by Paul Gilster | Mar 11, 2021 | Exotic Physics |
I see that Erik Lentz (Göttingen University) has just begun a personal blog, something that may begin to attract attention given that Dr. Lentz has offered up a new paper on faster than light travel. At the moment, the blog is bare-bones, listing only the paper itself (citation below) and an upcoming online talk that may be of interest. Here’s what the Lentz blog has on this:
Upcoming online talk to be given on 18 March 2021 at 3pm Eastern Standard Time for the Science Speaker Series at the Jim and Linda Lee Planetarium: https://youtu.be/6O8ji46VBK0
I checked the URL and found the page with a countdown timer, so I assume the event is publicly accessible. I would imagine it will draw a number of curious scientists and lay-people.
On the subject of faster than light travel, much of the work in the journals has evolved from Miguel Alcubierre’s now well known paper “The Warp Drive: Hyper-fast travel within general relativity,” which presented the idea of a ‘bubble’ of spacetime within which a volume of flat space could exist. In other words, it might be possible to enclose a spacecraft within such a bubble. While there is a physical restriction on objects within spacetime moving faster than the speed of light, spacetime itself is theoretically capable of expansion without limit — this is essentially the notion of ‘inflation’ that drives most current thinking about the earliest moments of the universe.
Alcubierre’s paper ran in May, 1994 in the prestigious journal Classical and Quantum Gravity, a venue whose demanding standards of peer review and acceptance give it high credibility. In other words, papers in this journal rightfully attract attention because of the demanding requirements of publication. I had more or less overlooked the new paper by Dr. Lentz until I realized that it was published here, after which I began to take notice.
This does not mean, of course, that either the Alcubierre ‘warp drive’ concept or the much different ideas of Erik Lentz can ever be engineered, but it does offer a great deal of interest from the standpoint of the mathematics of warped spacetime. After the Alcubierre paper, much of the ongoing work has been involved in exploring how negative energy operates, for ‘negative energy density’ is exotic and vast amounts would be required to form the needed ‘bubble’ of spacetime. The Lentz paper does away with negative energy. I’m hearing it described as an idea more in conformance with conventional physics, though that may also need clarification.
The essential notion put forward by Dr. Lentz is that there are configurations of spacetime curvature that can be explored as ‘solitons,’ which are a solution he deems physically viable, and thus not dependent on negative energy at all. Here we’re already in deep water. A soliton, as I have been learning, is a wave that can retain its shape and move at constant velocity. That such curiosities are within the realm of physical possibility is made clear by the origin of the study of solitons. They actually go back to an observation by British engineer John S. Russell in 1834. In a famous and oft-quoted passage delivered ten years later to the British Association for the Advancement of Science, Russell had this to say about what he called a ‘wave of translation’:
I was observing the motion of a boat which was rapidly drawn along a narrow channel by a pair of horses, when the boat suddenly stopped – not so the mass of water in the channel which it had put in motion; it accumulated round the prow of the vessel in a state of violent agitation, then suddenly leaving it behind, rolled forward with great velocity, assuming the form of a large solitary elevation, a rounded, smooth and well-defined heap of water, which continued its course along the channel apparently without change of form or diminution of speed. I followed it on horseback, and overtook it still rolling on at a rate of some eight or nine miles an hour, preserving its original figure some thirty feet long and a foot to a foot and a half in height. Its height gradually diminished, and after a chase of one or two miles I lost it in the windings of the channel. Such, in the month of August 1834, was my first chance interview with that singular and beautiful phenomenon which I have called the Wave of Translation.
Thus was born the study of solitons, which now extends into nuclear physics, optics and other fields, now including exotic propulsion. Notice that what Russell describes is a wave that is stable and can travel. His use of the word ‘translation’ means that this is not a wave made up of the same water that travels the length of the channel he was observing, but rather a wave that moves through the medium. Water is moving but being displaced in the process. We can think of the wave of translation — or at least I’ve seen it referred to this way — as a ‘wave packet’ that can maintain its shape, as it did in Scotland’s Union Canal for Russell.
I turned to Hilborn and Cross’ Chaos and Nonlinear Dynamics (Oxford University Press, 2000) to see solitons described as ‘nonlinear wave phenomena.’ Thus:
A soliton is a spatially localized wave disturbance that can propagate over long distances without changing itsshape. In brief, many nonlinear spatial modes become synchronized to produce a stable localized disturbance.
Solitons turn out to be remarkably stable. A great deal of mathematics has gone on since as solition concepts evolved, all much beyond my pay grade. I looked again at Dr. Lentz’ website to get a notion of what he was proposing in his own words, because I find it hard to make the considerable jump from the early observations of Russell to today’s understanding of solitons. Here’s Lentz with a vest-pocket description of faster than light travel that does not violate Einsteinian relativity:
Hyper-fast (as in faster than light) solitons within modern theories of gravity have been a topic of energetic speculation for the past three decades. One of the most prominent critiques of compact mechanisms of superluminal motion within general relativity is that the geometry must largely be sourced from a form of negative energy density, though there are no such known macroscopic sources in particle physics. I was recently able [to] disprove this position by constructing a new class of hyper-fast soliton solutions within general relativity that are sourced purely from positive energy densities, thus removing the need for exotic negative-energy-density sources. This is made possible through considering hyperbolic relations between components of the space-time metric’s shift vector. Further, these solutions are sourceable by a classical electronic plasma, placing superluminal phenomena into the purview of known physics. This is a very exciting breakthrough that I hope to have more [to] report on soon.
I take this to mean that there are mathematical solutions for spacetime curvature that use solitons as the mode of organization. Alcubierre’s ‘warp bubble’ becomes, in soliton mode, a wave that maintains its shape and moves at constant velocity. The key here, Lentz believes, is that this is a way of altering spacetime geometry without the use of exotic negative energy. Moreover, Lentz’ equations evidently show that tidal forces within the bubble can be minimized. The passage of time inside the soliton can be adjusted to match the time outside the bubble.
Image: Artistic impression of different spacecraft designs considering theoretical shapes of different kinds of “warp bubbles.” Credit: E Lentz.
We would still need enormous amounts of energy, but we are dealing with the kind of energy we understand rather than the far more amorphous ‘negative energy.’ Here’s Lentz again:
“The energy required for this drive travelling at light speed encompassing a spacecraft of 100 meters in radius is on the order of hundreds of times of the mass of the planet Jupiter. The energy savings would need to be drastic, of approximately 30 orders of magnitude to be in range of modern nuclear fission reactors… Fortunately, several energy-saving mechanisms have been proposed in earlier research that can potentially lower the energy required by nearly 60 orders of magnitude.”
Such energy savings methods would be prodigious indeed and it is to these that Dr. Lentz apparently turns next. The paper is Lentz, “Breaking the Warp Barrier: Hyper-Fast Solitons in Einstein-Maxwell-Plasma Theory,” Classical and Quantum Gravity Vol. 38, No. 7 (March, 2021). Abstract. We are in very deep mathematical waters here, so all I want to do is point to the paper and urge those interested to take in Dr. Lentz’ talk on the 18th.
by Paul Gilster | Mar 10, 2021 | Exoplanetary Science |
Gliese 486b is, in the words of astronomer Ben Montet, “the type of planet we’ll be studying for the next 20 years.” Montet (University of New South Wales) is excited about this hot super-Earth because it’s the closest such planet we’ve found to our own Solar System, at about 26 light years away. That has implications for studying its atmosphere, if it has one, and by extension sharpening our techniques for atmospheric analysis of other nearby worlds. The goal we’re moving toward is being able to examine smaller rocky planets for biosignatures.
But we’re not there yet, and what we have in Gliese 486b is an exoplanet that has now been identified as a prime target for future space- and ground-based instruments, one that, given its proximity, is an ideal next step to push our methods forward. The paper on this work shows that two techniques can be deployed here, the first being transmission spectroscopy, when this transiting world passes in front of its star and starlight filters through the atmosphere.
So-called emission spectroscopy happens when the planet orbits around to the other side of the star, making parts of the illuminated surface visible (think phases of the Moon as an analogy). Astronomers can deploy spectrographic tools in both methods to work out the chemical composition of the atmosphere, and according to Montet, Gliese 486b is the best single planet yet found for emission spectroscopy out of all the rocky planets we know. Moreover, says the astronomer, it’s the second best for transmission spectroscopy.
I asked Dr. Montet about this, wondering about the absolute best planet for transmission spectroscopy. His reply:
The best planet for transmission spectroscopy is TRAPPIST-1 b. Our new planet is #2, and the third best is L98-59 d, a planet discovered by TESS in 2019. We’re quantifying relative goodness using the Transmission Spectroscopy Metric from Eliza Kempton’s work in 2018.
Image: The graph illustrates the orbit of a transiting rocky exoplanet like Gliese 486b around its host star. During transit, the planet obscures the stellar disk. Simultaneously, a tiny portion of the starlight passes through the planet’s atmospheric layer. While Gliese 486b continues to orbit, parts of the illuminated hemisphere become visible like lunar phases until the planet vanishes behind the star. Credit: © MPIA graphics department.
430 degrees Celsius make Gliese 486b a nightmarish place, perhaps one with rivers of lava and, moreover, gravity that is 70 percent stronger than Earth’s. The planet orbits its star, an M-dwarf, every 36 hours. We can only imagine what an orbit this tight means for flares and coronal mass ejections on the surface, and it’s likely that the atmosphere itself could be threatened. What we find here in future studies will help us calibrate atmospheric survival and composition on planets orbiting red dwarfs.
The work on Gliese 486b comes out of the CARMENES Consortium, which is leading an effort that includes over 200 scientists and engineers from eleven institutions in Spain and Germany who have designed the 3.5 meter telescope at Calar Alto in southern Spain. The purpose is to monitor 350 M-dwarf stars in search of low-mass planets using a spectrograph mounted on the instrument. The word CARMENES is actually an acronym, and one that takes us into epic territory in terms of length: Calar Alto high-Resolution search for M dwarfs with Exoearths with Near-infrared and optical Échelle Spectrographs.
So far, what we know about Gliese 486b — using transit photometry and radial velocity spectroscopic data from a variety of Earth-based instruments as well as space-based TESS — is that it is about 2.8 times as massive as the Earth and about 30 percent larger. Calculating from mass and radius measurements, the astronomers on the team led by Trifon Trifonov (Max Planck Institute for Astronomy) find a mean density that indicates a rocky world with a metallic core, and as mentioned above, a gravitational pull about 70 percent stronger than Earth’s.
But is this planet’s tight orbital distance (2.5 million kilometers) too close for any atmosphere to survive? The paper makes it clear that this is possible but by no means certain:
With a radius of 1.31 RE, Gliese 486b is located well below the radius range of 1.4 to 1.8 RE, under which planets are expected to have lost their primordial hydrogen-helium atmospheres owing to photoevaporation processes. It remains unknown how stellar irradiation and planet surface gravity affect the formation and retention of secondary atmospheres.
Which makes this an interesting test case, because the numbers are provocative:
Planets with Teq > 880 K, such as 55 Cancri e, are expected to have molten (lava) surfaces and no atmospheres, except for vaporized rock. Gliese 486 b is not hot enough to be a lava world, but its temperature of ~700 K makes it suitable for emission spectroscopy and phase curve studies in search of an atmosphere. Our orbital model constrains the secondary eclipse time to within 13 min (at 1σ uncertainty), which is necessary for efficient scheduling of observations. Compared with other known nearby rocky planets around M dwarfs, Gliese 486 b has a shorter orbital period and correspondingly higher equilibrium temperature of ~700 K and orbits a brighter, cooler, and less active stellar host.
Image: The diagram provides an estimate of the interior compositions of selected exoplanets based on their masses and radii in Earth units. The red marker represents Gliese 486b, and orange symbols depict planets around cool stars like Gliese 486. Grey dots show planets hosted by hotter stars. The coloured curves indicate the theoretical mass-radius relationships for pure water at 700 Kelvin (blue), for the mineral enstatite (orange), for the Earth (green), and pure iron (red). For comparison, the diagram also highlights Venus and the Earth. Credit: Trifonov et al./MPIA graphics department.
We’re going to be learning a lot more about Gliese 486b as the effort to investigate it continues. How well rocky planets retain their atmospheres under extreme conditions will help us understand possible atmospheric processes going on in their stars’ presumably more clement habitable zones. Given their ubiquity, red dwarfs could be interesting places to look for life, but as this planet shows us, that investigation is in its early stages. For now, hot super-Earths are the best way to proceed.
The paper is Trifonov et al., “A nearby transiting rocky exoplanet that is suitable for atmospheric investigation,” Science Vol. 371, Issue 6533 (5 March 2021), pp. 1038-1041 (abstract).
by Paul Gilster | Mar 9, 2021 | Asteroid and Comet Deflection |
Were the rocky worlds of the inner Solar System depleted in carbon as they formed, the so-called ‘carbon deficit problem’? There is evidence for a system-wide carbon gradient in that era, which makes for interesting interactions between our Sun’s habitable zone and the far reaches of the system, for as the planets gradually cooled, the carbon so necessary for life as we know it would have been available only far from the Sun.
How much of a factor were early comets in bringing carbon into the inner system? This question underlies new work by Charles Woodward and colleagues. Woodward (University of Minnesota Twin Cities / Minnesota Institute of Astrophysics) focuses on Comet Catalina, which was discovered in early 2016. He sees carbon in the context of life:
“Carbon is key to learning about the origins of life. We’re still not sure if Earth could have trapped enough carbon on its own during its formation, so carbon-rich comets could have been an important source delivering this essential element that led to life as we know it.”
Image: Illustration of a comet from the Oort Cloud as it passes through the inner Solar System with dust and gas evaporating into its tail. SOFIA’s observations of Comet Catalina reveal that it is carbon-rich, suggesting that comets delivered carbon to the terrestrial planets like Earth and Mars as they formed in the early system. Credit: NASA/SOFIA/ Lynette Cook.
Let’s zoom in on this a little more closely. Volatile ices of water, carbon monoxide and carbon dioxide are found mixing with dust grains in the outer system, an indication that the young Solar System beyond the snowline was, in the authors’ words, “not entirely ‘primordial’ but was ‘polluted’ with the processed materials from the inner disk, the ‘hot nebular product.'” Or to slip the metaphor slightly, we can say that comets were salted with materials that were originally produced at higher temperatures. Comets can offer a window into this process.
The work is anything but straightforward, for although we’ve learned a lot through missions like Giotto, Rosetta/Philae and Deep Impact (including, of course, abundant telescope observations from Earth and a sample return mission called Stardust), the interplanetary dust particles we’ve been able to analyze from comets 81P/Wild 2 and 26P/Grigg-Skjellerup differ considerably. The paper explains:
The former contains material processed at high temperature (Zolensky et al. 2006), while the latter is very “primitive” (Busemann et al. 2009). For these reasons, it is necessary to determine as best as we can the properties of dust grains from a large sample of comets using remote techniques (Cochran et al. 2015). These include observations of both the thermal (spectrophotometric) and scattered light (spectrophotometric and polarimetric). The former technique provides our most direct link to the composition (mineral content) of the grains.
The research team drew on data from the Stratospheric Observatory for Infrared Astronomy (SOFIA), a Boeing 747 aircraft carrying a 2.7-meter reflecting telescope with an effective diameter of 2.5 meters. At altitude (SOFIA generally operates between 38,000 and 45,000 feet), the observatory is above 99 percent of Earth’s atmosphere, which can block infrared wavelengths. SOFIA data show Catalina as a carbon-rich object.
The paper points out that carbon dominates as well in other comets we’ve seen, both those in closer orbits (103P/Hartley 2) and Oort Cloud comets like C/2007 N3 and C/2001 HT50. It also turns out that dusty material from comet 67P/Churyumov-Gerasimenko was rich in carbon, although the authors note that comets can show changes in their silicate-to-carbon ratio, sometimes even during the course of a single night’s observations. The paper adds:
A dark refractory carbonaceous material darkens and reddens the surface of the nucleus of 67P/Churyumov-Gerasimenko. Comet C/2013 US10 (Catalina) is carbon rich. Analysis of comet C/2013 US10 (Catalina)’s grain composition and observed infrared spectral features compared to interplanetary dust particles, chondritic materials, and Stardust samples suggest that the dark carbonaceous material is well represented by the optical properties of amorphous carbon. We argue that this dark material is endemic to comets.
All this suggests that carbon delivered by comets is a part of the evolution of the early Solar System. Each carbon-rich comet we study has implications for how life may have been spurred by impacts, making the investigation of carbon-rich Oort Cloud comets a continuing priority for SOFIA, which can be deployed quickly when comets are found entering the inner system.
The paper is Woodward et al, “The Coma Dust of Comet C/2013 US10 (Catalina): A Window into Carbon in the Solar System,” The Planetary Science Journal (2021). Abstract / Full Text.
by Paul Gilster | Mar 4, 2021 | Deep Sky Astronomy & Telescopes |
As we follow the progress of the James Webb Space Telescope through performance tests in preparation for launch, Robert Zubrin has been thinking of far larger instruments. The president of Pioneer Astronautics and founder of the Mars Society thinks we can create telescopes of extremely large aperture — and sharply lower cost — by using the physics of spinning gossamer membranes, a method suitable for early testing as a CubeSat demonstration mission. In today’s essay, Dr. Zubrin explains the concept and considers how best to deploy next generation space telescopes reaching apertures as large as 1000 meters. We can’t know what new phenomena such an instrument would find, but the Enormous Space Telescope fits the theme of breakthrough discovery outlined in his latest book, The Case for Space: How the Revolution in Spaceflight Opens Up a Future of Limitless Possibility (Prometheus, 2020).
by Robert Zubrin
Abstract
This paper presents a method for creating Enormous Space Telescopes (ESTs). The EST employs a hoop to deploy a slack reflector membrane, such as solar sail material or radio dish. When the EST is simultaneously rotated around its center and accelerated along its axis of rotation, the membrane will assume a parabolic shape, thereby creating a reflector for a very large aperture telescope. The EST reflector can be accelerated along its linear axis by tethering its deployment hoop to a tug spacecraft. The tug can exert force on the hoop several methods, including direct thrust, centrifugal rotation of the tethered tug-reflector assembly, or by lowering the reflector from a high altitude balloon or more massive tug positioned in a higher orbit. A force equivalent to linear acceleration can also be generated to shape an EST without a tug using electrostatic means. ESTs can be used for astronomy across a wide spectrum of frequencies, ranging from the ultraviolet, through optical and infrared, down to radio. A demonstration EST with an aperture larger than the Webb Space Telescope could be flown on a CubeSat mission in low Earth orbit. ESTs with apertures of hundreds of meters could be delivered to heliocentric space in single flights of existing launch vehicles.
Background
There is no better place to do astronomy than space. Therefore, since the dawn of the space age, it has been the ardent ambition of astronomers to place ever more capable telescopes there. The largest such operational instrument, the 2.4 m diameter aperture Hubble Space Telescope, has benefitted from its location above the Earth’s atmosphere to make many great discoveries, and astronomers hold high hopes for more breakthroughs from the long-awaited 6.5 m diameter Webb Space Telescope. As the light gathering power of a telescope increases with the square of their aperture, still larger space telescopes are greatly to be desired. However, as the cost (>$10 billion) and quarter century long development schedule of the Webb telescope have demonstrated, new techniques will be required if construction of much larger observatories is to be made practical. This is the purpose of the Enormous Space Telescope (EST) concept.
The Enormous Space Telescope (EST)
The EST exploits the principle that if a flexible material subject to a linear acceleration is spun, the balance of linear acceleration and centrifugal acceleration forces will shape the material into a parabolic geometry. This technique has been used on Earth to spin cast liquid glass into parabolic dishes for use in telescopes up to several meters in diameter. The EST can employ similar physics with a properly tailored sheet of gossamer material in space to create parabolic reflector dishes with dimensions of hundreds of meters while keeping system masses well within existing launch vehicle limits.
In order to understand how the EST works, let us start by considering it in its smallest and possibly initial form, as a CubeSat demonstration mission. Consider a 13 kg, 12U CubeSat in a circular orbit 400 km above Earth. A one kilometer long tether is extended down from the satellite, and used to suspend a 13 m diameter (twice that of Webb) hoop, whose central axis aligns with the tether. Lines from the circumference of the hoop attach to the tether by a frictionless magnetic bearing, allowing the hoop to rotate freely. The interior of the hoop contains a slack solar sail material, properly tailored to accept a parabolic shape without folds, which is attached to the hoop like the skin of a slack-topped drum. Aluminized balloon film can be used to create solar sail material with a mass density of 6 grams/m2. Taking the hoop mass into account, we will assume 10 gm/m2 as the net mass density for our hoop/film combination, resulting in a mass estimate of 1.3 kg for that subsystem.
At an altitude of 400 km, the CubeSat will be moving with a velocity of 7668.63 m/s, generating a centrifugal acceleration of 8.6762 m/s2, exactly matching the Earth’s gravitational acceleration at that altitude. The reflector, however, hanging 1 km below the CubeSat will only be moving at 7667.50 km/s, generating a centrifugal acceleration of 8.675 m/s2. The Earth’s gravitational acceleration at that altitude will be 8.679 m/s. Thus the hoop will experience a downward acceleration of 0.004 m/s2, or 0.4 milliGees. This will make the sail film in the hoop sag. But if we rotate the hoop with an edge velocity of 0.1 m/s, the film material will also experience an outward acceleration, ranging from 0 at its center to 0.0015 m/s2 at its edge. Taken in combination with the linear acceleration, this will shape the film into a perfect parabola.
Fig. 1 An EST suspended by a tether in LEO. The telescope parabolic dish is spinning around the axis of the tether. Earth (down) is on the right.
This little demonstration EST, with a total mass less than 20 kg, including optics that would be positioned along or suspended from the tether at the parabola focal point, would have four times the light gathering capacity of Webb (about thirty times that of Hubble), while costing on the order of 1/1000th as much.
An even cheaper flight demonstration could be done suspending an EST from a high altitude balloon. Since a balloon moves with the wind, the payload would feel no wind. At 100,000 ft it would be above 99% of Earth’s atmosphere. A triangle of long spars could be employed with a balloon attached to each vertex, to keep the balloons out of the field of view of the telescope.
Such systems would have limitations, since they would be constantly pointing directly away from the center of the Earth. But we can do better.
Let us therefore scale our unit up in diameter by a factor of ten, to a 130 m diameter reflector dish, increasing the mass of the hoop, the optics and spacecraft by a factor of 100. It would still be a quite manageable mass though, about 2000 kg, easily launchable into interplanetary space by a Falcon 9 medium lift booster. In this case. there would be no gravity gradient available to stretch the tether. So we need to use an alternative technique.
One approach might be to spin the hoop around the spacecraft, in the manner of a tethered artificial gravity system, having a second hoop counter-rotating along with the one suspending the dish in order to neutralize gyroscopic effects. But such a system would still need to constantly change its pointing direction, making long duration exposures impossible.
Tugs for ESTs
A more effective approach would be to simply employ a spacecraft as a tug. Sunlight has a pressure of 9 micronewtons per square meter, which would add up to 0.12 Newtons over the whole body of the 130 m diameter sail. If that were the only linear acceleration of the sail, it would shape it into a parabolic reflector with its concave side pointing towards the Sun. As we want to be able to point the telescope the other way, we need to generate more thrust than that. This could be done using either electric propulsion or larger solar sails with a lower mass density then the hoop, or magnetic or electric sails, pulling its tether outward from the Sun.
Let us first consider electric propulsion. If we had an 70% efficient ion engine using argon propellant and a Isp of 7000 s, 50 kWe would be required to produce 1 N of thrust. Assuming a typical solar electric propulsion system mass to power ratio of 20 kg/kWe, that would require 1000 kg. The tug would thus accelerate at a rate of 0.001 m/s2. If the reflector was made of sail material with the minimum mass density of 6 gm/m2, its material would self-accelerate away from the Sun with an acceleration of 9e-6/0.006 =0.0015 m/s2, which is greater than the self-acceleration of the tug, and therefore unsatisfactory. However, the remedy for this is simple: just make the reflector material much thicker. For example, if we tripled its thickness to 18 gm/m2, its self-acceleration would only be 0.0005 m/s2, i.e. half that of the tug. So it would lag behind the tug and the net pull on it of 0.0005 m/s2 would make its center sag back towards the Sun. If we then set it spinning with a velocity of 0.1 m/s at its edge, an edge centrifugal acceleration of 0.00015 m/s2 would be created, shaping it into a 130 m diameter parabolic dish.
Fig. 2 Electric Propulsion tug pulling on an EST.
Operating at 1 N thrust, the thruster would consume 0.014 gm/s of propellant, or about 1.2 kg per day of thrusting (i.e. observing time). Thrust and thus propellant requirements would drop if the telescope were positioned further out in the solar system, since solar light pressure would drop as the inverse square of the telescope’s distance from the Sun. Thus, for example at 3.1 AU, it would only need to use 0.12 kg/day of propellant to generate adequate acceleration.
We could also use solar sails as tugs. In this case no propellant would be needed. Positioning the tug behind the EST would allow it to eclipse solar pressure, as shown if Fig. 3. If the tug is pulling the EST, making tug acceleration greater than reflector material self-acceleration could be assured simply by having the tug sails be larger than the reflector sail, and using a heavy gauge material for the reflector sail.
Fig. 3 Using solar sail tugs to accelerate an EST, by pushing from behind. The EST spins around the central axis. The Sun is on the left. An alternative design would send the mast through the sunward pusher sail, allowing it to deliver its thrust to the base of the mast by a set of shrouds.
A pusher sail telescope would need to point (generally, but not necessarily exactly) outward from the Sun all the time. However if a nuclear electric tug were used, and the telescope were positioned in the shadow of a planet, sunlight impinging on the rear side of the reflector would not be an issue and the telescope could be pointed in any direction.
In the case of radio telescopes, all of this becomes much easier, as there would be no solar light pressure on the rear face of the dish. In that case any kind of tug – solar electric, nuclear electric, or solar sail- could be used, with the EST pointable in any direction simply by maneuvering the tug. The amount of acceleration required from the tug could also be much less.
The Electrostatic EST
An alternative to physical acceleration to impose linear force on the dish is to use electrostatic attraction, In this case the reflector sail would be charged one way, while another sail positioned behind it and held off at a distance by a structural system would be given an opposite charge. The sails would thus attract each other, much as if by gravity, and when the assembly was spun up, both sails would assume parabolic shapes, with their concave sides pointing in opposite directions.
Let us consider the case of two 50 m radius dishes held 25 m apart by structure, with a potential difference between the two of 10 kV, creating a field of 400 volts/m. From electrostatics we have EA = Q/?, so Q, the charge on each dish will be given by Q=(400)(7854 m2)(8.85e-12) = 2.8e-5 coulombs. The electrostatic force on each sail will be given by F=QE, so the total electrostatic force between the sails will be F=400(2.8e-5) = 0.0112 N. Assuming the sail materials have a mass density of 6 gm/m2, this will result in a self-acceleration each sail towards the other of 0.0112/(0,006)(7854) = 0.00024 m/s2. It may be observed that the field will actually be greater near the center because the dishes would sag towards each other. This, however, could be compensated for by varying the thickness of the sail material, making it thicker towards the center and thinner towards the edge, thereby keeping the linear self-acceleration of the two sails towards each other equal over their entire surfaces.
Fig. 4. An Electrostatic EST. The sails have opposite charges and are held separate from each other by a compressive structure. The mutual attraction of the sails can substitute for linear acceleration of the system
Size Limits of ESTs
There does not seem to be any theoretical limit to the potential size of an EST. However, as we have seen, using current materials, the mass required the create an EST system goes approximately as:
Where M is the EST system mass in kilograms and R is the aperture radius in meters. Thus our 6.5 m radius EST demo unit has an estimated mass, including its associated spacecraft, on the order of 20 kg, while our 65 m radius operational EST would be expected to have a mass on the order of 2000 kg.
Currently the largest operational launch vehicle is the Falcon Heavy, with a capability of about 60,000 kg to low Earth orbit (LEO). More powerful vehicles, including the NASA SLS and the SpaceX Starship system are expected to become operational within the next few years, with capabilities of up to 120,000 kilograms to LEO. Since an EST tug could propel itself out of LEO and into heliocentric space, this may also be taken as the limit of the size of an EST system, deliverable into space with a single launch. If we plus 120,000 kg into equation (1), we find that a practical size limit for relatively near-term EST systems would be an aperture diameter of about 1000 meters. The discoveries that might be enabled by such systems are beyond reckoning.
Conclusion
We find that the EST concept offers a practical path towards creating space telescopes with capabilities dwarfing conventional systems by many orders of magnitude. We also find that ESTs could be used to create space telescopes with comparable capabilities to conventional systems, but with several orders of magnitude lower cost. Furthermore, the EST concept holds these benefits for space astronomy across a wide range of frequencies, from ultraviolet down to radio. We therefore recommend that the concept be studied further, and that a demonstration mission be flown at an early date.
Acknowledgement; The author wishes to acknowledge the assistance of Heather Rose, who provided the illustrations for this paper.
by Paul Gilster | Mar 3, 2021 | Outer Solar System |
The orbital interactions between objects in a stellar system result in all kinds of interesting effects, a celestial pinball machine that sometimes flings planets outward to wander alone among the stars. Gas giants can be pulled from more distant orbits into a broiling proximity to their star. But the object known as P/2019 LD2 has a special interest because its interactions are happening in a tight time frame even as we observe them.
We could call P/2019 LD2 a ‘comet-like object,’ because it sometimes acts like an asteroid, sometimes like a comet. It is in fact a Centaur, one of that group of outer system objects that only become active as they move into the inner system. We’re watching a transition from Centaur to Jupiter family comet mediated by the gradually warming environment. This one evidently swung close to Jupiter roughly two years ago, to be flung by the giant planet’s gravity toward the Trojan asteroid group that leads Jupiter in its orbit by some 700 million kilometers.
At least that’s the thinking of lead author Bryce Bolin (Caltech), who used Hubble imagery as a follow-up to Spitzer data by way of identifying comet-like activity on P/2019 LD2. The Hubble work showed a cometary tail some 650,000 kilometers long, and could also resolve features near the nucleus at high resolution. The object’s coma and jets are clearly visible. Bolin believes that the gravitational interactions that put LD2 where it is today are comparatively rare:
“The visitor had to have come into the orbit of Jupiter at just the right trajectory to have this kind of configuration that gives it the appearance of sharing its orbit with the planet. We’re investigating how it was captured by Jupiter and landed among the Trojans. But we think it could be related to the fact that it had a somewhat close encounter with Jupiter.”
Image: NASA’s Hubble Space Telescope snapped this image of the young comet P/2019 LD2 as it orbits near Jupiter’s captured ancient asteroids, which are called Trojans. The Hubble view reveals a 650,000-kilometer-long tail of dust and gas flowing from the wayward comet’s bright solid nucleus. Credit: NASA/ESA/J. Olmsted/STScI.
So this is a new interaction involving an object that was itself discovered only in June of 2019 by the University of Hawaii’s Asteroid Terrestrial-impact Last Alert System (ATLAS) telescopes and then identified in archival data from the Zwicky Transient Facility at Palomar Observatory. Moreover, this particular interaction is fleeting, because Bolin and team used computer simulations to show that another close encounter with Jupiter will occur in two years, one that should push the comet away from the Trojans and fling it into the inner Solar System.
Assuming LD2 has its origin in the Kuiper Belt, it would have been bumped out of its location there by other gravitational interactions with another KBO, warming as it moved closer to the Sun in a process thought to nudge a new short-period comet inward about once every century. LD2 also reminds us how we proceed with the identification of an object as a comet. This one began to show outgassing activity fully 750 million kilometers away from the Sun, which is interesting because at that distance water ice is only beginning to be able to sublimate.
That would imply that outgassing in the form of jets escaping from the nucleus is caused by carbon monoxide and carbon dioxide, which can be converted into gaseous form at lower temperatures. The team’s observations using Spitzer identified gas and dust around the nucleus and motivated the Hubble investigation at visible light wavelengths, as did contact from Japanese amateur astronomer Seiichi Yoshida, who had also seen activity on the object.
Image: The main asteroid belt lies between Mars and Jupiter, whereas Trojan asteroids both lead and follow Jupiter. Credit: NASA/ESA/J. Olmsted/STScI.
Where to from here? Exiting the Trojans and interacting again with Jupiter in two years, the object will likely head for deep space. But that could take some time. Carey Lisse of the Johns Hopkins University Applied Physics Laboratory (APL) describes the possibilities:
“Short-period comets like LD2 meet their fate by being thrown into the Sun and totally disintegrating, hitting a planet, or venturing too close to Jupiter once again and getting thrown out of the solar system, which is the usual fate. Simulations show that in about 500,000 years, there’s a 90% probability that this object will be ejected from the solar system and become an interstellar comet.”
But we should keep in mind what happens with comets as they move into warmer regions — they begin to change. Let me quote the paper on this:
Another consequence of the increased heating from closer proximity of the Sun is that large-scale ablation of the comet’s structure due to thermal stress can occur resulting in it becoming partially or completely disrupted (Fernandez 2009). Since P/2019 LD2 is now in transition between the Centaur and Jupiter Family Comet populations, it seems likely that it has become active for the first time, and as such, its activity will be rapidly evolving in response to the new epoch of increased Solar heating.
It will be fascinating to see that change as LD2 evolves, reminding us of the activity of icy objects nudged out of their distant Kuiper Belt orbits into the giant planet interactions that await them, a process that can take several million years as they move toward the inner system.
The paper is Bolin et al., “Initial Characterization of Active Transitioning Centaur, P/2019 LD2 (ATLAS), Using Hubble, Spitzer, ZTF, Keck, Apache Point Observatory, and GROWTH Visible and Infrared Imaging and Spectroscopy,” Astronomical Journal Vol. 161, No. 3 (11 February 2021). Abstract / Preprint.
