Centauri Dreams
Imagining and Planning Interstellar Exploration
Exoplanet Prospects at Earth-based Observatories
Although I often write about upcoming space missions that will advance exoplanet research, we’re also seeing a good deal of progress in Earth-based installations. In the Atacama Desert of northern Chile, the Extremely Large Telescope is under construction, with first light planned for 2024. With 256 times the light gathering area of the Hubble instrument, the ELT is clearly going to be a factor in not just exoplanet work but our studies of numerous other astronomical phenomena, from the earliest galaxies in the cosmos to the question of dark energy.
Today we learn that the first six hexagonal segments for the ELT’s main mirror have been cast by the German company SCHOTT at their facility in Mainz, Germany. We’re just at the beginning of the process here, for the primary mirror is to be, at 39 meters, the largest ever made for an optical-infrared telescope. 798 individual segments — each 1.4 meters across and 5 centimeters thick — will go into it, working together as a single gigantic mirror.
Image: The first six hexagonal segments for the main mirror of ESO’s Extremely Large Telescope (ELT) have been successfully cast by the German company SCHOTT at their facility in Mainz. These segments will form parts of the ELT’s 39-metre main mirror, which will have 798 segments in total when completed. The ELT will be the largest optical telescope in the world when it sees first light in 2024. Credit: ESO.
SCHOTT will embark on making a total of 900 segments, 798 for the primary mirror plus a spare set, with production rates when up to speed of about one segment per day. After cooling and a heat treatment sequence, the mirror segment blanks will be ground and polished to a precision of 15 nanometers, with shaping and polishing performed by the French company Safran Reosc, which will mount and test the individual segments.
Binocular Vision
Meanwhile, we also get word of two new instruments that will be mounted on the Large Binocular Telescope (LBT), located on Mount Graham in Arizona. The SHARK instruments (System for coronagraphy with High order Adaptive optics from R to K band) are designed with an explicit exoplanet purpose, to conduct direct imaging of distant worlds.
What makes the SHARK effort intriguing is that it comprises two instruments. SHARK-VIS works in visible light, SHARK-NIR in near-infrared, and on the LBT platform, the two will be operated in parallel, using the two 8.4-meter mirrors of the observatory. Built by an international consortium led by INAF, the Italian National Institute for Astrophysics, the two SHARK instruments will likewise take advantage of the observatory’s adaptive optics system, also developed by INAF. Adaptive optics corrects for distortions caused by turbulence in the Earth’s atmosphere.
Image: Each SHARK will be installed on one side of the LBT Interferometer (LBTI), the green structure seen in the middle of the picture between the two main mirrors of LBT. Credit: SHARK Consortium/INAF.
Notice what’s happening here: The LBT, once equipped with the two SHARK add-ons, becomes the first telescope in the world that can observe exoplanets simultaneously over such a wide range of wavelengths, helping astronomers tease out planets that would otherwise be drowned in the glare of the host star. The installation, which is expected to be completed in 2019, also points the way toward the upcoming Giant Magellan Telescope, which will deploy seven 8.4-meter mirrors on the same mount instead of the LBT’s two. The GMT facility is under construction at the Las Campanas Observatory in Chile’s southern Atacama Desert.
“With SHARK, we will observe exoplanets at unprecedented angular resolution and contrast, so that we will be able to go closer to their host stars than what has been achieved up to now with direct imaging,” says Valentina D’Orazi of the INAF-Osservatorio Astronomico di Padova, instrument scientist for SHARK-NIR. “This will be possible thanks to the use of coronagraphy, which blocks out the light from the central star and highly improves the contrast in the region around the source, thus allowing us to detect the planetary objects we want to study, which otherwise would remain hidden in the star light.”
Clearly we’re moving into an era where Earth-based observatories will be capable of major advances in the exoplanet hunt, complementing the upcoming space missions that will expand the planetary census and begin the analysis of smaller exoplanet atmospheres, particularly those around red dwarf stars. Both the Extremely Large Telescope and the Giant Magellan Telescope could be completed, if current schedules are realistic, by 2025.
An Ancient Planetary System
I just noticed that the team behind PEPSI (Potsdam Echelle Polarimetric and Spectroscopic Instrument) at the Large Binocular Telescope has released three papers analyzing high spectral resolution data from the site. Because I’ve only had the chance to skim the papers, let me just quote the news release on one of these, examining the 10-billion year old system Kepler-444:
…the star “Kepler-444”, hosting five sub-terrestrial planets, was confirmed to be 10.5 billion years old, more than twice the age of our Sun and just a little bit younger than the universe as a whole. The star is also found being poor on metals. The chemical abundance pattern from the PEPSI spectrum indicates an unusually small iron-core mass fraction of 24% for its planets if star and planets were formed together. For comparison, terrestrial planets in the solar system have typically a 30% iron-core mass fraction. “This indicates that planets around metal-poor host stars are less dense than rocky planets of comparable size around more metal-rich host stars like the Sun”, explains Claude “Trey” Mack, project scientist for the Kepler-444 observation.
The paper is Mack et al., “PEPSI deep spectra. III. A chemical analysis of the ancient planet-host star Kepler-444,” in press at Astronomy &Astrophysics (preprint).
Tightening the Focus on Brown Dwarfs
Among the many indicators that we have much to learn about brown dwarfs is the fact that we don’t yet know how frequently they form. Recent work from Koraljka Muzic (University of Lisbon) and colleagues has pointed, however, to quite a robust galactic population (see How Many Brown Dwarfs in the Milky Way?). Working with observations at the Very Large Telescope, the study pegged the brown dwarf population at 25 billion, with a potential of as many as 100 billion.
Image: Stellar cluster NGC 1333 is home to a large number of brown dwarfs. Astronomers will use Webb’s powerful infrared instruments to learn more about these dim cousins to the cluster’s bright newborn stars. Credit: NASA/CXC/JPL.
Likewise in need of further data is our understanding of how brown dwarfs form, especially in the region where planet and star overlap. Recall that brown dwarfs are not main sequence stars, as they are not massive enough to ignite hydrogen fusion, even if deuterium and lithium fusion may occur. If we get down to brown dwarfs less than 13 times the mass of Jupiter, are we dealing with planets or stars? And do these objects form as planets (within a circumstellar disk) or as stars (via the collapse of interstellar gas)?
Aleks Scholz (University of St Andrews, UK) will be using the James Webb Space Telescope (assuming successful launch and deployment next year, fingers crossed) to study the cluster in the image above, NGC 1333 in Perseus. A number of brown dwarfs have been located within the cloud, which is itself considered to be a stellar birthing ground for young stars. Usefully, NGC 1333 also appears to contain brown dwarfs at the very low end of the mass distribution.
“In more than a decade of searching, our team has found it is very difficult to locate brown dwarfs that are less than five Jupiter-masses – the mass where star and planet formation overlap. That is a job for the Webb telescope,” Scholz says. “It has been a long wait for Webb, but we are very excited to get an opportunity to break new ground and potentially discover an entirely new type of planets, unbound, roaming the Galaxy like stars.”
Note the the Substellar Objects in Nearby Young Clusters (SONYC) project, which Scholz leads. The goal is to study the frequency and properties of brown dwarfs in star-forming regions (Koraljka Muzic is part of this collaboration, which also includes Toronto’s Ray Jayawardhana). Below is an image showing brown dwarfs in NGC 1333 as identified by the survey.
Image: Brown dwarfs in the young star cluster NGC 1333. This photograph combines optical and infrared images taken with the Subaru Telescope. Brown dwarfs newly identified by our SONYC Survey are circled in yellow, while previously known brown dwarfs are circled in white. The arrow points to the least massive brown dwarf known in NGC 1333; it is only about six times heftier than Jupiter. Credit: SONYC Team/Subaru Telescope.
Likewise planning to use JWST to address brown dwarf issues is Étienne Artigau (Université de Montréal), who will be looking at an interesting low-mass brown dwarf called SIMP0136. We’ve looked previously at the work Jonathan Gagné (Carnegie Institution for Science) has performed on this one, a brown dwarf whose variability in brightness has been attributed to weather patterns moving into view during its short (2.4 hour) rotation period. Gagné’s team, studying SIMP0136’s membership in a nearby moving group, has pegged its mass at 12.7 Jupiter masses. See Exploring the Planet/Brown Dwarf Boundary for more.
This is an interesting object on a number of counts, not the least of which is the fact that it is a bit less than 20 light years out, making it one of the 100 nearest systems to the Sun. Also noteworthy is the fact that it is a free-floating object not associated with any star, a low-mass brown dwarf on the planet/brown dwarf boundary that in many ways resembles a gas giant. The Webb instrument should be just the ticket for pushing our understanding further.
“Very accurate spectroscopic measurements are challenging to obtain from the ground in the infrared due to variable absorption in our own atmosphere, hence the need for space-based infrared observation. Also, Webb allows us to probe features, such as water absorption, that are inaccessible from the ground at this level of precision,” Artigau explains.
Let’s place SIMP0136 into context, then, as we look toward the next generation of space-based exoplanet work. We know that a potent tool for atmospheric analysis will be transmission spectroscopy, conducted as a planet moves in front of its star as seen from Earth. In SIMP0136, we have an object with gas giant characteristics unhampered by proximity to a star. Artigau goes on to point out in this NASA news item that we can use it to better understand cloud decks in brown dwarf and planet atmospheres as we contrast the two kinds of observation.
One day we’ll have a better idea of how many unbound planetary-mass objects are out there. That they are hard to discover goes without saying, and we’ve only located a few on the brown dwarf/planet boundary. But it’s clear that the attention now being devoted to them through efforts like the Substellar Objects in Nearby Young Clusters project as well as the BANYAN All-Sky Survey-Ultracool (BASS-Ultracool) will, with the aid of space-based instruments, tell us much more. The resource list on the BASS-Ultracool page offers abundant references.
2017 from an Interstellar Perspective
The recent burst of interest in interstellar flight has surely been enhanced by the exoplanet discoveries that have become almost daily news. Finding interesting planets, some of them with the potential for water on their surfaces, inevitably raises the question of how we might find a way to get there. We can only imagine this accelerating as missions like the Transiting Exoplanet Survey Satellite (TESS) and the James Webb Space Telescope begin to fill in not just our inventory of nearby planets but our understanding of their compositions.
Find a terrestrial class planet around another star — we may find that there is more than one around the Alpha Centauri stars — and the interstellar probe again becomes a topic of lively conversation. Breakthrough Starshot, the hugely ambitious attempt to develop a concept for tiny payloads being delivered through beamed laser propulsion to a nearby star, is by now a major part of the discussion. And as I said in my closing remarks at the recent Tennessee Valley Interstellar Workshop in Huntsville, there is a synergy among these developments.
Here’s a bit of what I said in Huntsville:
The emergence of Breakthrough Starshot clearly changes the game for everyone in the interstellar community. We have a congressional subcommittee report that ‘encourages NASA to study the feasibility and develop propulsion concepts that could enable an interstellar scientific probe with the capability of achieving a cruise velocity of 10 percent of the speed of light.’ I doubt seriously that that phrasing would have emerged without the powerful incentive of the funding provided by Breakthrough, nor would the Tau Zero Foundation’s recent grant.
Let’s take this apart and look at the pieces. We all know that Breakthrough Starshot lit up media coverage of the interstellar idea at the same time that we were finding an interesting planet not so much larger than Earth in what appeared to be a habitable zone orbit around Proxima Centauri — being at one of the Breakthrough Starshot sessions when the announcement was made was an energizing experience, and I remember staying up late one night in Palo Alto writing the article on the Proxima Centauri discovery that I would post when the embargo lifted.
Image credit: Manchu.
The subcommittee report I referred to was the work of representative John Culberson (R-TX), long known for his interest in the space program and a panelist at the TVIW 2017 gathering. Culberson submitted a report to the Committee on Appropriations to accompany a bill setting NASA’s budget for the 2017 fiscal year, which began on October 1 of that year.
The bill sets down a futuristic agenda:
Interstellar propulsion research.—Current NASA propulsion investments include advancements in chemical, solar electric, and nuclear thermal propulsion. However, even in their ultimate theoretically achievable implementations, none of these could approach cruise velocities of one-tenth the speed of light (0.1c), nor could any other fission-based approach (including nuclear electric or pulsed fission). The Committee encourages NASA to study and develop propulsion concepts that could enable an interstellar scientific probe with the capability of achieving a cruise velocity of 0.1c.
Part of this study would be focused on Alpha Centauri, as the report makes clear:
These efforts shall be centered on enabling such a mission to Alpha Centauri, which can be launched by the one-hundredth anniversary, 2069, of the Apollo 11 moon landing.
And there is this about propulsion prospects:
Propulsion concepts may include, but are not limited to fusion-based implementations (including antimatter-catalyzed fusion and the Bussard interstellar ramjet); matter-antimatter annihilation reactions; multiple forms of beamed energy approaches; and immense ‘sails’ that intercept solar photons or the solar wind. At the present time, none of these are beyond technology readiness level (TRL) 1 or 2. The NASA Innovative Advanced Concepts (NIAC) program is currently funding concept studies of directed energy propulsion for wafer-sized spacecraft that in principle could achieve velocities exceeding 0.1c and an electric sail that intercepts solar wind protons.
The report notes work at the NASA Innovative Advanced Concepts program, pointing to studies Phil Lubin (UC-Santa Barbara) has performed on the whole issue of beamed propulsion using lasers. This work is repeatedly cited by Breakthrough Starshot and Lubin is actively involved in Breakthrough’s work on laser technologies. Thus there is some overlap even here between NASA and a privately funded venture that is putting the beamed sail idea to the test and examining the infrastructure needed.
What Culberson’s report went on to do was to tell NASA to submit an “interstellar propulsion technology assessment report” with a draft roadmap that could include an overview of the propulsion concepts considered viable, one that would include the technical challenges, assessments of technology readiness levels, near-term goals and funding requirements.
If this sounds familiar, it is because of the tie-in with the grant recently awarded to the Tau Zero Foundation to compile just such a technology roadmap, work which is now in progress. But despite overstatements in many media outlets (along the lines of ‘NASA Planning Interstellar Mission’ and the like), funding breakthrough propulsion ideas is difficult at the best of times, as Tau Zero founder Marc Millis knows all too well. The former head of NASA’s Breakthrough Propulsion Physics project, Millis told me that acquiring the Tau Zero grant was an extended process that took a number of years to complete. From a recent email:
“A part of this story is the funding process. Those processes are not as singular or straight forward (or fast) as many envision. For example, the grant awarded to Tau Zero in January 2017 was proposed to NASA five years earlier, in February 2012. At that time NASA agreed that such work was needed, but was out of scope for its current funding categories. As those five years passed, the details of the work were iterated with NASA four times, each time getting closer to being funded. The last requested revision was December 2016, where Culberson’s interest added the last nudge. The other part of this story is that funding can vanish faster than it is awarded. In multi-year grants, like the one to Tau Zero, there is no guarantee that funding will exist for its second and third years. That is all part of the realities that we have to deal with.”
In other words, although I’ve seen the ‘NASA to the stars’ story pitched as a reprise of the Apollo program, it is actually a very small step in the direction of assessing what would be required to get an interstellar option in motion. This is certainly not a funded effort to build and launch specific hardware, or even a detailed mission study of the sort Breakthrough Starshot will be creating. But we do have recent reports that a small team based at the Jet Propulsion Laboratory is working on further ideas. JPL’s Anthony Freeman spoke of the possibilities at the 2017 American Geophysical Union conference. At the Huntsville TVIW meeting, JPL’s Stacy Weinstein-Weiss discussed the science prospects for an interstellar probe.
Obviously, we’ll follow such efforts with great interest. Meanwhile, my assumption on the background of all this is that Breakthrough Starshot’s sudden emergence prompted questions about NASA’s interest in interstellar matters on the part of Rep. Culberson, who off-loaded the idea to the committee report, which led to the awarding of the Tau Zero grant, perhaps intensifying the JPL investigations as well. A cynic might question whether the whole story hasn’t received far too much attention, given the excesses of many headline writers. But I have a different take.
In my view, keeping deep space in front of the public is helpful as long as we are pointing to legitimate research that moves the ball forward. The idea that NASA has a large interstellar program in place is incorrect, but that it takes even small steps in this direction by way of early conceptualizations and roadmaps is encouraging. Meanwhile, a vigorous private effort to put theoretical technologies to actual prototype and testing is all to the good, perhaps pointing toward future synergies between space agencies and non-traditional space organizations.
Everything gets blown out of proportion somewhere on the Internet, a challenge we all have to live with as we pursue ideas as futuristic as travel to other stars. But on balance, I’d say that 2017’s flurry of media attention was a good thing, and one that may remind us how much it would take to actually build serious interstellar hardware by 2069 or sooner. Technologies need development at every level, but there is nothing wrong with the Starshot model, beginning with conceptual studies and progressing to laboratory work that could point to eventual starflight.
3200 Phaethon: Arecibo Back at Work
With the holidays behind us (alas), I want to be sure to cover the Arecibo observations of asteroid 3200 Phaethon, not only for their intrinsic interest but as a nod to the restoration of operations at the Puerto Rico observatory. We are fortunate indeed that the structural damage Arecibo suffered on September 20 because of hurricane Maria was relatively minor. Radio astronomy work was back in progress within days of the storm, though it took until early December before commercial power was restored and radar work could resume.
If you’re interested in radar astronomy, have a look at Alessondra Springmann’s How Radar Really Works: The Steps Involved Before Getting an Image, which is available via The Planetary Society. Springmann offers a detailed overview of radar operations with a splash of humor:
Arecibo Observatory is known for its 305-meter (1000-foot) diameter telescope and its appearances in Goldeneye and Contact. Aside from battling Bond villains and driving red diesel Jeeps around the telescope (grousing at the site director about the funding status of projects is optional), several hundred hours a year of telescope time at Arecibo go toward radar studies of asteroids. A group of four planetary radar astronomers at Arecibo (as well as collaborators and colleagues at institutions outside of Puerto Rico) is tasked with “finding them before they find us” by NASA’s Near-Earth Object Observation program. We study the orbits and surface properties of our rowdy neighbors, near-Earth asteroids.
‘Finding them before they find us’ indeed. Arecibo contains the most powerful radar system on the planet, making it a key asset for planetary defense, a great part of which is keeping an eye on asteroids close enough to be problematic. This is a facility we need to keep in action as we continue to build the catalog of near-Earth asteroids and examine their properties. The recently released observations of 3200 Phaethon were conducted from December 15 to 19, 2017.
Image: These radar images of near-Earth asteroid 3200 Phaethon were generated by astronomers at the National Science Foundation’s Arecibo Observatory on Dec. 17, 2017. Observations of Phaethon were conducted at Arecibo from Dec.15 through 19, 2017. At time of closest approach on Dec. 16 at 3 p.m. PST (6 p.m. EST, 11 p.m. UTC) the asteroid was about 10.3 million kilometers away, or about 27 times the distance from Earth to the moon. The encounter is the closest the object will come to Earth until 2093. Credit: Arecibo Observatory/NASA/NSF.
About Phaethon itself, we know that it is about 6 kilometers in diameter, larger than previously thought, and is the second largest near-Earth asteroid in the category classified as ‘potentially hazardous.’ Size and close approach are determinants of which asteroid fits into that category, but work on any passing asteroid is useful because radar can study its size, shape, rotation, and surface features, while adding to our knowledge of its trajectory. Joan Schmelz, Arecibo’s deputy director, calls the facility “crucial for planetary defense work,” and that it is.
The images of 3200 Phaethon are the best we’ve yet taken, showing a spheroidal object with a large concave depression near its equator and a dark circular feature near one of its poles. The resolution in these images is about 75 meters per pixel.
“These new observations of Phaethon show it may be similar in shape to asteroid Bennu, the target of NASA’s OSIRIS-REx spacecraft, but more than 1,000 Bennus could fit inside of Phaethon,” said Patrick Taylor, a Universities Space Research Association (USRA), Columbia, Maryland, scientist and group leader for Planetary Radar at Arecibo Observatory. “The dark feature could be a crater or some other topographic depression that did not reflect the radar beam back to Earth.”
This asteroid is, by the way, the first to be discovered through images taken from a spacecraft, the craft in question being the Infrared Astronomical Satellite (IRAS), which produced data that Simon Green (Open University) and John K. Davies (then at Leicester University) examined to find the object. It is the parent body of the Geminids meteor shower and, given over thirty years of observations, is an asteroid with a well determined orbit, one well constrained for the next 400 years. The closest approach to Earth in 2017 was known with an accuracy of ±40 kilometers.
KIC 8462852: A Dusty Solution?
Research into Boyajian’s Star, otherwise known as KIC 8462852 or ‘Tabby’s Star,’ has continued in robust fashion even as many of us were distracted by that other curiosity with a faint SETI potential, the interstellar asteroid `Oumuamua. In both cases, a highly interesting object provoked speculation as to its origins, with Boyajian’s Star getting the lion’s share of attention because the unusual dips in its lightcurve proved hard to explain.
Now a team of more than 200 researchers led by Tabetha Boyajian herself is drawing useful conclusions about the star. Also on the team is Penn State’s Jason Wright, whose interest in possible SETI signatures led him to point out that engineering on a vast scale could not immediately be ruled out. The paper now being made available in The Astrophysical Journal Letters shows that the star dims more at some wavelengths than at others.
And that is, to say the least, problematic for the idea that an artificial megastructure orbits Boyajian’s Star. The paper, titled “The First Post-Kepler Brightness Dips of KIC 8462852,” draws on data collected by Boyajian (Louisiana State) and colleagues as a result of a Kickstarter campaign in which some 1700 contributors donated money to observations through the Las Cumbres Observatory, a network of robotic telescopes with northern hemisphere sites in the Canary Islands and Hawaii. Follow-up data were acquired from a number of other instruments.
The observations ran from March 2016 to December 2017, with four main 1-2.5% dips, beginning in May of 2017 and named “Elsie,” “Celeste,” “Skara Brae,” and “Angkor,” persisting on timescales from several days to weeks. What analysis of these dips shows is not consistent with any solid structure around the F3-class Star. As Wright explains on his PSU site:
Eva Bodman has done a lot of work to characterize how much deeper the dips are at blue wavelengths than red ones. If there were opaque objects blocking our view of the light, the star should get equally dim at all wavelengths. Instead, Eva finds that the blue (B) dips are much deeper—about twice as deep—as they are when we look at infrared wavelengths (i’ band, just beyond human vision).
This is consistent with ordinary astrophysical dust, and a major conclusion of our paper: the dips are not caused by opaque macroscopic objects (like megastructures or planets or stars) but by clouds of very small particles of dust (less than 1 micron in typical size). We can also say that these clouds are mostly transparent (“optically thin” in astrophysics parlance).
Image: Analysis of LCO data by Eva Bodman.
This work marks the first real-time detection of a dip in brightness for this unusual star, and as the paper notes, “Triggered spectroscopic and polarmetric observations taken during the dips reveal no large, obvious changes compared to out of dip observations.”
The paper goes on to say:
Invoking dust still challenges our creativity in developing a unified theory to explain all the observations; however, the models of Wyatt et al. (2017) give hope to a swarm of yet unspecified objects in an eccentric orbit (in this case, exocomets, with an alternative being dust-enshrouded planetesimals as proposed by Neslušan & Budaj 2017) causing the brightness fluctuations. Continued monitoring to detect events in the future will help narrow down any periodicity within the dip occurrence, which would strengthen the argument that the source of the obscuring material was in orbit around the star, as opposed to density fluctuations in the ISM, etc.
It would have been exciting, to say the least, to find evidence for an artificial cause of Boyajian’s Star’s peculiarities, but I find this work exhilarating in its own right. What we have here is a highly publicized, privately funded investigation into an enigmatic phenomenon that now seems to be closer to a solution. That extraterrestrial engineering is not involved doesn’t diminish the power of the process, in which scientists examined an observational anomaly from all angles and counted an ETI hypothesis among the possibilities.
Bear in mind that using through the Kepler data on Boyajian’s Star alone would not have been sufficient because ground-based follow-up observations were not contemporaneous. That made the ability to summon up a crowd-funded campaign to observe with various instruments at differing sensitivities, resolutions and wavelengths an essential component in this result.
A friend asked not long after the Boyajian’s Star story broke whether I would be disappointed if it turned out to have a ‘boring natural cause.’ But that’s just it. I don’t find natural causes boring, especially when they push us to the limit to explain them. We still have a mystery here, because the original comet hypothesis — or the idea that some kind of circumstellar material is responsible — gains new life at the same time that regular dimming of the star itself — through mechanisms not yet understood — cannot be ruled out. Getting a handle on unusual astrophysical phenomena has a deep allure as we continue to learn about the cosmos.
Where next, then, with Boyajian’s Star? The paper concludes:
We emphasize the importance that continued monitoring will bring to our understanding of the physical processes responsible for the light curve features. In general, precise, long-term photometric monitoring to detect future dips is a level-zero requirement. These data also provide the means of informing planned triggered observations such as high-resolution spectroscopy to study the events in more detail. Furthermore, extended photometric monitoring will enable us to characterize the star’s long-term variability (Schaefer 2016; Montet & Simon 2016; Meng et al. 2017; Simon et al. 2017), which is thought to be linked to the dips in some way. All-in-all, the apparent low duty cycle of the dips, unclear predictions on when they will recur, and fairly unconstrained multiyear timescales of the long-term variability will require a committed, intensive monitoring program spanning the next decade and beyond.
The paper is Boyajian et al., “The First Post-Kepler Brightness Dips of KIC 8462852,” Astrophysical Journal Letters 2018 January 3 (preprint).
The Plasma Magnet Drive: A Simple, Cheap Drive for the Solar System and Beyond
Can we use the outflow of particles from the Sun to drive spacecraft, helping us build the Solar System infrastructure we’ll one day use as the base for deeper journeys into the cosmos? Jeff Greason, chairman of the board of the Tau Zero Foundation, presented his take on the idea at the recent Tennessee Valley Interstellar Workshop. The concept captured the attention of Centauri Dreams regular Alex Tolley, who here analyzes the notion, explains its differences from the conventional magnetic sail, and explores the implications of its development. Alex is co-author (with Brian McConnell) of A Design for a Reusable Water-Based Spacecraft Known as the Spacecoach (Springer, 2016), focusing on a new technology for Solar System expansion. A lecturer in biology at the University of California, he now takes us into a different propulsion strategy, one that could be an enabler for human missions near and far.
by Alex Tolley
Suppose I told you that a device you could make yourself would be a more energy efficient space drive than an ion engine with a far better thrust to weight ratio? Fantasy? No!
Such a drive exists. Called the plasma magnet, it is a development of the magnetic sail but with orders of magnitude less mass and a performance that offers, with constant supplied power, constant acceleration regardless of its distance from the sun.
At the recent Tennessee Valley Interstellar Workshop (TVIW), Jeff Greason presented this technology in his talk [1]. What caught my attention was the simplicity of this technology for propulsion, with a performance that exceeded more complex low thrust systems like ion engines and solar sails.
What is a plasma magnet?
The plasma magnet is a type of magsail that creates a kilometers wide, artificial magnetosphere that deflects the charged solar wind to provide thrust.
Unlike a classic magsail [9] (figure 1) that generates the magnetic field with a large diameter electrical circuit, the plasma magnet replaces the circular superconducting coil by inducing the current flow with the charged particles of the solar wind. It is an upgraded development of Robert Winglee’s Mini-Magnetospheric Plasma Propulsion (M2P2) [7, 8], a drive that required injection of charged particles to generate the magnetosphere. The plasma magnet requires no such injection of particles and is therefore potentially propellantless.
Figure 1. A triple loop magsail is accelerated near Jupiter. Three separate boost beams transfer momentum to the rig, carefully avoiding the spacecraft itself, which is attached to the drive sail by a tether. Artwork: Steve Bowers, Orion’s Arm.
Developed by John Slough and others [5, 6], the plasma magnet drive has been validated by experimental results in a vacuum chamber and was a NIAC phase 1 project in the mid-2000s [6]. The drive works by initially creating a rotating magnetic field that in turns traps and entrains the charged solar wind to create a large diameter ring current, inducing a large scale magnetosphere. The drive coils of the reference design are small, about 10 centimeters in diameter. With 10 kW of electric power, the magnetosphere expands to about 30 kilometers in diameter at 1 AU, with enough magnetic force to deflect the solar wind pressure of about 1 nPa (1 nN/m2) which produces a thrust in the direction of the wind of about 1 newton (1N). Thrust is transmitted to the device by the magnetic fields, just as with the coupling of rotation in an electric motor (figure 2).
For a fixed system, the size of the induced magnetosphere depends on the local solar wind pressure. The magnetosphere expands in size as the solar wind density decreases further from the sun. This is similar to the effect of Janhunen’s electric sail [2] where the deflection area around the charged conducting wires increases as the solar wind density decreases. The plasma magnet’s thrust is the force of the solar wind pushing against the magnetosphere as it is deflected around it. It functions like a square-rigged sail running before the wind.
Figure 2. Plasma magnetic sail based on rotating magnetic field generated plasma currents. Two polyphase magnetic coils (stator) are used to drive steady ring currents in the local plasma (rotor) creating an expanding magnetized bubble. The expansion is halted by solar wind pressure in balance with the magnetic pressure from the driven currents (R >= 10 km). The antennas (radius ~ 0.1 m) are shown expanded for clarity. [6]
The engine is little more than 2 pairs of charged rotating coils and is therefore extremely simple and inexpensive. The mass of the reference engine is about 10 kg. Table 1 shows that the plasma magnet has an order higher thrust to weight ratio than an ion engine and 2 orders better than a solar sail. However, as the plasma magnet requires a power source, like the ion engine, the comparison to the solar sail should be made when the power supply is added, reducing is performance to a 10-fold improvement. [ A solar PV array of contemporary technology requires about 10 kg/kW, so the appropriate thrust/mass ratio of the plasma magnet is about 1 order of magnitude better than a solar sail at 1 AU]
The plasma magnet drive offers a “ridiculously high” thrust to weight ratio
The plasma magnet, as a space drive, has much better thrust to weight ratio than even the new X-3 Hall Effect ion engine currently in development. This ratio remains high when the power supply from solar array is added. Of more importance is that the plasma magnet is theoretically propellantless, providing thrust as long as the solar wind is flowing past the craft and power is supplied.
| Name | Type | Thrust/weight (N/kg) Engine mass only | Thrust/weight (N/kg) with power supply |
|---|---|---|---|
| SSME | Chemical | 717 | N/A |
| RD-180 | Chemical | 769 | N/A |
| plasma magnetosphere | Electro-magnetic | 0.1 | .01 |
| NSTAR-1 | Ion (Gridded) | 0.004 | 0.002 |
| X-3 | Ion (Hall Effect) | 0.02 | 0.004 |
| Solar Sail | Photon Sail | 0.001 (at 1 AU) | N/A |
Table 1. Comparison of thrust to mass ratios of various types of propulsion systems. The power supply is assumed to be solar array with a 10 kg/kW performance.
The downside with the plasma magnet is that it can only produce thrust in the direction of the solar wind, away from the sun, and therefore can only climb up the gravity well. Unlike other propulsion systems, there is little capability to sail against the sun. While solar sails can tack by directing thrust against the orbital direction, allowing a return trajectory, this is not possible with the basic plasma magnet, requiring other propulsion systems for return trips.
Plasma magnet applications
1. Propulsion Assist
The most obvious use of the plasma magnet that can only be used to spiral out from the sun is as a propellantless assist. The drive is lightweight and inexpensive, and because it is propellantless, it can make a useful drive for small space probes. Because the drive creates a kilometers sized magnetosphere, scaling up the thrust involves increased power or using multiple drives that would need to be kept 10s of kilometers apart. Figure 3 shows a hypothetical gridded array. Alternatively, the plasma magnets might be separated by thrusters and individually attached to the payload by tethers.
Figure 3. Plasma magnets attached to the nodes in a 2D grid could be used to scale up the thrust. The spacecraft would be attached by shroud lines as in a solar sail with a trailing payload. Scaling up the power supply to create a larger magnetosphere is also possible.
For a mixed mode mission, the plasma magnet engine is turned on for the outward bound flight, with or without the main propulsion system turned on. The use of power to generate thrust without propellant improves the performance of propellant propulsion systems where the accumulated velocity exceeds the performance cost of the power supply mass or reduced propellant. For an ion engine as the main drive, the plasma magnet would use the same power as 4 NSTAR ion engines but provide 3x the thrust.
2. Moving Asteroids for Planetary Defense
The propellantless nature of the plasma magnet drive makes it very suitable for pushing asteroids for planetary defense. Once turned on, the drive provides steady thrust to the asteroid, propelling it away from the sun and raising its orbit. Because the drive does not need to be facing any particular direction, it can be attached to a tumbling asteroid without any impact on the thrust direction.
3. Charged particle radiation shield for crewed flights
The magnetosphere generated by the engine makes a good radiation shield for the charged particles of the solar wind. It should prove to be a good solution for the solar wind, solar flares and even coronal mass ejections (CME). This device could, therefore, be used for human flight to reduce radiation effects. For human crewed flights, the 1N of thrust is insufficient for the size of the spacecraft and would have a marginal propulsion compared to the main engines. Given the plasma magnet’s small size and mass, and relatively low power requirements, the device provides a cost-effective means to protect the crew without resorting to large masses of physical shielding. The plasma magnet would appear to be only effective for the charged solar wind, leaving the neutral GCRs to enter the craft. However, when an auxiliary device is used in the mode of aerobraking, the charge exchange mechanism should reduce the galactic cosmic ray (GCR) penetration (see item 8 below).
4. Asteroid mining
The plasma magnet thruster might be a very useful part of a hybrid solution for automated mining craft. The hybrid propulsion would ally the plasma magnet thruster with a propellant system, such as a chemical or ion engine. The outward bound trip would use the plasma magnet thruster to reach the target asteroid. The propellant tanks would be empty saving mass and therefore improving performance. The propellant tanks would be filled with the appropriate resource, e.g. water for an electrothermal engine, or for a L2/O2 chemical engine. This engine would be turned on for the return trip towards the inner system. The reverse would be used for outward bound trips to the inner system
5. Interstellar precursor using nuclear power
A key feature of the plasma magnet is that the diameter of the magnetosphere increases as the density of the solar wind decreases as it expands away from the sun. The resulting expansion exactly matches the decrease in density, ensuring constant thrust. Therefore the plasma magnet has a constant acceleration irrespective of its position in the solar system.
As the solar wind operates out to the heliopause, about 80 AU from the sun, the acceleration from a nuclear powered craft is constant and the craft continues to accelerate without the tyranny of the rocket equation. Assuming a craft with an all up mass of 1 MT (700 kg nuclear power unit, 10 kg engine, and the remaining in payload), the terminal velocity at the heliopause is 150 km/s. The flight time is 4.75 years, which is a considerably faster flight time than the New Horizons and Voyager probes.
Slough assumed a solar array power supply, functional out to the orbit of Jupiter at 5 AU. This limited the velocity of the drive, although the electrical power output of a solar array at 1 AU is about 10-fold better than a nuclear power source, but rapidly decreases with distance from the sun. Assuming a 10 kW PV array, generating decreasing power out to Jupiter, the final velocity of the 1 MT craft is somewhere between 5 and 10 km/s, but with a much larger payload.
In his TVIW talk [1], Greason suggested that the 10kW power supply could propel a 2500 kg craft with an acceleration of 0.5g, reaching 400-700 km/s in just half a day. Greason [i] suggested that with this acceleration, the FOCAL mission for gravitational lens telescopes requiring many craft should be achievable. *
6. Mars Cycler
Greason suggested that the plasma magnet might well be useful for a Mars cycler, as the small delta v impulse needed for each trip could be easily met.[1]
7. Deceleration at target star for interstellar flight
For interstellar flights, deploying the plasma magnet as the craft approaches the target star should be enough to decelerate the craft to allow loitering in the system, rather than a fast flyby. Again, the high performance and modest mass and power requirements might make this a good way to decelerate a fast interstellar craft, like a laser propelled photon sail.[1]
8. Magnetoshell Aerocapture (MAC)
While the studies on the plasma magnet seemed to have stalled by the late 2000s, a very similar technology development was gaining attention. A simple dipole magnet magnetosphere can be used as a very effective aerocapture shield. The shield is just the plasma magnet with coils that do not rotate, creating a magnetosphere of a diameter in meters, one that requires the injection of gram quantities of plasma to be trapped in the magnetic field. As the magnetosphere impacts the atmosphere, the neutral atmosphere molecules are trapped by charge exchange. The stopping power is on the order of kilonewtons, allowing the craft to achieve orbit and even land without a heavy, physical shield. The saving in mass and hence propellant is enormous. Such aerobraking allows larger payloads, or alternatively faster transit times. Because the magnetoshell is immaterial, heat transmission to the shield is not an issue. The mass saving is considerable and offers a very cost-effective approach for any craft to reduce mass, propellant requirements or increase payloads. This approach is suitable for Earth return, Mars, outer planets, and Venus capture. Conceivably aerocapture might be possible with Pluto.
Figure 4. A dipole magnet creating a small diameter magnetic field is injected with plasma. As the magnetosphere impacts the atmosphere, charge exchange result in kilonewton braking forces. The diagram at left shows the craft with the training magnetosphere impacting the atmosphere. The painting on the right shows what such a craft might look like during an aerobraking maneuver. Source: Kirtley et al [3].
Making the plasma magnet thrust directional
A single magnetosphere cannot deflect the solar wind in any significant directional way, which limits this drive’s navigational capability. However, if the magnetosphere could be shaped so that its surface could result in an asymmetric deflection, it should be possible to use the drive for tacking back to the inner system.
Figure 5 shows an array of plasma magnets orientated at an angle to the solar wind. The deflection of the solar wind is no longer symmetric, with the main flow across the forward face of the array. Under those conditions, there should be a net force against the grid. This suggests that like a solar sail, orientating the grid so that the force retards the orbital velocity, the craft should be able to spiral down towards the Sun, offering the possibility of a drive that could navigate the solar system.
Figure 5. A grid of plasma magnets deflects the flow of the solar wind, creating a force with a component that pushes against the grid. If the grid is in orbit with a velocity from right to left, the force will reduce the grid’s velocity and result in a spiral towards the Sun.
Pushing the Boundaries
The size of the magnetic sail can be increased with higher power inputs, or increasing the antenna size. Optimization will depend on the size of the craft and the mass of the antenna. Truly powerful drives can be considered. Greason [12] has calculated that a 2 MT craft, using a superconducting antenna with a radius of 30 meters, fed with a peak current of 90 kA, would have a useful sail with a radius of 1130 km and an acceleration of 2 m/s2, or about 0.2g. As the sail has a maximum velocity of that of the solar wind, a probe accelerating at 0.2g would reach maximum velocity in a few days, and pass by Mars within a week. To reach a velocity of 20 km/s, faster than New Horizons, the Plasma magnet would only need to be turned on for a few hours. Clearly, the scope for using this drive to accelerate probes and even crewed ships is quite exciting.
Coupling a more modest velocity of just 10’s of km/s with the function of a MAC, a craft could reach Mars in less than 2 months and aerobrake to reach orbit and even descend to the surface. All this without propellant and a very modest solar array for a power supply.
An Asteroid, a tether and a Round Trip Flight
As we’ve seen, the plasma magnet can only propel a craft downwind from the Sun. So far I have postulated that aerobraking and conventional drives would be needed for return flights. One outlandish possibility for use in asteroid mining might be the use of a tether to redirect the craft. On the outward bound flight, the craft driven by the plasma magnet makes a rapid approach to the target asteroid which is being mined. The mined resources are attached to a tether that is anchored to the asteroid. As the craft approaches, it captures the end of the tether to acquire the new payload, and is swung around the asteroid. On the opposite side of the asteroid, the tether is released and the craft is now traveling back towards the Sun. No propellant needed, although the tether might cause some consternation as it wraps itself around the asteroid.
Conclusion
The plasma magnet as a propulsion device, and the same hardware applied for aerocapture, would drastically reduce the costs and propellant requirements for a variety of missions. Coupled with another drive such as an ion engine, a craft could reach a target body with an atmosphere and be injected into orbit with almost no propellant mass. The return journey would require an engine delivering just enough delta V to escape that body and return to Earth, where aerocapture again would allow injection into Earth orbit with no extra propellant. If direction deflection can be achieved, then the plasma magnet might be used to navigate the Solar System more like a solar sail, but with a far higher performance, and far easier deployment.
Using a steady, nuclear power or beamed power source, such a craft could accelerate to the heliopause, allowing interstellar precursor missions, such as Kuiper belt exploration and the FOCAL mission within a short time frame.
The technology of the plasma magnet combined with a MAC could be used to decelerate a slowish interstellar ship and allow it to achieve orbit and even land on a promising exoplanet.
The size of the magnetic sail can be extended with few constraints, allowing for considerably increased thrust that can be applied to robotic probes and crewed spacecraft. For crewed craft, the magnetosphere also provides protection from the particle radiation from the sun, and possibly galactic cosmic rays.
Given the potential of this drive and relatively trivial cost, it seems that testing such a device in space should perhaps be attempted. Can a NewSpace billionaire be enticed?
* These numbers are far higher than those provided by Winglee and Slough in their papers and so I have used their much more conservative values for all my calculations.
References
Greason, Jeff “Missions Enabled by plasma magnet Sails”, Presentation at the Tennessee Valley Interstellar Workshop, 2017. https://www.youtube.com/watch?v=0vVOtrAnIxM
Janhunen, P., The electric sail – a new propulsion method which may enable fast missions to the outer solar system, J. British Interpl. Soc., 61, 8, 322-325, 2008.
Kelly, Charles and Shimazu, Akihisa “Revolutionizing Orbit Insertion with Active Magnetoshell Aerocapture,” University of Washington, Seattle, WA, 98195, USA.
Kirtley, David, Slough, John, and Pancotti, Anthony “Magnetoshells Plasma Aerocapture for Manned Missions and Planetary Deep Space Orbiters”, NIAC Spring Symposium, Chicago, Il., March 12, 2013
Slough, John. “The plasma magnet for Sailing the Solar Wind.” AIP Conference Proceedings, 2005, doi:10.1063/1.1867244.
Slough, John “The plasma magnet” (2006). NASA Institute for Advanced Concepts Phase 1 Final Report.
Winglee, Robert. “Mini-Magnetospheric Plasma Propulsion (M2P2): High Speed Propulsion Sailing the Solar Wind.” AIP Conference Proceedings, 2000, doi:10.1063/1.1290892.
Winglee, R. M., et al. “Mini-Magnetospheric Plasma Propulsion: Tapping the Energy of the Solar Wind for Spacecraft Propulsion.” Journal of Geophysical Research: Space Physics, vol. 105, no. A9, Jan. 2000, pp. 21067-21077., doi:10.1029/1999ja000334.
Zubrin, Robert, and Dana Andrews. “Magnetic Sails and Interplanetary Travel.” 25th Joint Propulsion Conference, Dec. 1989, doi:10.2514/6.1989-2441.
Greason, Jeff. Personal communication.
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