by Paul Gilster | Apr 20, 2018 | Sail Concepts |
One of the reasons I described Greg Matloff as the ‘renaissance man of interstellar studies’ in my Centauri Dreams book is the continuing stream of ingenious ideas that he develops and delivers through papers and conference presentations. I found the holographic sail concept below fascinating, and would have referenced Bob Forward myself if Greg hadn’t already done it in the text. These two must have been great to hear in conversation! Read on to learn how Greg, a physicist at New York City College of Technology (CUNY) came up with the idea, a process that deftly blended science and art and may provide solutions to some of the more intractable problems posed by Breakthrough Starshot. The author of The Starflight Handbook among many other books (volumes whose pages have often been graced by the artwork of the gifted C Bangs), Greg has been inspiring this writer since 1989.
By Greg Matloff
It was perhaps inevitable that I would be asked to serve on the Advisor’s Board of Yuri Milner’s Breakthrough Starshot, because of my long experience in the analysis of interstellar travel techniques. According to Phil Lubin’s paper on this technology development project, a 50-70 GW laser array mounted atop a southern hemisphere mountain would generate a beam that would be projected against an Earth orbiting ~1 m photon sail for a period of minutes [1]. The sail would be a major component of a ~1 gram wafer-scale spacecraft with a ~0.1-gram payload that would exit the beam after experiencing average accelerations of ~5,000 g. The planned interstellar cruise velocity of the tiny spacecraft is ~0.2c and the voyage time to the Proxima/Alpha Centauri system is approximately two decades.
Image: Gregory Matloff (left) being inducted into the International Academy of Astronautics by JPL’s Ed Stone.
So I attended the first Starshot Advisors Meeting in August 2016 and left with a non-optimistic attitude. Yes, it is possible to design very-high efficiency optical reflectors at the laser wavelength (about 1 micron) to tolerate the enormous thermal load while maximizing acceleration [1]. But these devices tend to be physically thick and massive.
A major problem turned out to be beam-riding sail stability. The sail must be configured to remain in the beam, with its source located on the moving Earth and its terminus directed towards the Centauri system, for a period of minutes. Analysis discussed during the August 2016 meeting and later published revealed that a spherical sail curvature was the best approach to address the beam-riding stability issue [2]. But how would the sail maintain its required spherical curvature during the minutes-duration high-acceleration run?
Finally, rare ~1-micron interstellar dust grains impacting a sail moving through the interstellar medium at ~0.2c pack quite a wallop [3]. So if the spherical sail somehow survived acceleration, it would be a good idea to straighten it post-acceleration to a flat sheet and reorient the spacecraft edge-on to the direction of travel.
Initially, I could think of no way to satisfy all of these requirements. So I encouraged theoretical physicists associated with CUNY to think about ways of increasing graphene reflectivity in response to an expected Request for Proposals (RFP) from the Starshot management team. Because such a development is not impossible, I delivered several papers on the utility of reflective graphene in interstellar solar-photon sailing. In collaboration with other researchers, I also considered toned-down space applications of wafer-scale spacecraft and less intense collimated power beams. Even if the Starshot goal could not be met, I hoped that some major technological advances might come from the decade-duration, $100 million research effort.
But the goals of robotically exploring the planetary systems of our nearest stellar neighbors on voyages requiring a few decades seemed too enticing to simply abandon. So when my partner, artist C Bangs, suggested that I reconsider holographic photon sail coatings, a concept we had collaborated on in 2000-2001, I agreed.
Bob Forward and Holographic Photon Sails
Long before C and I married, we were collaborators. She has generated chapter frontispiece art for most of my books, including The Starflight Handbook. During the summer of 2000, my second year as a NASA Faculty Fellow at Marshall Spaceflight Center in Huntsville Alabama, we attended an International Academy of Astronautics symposium organized by Giancarlo Genta of Politechnico di Torino in Aosta, an Italian alpine city. My participation was concerned with extrasolar and interstellar solar-photon sailing, since NASA had funded my research in the Heliopause Sail Technology Project, under the direction of Les Johnson. C’s role was to curate and hang an art show, “Messages from Earth”, in a medieval Aosta chapel. About 30 international artists contributed work presenting their conceptual message plaques that could be mounted on a solar-photon sail bound for the stars.
During the reception associated with the art show, C was approached by the late Robert Forward. As many Centauri-Dreams readers will remember, Bob pioneered numerous approaches to interstellar travel during the last few decades of the twentieth century. When Bob reached into his wallet and withdrew a credit card, on-lookers expected that he might be making a purchase. Instead, he asked C how she would affix a message plaque to the sail. She responded that a physical plaque (as was done in Pioneer 10/11), a long-playing phonographic record (as was done in Voyager 1/2) or a computer chip were possible approaches. Bob drew her attention to the white-light hologram on the credit card and expressed the opinion that a low-mass, thin-film holographic plaque could contain a huge amount of information.
After the symposium, C returned to Brooklyn and I rejoined the Marshall sail team. A few weeks later, while C was coincidentally visiting me in Huntsville, we were invited to a lecture by Bob. When Les Johnson introduced him, Bob pointed to C and said: “Fund that woman to do a prototype holographic interstellar message plaque”.
So my small NASA University Challenge Grant through Pace University, where I taught at the time, was reconfigured to support the creation of the hologram. I received no salary from this grant so that the project could be financed. We contacted the Center for Holographic Arts (then located in Long Island City) and the rainbow hologram was completed at that facility with two sculpted figures and four line drawings by C with a transparency of the Apollo 17 image of Earth from deep space that is always visible and supports the holographic images.
We delivered one of the three resulting holograms to NASA Marshall in mid-2001. Another was later purchased by a collector and donated to New York City College of Technology. We use the third for display purposes.
There are seven 2D and 3D monochromatic images on this hologram, representing our solar system, probe trajectory and the human form in a similar manner to the Pioneer plaque. During the summer of 2001, we showed it to many NASA and contractor employees. As part of the research effort associated with the plaque, we participated in simulated space-radiation tests of holographic wrapping paper samples. Holograms are apparently immune to image degradation caused by intense solar flares [4-6].
Image: A view of the Rainbow Hologram created by C Bangs. The hologram contains six images. As the viewer moves from left to right the images transition from one frame to the next. On the extreme left side is a line drawing that places our home solar system on the edge of the Milky Way Galaxy and our planet, third from our sun. The second frame is a line drawing of the female figure holding the payload of the solar sail to demonstrate her size relative to it. The third frame is the sculpted female figure. The fourth frame is the sculpted male figure with his hand raised in what is believed to be a universal greeting. The fifth frame is a line drawing of the male figure. The last frame contains equations that describe the acceleration of the solar sail that the hologram would hypothetically be traveling on. In front of all the images is an photograph of the full Earth visually demonstrating the beauty of our home planet. Credit: C Bangs.
It became apparent to the team that Bob Forward was interested in other applications of holographic solar sail applications than message plaques. As discussed in Ref. 5, it is possible to change the reflectivity of a white-light hologram with a slight rotation. It is therefore conceptually possible to accelerate a solar photon sail from Low Earth Orbit by rotating the sail to reflect sunlight when the Sun is behind the spacecraft and transmit sunlight when the sail faces the Sun.
Current Technology Holography and Project Starshot
During 2017, C and I had several meetings with Dr. Martina Mrongovius, Creative Director of the NYC HoloCenter. The HoloCenter is the outgrowth of the Center for Holographic Arts.
The art and science of holography has advanced at a rapid pace during the past few decades. Holograms as thin as 25 nm have been produced by an Australian-Chinese team [7]. Highly efficient wavelength-selective holographic filters and reflectors have been produced and evaluated [8-10].
Color of contemporary holograms displayed at the HoloCenter seems true to life. If the observer slowly changes position to view an experimental 3D holographic movie, action seems continuous with no breaks. Clearly, a vast amount of information can be stored on a single thin-film hologram. According to the Wikipedia article on holography, thousands of images can be produced and stored per second. Martina reports in a YouTube video that some modern holograms contain 10,000 holographic layers.
It no longer seems impossible to me that the Project Starshot goals can be achieved. One would use a holographic film and expose the image of a filter or mirror that is highly reflective in the laser’s wavelength range. My colleague at Citytech, Lufeng Leng teaches optics. She is quite sure that a hologram of a spherical surface will behave optically like a spherical surface. So the filter or mirror should ideally have a convex spherical shape, from the point of view of the observer (or laser).
If the holographic filter or mirror sail is sufficiently reflective to laser light, the thermal issue should be resolvable. Since the hologram is a flat sheet, it should be tolerant to high accelerations. If the spherical filter/mirror 3D image behaves as discussed in Ref. 2, the sail should self-correct its position and remain in the moving laser beam. If a pair of tiny thrusters are mounted on the anti-laser face of the sail, it should be possible to rotate the flat sail by 90 degrees after acceleration terminates to minimize damage by the interstellar medium.
Image: An artist’s conception of a laser-beamed sail. Credit: Adrian Mann.
The April 2018 Breakthrough Committee Meeting
On April 11, the Starshot advisors met at a Breakthrough facility in the NASA Ames Space Flight Center. While C displayed the prototype holographic message plaque, I presented the case for a holographic sail. We learned that Harry Atwater of the California Institute of Technology and his team are investigating technologies that combine aspects of engineered metamaterials and holography. Most participants agreed that the idea of a holographic sail is promising. Some, including Avi Loeb of Harvard, suggested that experimental validation is required.
A number of experiments should be possible. Some of these could be addressed in response to the Starshot Sail RFP, which is scheduled for release in the near future. Jason Wentworth, a frequent contributor to Centauri Dreams, has informed me that projectiles fired by large naval guns routinely survive very high accelerations. A small thin-film hologram mounted on or in a suitable projectile might demonstrate whether a hologram can survive the requisite ~5,000 g acceleration.
It is not possible today to test a holographic filter’s reflectance and survival in a continuous ~50 GW laser beam. But according to Wikipedia, the inertial-fusion confinement lasers at the National Ignition Facility located at Lawrence Livermore can deliver 500 terawatts for a few picoseconds. Perhaps a test of a holographic sail could be performed at that facility.
If a prototype thin-film holographic spherical filter or mirror is engineered to reflect in the microwave region rather than at the laser wavelength, another test is possible using existing facilities. Beam-riding stability could be demonstrated using the equipment applied by Jim Benford, Greg Benford and colleagues to examine beam-riding stability of a number of sail shapes during 2001 [11].
In any event, the situation is hopeful. Both C and I felt that we were channeling Bob Forward during our presentation. It’s nice to imagine that his shade is smiling and cheering on the efforts of the Project Starshot team.
References
1. P. Lubin, “A Roadmap to Interstellar Flight”, JBIS, 69, 40-72 (2016).
2. Z. Manchester and A. Loeb, “Stability of a Light Sail Riding on a Laser Beam”, arXiv:submit/1680014 [astro-ph.IM] 29 Sep 2016.
3. T. Hoang, A. Lazarian, B. Burkhart, and A. Loeb, “The Interaction of Relativistic Spacecrafts with the Interstellar Medium”, arX1v: 1608.05284v1 [astro-ph.GA] a8 Aug 2016.
4. G. L. Matloff, G. Vulpetti, C Bangs and R. Haggerty, “The Interstellar Probe (ISP). Pre-Perihelion Trajectories and Application of Holography”, NASA/CR-2002-211730, NASA Marshall Space Flight Center, Huntsville, AL (June, 2002).
5. R. Haggerty and T. Stanaland, “Applications of Holographic Films in Solar Sails”, presented at STAIF-2002 Conference, University of New Mexico, Albuquerque NM (January, 2002).
6. G. L. Matloff, Deep Space Probes: To the Outer Solar System and Beyond, 2nd. ed. Springer-Praxis, Chichester, UK (2005).
7. M. Irving, “World’s Thinnest Holograms Could Lead to Thin-Film 3C displays”, New Atlas (May 18, 2017).
8. W. Wang, “Reflection and Transmission Properties of Holographic Mirrors and Holographic Fabry-Perot Filters. 1. Holographic Mirrors with Monochromatic Light”, Applied Optics, 1994, May 1;33:2560-6. doi: 10.1364/AO.33.002560.
9. P. Sharlandjiev and Ts Mateeva, “Normal incidence Holographic Mirrors by the Characteristic Matrix Method”, Journal of Optics, 16, 185-190 (1985).
10. D. W. Diehl, “Holographic Interference Filters”, Ph.D. Thesis, Institute of Optics, Schoolof Engineering and Applied Science, University of Rochester, Rochester, NY (2003).
11. James Benford, Gregory Benford, Olga Gornostaeva, Eusebio Garate, Michael Anderson, Alan Prichard, and Henry Harris, “Experimental Tests of Beam-Riding Sail Dynamics”, Proc. Space Technology and Applications International Forum (STAIF-2002), Space Exploration Technology Conf, AIP Conf. Proc. 608, ISBN 0-7354-0052-0, pg. 457, (2002).
by Paul Gilster | Apr 19, 2018 | Deep Sky Astronomy & Telescopes |
With TESS going into a 60-day period of calibration and testing, I’ll turn this morning to a different kind of survey. GALAH is an acronym for Galactic Archaeology, a term I’ve generally associated with so-called Dysonian SETI, where data is mined in a search for signs of advanced engineering or any anomalies that could signify an extraterrestrial civilization at work. But GALAH has a different object: It has examined some 340,000 stars enroute to 1 million.
A just published paper on GALAH states the goal succinctly:
The overarching goal of the GALAH survey is to acquire high-resolution spectra of a million stars for chemical tagging, in order to investigate the assembly history of the Galaxy.
The survey was launched in 2013 as a deep study of galactic formation and evolution, using the HERMES spectrograph at the Australian Astronomical Observatory’s 3.9-meter Anglo-Australian Telescope near Coonabarabran, NSW. Now coming online is a major data release, the second from GALAH, that has interesting implications not only for the broad field of galactic evolution but also the history of our own Sun. Thus Gayandhi De Silva (University of Sydney and AAO), who oversaw the work on Hermes and coordinated the effort leading to the release:
“No other survey has been able to measure as many elements for as many stars as GALAH. This data will enable such discoveries as the original star clusters of the Galaxy, including the Sun’s birth cluster and solar siblings — there is no other dataset like this ever collected anywhere else in the world.”
Image: A schematic of the HERMES instrument showing the path of star light from the telescope and demonstrating how it is split into four different channels. Credit: AAO.
Our recent look at the possibility of a prior civilization on our own planet is now superseded by a quest to dig much further back in time. Finding the Sun’s birth cluster leverages the fact that stars from the same originating cluster should have the same chemical composition. The problem is that clusters within the Milky Way are quickly pulled apart and are scattered. GALAH is looking to match compositional traits — the researchers call this the stars’ DNA — to parse for each star about two dozen chemical elements like oxygen, aluminium and iron.
So the day may come when we can point to stars that were born in the same cluster as the Sun. While an hour is needed to collect enough photons of light from each of the stars in the GALAH survey, the project is able to observe 360 stars at the same time, an effort that so far has involved more than 280 nights at the AAO instrument since 2014 for data collection. The HERMES spectrograph was designed by the AAO specifically for the GALAH survey.
At work here is computer code that PhD student Sven Buder, lead author of a paper on the data release, calls ‘The Cannon,’ a nod to astronomer Annie Jump Cannon, whose work on stellar spectra of several hundred thousand stars in the early 20th Century was groundbreaking. Says Buder:
“We train The Cannon to recognize patterns in the spectra of a subset of stars that we have analysed very carefully, and then use The Cannon’s machine learning algorithms to determine the amount of each element for all of the 340,000 stars.”
Image: Hubble has captured the most detailed image to date of the open star cluster NGC 265 in the Small Magellanic Cloud. The image taken with the Advanced Camera for Surveys onboard the NASA/ESA Hubble Space Telescope show a myriad of stars in crystal clear detail. The brilliant open star cluster, NGC 265, is located about 200,000 light-years away and is roughly 65 light-years across. Our investigations into open clusters like this may help us learn more about our Sun’s birth. Credit: ESA / NASA / E. Olszewski (University of Arizona).
Be aware that on the 25th of this month, the European Gaia satellite will likewise offer a major data release. Gaia, with its mapping of more than 1.6 billion stars in our galaxy, will mesh with the GALAH findings, using the latter’s calculations of stellar velocities to interpret Gaia data in what is becoming the most accurate atlas of the night sky ever available. From the paper:
The stellar parameter and abundance information contained in GALAH DR2 [Data Release 2] will enable major steps forward in Galactic Archaeology, including detailed work to identify clusters within the chemical space and characterize its structure and dimensionality. In combination with the dynamical information provided by Gaia DR2, we will work toward a reliable narrative of how the Milky Way was assembled and how it has evolved since, using chemodynamics and chemical tagging.
Incorporating parallax information from GAIA and broadening GALAH toward the target of one million stars is the task that lies ahead for the researchers, who plan subsequent data releases with re-analyses of all the stars from DR2 and the new stars subsequently observed.
The overview paper on GALAH, one of eleven science papers that will accompany the data release, is Buder, “The GALAH Survey: Second Data Release,” Monthly Notices of the Royal Astronomical Society Volume 476, Issue 4 (1 June 2018), pp. 5216-5232 (abstract / preprint).
by Paul Gilster | Apr 18, 2018 | Culture and Society |
My doctor is a long-time friend who always stops during my annual physical to ask about what’s going on in the hunt for exoplanets. Last week he surprised me when, after I had described ways of analyzing a transiting planet’s atmosphere, he asked whether planets could give rise to civilizations in different epochs. Why just one, in other words, given that homo sapiens has only been around for several hundred thousand years? Our technological civilization is a very recent, and to this point a short-lived phenomenon. Were there others?
I was startled because Adam Frank (University of Rochester) and Gavin Schmidt (NASA Goddard Institute for Space Studies) have recently raised a stir with a paper on what they call the ‘Silurian Hypothesis,’ the name deriving from a Doctor Who TV episode referencing intelligent reptiles called Silurians who come to life when accidentally awakened. As the authors point out in their paper:
We are not however suggesting that intelligent reptiles actually existed in the Silurian age, nor that experimental nuclear physics is liable to wake them from hibernation. Other authors have dealt with this possibility in various incarnations (for instance, Hogan (1977), but it is a rarer theme than we initially assumed.
True enough, although it does pop up in science fiction from time to time. I’m remembering a 1989 tale from Barry Longyear that involved a fleet of starships returning to their home world to find that it is now being managed by humans. The starship crews — essentially intelligent dinosaurs — have been gone 70 million years, their civilization long obliterated on their home world. The novel Frank and Schmidt reference is James Hogan’s Inherit the Stars, in which evidence of an unknown early human technology is found on the Moon. Maybe readers can supply some other stories involving civilizations from deep time.
Digging Out the Evidence
Frank comes to this topic as a natural outgrowth, I think, of his earlier investigations of how industrial civilizations affect their home planets. All this involves issues of sustainability and climate alteration, using dynamical systems theory as a methodology to examine how species with energy-intensive technology alter planetary evolution (you can read more about this in my Astrobiology and Sustainability). Does an industrial civilization invariably cause detectable climate shift? Gavin Schmidt jogged Frank’s thinking on the topic by bringing up the question of prior civilizations, which raised the issue of just how we might detect such a culture.
Image: The inland seas in North America (Western Interior Seaway) and Europe had receded by the beginning of the Paleocene, making way for new land-based flora and fauna. If an early mammal had produced a civilization in this era, would we be able to find traces of it? Credit: Paleontology World.
We’re talking non-human cultures if we go back far enough, and with the passage of hundreds of millions of years, evidence becomes more than a little problematic. Frank has just written a piece for The Atlantic that pulls out the major themes of the paper. Was There a Civilization On Earth Before Humans? puts the matter succinctly. Because when you talk about direct evidence of an industrial civilization, we’re dealing with a geologic record that makes it all but impossible to probe past the Quaternary period, some 2.6 million years ago.
We can obviously study earlier eras, but as the authors point out, the largest extant surface area is in the Negev Desert, in southern Israel, which can take us back 1.8 million years. Earlier than the Quaternary, we rely on exposed surfaces unearthed through excavation, drilling or mining, while our study of ocean sediments faces the fact of recycling ocean crust, so that our evidence is for periods that post-date the Jurassic; i.e., we can go back some 170 million years.
This gets seriously intriguing, and I found myself reading The Atlantic piece and the original paper (citation below) with a compulsive fascination, maybe because when I was a kid, I used to think about becoming a paleontologist, digging up the remains of creatures from the days of the brontosaurus. But how much of that past world can we recover given how sparse the fossil record is? For Frank and Schmidt point out how little of life is captured this way. From the paper;
The fraction of life that gets fossilized is always extremely small and varies widely as a function of time, habitat and degree of soft tissue versus hard shells or bones (Behrensmeyer et al., 2000). Fossilization rates are very low in tropical, forested environments, but are higher in arid environments and fluvial systems. As an example, for all the dinosaurs that ever lived, there are only a few thousand near-complete specimens, or equivalently only a handful of individual animals across thousands of taxa per 100,000 years. Given the rate of new discovery of taxa of this age, it is clear that species as short-lived as Homo Sapiens (so far) might not be represented in the existing fossil record at all.
The survival of actual objects produced by such a civilization — think the Antikythera Mechanism from ancient Greece — is unlikely indeed. Our species has left countless artifacts that have yet to be recovered, if they ever will be. On a wider scale, we are learning much about detecting the effects of civilizations on the landscapes around them (here I think of aerial surveys finding building sites or burial mounds), but Frank and Schmidt note that the current rate of urbanization is less than one percent of the Earth’s surface. We don’t know where to look, and the likelihood of finding direct evidence of artifacts is remote in the extreme.
Would we know it, then, if an early mammal built a civilization in the Paleocene (60 million years ago)? Let’s assume a civilization lasting no more than 100,000 years, which turns out to be 500 times longer than our own civilization to this point. You would think that specific markers of industrial acthttps://arxiv.org/abs/1804.03748ivity would get through — these would include, perhaps, plastics, which seem to live forever. They do break down eventually, as the authors note, but the results are unclear:
The densification of small plastic particles by fouling organisms, ingestion and incorporation into organic ‘rains’ that sink to the seafloor is an effective delivery mechanism to the seafloor, leading to increasing accumulation in ocean sediment where degradation rates are much slower (Andrady, 2015). Once in the sediment, microbial activity is a possible degradation pathway (Shah et al., 2008) but rates are sensitive to oxygen availability and suitable microbial communities. As above, the ultimate long-term fate of these plastics in sediment is unclear, but the potential for very long term persistence and detectability is high.
We might likewise find evidence like increased concentrations of metals. Maybe our relentless production of electronics will leave a trace in the concentration of rare-Earth elements in sediments for some successor species to identify. Of course, we can’t be comfortable about generalizing from our own activities to those of some hypothetical primeval civilization, but there is room for speculation nonetheless, and Frank and Schmidt look hard at fossil fuels, the burning of which releases carbon into the atmosphere, causing the balance of carbon isotopes to shift in what atmospheric scientists call the ‘Suess effect,’ a change in isotope ratios of carbon that is readily traced in the last century.
We do see ‘spikes’ in the geological record, though none that are ‘spiky enough’ to fit into the hypothesis of a Silurian civilization. Using our own ‘Anthropocene’ era as a guide, we are seeing a huge increase in atmospheric carbon levels much unlike the slower spikes of the Paleocene-Eocene Thermal Maximum (PETM), when the planet’s average temperature rose well above what we have today. Those much earlier spikes (56 million years ago) took hundreds of thousands of years to play out. A civilization’s signal in terms of carbon output is, at least judging from our own, much more sudden, though we have yet to learn how it will end.
We can’t rule out detection methods that could trace extremely short-lived events in ancient sediments, the authors conclude, but they would be extraordinarily hard to detect. Frank and Schmidt don’t believe any such civilization existed, but their paper asks a broader question that is relevant to exoplanet studies. What kind of effects does the collection of energy for building a civilization leave on its home world? Assuming there is feedback into planetary systems, we may be able to build a set of markers that could help us identify the process at work.
As Frank concludes in his Atlantic essay:
…our work also opened up the speculative possibility that some planets might have fossil-fuel-driven cycles of civilization building and collapse. If a civilization uses fossil fuels, the climate change they trigger can lead to a large decrease in ocean oxygen levels. These low oxygen levels (called ocean anoxia) help trigger the conditions needed for making fossil fuels like oil and coal in the first place. In this way, a civilization and its demise might sow the seed for new civilizations in the future.
Cycles of civilizations could thus occur, even if we have no evidence that they have previously taken place on Earth. The broader question is whether we can deduce a set of maxims telling us how biospheres evolve, and how the activities of their societies re-shape their world. Again we are seeding the debate over differing kinds of biosignatures and technosignatures that will inform our studies of the data gathered by the next generation of space and ground instruments.
The paper is Schmidt and Frank, “The Silurian Hypothesis: Would it be possible to detect an industrial civilization in the geological record?” published online by the International Journal of Astrobiology 16 April 2018 (abstract / preprint).
by Paul Gilster | Apr 17, 2018 | Missions |
The launch of TESS aboard a SpaceX Falcon 9 looks to be on track for Wednesday after yesterday’s delay, which the company attributed to the need for “additional GNC [guidance, navigation and control] analysis.” So we wait just a bit more, knowing that the payoff justifies the caution. We should be identifying planets in the thousands, and around bright, nearby stars.
Principal investigator George Ricker and team have been through the process of designing, building and launching a mission before. It was in 2000 that NASA launched the MIT-built High Energy Transient Explorer 2, or HETE-2, that studied gamma-ray bursts for seven years in Earth orbit. A key technology for HETE-2 was the CCD — charge-coupled device — which allowed the satellite’s optical and X-ray cameras to record bursts in electronic format.
“With the advent of CCDs in the 1970s, you had this fantastic device … which made a lot of things easier for astronomers,” says HETE-2 team member Joel Villasenor, who is now also instrument scientist for TESS. “You just sum up all the pixels on a CCD, which gives you the intensity, or magnitude, of light. So CCDs really broke things open for astronomy.”
Image: A set of flight camera electronics on one of the TESS cameras, developed by the MIT Kavli Institute for Astrophysics and Space Research (MKI), will transmit exoplanet data from the camera to a computer aboard the spacecraft that will process it before transmitting it back to scientists on Earth. Credit: MIT Kavli Institute.
HETE-2’s operations led to an obvious question: Could the satellite use its optical cameras to study exoplanets? The spacecraft’s photometry proved to be insufficient for the task of identifying transits, found by the kind of dips in a star’s light that the Kepler mission would use to such success. But transit hunting stayed on Ricker’s mind, and by 2006 his team had proposed HETE-S to NASA, pitching it as a Discovery class mission. It would later become a proposal for a Small Explorer Class mission under its current name, Transiting Exoplanet Survey Satellite. A new plan emerged when NASA passed on the first TESS proposal.
What we have now is a spacecraft that will use a ‘lunar-resonant’ orbit, as this MIT news release explains. Growing out of work at NASA GSFC as well as Orbital ATK, the orbit will take TESS on a highly elliptical path between and Earth and the Moon, a stable configuration Villasenor describes this way:
“The moon and the satellite are in a sort of dance,” Villasenor says. “The moon pulls the satellite on one side, and by the time TESS completes one orbit, the moon is on the other side tugging in the opposite direction. The overall effect is the moon’s pull is evened out, and it’s a very stable configuration over many years. Nobody’s done this before, and I suspect other programs will try to use this orbit later on.”
The planned trajectory takes the spacecraft on a swing toward the Moon (with apogee near the Moon’s distance) and then a swing back toward the Earth, a stable orbit one benefit of which is that TESS will not need to perform regular thruster burns to maintain its orbit. With Kepler ending its original exoplanet survey, NASA approved the revamped TESS in 2013 as an Explorer class mission. Kepler’s success was obviously a huge motivator, demonstrating the ubiquity of planets around stars and highlighting the good science that could be done on a mission with a wider view that could scan the nearest stars.
Because many of the nearby stars of high interest are red dwarfs, TESS is built around ‘deep depletion’ CCDs that can detect light in a wide range of wavelengths into the near infrared. Once launched, TESS will begin observations in the southern hemisphere and will divide the sky into thirteen ‘stripes,’ with each of these being monitored for 27 days before the cameras are turned to the next. This method should allow almost the entire southern hemisphere sky to be monitored in the first year, after which TESS will turn its attention to the northern hemisphere.
As to data collection, here’s what NASA says:
TESS has two data collection modes: “postage stamp” images that capture light from individual stars and full-frame images that cover the entire field of view. During an observation sector, 15,000 stars selected from a carefully curated list of 200,000 stars make up the primary targets for exoplanet detection, and TESS will record their brightness every two minutes. Images covering the entire 24-by-96-degree field of view will be acquired at 30-minute intervals. Exoplanets will be found using both data products.
Image: From the transit data alone, scientists will be able to determine the size of the planets and orbital parameters. Ground-based follow-up observations of these objects, possible because of the brightness of the host stars, will allow the determination of the planetary masses. Combining the two, radius and mass, will allow astronomers to determine the density of planets, and hence their bulk composition (are they gas giants? water worlds? big rocks, like Earth?). In addition, transit observations can be used to study the dynamics of planetary systems, such as planet-planet interactions and mutual inclinations. Additional follow-up observations, largely from space with HST and JWST, will allow direct measurement of the atmospheric composition and structure of some planets. This will open the door for a host of new discoveries about exoplanets, and perhaps of the processes behind the formation and evolution of planetary systems. Credit: NASA GSFC.
Once in space, TESS will undergo a 60-day commissioning phase involving instrument calibration and calculations of the spacecraft’s trajectory and performance. After that, data collection commences as TESS produces what the science team will need to generate the necessary light curves. MIT’s Sara Seager and the TESS science team will go to work on the light curves, with mass as a key determinant. The TESS stars are close and bright enough to allow mass determination via ground-based radial velocity methods. As Seager says:
“Mass is a defining planetary characteristic. If you just know that a planet is twice the size of Earth, it could be a lot of things: a rocky world with a thin atmosphere, or what we call a “mini-Neptune” — a rocky world with a giant gas envelope, where it would be a huge greenhouse blanket, and there would be no life on the surface. So mass and size together give us an average planet density, which tells us a huge amount about what the planet is.”
If all goes as planned, TESS should discover, among its thousands of exoplanets, hundreds that are less than twice the size of the Earth. The primary goal here is to identify small worlds where follow-up observations can be made with current or planned telescopes. The James Webb Space Telescope will then have the opportunity to use the TESS target list for deeper investigation, and there will also be useful synergies with the European Space Agency’s CHaracterising ExOPlanets Satellite (CHEOPS), scheduled for launch next year.
But first we have to get the mission off. All eyes on Florida for tomorrow’s attempt.
