no other purpose, and didn’t turn on the comments function until a year after the site went live. My plans were for a second edition of the book, but I began to realize as the website grew that to avoid instant obsolescence, the Web was my best friend. This site, then, began serving as a de facto second edition and I’ve kept it running now for 15 years.

Sometimes I’m asked how long I plan to keep the site going, and the answer is simply that I plan to be here for years to come. I have no thoughts about closing down Centauri Dreams. But as my work in the space community has grown, I’ve also become involved in various other aerospace efforts to which I’ve contributed, and right now I’m in the midst of a report on a particular kind of interstellar mission that demands a lot of transcription of talks, extensive note-taking along the way, and drawing together a lot of different viewpoints and research.

In practical terms, while Centauri Dreams isn’t going away, this also means that my posts are occasionally going to become sporadic, as they will be in the next few weeks. I’ll post when I can, but I have to devote the bulk of my attention to this particular project, which leaves little time for anything else in my workday. So bear with me, please. The comments are still live and I’ll do moderation on them at least twice a day. Feel free to comment when you have the urge. Once I get the current effort wrapped up, things should return to something like normalcy.

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2I/Borisov: Naming the Interstellar Visitor

Congratulations to Gennady Borisov, the Crimean amateur who discovered the object now officially designated as 2I/Borisov (with a 0.65-metre telescope he built himself!). That ‘I’ in the designation points to the object’s interstellar origins, and picks up the nomenclature used with the first interstellar object in our system, 1I/‘Oumuamua. We’ve examined thousands of comets over the years but have found none with an orbit as hyperbolic as 2I/Borisov. That means that while the comet’s trajectory is being affected by the Sun, it’s not going to be captured by it.

What’s ahead: 2I/Borisov reaches perihelion on 7 December 2019, at which point it will be 2 astronomical units from the Sun and also 2 AU from Earth. It reaches its brightest levels in the southern sky in December and January and then heads back out toward the interstellar deep. So far, it appears that 2I/Borisov is a few kilometers in diameter, and we’ve also learned — via the Gran Telescopio Canarias (Canary Islands) — that its spectrum resembles typical cometary nuclei. The new interstellar visitor appears to be more straightforward than 1I/‘Oumuamua.

But considering that 1I/‘Oumuamua appeared a scant two years before 2I/Borisov, the inference is clear that such objects may be fairly numerous. Keep in mind that new instrumentation about to come online will expand our catalog, allowing us to investigate exoplanetary systems in ways beyond radial velocity and transits, through the spectra of objects within our own system.

Image: A comet from beyond our Solar System, as imaged by the Gemini Observatory. The image of the newly discovered object, named 2I/Borisov, was obtained on the night of 9–10 September 2019 using the Gemini Multi-Object Spectrograph on the Gemini North Telescope on Hawaii’s Mauna Kea. Credit: Gemini Observatory/NSF/AURA.

There is little doubt about 2I/Borisov’s cometary nature, given that observations have shown a condensed coma and a short tail. The IAU’s decision to give it a designation as an interstellar object follows the computation of its orbit by the IAU Minor Planet Center, with confirmation of the hyperbolic orbit from JPL’s Solar System Dynamics Group. Now we wait to learn how often to expect such objects and how much information we will be able to tease out of them.

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Marc Millis: Testing Possible Spacedrives

Marc Millis, former head of NASA’s Breakthrough Propulsion Physics project, recently returned from another trip to Germany, where he worked with Martin Tajmar’s SpaceDrive project at Germany’s Technische Universität Dresden. Recent coverage of the ongoing experimental work into spacedrives in both the popular and scientific press has raised public interest, leading Millis to explain in today’s essay why and how the techniques for studying these matters are improving, and how far we have to go before we have something definitive. Millis is in the midst of developing an interstellar propulsion study from a NASA grant even as he continues to examine advanced propulsion concepts and the methodologies with which to approach them.

by Marc Millis

Two recent articles, one in Scientific American [1] and the other in Acta Astronautica [2], prompted this update about the experimental tests of possible spacedrives. In short, the experimental methods are improving, but definitive results are not yet in hand. While this update is mostly on the “Mach Effect Thruster,” it also touches on the infamous “EmDrive,” as well as a refresher on the general quest for spacedrive physics.

First, what is a spacedrive? Presently, a spacedrive is still a goal rather than a proven device. The ambition is to find a fundamentally different way to propel spacecraft rather than rockets or sails. Rockets are limited by having to carry their entire journey’s reaction mass with them (propellant). Sails are limited by one-directional photons (or particles) from an external source. Imagine, instead, if there was some way for a spacecraft to interact with its surrounding spacetime to move in any direction and be limited only by the amount of available energy. That ambition is the essence of a spacedrive.

That detail – of interacting with spacetime to induce motion – is a matter of undiscovered physics. That makes it harder to grasp, harder to explain, and harder to solve. It’s easier to grasp engineering challenges that are based on known physics, since there are already operating principles to cite. With spacedrives, the operating principles are works-in-progress – more akin to lines of inquiry than having complete packages ready for scrutiny. Though theories for faster-than-light warp drives do exist (one type of spacedrive), the physics of the required negative energy is still debated – which itself is a prerequisite to devising how to engineer a warp drive. In addition, though there are experimental replications of thrusts from possible spacedrives, separating experimental artifacts from actual thrusts is also, still, a work in progress – and the main point of this update.

Before getting to the latest experiments, here is a bit more background behind the challenges of a spacedrive. At first blush, such wishful thinking might seem to violate conservation of momentum – a crucial detail. Conservation of momentum is easy to grasp for a rocket; the rearward-blasted propellant matches the forward momentum of the spacecraft. The situation is less obvious with spacedrives. There are a least 3 approaches to address conservation of momentum: 1) using a reaction mass indigenous to space or spacetime, 2) negative inertia, or 3) exploring the physics about inertial reference frames – the backdrop upon which the conservation laws are defined.

The majority of this update is related to the 3rd option – inertial frames. For new readers, a more complete introduction to various approaches and issues of both spacedrives and faster-than-light flight are spelled out in the book Frontiers of Propulsion Science [3]. If you’re curious about that broader coverage, that book and subsequent papers are one starting point.

Back to inertial frames and conservation laws: An inertial frame is such a ubiquitous property of spacetime that it is often taken for granted. It is what allows accelerated motion to be felt – the reference frame for Newton’s F=ma and the subsequent conservation laws. If you’ve never thought about it before, this can be hard to grasp because it’s so foundational. One useful book is Mach’s Principle: From Newton’s Bucket to Quantum Gravity [4], which articulates several different attempts to represent how inertial frames exist. What makes this book particularly useful is that it compiled workshop discussions about the differing approaches. Those discussions are illuminating.

One of those attempts is called “Mach’s Principle,” which asserts that the surrounding matter of the universe gives rise to the inertial frame properties of space. Or stated differently, “inertial here, because of matter, out there.” A similar perspective is something called “inertial induction.” The implication of these is that inertia is more than just a property of mass. Inertia is an interaction between mass and spacetime – and perhaps with undiscovered nuances.

Perhaps an analogy might help. When you plot trajectories on graph paper, you usually don’t give much thought to the paper. The paper is just some fixed, reliable background upon which the more interesting details are plotted. But what if the paper was not uniform nor constant over time? What if the trajectories might vary because of the properties of the paper itself? In this case, the rules for plotting on graph paper would have to be updated to account for the rules about the paper itself. Here, the graph paper is analogous to an inertial frame and “plotting trajectories” is analogous to Newton’s F=ma and subsequent conservation laws. If there are deeper details about inertial frames and their effect on inertia, then Newton’s F=ma and the conservation laws would have to be refined to incorporate those finer details.

In terms of Einstein’s general relativity – an established refinement of Newton’s laws – inertial frames and momentum conservation are treated only locally. I’m not sure quite how to put this in words, so I’ll defer to examples. With the warp drive, Einstein’s equations describe the local effects on spacetime from the warp drive itself, but cannot describe how (or if) momentum is conserved across a whole journey, encompassing the departure and arrival points as a total picture. Similarly, the momentum conservation of traveling through a wormhole cannot be described. While the local effects at each throat can be described, the bigger picture encompassing both the entry and exit throats and the mass that went through, cannot. There is room for more advances in physics.

Mach’s Principle and Inertial Induction are still open investigations in general physics, though not a dominant theme. Their relevance to spacedrives is because Mach’s Principle was a starting point for what is now called the “Mach Effect Thruster.” It began around 1990, when a reexamination of Mach’s Principle led to new hypotheses about fluctuating inertia, which then led to a 1994 patent for a propulsion concept [5]. Experiments followed. By 2016, three other labs were observing similar thrusts, which led NASA to award a 2017 NIAC grant for further investigations.

The original theory, from James Woodward of the University of California at Fullerton, showed that the inertia of a mass would fluctuate with a change of power of that mass. At first, varying the power of the mass took the form of charging and discharging a capacitor – where the capacitor was that mass. By doing this with two capacitors, while also changing the distance between them (via a piezoelectric actuator), a propulsive force was claimed to be generated (see figure and caption).

Figure 1. Transient inertia applied for propulsion: While the rear capacitor’s inertia is higher and the forward capacitor lower, the piezoelectric separator is extended. The front capacitor moves forward more than the rear one moves rearward. Then, while the rear capacitor’s inertia is lower and the forward capacitor higher, the piezoelectric separator is contracted. The front capacitor moves backward less than the rear one moves forward. Repeating this cycle shifts the center of mass of the system forward.

Since the center of mass of such a system moves without the opposite motion of a reaction mass, it appears to violate conservation of momentum, but does it? Since inertia is no longer constant, the usual equations do not fit without some reconsideration. This is a debated issue – debated in a constructive way. One version asserts how momentum conservation is indeed satisfied [6]. Others would prefer that the original fluctuating inertia equation be further advanced to explicitly address the conservation laws. Another desired refinement is to have the original equations explicitly connected to the experimental hardware – to show what dimensions of that hardware are the most critical.

Armed with an apparently working device, Woodward and his team concentrated on improving the experiments rather than that additional theoretical work. Over the years of making modifications to the device to amplify the effect, the ‘fluctuating inertia’ capacitors and the piezoelectric actuator were merged. Now a stack of piezoelectric disks serves both the functions of the inertial fluctuations and the oscillating motion. The power that affects the inertia now includes the mechanical motion too.

This is where the Scientific American article is worth mentioning. That article gives a decent review of the history and status of the Mach Effect Thruster (which also goes by the name “Mach Effect Gravity Assist (MEGA) Device”) as conducted by Jim Woodward and Heidi Fearn. It includes some perspectives that are useful to read separately, instead of needing to repeat those here. It addresses other aspects of the bigger picture of pursuing these kinds of research inquires.

The other article that prompted this update is in the journal Acta Astronautica. In addition to Woodward’s team, a group at the Technical University of Dresden, Germany, led by Martin Tajmar, secured funding for a broader project to research spacedrives in 2017. That group is one of the 3 labs that replicated the Woodward results in 2016. The recent Acta Astronautica article is an update on their experimental hardware and procedures, in preparation for careful testing of the Mach Effect Thruster, the EmDrive, and other possible spacedrive effects.

A preceding work by Tajmar that fed into this latest update was an attempt to advance Woodward’s original fluctuating inertia equations into a form that mapped to the experimental hardware [7]. With such equations a new thruster could be designed to maximize the thrust and experimental predictions could be made for the existing hardware. To span the possibility of debated assumptions (such as what kind of power affects the inertia; mechanical, electrical, other?), more than one version of such equations was derived for future tests.

Though this paper is more about the testing methods, in the course of that preparatory work, it became evident that none of the analytical models match the data. The models predicted correlations between the thrust and operating frequencies that was not observed. If the Mach Effect Thruster is indeed working, it is not producing thrust per these models derived from the original theory. Hence, that thruster is now considered a “black box” – a term used to denote a device whose operating principles are unknown, and where the test program concentrates on seeing if, and under what circumstances, it functions.

To test the thrusters, they are placed on the end of a torsion beam that can twist horizontally (vertical axis). The term “torsion” means that the beam is sprung, its rotation is limited and proportional to how much thrust occurs at the tip. This is the same concept as the Cavendish balance that measured Newton’s gravitational constant. When the thruster is pointed one way, the beam deflects one direction. When pointed in the other direction, the beam deflects in the other direction. And the third important orientation is when the thruster is pointed in a direction where it should not deflect the beam. By comparing the actual deflections in each direction (and under different operating conditions), the performance of the thruster can be assessed.

Deciphering actual thrust from all the other things that can look like thrust is difficult. A major clue for a false positive is if the beam is deflected when the thruster is not pointed in a thrusting direction. Another major clue is revealed when the power is delivered to a dummy device instead of to the thruster – to see if simply delivering the power through the apparatus affects the apparatus. Another possible effect is from the peculiarities of the balance beam itself while powered up (e.g. thermal drift of the electronics). When testing the thruster in a thrusting direction, there might be slight shifts in the center of mass as the thruster warms up – where that thermal effect might look like thrust. And then there is the challenge of how vibration might shift the position of the balance beam. There are more possible side-effects than these, but these are the major ones.

Another false positive that merits separate mention is confirmation bias. Confirmation bias is not an instrumentation phenomenon, but a psychological phenomenon. After people reach a conclusion, they tend to filter evidence to fit their preconceived notion, rather than letting the data speak for itself. It happens way more often than it should. It is so insidious that we seldom know when we are guilty of it ourselves. Our bias skews, well, our bias. The important lesson here, for you the audience, is how to spot those biases when you come across new articles. If an article sounds like they it’s trying to prove or disprove, rather than decipher and conclude, then its findings are likely skewed.

The Acta Astronautica article comes across like an investigation in progress, rather than a conclusion in search of evidence (or advocacy). The article outlines the performance limits of their hardware and the procedures used to distinguish the aforementioned side-effects from potential genuine thrust. To measure a claimed thrust of 2 µN, the thrust stand has demonstrated a sensitivity of 0.1 µN, as well as plots of the background noise showing less than ± 0.02 µN. The procedures include calibration with known forces before and after each run, measuring the thermal drift of the electronics, and automated operation that repeats a set of runs 140 times to get ample data to average. The tests are conducted in vacuum and the thrusting directions can be changed during a test sequence remotely without having to break vacuum or risk affecting other configuration settings.

Other than the aforementioned conclusion that the Mach Effect Thruster is not following analytical models, there are no other conclusions to report. Sample data is shown for the Mach Effect Thruster (more than one version) and the EmDrive, but only to illustrate the measurements that can be made, rather than any attempt to report on the viability of either of those thrusters.

In closing

Conferences are coming up where more progress will be reported. Consider this article a preparation for interpreting these next series of papers. Carl Sagan’s adage, “Extraordinary claims require extraordinary evidence,” is exactly the tactic here. The results only have as much substance as the fidelity of the tests. This most recent progress bodes well for that fidelity. The prior tactic of “quick and cheap” experiments to test other claimed devices turned out to be neither quick nor cheap. Promotional material and sensationalistic articles are easy to create. Reliable findings are harder, less glamorous, and take longer.

The implications of a genuine new propulsion method, plus the independent replications, are driving the perseverance to wade through these complications. If it turns out that a new propulsion method is discovered, then not only will we have a more effective way to propel spacecraft, but also a new window into the lingering mysteries of physics. The less obvious value is if it turns out to be a false positive. In that case the years-long ambiguity will be resolved, and the lessons learned will make it easier to assess future claims of new thrusters.

References

1. Scoles, S. (2019). The Good Kind of Crazy. Scientific American, 321, 59-65.

2. Kößling, M., Monette, M., Weikert, M., & Tajmar, M. (2019). The SpaceDrive project-Thrust balance development and new measurements of the Mach-Effect and EMDrive Thrusters. Acta Astronautica, 161, 139-152.

3. Millis, M. G., & Davis, E. W. (Eds.). (2009). Frontiers of propulsion science. American Institute of Aeronautics and Astronautics.

4. Barbour, J. B., & Pfister, H. (Eds.). (1995). Mach’s principle: from Newton’s bucket to quantum gravity (Vol. 6). Springer Science & Business Media.

5. Woodward, J. F. (1994). U.S. Patent No. 5,280,864. Washington, DC: U.S. Patent and Trademark Office.

6. Wanser, K. H. (2013). Center of mass acceleration of an isolated system of two particles with time variable masses interacting with each other via Newton’s third law internal forces: Mach effect thrust. J. of Space Exploration, 2(2).

7. Tajmar, M. (2017). Mach-Effect thruster model. Acta Astronautica, 141, 8-16.

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Enceladus Lights Up Saturn’s Inner Moons

In his wonderful account of the rocket that never was (Project Orion: The True Story of the Atomic Spaceship, 2002), George Dyson discusses his father’s thoughts on taking the craft to the moons of Saturn. Freeman Dyson and other Orion colleagues wanted to land on a moon to pick up propellant, but thought the moons of Jupiter were trickier than Saturn’s because of the depth of the Jovian gravity well. Anyway, Enceladus was a kind of beacon, and it was there Dyson fixed his attention. George Dyson quotes Freeman on the matter:

“We knew very little about the satellites in those days. Enceladus looked particularly good. It was known to have a density of .618, so it clearly had to be made of ice plus hydrocarbons, really light things; which were what you need both for biology and for propellant, so you could imagine growing your vegetables there. Five-one-thousandths g on Enceladus is a very gentle gravity, just enough so that you won’t jump off.”

As George noted, Enceladus was a long way from La Jolla, CA (where General Atomic had moved in 1958 to a 300-acre facility above the beaches near Torrey Pines), some 9 astronomical units, but the views would be spectacular. Imagine standing on Enceladus, where Saturn would appear vastly larger than Earth’s moon appears to us. Planet and rings would fill the sky, “changing phase from hour to hour, illuminated by the pale light of a distant sun.”

Image: Mosaic of the surface of Enceladus captured by Cassini on 9th October 2008 from an altitude of 25 kilometres. Credit: NASA/JPL/Space Science Institute.

We now know just how prescient Freeman Dyson was about the virtues of Enceladus, armed with Cassini data about geysers and an internal ocean. I always keep an eye on further insights from Cassini about this intriguing moon, and notice that at the EPSC-DPS joint meeting in Geneva this week, a team of researchers under Alice Le Gall (LATMOS-UVSQ, Paris) has analyzed some 60 radar observations of Saturn’s inner moons taken by Cassini. The result: Previous reports from these data underestimated their radar brightness by a factor of two.

All of Saturn’s inner moons show high radar reflectivity, with Enceladus itself being twice as bright as Europa, which is the brightest of Jupiter’s satellites. Significant variation turns up among the moons of Saturn, with Enceladus having the highest radar albedo, Titan the lowest.

Image: This is Figure 1 from the precis of Dr. Le Gall’s presentation. Caption: Averaged 2.2-cm disk-integrated radar albedos of Saturn’s major satellites. The error bars show the dispersion of the dataset for each satellite. Credit: Alice Le Gall.

What we’re witnessing among these inner moons is a fascinating interplay of material being pumped from the internal ocean of Enceladus into nearby space, affecting Tethys and Mimas. These three moons also interact with Saturn’s E-ring, which delivers water ice to their surfaces. The dazzling reflectivity that attracted the Orion team is largely the result of fresh water-ice particles from Enceladus, which acts as what Dr. Le Gall calls a ‘snow-cannon’ to coat all three surfaces. In other words, it snows on these worlds, in the sense that the particles fall back onto Enceladus and precipitate onto the surfaces of the other moons as well. Says Le Gall:

“The super-bright radar signals that we observe require a snow cover that is at least a few tens of centimetres thick. However, the composition alone cannot explain the extremely bright levels recorded. Radar waves can penetrate transparent ice down to [a] few meters and therefore have more opportunities to bounce off buried structures. The sub-surfaces of Saturn’s inner moons must contain highly efficient retro-reflectors that preferentially backscatter radar waves towards their source.”

Just what these ‘structures’ are is not known, though radar observations of Enceladus have shown subsurface features including pinnacles and ice blocks, along with fractured terrain. Le Gall’s team has developed models to test what specific shapes can act as effective reflectors, also testing whether enhanced reflection is a matter of random scattering events. This is a work in progress:

“So far, we don’t have a definitive answer,” adds Le Gall. “However, understanding these radar measurements better will give us a clearer picture of the evolution of these moons and their interaction with Saturn’s unique ring environment. This work could also be useful for future missions to land on the moons.”

The presentation is Le Gall et al., “Saturn’s inner moons: why are they so radar-bright?” in EPSC Abstracts Vol. 13, EPSC-DPS 2019-454-2, 2019 EPSC-DPS Joint Meeting 2019 (abstract).

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eVscope: Supporting Lucy Mission to Jupiter Trojans

Spreading scientific investigation beyond the research lab and astronomical observatory is what citizen science is all about. As we saw yesterday, projects like ExoClock are enlisting amateur volunteers to time exoplanet transits in support of the upcoming ARIEL mission. Also among such projects discussed at the ongoing EPSC-DPS joint meeting in Geneva is the eVscope digital telescope, a crowd-sourced effort from Unistellar that raised more than $3 million in its development under the direction of co-founder Franck Marchis (SETI Institute).

Here again we have a useful mission tie-in. The eVscope is designed to be a compact digital instrument that can be folded into what will become an extensive network of connected telescopes. The SETI Institute recently signed an agreement with Unistellar, which Marchis now serves as chief scientific officer, to make citizen astronomy a full-fledged effort that can contribute to the Lucy mission, which will launch in 2027 to the Jupiter Trojan asteroids.

One of the six asteroids targeted is Orus, but for each, learning the shape and precise size of the asteroid will increase the science return while helping mission engineers tune the exploration schedule. I mention Orus because we know so little about it — its shape is only vaguely known, and we have no knowledge of possible satellites. Thus it’s exciting news that the eVscope was used this month to observe an occultation of Orus for the first time.

Video: The Orus asteroid occulting a star, in the middle of the screen. Credit: F Marchis/Unistellar/SETI Institute.

The Unistellar team flew to Oman to make the observation, which involved the occultation of a magnitude 11 star. You’ll recall how this works from the extensive campaign to study Ultima Thule through occultations while the New Horizons spacecraft was on its long approach to the Kuiper Belt object. The occultation data proved strikingly accurate, as we would see when New Horizons flew past Ultima Thule, and the Orus occultation likewise generated a light curve that should help scientists refine its position and make a better assessment of its size.

“This is the first time an Orus occultation has been observed,” said Marchis. “We were in the right spot, and everything worked perfectly. One station located near Khalil, Oman detected and recorded this occultation while the other one only 30 km away did not see anything. In the future, it will be as simple as pushing a button on the Unistellar eVscope’s app for users to participate in these campaigns.”

Marchis presented the work at EPSC-DPS 2019 on September 16 in a session on citizen science, describing all the factors that go into making an occultation observation possible. In the case of Orus, the effort involved data from ESA’s Gaia space telescope, along with robotic telescopes on the ground as well as the PanSTARRS facility at Haleakala Observatory, Hawaii. Another occultation of Orus should be visible in Australia in November of this year, one of a series of observations within the next few years that will reveal Orus’ shape and rotation.

To support the Lucy mission, the plan is to utilize more than 2,500 connected eVscope instruments worldwide, telescopes that are shipping right now. Lucy is scheduled for launch in October of 2021 to study the Trojans, which orbit at the L4 and L5 Lagrange points 60° ahead and behind Jupiter. These are primitive objects that should contribute to our understanding of the history and development of the Solar System.

More than 6,000 Trojans have been identified in a mix of types (D-, C- and P-type) that Lucy will study as it moves into both clusters of Trojans. While C-type asteroids are primarily found in the outer regions of the Main Belt, the darker P- and D-type objects have similarities to Kuiper Belt objects beyond the orbit of Neptune. Evidently abundant in dark carbon compounds, all are thought to be rich in volatiles.

Image: This diagram illustrates Lucy’s orbital path. The spacecraft’s path (green) is shown in a frame of reference where Jupiter remains stationary, giving the trajectory its pretzel-like shape. After launch in October 2021, Lucy has two close Earth flybys before encountering its Trojan targets. In the L4 cloud Lucy will fly by (3548) Eurybates (white), (15094) Polymele (pink), (11351) Leucus (red), and (21900) Orus (red) from 2027-2028. After diving past Earth again Lucy will visit the L5 cloud and encounter the (617) Patroclus-Menoetius binary (pink) in 2033. As a bonus, in 2025 on the way to the L4, Lucy flies by a small Main Belt asteroid, (52246) Donaldjohanson (white), named for the discoverer of the Lucy fossil. After flying by the Patroclus-Menoetius binary in 2033, Lucy will continue cycling between the two Trojan clouds every six years. Credits: Southwest Research Institute.

“Calculations based previous to our observations from Oman give a diameter of about 54.8 kilometres for Orus, which is in line with estimations,” adds Marchis. “We don’t yet know much about Orus, such as its shape and whether it possesses one or several satellites. The observation has proved that our predictions of the orbit of Orus are accurate and we can now plan a campaign to make multiple occultation observations.”

Lucy should reach its first targets, the L4 trojans, in 2025, followed by a return to Earth and gravity assist there to move on to the L5 trojan cluster in 2033. Unlike the Dawn mission to Vesta and Ceres, Lucy will use chemical propulsion with gravitational assists, drawing on solar panels for power. The mission will give us a window to study not just the conditions of the Trojans’ formation but also their dynamical evolution into the orbits we see today.

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Introducing ExoClock: An Open Call for Participation

Ongoing in Geneva is the joint meeting of the European Planetary Science Congress and the Division for Planetary Sciences of the American Astronomical Society. We can abbreviate the whole thing as EPSC-DPS 2019, and you can read more about it here. We’ll track several stories here as they develop, but I notice that the European Space Agency’s ARIEL mission, which is slated to make the first large-scale survey of exoplanet atmospheres, has been supporting a Data Challenge involving removing noise from exoplanet observations. So let’s start there.

The slant here is training computers to filter out errors in collecting exoplanet data caused by starspots and by instrumentation, with two winners, James Dawson (Team SpaceMeerkat), and Vadim Borisov (Team major_tom), announced yesterday in Geneva. All told, 112 teams registered for the competition, a heartening number illustrative of the growing interest in computational statistics and machine learning among exoplanet researchers. The top five teams in this first international Machine Learning Data Challenge will present their methodologies to the European Conference on Machine Learning (ECML-PKDD 2019) on Friday.

Says Nikos Nikolaou (UCL Centre for Exochemistry Data), who created the competition:

“The outcomes of the competition exceeded our expectations, both in terms of the quality of the technical solutions submitted and in the massive numbers of entries for the challenge, which rivalled participation in open machine learning competitions with large monetary prizes.”

Image: Space mission data analysis is not easy, especially if you need to observe a planet passing in front of its star that is often 100s of lightyears away. At such distances, one of the main issues is differentiating what is planet and what is star. The Machine Learning ARIEL Data Challenge tackled the problem of identifying and correcting for the effects of spots on the star from the faint signals of the exoplanets’ atmospheres. This image shows a transiting planet passing in front of a star with stellar spots. Credit: ESO/L. Calçada.

ARIEL, which I mentioned last week, stands for Atmospheric Remote-sensing Infrared Exoplanet Large-survey. ESA is also launching what it calls the ‘ExoClock’ project to collect transit light curves. Here we turn to the community of amateur astronomers for assistance, for transits can be measured by small telescopes when properly equipped, and a growing number of amateurs have the needed hardware. The idea is to enlist a global community of observers to gather light curves that will be used by the ARIEL mission to improve the accuracy of transit timings, so the spacecraft will be armed with the best data on the exoplanets on its target list.

“This is the first open call to join the ExoClock project and we encourage all interested observers to become part of ESA’s ARIEL mission. Every transit observation is unique and important. By participating in ExoClock, citizens all over the world can contribute to the success of the ARIEL mission,” says Anastasia Kokori, who announced the launch of ExoClock at EPSC-DPS 2019.

Online resources are being set up to support the effort with a platform that includes an alert system to maximize the coverage of individual exoplanet targets and a system of target prioritization that allows individual users to have a personalized schedule based on their equipment and their geographical location. The light curves will be submitted and published along with analysis on the ExoClock website and may be included in scientific papers.

To learn more or to receive training in transit observations, go to the ExoWorlds Spies project (exoworldsspies.com). Experienced observers can register directly at exoclock.space. This citizen science opportunity is one that factors directly into the upcoming ARIEL mission, scheduled for a 2028 launch. Principal investigator Giovanna Tinetti comments on both the machine learning data challenge and the Exoclock effort:

“ARIEL is a challenging mission that’s pushing the boundaries of exoplanet research. The Data Challenges and ExoClock project are enabling us to build a global community of collaborators with a diverse mix of skills and backgrounds. We look forward to working with them over the next few years to develop networks, tools and analysis techniques in preparation for the mission’s launch in 2028.”

Image: Artist’s impression of ARIEL on its way to Lagrange Point 2 (L2). Here, the spacecraft is shielded from the Sun and has a clear view of the whole sky. Credit: ARIEL space mission/Science Office.

ARIEL will be the first mission dedicated to making a survey of the chemical composition and thermal structures of up to 1000 transiting exoplanets, bringing a new level of planetary science into the realm of other stars. The mission designers note that while we’ve discovered thousands of exoplanets, we have as yet no pattern that links their masses, sizes and orbital parameters to the nature of the parent star. How is a planet’s chemistry determined by its place of formation, and how does its evolution depend upon the properties of the star it orbits?

The mission will home in on warm and hot planets, ranging from super-Earths to hot Jupiters, to parse the chemistry of their atmospheres as they transit, making a deep survey of cloud systems as well as seasonal and even daily atmospheric variations for a subset of its targets. ARIEL will operate at visible and infrared wavelengths as it orbits the L2 Lagrange point 1.5 million kilometers behind the Earth as viewed from the Sun, with a mission duration of four years.

For more on ARIEL’s target list, see Edwards et al., “An Updated Study of Potential Targets for Ariel,” The Astronomical Journal Vol. 157, No. 6 (30 May 2019). Abstract / preprint.

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