Centauri Dreams
Imagining and Planning Interstellar Exploration
The Physics of Starship Catastrophe
Now that gravitational wave astronomy is a viable means of investigating the cosmos, we’re capable of studying extreme events like the merger of black holes and even neutron stars. Anything that generates ripples in spacetime large enough to spot is fair game, and that would include supernovae events and individual neutron stars with surface irregularities. If we really want to push the envelope, we could conceivably detect the proposed defects in spacetime called cosmic strings, which may or may not have been formed in the early universe.
The latter is an intriguing thought, a conceivably observable one-dimensional relic of phase transitions from the beginning of the cosmos that would be on the order of the Planck length (about 10-35 meters) in width but lengthy enough to encompass light years. Oscillations in these strings, if indeed they exist, would theoretically generate gravitational waves that could be involved in the large-scale structure of the universe. Because new physics could well lurk in any detection, cosmic strings remain a tantalizing subject for speculation in gravitational wave astronomy.
Remember the resources that are coming into play in this field. In addition to LIGO (Laser Interferometer Gravitational-Wave Observatory), we have KAGRA (Kamioka Gravitational Wave Detector) in Japan and Virgo (VIRgo interferometer for Gravitational-wave Observations) in Italy. The LISA observatory (Laser Interferometer Space Antenna) is currently scheduled for a launch some time in the 2030s.
For that matter, could a cosmic string be detected in other ways? One possibility is in any signature it might leave in the cosmic microwave background (CMB). Another, and this seems promising, is the potential for gravitational lensing as light from background objects travels through the distorted spacetime produced by the string. That would be an interesting signature to find, and indeed, one of the exciting aspects of gravitational wave astronomy is speculation on what new phenomena it would allow us to detect.
As witness a new paper from Katy Clough (Queen Mary University, London) and colleagues, who ask whether an artificial gravitational event could generate a signal that an observatory like LIGO could detect. Now we nudge comfortably into science fiction, for at issue is what would happen if a starship powered by a warp drive were to experience a malfunction. Given the curvature of spacetime induced by an Alcubierre-style drive, a problem in its operations could be detectable, although not, the team points out, at the frequencies currently observed by LIGO.
An Alcubierre warp drive would produce a spacetime that is truly exotic, but one that can be described within the theory of General Relativity. The speed of light is never exceeded by our starship, thus satisfying Special Relativity, but a craft that can contract spacetime in front of it and expand spacetime behind it would theoretically cross distances faster than the speed of light as witnessed by an outside observer.
Huge problems would be created by such a craft, including some that may be insurmountable. It seems to violate what is known as the Null Energy Condition, for one thing, which demands negative energy seemingly not allowed in standard theories of spacetime. But the authors note that “The requirement that warp drives violate the NEC may be considered a practical rather than fundamental barrier to their construction since NEC violation can be achieved by quantum effects and effective descriptions of modifications to gravity, albeit subject to quantum inequality bounds and other semiclassical considerations that seem likely to prove problematic.”
Image: Two-dimensional visualization of an Alcubierre drive, showing the opposing regions of expanding and contracting spacetime that displace the central region. Credit: AllenMcC., CC BY-SA 3.0
Problematic is a useful word, and it seems appropriate here. It’s also appropriate when we consider that a functioning warp drive raises paradoxical issues with regard to time travel, allowing closed time-like curves (in other words, the possibility of traveling into the past, with all the headaches that causes for causality and our view of reality). That puts us in the realm of rotating black holes and wormholes, powerful gravitational wave generators. The authors also point out that a warp drive would be a difficult thing to control and deactivate, as Miguel Alcubierre himself pointed out in a 2017 paper.
So how would we detect a starship of this variety? The authors note that at constant velocity, an Alcubierre drive spacecraft would not generate gravitational waves, but interesting phenomena would be observed if the drive bubble were to collapse, accelerate or decelerate:
There is (to our knowledge) no known equation of state that would maintain the warp drive metric in a stable configuration over time – therefore, whilst one can require that initially, the warp bubble is constant, it will quickly evolve away from that state and, in most cases, the warp fluid and spacetime deformations will disperse or collapse into a central point….This instability, whilst undesirable for the warp ship’s occupants, gives rise to the possibility of generating gravitational waves.
In other words, a working warp drive craft may well be undetectable, but a prototype that fails could throw an observable signature. The paper homes in on the collapse of a warp drive bubble, which could be created by the breakdown of the containment field that the makers of the starship use to support it. So we have a potential gravitational wave signature for a technological catastrophe as an advanced civilization experiments with the distortion of spacetime for interstellar travel.
Such events are presumably rare. I’m reminded of Greg Benford’s story “Bow Shock,” in which as astronomer studying what he thinks is a runaway neutron star – “a faint finger in maps centered on the plane of the galaxy, just a dim scratch” – is in fact a technological object. Here’s a clip:
“What you wrote,” she said wonderingly. “It’s a…star ship?”
“Was. It got into trouble of some kind these last few days. That’s why the wake behind it – ” he tapped the Fantis’ image – “got longer. Then, hours later, it got turbulent, and—it exploded.”
She sipped her coffee. “This is…was…light years away?”
“Yes, and headed somewhere else. It was sending out a regular beamed transmission, one that swept around as the ship rotated, every 47 seconds.”
Her eyes widened. “You’re sure?”
“Let’s say it’s a working hypothesis.”
Great scenario for a science fiction story, and there are a number of papers on starship detection from other angles in the scientific literature. In Benford’s case, the starship is thought to be of the Bussard ramjet variety, definitely not moving through warp drive methods. All this reminds me that a survey of starship detection papers is overdue in these pages, and I’ll plan to get to that in coming weeks. But back to warp drives.
Let’s assume things occasionally go wrong at whatever level of technology we’re looking at. We’re witnessing SpaceX actively developing Starship, a craft that gets a little better, and sometimes a lot better, each time it is launched, but development is hard and there are errors along the way. Throw an error into an Alcubierre-style starship and gravitational effects should show up involving nasty tidal outcomes.
To investigate these, Clough and colleagues develop a structured framework to simulate warp bubble collapse and analyze the gravitational wave signatures that would be produced at the point of collapse. Other types of signal may also be produced, but the paper notes: “Since we do not know the type of matter used to construct the warp ship, we do not know whether it would interact (apart from gravitationally) with normal matter as it propagates through the Universe.”
We don’t have equipment tuned to pick up such signals. We have the needed sensitivity in observatories like LIGO, but we would need to tune it to a different range of gravitational waves. The paper continues:
…for a 1km-sized ship, the frequency of the signal is much higher than the range probed by existing detectors, and so current observations cannot constrain the occurrence of such events. However, the amplitude of the strain signal would be significant for any such event within our galaxy and even beyond, and so within the reach of future detectors targeting higher frequencies… We caution that the waveforms obtained are likely to be highly specific to the model employed, which has several known theoretical problems, as discussed in the Introduction. Further work would be required to understand how generic the signatures are, and properly characterise their detectability.
A funding request to study starships undergoing catastrophic failure is going to be a tough sell. But probing the question produces the formalism developed by the Clough team and gives us further insights into warp drive prospects. Fascinating.
The paper is Clough et al, “What no one has seen before: gravitational waveforms from warp drive collapse” (preprint).
An X-Ray Study of Exoplanet Habitability
Great observatories work together to stretch the boundaries of what is possible for each. Data from the Chandra X-ray Observatory were used in tandem with the James Webb Space Telescope, for example, to observe the death of a star as it was consumed by a black hole. JWST’s infrared look at this Tidal Disruption Event (TDE) helped show the structure of stellar debris in the accretion disk of the black hole, while Chandra charted the high-energy processes at play in the cataclysmic event.
Or have a look at the image below, combining X-ray and infrared data from these two instruments along with the European Space Agency’s XMM-Newton, the Spitzer Space Telescope and optical data from Hubble and the European Southern Observatory’s New Technology Telescope to study a range of targets.
Image: Four composite images deliver dazzling views from NASA’s Chandra X-ray Observatory and James Webb Space Telescope of two galaxies, a nebula, and a star cluster. Each image combines Chandra’s X-rays — a form of high-energy light — with infrared data from previously released Webb images, both of which are invisible to the unaided eye. Data from NASA’s Hubble Space Telescope (optical light) and retired Spitzer Space Telescope (infrared), plus the European Space Agency’s XMM-Newton (X-ray) and the European Southern Observatory’s New Technology Telescope (optical) is also used. These cosmic wonders and details are made available by mapping the data to colors that humans can perceive. Credit: X-ray: Chandra: NASA/CXC/SAO, XMM: ESA/XMM-Newton; IR: JWST: NASA/ESA/CSA/STScI, Spitzer: NASA/JPL/CalTech; Optical: Hubble: NASA/ESA/STScI, ESO; Image Processing: L. Frattare, J. Major, and K. Arcand.
Working at multiple wavelengths obviously pays dividends, and with Chandra in the news because of proposed budget cuts, it’s worth noting that its observations have a role to play in the search for habitable worlds in our own stellar neighborhood. Currently the observatory is being used in conjunction with XMM-Newton in an ongoing exploration of habitability in terms of radiation. Which stars close enough to Earth for us to image planets in their habitable zones are also benign in terms of the radiation bath to which these same planets would be exposed?
High levels of ultraviolet and X-ray radiation can break chemical bonds and damage DNA in biological systems, not to mention their effect in stripping away planetary atmospheres. Thus ionizing radiation becomes a key factor in habitability in the kind of planets whose atmospheres we are first going to be able to study, those orbiting nearby M-dwarfs where the habitable zone is dauntingly close to the parent star.
The new study is led by Breanna Binder (California State Polytechnic University), who told the American Astronomical Society’s recent meeting in Madison, Wisconsin that of some 200 promising target stars, only a third have previously been examined by X-ray telescopes. Of these, many appeared quiet at visible wavelengths but quite active in X-ray emissions. Clearly we need to learn more about this critical variable.
With a generation of Extremely Large Telescopes soon to come online and future space observatories like the Habitable Worlds Observatory in the works, we will be able to take apart light from individual planets in search of biosignatures. Binder’s team will try to identify stars where the X-ray background is not dissimilar to Earth’s, so that the chances of life evolving within a protective atmosphere are enhanced. Ten days of Chandra observations along with 26 days of data from XMM-Newton were collected to home in on the X-ray behavior of some 57 nearby stars, ranging from 4.2 to 62 light years away.
This continuing effort is in a sense a shot in the dark, for while some of the stars are known to have planets, many are not. The necessary geometry of planetary transits means that many stars are likely orbited by habitable zone planets we can’t yet detect, and radial velocity methods are likewise not tuned for planets of this size. Edward Schwieterman (University of California Riverside) points to what comes next:
“We don’t know how many planets similar to Earth will be discovered in images with the next generation of telescopes, but we do know that observing time on them will be precious and extremely difficult to obtain. These X-ray data are helping to refine and prioritize the list of targets and may allow the first image of a planet similar to Earth to be obtained more quickly.”
Image: A three-dimensional map of stars near the Sun that are close enough to Earth for planets in their habitable zones to be directly imaged using future telescopes. Planets in their stars’ habitable zones likely have liquid water on their surfaces. A study with Chandra and XMM-Newton of some of these stars (shown in blue haloes) indicates those that would most likely have habitable exoplanets around them based on a second condition — whether they receive lethal radiation from the stars they orbit. Credit: Cal Poly Pomona/B. Binder; Illustration: NASA/CXC/M.Weiss.
More on this continuing study as results become available. Meanwhile, note in relation to the budgetary woes Chandra is experiencing that a letter to NASA administrator Bill Nelson went out on June 6 from several U.S. senators and members of the House of Representatives urging that restrictions on its funding be rescinded. The letter bears all too few signatures, but it’s at least a step in the right direction. Quoting from it:
Premature termination of the Chandra mission would jeopardize this critical workforce, potentially driving talent to other countries. Chandra should serve as a bridge to a promising future in high energy astrophysics at NASA, including the development of its eventual flagship-scale successor, as recommended by the 2020 Decadal Survey in Astronomy and Astrophysics.
We strongly urge NASA to maintain full FY25 funding for the Chandra mission at the $68.7 million level, as outlined in NASA’s FY24 budget request, and to halt plans for significant reductions in FY25 until Congress determines Chandra’s appropriations. The proposed budget cuts would cause damage to U.S. leadership in high energy astrophysics and prematurely end the mission of a national treasure whose most significant discoveries may still be ahead.
Shutting Down Chandra: Will We Lose Our Best Window into the X-ray Universe?
Our recent discussions of X-ray beaming to propel interstellar lightsails seem a good segue into Don Wilkins’ thoughts on the Chandra mission. Chandra, of course, is not a deep space probe but an observatory, and a revolutionary one at that, with the capability of working at the X-ray wavelengths that allow us to explore supernovae remnants, pulsars and black holes, as well as making observations that advance our investigation of dark matter and dark energy. This great instrument swims into focus today because it faces a funding challenge that may result in its shutdown. It’s a good time, then, to take a look at what Chandra has given us since launch, and to consider its significance as efforts to save the mission continue. We should get behind this effort. Let’s save Chandra.
by Don Wilkins
On July 23, 1999, the Chandra X-ray Observatory deployed from Space Shuttle Columbia. Chandra along with the Hubble Space Telescope, Spitzer Space Telescope (decommissioned when its liquid helium ran out) and Compton Gamma Ray Observatory (de-orbited after gyroscope failure), composed NASA’s fleet of ‘Great Observatories,’ so labeled because of their coverage from gamma rays through X-rays, visible light and into the infrared, spanning the electromagnetic spectrum. This scientific fleet expanded – and continues to expand – our understanding of the cosmos.
A two-stage Inertial Upper Stage booster put the observatory into an elliptical orbit. Apogee is approximately 139,000 kilometers (86,500 miles) — more than a third of the distance to the Moon – with perigee at 16,000 kilometers (9,942 miles) and an orbital period of 64 hours and 18 minutes. The numbers are significant: The consequence of this orbit is that about 85 percent of the time, Chandra orbits above the charged particle belts surrounding the Earth, allowing periods of uninterrupted observing time up to 55 hours.
Figure 1. The Chandra Observatory monitors X-rays, a critical and unique window into the hottest and most energetic places in the Universe. Credit: NASA.
A Huge Return on Investment
Chandra’s achievements include:
- The first image of the compact object, possibly a neutron star or black hole, at the center of the supernova remnant Cassiopeia A.
- The detection of jets from and a ring around the Crab Nebula.
- The detection of X-ray emitting loops, rings and filaments encircling Messier 87’s supermassive black hole.
- The discovery, by high school students, of a neutron star in the supernova remnant IC 443.
- The detection of X-ray emissions from Sagittarius A*, the black hole at the center of the Milky Way. Chandra observed a Sagittarius A* X-ray flare 400 times brighter than normal, and found a large halo of hot gas surrounding the Milky Way.
- The X-ray observation of a supernova, SN 1987A, shock wave.
- The observation of streams of high-energy particles light years in length flowing from neutron stars.
- Observations of planets within the Solar System found X-rays associated with Uranus that may not be the reflection of solar X-rays. Chandra also found that Jupiter’s X-ray emissions occur at its poles, and it detected X-ray emissions from Pluto.
- Found a dip in X-ray intensity as the planet HD 189733b transited its parent star. This is the first time a transit of an exoplanet was observed using X-rays.
- Detected the shadow of a small galaxy as it is cannibalized by a larger one.
- Found a mid-mass black hole, between stellar-sized black holes and galactic core black holes, in M82.
- Associated X-rays with the gamma ray burst GRB 91216 (Beethoven Burst). Chandra observations also associated X-rays with a gravitational source (Figure 2).
- Produced controversial data suggesting RX J1856.5-3754 and 3C58 are quark stars.
- Measured the Hubble constant as 76.9 km/s per megaparsec (a megaparsec is equal to 3.26 million light years).
- Found strong evidence dark matter exists by observing super cluster collisions. Observations placed limits on dark matter self-interaction cross-sections.
- Combined with the outputs of other telescopes yielded insights into astronomical phenomena, Figure 3.
Building Chandra was a significant technical challenge. Mirror alignment accuracy, from one end of the mirror assembly to the other, a distance of 2.7 meters or 9 feet, is accurate to 1.3 micrometers (50 millionths of an inch). X-rays are focused using complex geometrical shapes which must be precisely formed and kept ultraclean. Even today, no X-ray telescope provides the sharpness of image that Chandra does.
Figure 2 Chandra made the first X-ray detection of a gravitational wave source. The inset shows both the Chandra non-detection, or upper limit, of X-rays from GW170817 and its subsequent detection. The main panel is the Hubble image of NGC 4993. Credit: NASA.
Figure 3. The image is a combination of the outputs of three telescopes. Webb highlights infrared emission from dust; Chandra the X-ray emissions and Hubble shows stars in the field. Credit: NASA/CXC/SAO
Chandra’s Demise?
This list of discoveries may be ending. Although it is estimated Chandra has another decade of operation, proposed budget cuts eliminate the $74 million necessary for Chandra operation, maintenance and analysis of data. Unless the funding is restored by way of a budget line item directing NASA to spend the funds on Chandra, the agency’s X-ray observation is stopped for years until a new telescope can be brought on line. The skills of highly talented scientists and engineers working on Chandra will be dissipated.
The problem is significant. NASA’s own NuStar instrument (Nuclear Spectroscopic Telescope Array) and the joint NASA/JAXA XRISM effort (X-Ray Imaging and Spectroscopy Mission) have their virtues, as does the European Space Agency’s XMM-Newton (X-ray Multi-Mirror Mission). But Chandra’s eyes are sharper, with imagery 10 times crisper than XMM-Newton’s, 30 times sharper than NuStar, and fully 150 times sharper than XRISM.
A possible successor is in the planning stages. With 100 times Chandra’s resolution, the Lynx X-ray Observatory could study individual stars more than 16,000 light-years away, or 12.5 times the range of Chandra, using an X-ray Mirror Assembly that is the most powerful X-ray optic ever conceived. Even so, it is probable three decades or more would elapse before the new observatory would orbit and it might well involve significant cost overruns. ESA plans the Athena mission (Advanced Telescope for High ENergy Astrophysics), a large-aperture grazing-incidence X-ray telescope, but Athena would not come online until the late 2030s. In the meantime, we have a healthy Chandra.
Although we are dealing with a 25 year old spacecraft, its efficiency is both high and stable, and enough fuel remains for another decade of operations, while the cost of operations has remained stable for decades. Considered as a bridge to Lynx, Chandra is at the mercy of Congress largely through the FY25 appropriations process. The particulars of how to support this great observatory are available on the Save Chandra website, which is the voice of a grassroots effort within the astronomical community.
Saving Chandra would avert what some are describing as “an extinction-level-event for X-ray astronomy in the US.” An open community letter with 87 pages of signatures recently submitted to NASA Science Leadership strongly supports this statement.
Why X-Rays Can’t Push Interstellar Sails
Although solar sails were making their way into the aerospace journals in the late 1950s, Robert Forward was the first scientist to consider using laser beams rather than sunlight to drive a space sail. That concept, which György Marx picked up on in his 1966 paper, opened the door to interstellar mission concepts. Late in life in an unpublished memoir, Forward recalled reading about Theodore Maiman’s work on lasers at Hughes Research Laboratories, and realizing that this was a way to create a starship. His 1962 article (citation below) laid out the idea for the journal Missiles and Rockets and was later reprinted in Science Digest. Marx surely knew the Forward article and his subsequent paper in Nature probed how to achieve this goal.
Image: One of the great figures of interstellar studies, Robert Forward among many other things introduced and explored the principles of beamed propulsion. Credit: UAH Library Robert L. Forward Collection.
Marx was at that time a professor of theoretical physics at Roland Eötvös University in Budapest. He was plugged into the difficulties of interstellar flight through Les Shepherd’s work in Britain, and he cites the latter’s Realities of Space Travel (1957) in the paper as one of many sources highlighting the depth of the problem. His paper “Interstellar Vehicle Propelled by Terrestrial Laser Beam” is a mere two pages built largely around equations, reporting on the “commonly accepted view that, apart from the technical difficulties involved, the laws of conservation of energy and momentum forbid the visiting of other planetary systems in the human lifespan.”
Marx had already explored energy issues for interstellar flight in a 1960 paper for Astronautica Acta (as it was then known) and a second for the same journal in 1963 (citations below). But the idea of laser beaming offered the physicist a glimmer of hope. In the 1966 paper, he cites the advantages of beamed sailing vs conventional rocket propulsion. The paper argues that “a vehicle can be accelerated almost to the speed of light if an emitter on the Earth can accurately project light onto its mirror.”
The ideal focusing mechanism would be the laser, and it is here that he runs into trouble. For Marx worried about the size of the transmitter aperture, which determines the size and initial beam diameter that will emerge. Remember we’re in the Robert Forward era of maxed out engineering, when the idea was simply to establish what was possible even if it required building capabilities far beyond those of the present day. So here’s what Marx comes up with, the best concept he thought feasible:
…the technical conditions are extremely challenging. A range of operation of 0.1 light year would require a coherently radiating surface of the order of 1 km2 which emits hard X-rays, and the vehicle would need an X-ray mirror with an effective cross-sectional area of several km2.
Marx talks about the absorbed energy of his sail ‘mirror’ radiating out isotropically into space, and here we run into serious problems. I took my questions to Jim Benford, CEO of Microwave Sciences and author of High Power Microwaves, a standard text which is about to go into its fourth edition at CRC Press. Jim is also a regular contributor to Centauri Dreams. And he was quick to point out that X-rays reflect from conducting surfaces in ways that defeat Marx’s purpose.
Image: György Marx in his office. Credit: REAL-I, the Image File Collection of the Academic Library.
As Jim told me, incoming X-rays reflect only at very low grazing angles. The efficiency of energy transfer is at stake here. Here’s a bit more of what he said, using one of the most important formulae in all of modern physics:
X-ray photons have far more energy than visible light or microwaves. Remember the relation E=hν, where E is the energy of the photon, h is Planck’s constant, ν the frequency. X-ray photons have energies about a thousand times that of visible light, a million times that of microwaves. If they come in normal to the surface [i.e., striking the sail head on], they ionize atoms, damaging the lattice of the material.
X-ray telescopes, as a matter of fact, work through a series of grazing incidence reflectors. In other words, we can’t direct Marx’s fantastic X-ray beam toward our sail without seriously damaging it, not unless we are willing to bring the beam to it at such a low angle that the intrinsic power of the beam is largely lost. Benford again:
There’s no way to accelerate a sail with X-rays. The cross-section of the sail must be at a slight angle to the beam, not perpendicular to it, for the X-rays to reflect. That’s hugely inefficient. Grazing incidence means that only the slight transverse component of the photon velocity vector is reversed, leaving the far larger axial component almost unchanged. Little energy is transferred to the inclined sail, and that drives it sideways to the beam, not antiparallel to it, as reflected photons do when they incident normally. So the sail is accelerated very little in the direction of the X-ray beam.
This is the coup de grâce for the X-ray sail. It’s interesting to see what Robert Forward thought of Marx’s idea. Here he is, writing in a 1984 paper called “Roundtrip Interstellar Travel Using Laser-Pushed Lightsails,” which is one of the classics of the field:
The concept of laser-pushed interstellar lightsails was reinvented by Marx in 1966. Since Marx was unwilling to consider a laser aperture greater than 1 km2, he was forced to assume the use of hard x-rays in order to obtain the operational ranges needed for interstellar flight. The impossibility of constructing both an x-ray laser and a lightweight sail to reflect those x-rays led to Marx’s highly pessimistic conclusion about the feasibility of the concept. If Marx had been willing to consider a larger transmitter aperture, then his laser frequencies and sail requirements would have been much easier.
J. L. Redding, then at Bishop’s University in Quebec, saw Marx’s paper and responded to it in the same year, offering corrections to Marx’s equations without challenging the X-ray concept. His telling remark that “…one does not need to consider the difficulties of arranging suitable deceleration and landing facilities” refers to what he saw as the overwhelming problems in making a beamed energy propulsion system work at all. Marx had commented on the deceleration problem and Forward would go on to offer a potential solution in his 1984 paper, one so baroque that it deserves a future post of its own.
I should also mention a little referenced paper by W. E. Moeckel, “Propulsion by Impinging Laser Beams,” which ran in the Journal of Spacecraft and Rockets in 1972. Moeckel (working at what was then NASA’s Lewis Research Center in Cleveland) analyzed laser beaming to 100 ton relativistic flyby probes, each of which would require 1012 watts of X-ray energy. Making specific reference to Marx, Moeckel found X-ray beaming promising but did not know if it was feasible. His conclusion would have warmed the hearts of science fiction writers of the time:
…some future generations of mankind, with a somewhat different ordering of priorities than ours and much more available power, could conceivably explore other stars and other solar systems with highly sophisticated unmanned spacecraft capable of relaying information in elapsed times of the order of decades.
If only it worked! Fortunately, we’re not restricted to X-rays when it comes to beamed propulsion.
References
The early Forward paper is “Pluto-Gateway to the Stars,” Missiles and Rockets 10, 26 ff. (2 April 1962); reprinted in Science Digest 52, 70-75 (August 1962). Forward’s “Roundtrip Interstellar Travel Using Laser-Pushed Lightsails” appeared in the Journal of Spacecraft and Rockets 21 (1984), pp. 187-195 (abstract).
György Marx’s paper on X-ray beaming is “Interstellar Vehicle Propelled by Terrestrial Laser Beam,” which ran in Nature on July 2, 1966 (abstract). His two other interstellar papers are “The mechanical efficiency of interstellar vehicles,” Astronautica Acta 9 (1963) 131–139, and “Über Energieprobleme der Interstellaren Raumfahrt,” Astronautica Acta 6 (1960) 366–372.
The Redding paper in response to Marx has the same title, “Interstellar Vehicle Propelled by Terrestrial Laser Beam,” Nature February 11, 1967 (abstract). W. E. Moeckel’s paper “Propulsion by Impinging Laser Beams” appeared in the Journal of Spacecraft and Rockets Vol. 9 (1972), 942-944 (abstract).
My thanks to Jim Benford, Greg Matloff and Al Jackson for invaluable references and commentary.
Going Interstellar via Budapest
Studying the rich history of interstellar concepts, I realized that I knew almost nothing about a figure who is always cited in the early days of beamed sail papers. Whereas Robert Forward is considered the source of so many sail concepts, the earliest follow-up to his 1962 paper on beamed sails for interstellar purposes is by one G. Marx. The paper is “Interstellar Vehicle Propelled by Terrestrial Laser Beam,” which ran in Nature on July 2, 1966. Who is this G. Marx?
My ever reliable sources quickly came through when I asked if any of them had known the man. None had, though all were familiar with the paper, but Al Jackson sent me a copy of it along with another by J. L. Redding (Bishop’s University, Canada), who published a correction to the Nature paper on February 11, 1967. It didn’t reduce my confusion that Redding’s short contribution bears the exact same title as Marx’s. My other contacts on Marx had no personal experience with him either but were curious to learn more.
Image: Astrophysicist and science historian György Marx. Credit: FOTO:Fortepan — ID 56238:Adományozó/Donor: Rádió és Televízió Újság. – This file has been extracted from another file, CC BY-SA 3.0.
This is worth digging into because it illustrates some useful facts about beamed propulsion. But first: György Marx (1927-2002) was Hungarian, an astrophysicist and historian of science who is evidently best known for his work on leptons, that class of subatomic particles that are not affected by the strong force and can carry electrical charge or be neutral (the charged leptons are the electrons, muons, and taus). As a science historian, Marx was clearly possessed of a sense of humor, authoring a study of Hungarian scientists in 2000 called The Voice of the Martians.
That title is itself worth unpacking. The reference is to that great influx of supremely gifted scientists and thinkers – among them Peter K. Goldmark, Nicholas Kaldor, Arthur Koestler, Nicholas Kurti, John von Neumann, Egon Orowan, Michael Polanyi, Leo Szilárd, Edward Teller, and Eugene P. Wigner – who were born between 1890 and 1910 and greatly influenced the growth of technology through their work in the U.S. Their gifts struck some as all but other-worldly. The Voice of the Martians has just been re-published in a new edition by Pallas Athéné Books. There’s a helpful review in The Budapest Times, available on Mary Murphy’s fine Unpacking My Bottom Drawer blog.
My friend Tibor Pacher and I joked about the extraterrestrial origin of Hungarians when we met up at a conference in Italy. Tibor is himself Hungarian, and he pointed to the old joke about the influx of Hungarian scientists being of off-planet origin. A Princeton professor upon learning that John Kemeny was Hungarian is said to have exclaimed about the mathematical prodigy “Not another one!” Leo Szilárd, asked about aliens, once quipped “They walk among us, but we call them Hungarians.” The fact that the Hungarian language is non-Indo European makes the joke even better.
Teller, by the way, when told of Szilárd’s statement, took on a worried mien and said, “Von Kármán must have been talking.” And Marx would push the notion even further, noting that Hungarians must have an extraterrestrial origin because the names of so many of them, like Szilárd, von Neumann, and Theodore von Kármán, show up on no maps of Budapest, but there is a Von Kármán crater on Mars, and another on the Moon, along with craters named for von Neumann and Szilárd. Case closed.
And then there is nuclear physicist Friedrich “Fritz” Houtermans, himself born near Danzig but knowledgeable about the Hungarian element, who opined:
“The galaxy of scientific minds, that worked on the liberation of nuclear power, were really visitors from Mars. They found it difficult to speak English without an alien accent, which would give them away, and therefore they chose to pretend to be Hungarian, whose inability to speak any language but Hungarian without a foreign accent is well known. It would be hard to check the above statement, because Hungary is so far away.”
What a fascinating bunch. Marx saw this bulge in the demographic of prodigies and scientific wizards known in Hungarian as a marslakók, the term for ‘Martians,’ as a cultural phenomenon. He interviews and writes about them with insight and affection. Here he is in The Voice of the Martians in conversation with Teller as the latter reminisces about Leo Szilárd. Marx as historian was clearly able to extract great anecdotes from very deep thinkers:
“[…] fortunately, there was a Hungarian in America, Leo Szilárd, who was a versatile person. He was even capable of explaining the concept of a nuclear chain reaction to the Americans! Yet there was one thing that even Szilárd could not do: drive a car. In the summer of 1939, I was working at Columbia University in New York, just like Szilárd. One day, he came up to me and said, ‘Mr. Teller, I am asking you to drive out with me to Einstein.’ […] So, out we drove on August 2. The only problem remaining was that Szilárd again did not know where Einstein was staying for the holidays. We started asking around but nobody knew. We asked an eight year–old girl—she had a nice ponytail—where Einstein was living. She did not know either. Finally, Szilárd said, ‘You know he is that old man with long, flowing white hair.’ Then the girl gave us the direction, ‘He’s staying in the second house!’ We entered; Einstein was cordial, offered tea to Szilárd, and—being democratic—he invited in the chauffeur as well. Szilárd pulled a letter from his pocket addressed to President Roosevelt […]”
Leo Szilárd’s own sense of humor was a kind of magic dust that affected other scientists working on nuclear topics. He once announced his intention to write down everything he remembered about working on nuclear fission, “not for anyone to read, just for God.” To which Hans Bethe replied, “Don’t you think God knows the facts?” And Szilárd replied, “Maybe he does, but he does not know my version of the facts.”
All this is fine stuff, and one reason why I wander off down sideroads when I start asking questions about scientists. But it’s time to get back to interstellar concepts, because György Marx had ideas about reaching other stars that helped to focus the attention of other scientists on what Forward had been saying for some time, that interstellar flight was possible at the extreme end of engineering, and that it behooved scientists to be studying the best ways to accomplish it. But Marx’s specific choices for beamed propulsion would turn out to be ill-advised, as we’ll see next time.
A Shifting, Seething Solar Wind
In search of ever-higher velocities leaving the Solar System, we need to keep in mind the options offered by the solar wind. This stream of charged plasma particles flowing outward from the Sun carves out the protective bubble of the heliosphere, and in doing so can generate ‘winds’ of more than 500 kilometers per second. Not bad if we’re thinking in terms of harnessing the effect, perhaps by a magnetic sail that can create the field needed to interact with the wind, or an electric sail whose myriad tethers, held taut by rotation, create an electric field that repels protons and produces thrust.
But like the winds that drove the great age of sail on Earth, the solar version is treacherous, as likely to becalm the ship as to cause its sails to billow. It’s a gusty, turbulent medium, one where those velocities of 500 kilometers and more per second can as likely fall well below that figure. Exactly how it produces squalls in the form of coronal mass ejections or calmer flows is a topic under active study, which is where missions like Solar Orbiter come into play. Studying the solar surface to pin down the origin of the wind and the mechanism that drives it is at the heart of the mission.
Launched in 2020, Solar Orbiter carries a panoply of instruments, ten in all, for the analysis, including an Electron Analyzer System (EAS), a Proton-Alpha Sensor (PAS) for measuring the speed of the wind, and a Heavy Ion Sensor (HIS) designed to measure the heavy ion flow. Critical to the analysis of this paper is the Spectral Imaging of the Coronal Environment (SPICE) instrument, as we’ll see below. Steph Yardley (Northumbria University) is lead author of the paper on this work, which has just appeared on Nature Astronomy:
“The variability of solar wind streams measured in situ at a spacecraft close to the Sun provide us with a lot of information on their sources, and although past studies have traced the origins of the solar wind, this was done much closer to Earth, by which time this variability is lost. Because Solar Orbiter travels so close to the Sun, we can capture the complex nature of the solar wind to get a much clearer picture of its origins and how this complexity is driven by the changes in different source regions.”
What the work is analyzing is a theory that the process of magnetic field lines breaking and reconnecting is critical to producing the slower solar wind. Different areas of the Sun’s corona are implicated in the origin of both the fast and slow winds, with the ‘open corona’ being those regions where magnetic field lines extend from the Sun into space, tethered to it at one end only and creating the pathway for solar material to flow out in the form of the fast solar wind. Closed coronal regions, on the other hand, are those where the magnetic field lines connect to the surface at both ends, forming loops.
As you would imagine, the process is wildly turbulent and marked by the frequent breakage of these closed magnetic loops and their subsequent reconnection. The researchers have probed the theory that the slow solar wind originates in the closed corona during these periods of breakage and reconnection by studying the composition of solar wind streams, for the heavy ions emitted vary depending on their origins in either the closed or open corona. Solar Orbiter’s Heavy Ion Sensor (HIS) is able to take the needed measurements to relate the effects of this activity on the surrounding plasma.
The image below is from the Solar Dynamics Observatory spacecraft rather than Solar Orbiter, reminding us of the different views we are gaining by our various missions to our star. The comparison of key datasets tells the story.
Image: This is part of Figure 1 from the paper. The caption reads: SDO/AIA [Solar Dynamics Observatory data using its Atmospheric Imaging Assembly] 193 Å image showing the source region from the perspective of an Earth observer. Open magnetic field lines that are constructed from the coronal potential field model are overplotted, coloured by their associated expansion factor F. The large equatorial CH [Coronal Hole] and AR [Active Region] complex are labeled in white. The FOVs [fields of view] of SO EUI/HRI and PHI/HRT [references to instruments aboard Solar Orbiter] are shown in cyan and pink, respectively. The back-projected trajectory of SO [Solar Orbiter] from 1 March 2022 until 9 March 2022 is shown by the olive dotted line (from right to left).
So because we have Solar Orbiter, we can now combine observations of the Sun from various sources including other space missions, like the Solar Dynamics Observatory, with the measurements of the solar wind actually flowing past the spacecraft. Susan Lepri (University of Michigan) is deputy principal investigator on the HIS system:
“Each region of the Sun can have a unique combination of heavy ions, which determines the chemical composition of a stream of solar wind. Because the chemical composition of the solar wind remains constant as it travels out into the solar system, we can use these ions as a fingerprint to determine the origin of a specific stream of the solar wind in the lower part of the Sun’s atmosphere.”
The results have been productive. The analysis gives us a precise breakdown of just what Solar Orbiter has encountered during the period studied. This is a thorny quotation but it includes a key finding. From the paper:
Combining the SO [Solar Orbiter] trajectory, coronal field model, magnetic connectivity tool, the SPICE composition analysis of the AR [Active Region] complex, and the in situ plasma and magnetic field parameters, we suggest that SO was immersed in three fast wind streams… originating from the three linked sections of the large equatorial CH [Coronal Hole]… These were followed by two slower streams associated with the negative polarities of the AR complex… The decrease of the solar wind speed can be explained by the expansion of the open magnetic field associated with the CH-AR complex, as the connectivity of SO transitioned across these regions. Credit: Yardley et al.
The findings described here are significant. We learn from this work just how complex the solar wind flow is, in this case involving three fast streams and two slower ones, all involving changes in magnetic field connectivity. Matching the composition of the solar wind streams to different areas on the corona gives us new insights into the turbulent mix found where the open and closed corona meet. The slow solar wind’s ‘breakout’ from closed magnetic field lines is demonstrated. The phenomenon of magnetic reconnection proves critical to the wind’s variability.
Demonstrating these linkages means that we can now use the findings to probe further into the origins of the solar wind. But this is a variability that is in no way predictable, making the prospect of riding the solar wind via electric or magnetic sail a daunting one. We’ll continue to learn more, though, as we bring in data from missions like the Parker Solar Probe. It will be fascinating to see one day how we use the solar wind to test out possible spacecraft designs in search of a faster route to the outer Solar System.
Addendum: In an earlier draft, I mistakenly criticized the authors for not initially clarifying some of the acronyms in this paper. I’ve removed that comment because a later reading showed I was mistaken about the two examples I cited.
The paper is Yardley et al., “Multi-source connectivity as the driver of solar wind variability in the heliosphere,” Nature Astronomy 28 May 2024 (full text).
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