by Paul Gilster | Apr 9, 2020 | Exoplanetary Science |
The brown dwarf 2MASS J10475385+2124234 is about the size of Jupiter, but maybe 40 times more massive. 33.2 light years from Earth, this object is in that category between planet and star, not massive enough to launch the same kind of nuclear reactions that power the Sun, but considerably more massive than any planet. Combining two tools — the Very Large Array (VLA) and NASA’s Spitzer Space Telescope — scientists have now measured the wind speed here.
Katelyn Allers (Bucknell University), who led the research team, realized that the combination of radio observations (VLA) and infrared (Spitzer) would make this kind of measurement possible, and expressed surprise that no one else had thought to do it before. After all, we already knew that the rotation period of Jupiter found through radio measurements differs from the period found at visible and infrared wavelengths. That disparity is key to the new measurement.
For the difference is the result of two separate phenomena. Radio emissions from Jupiter are caused by electrons interacting with the planet’s huge magnetic field, located deep in the interior. The infrared emission is measured at the top of the atmosphere. Because the atmosphere is rotating more quickly than the interior, we have a difference in velocity due to atmospheric winds. The same kind of measurement is now put to use at a brown dwarf.
“Since the magnetic field originates deep in the planet, or in this case brown dwarf, the radio data allows us to determine the interior period of rotation,” Allers says. “When you have an interior rotation rate and an atmospheric rotation rate, you can compare them to see how fast the wind is blowing.”
Image: Artist’s conception of a brown dwarf and its magnetic field. The magnetic field, rooted deep in its interior, rotates at a different rate than the top of the atmosphere. The difference allowed astronomers to determine the object’s wind speed. Credit: Bill Saxton, NRAO/AUI/NSF.
Allers and company observed 2MASS J10475385+2124234 with Spitzer in 2017 and 2018, noting the interesting fact that its infrared brightness varies regularly, indicating a long-lived feature of some sort in the upper atmosphere. Does this brown dwarf have something analogous to Jupiter’s Great Red Spot? That we can’t know at this point, but a set of VLA observations from 2018 measured the rotation period of the brown dwarf’s interior, so we do know about the winds.
The Jupiter analogy holds, for the brown dwarf’s atmosphere does indeed rotate faster than its interior, allowing the calculation of a wind speed of about 2300 kilometers per hour. By comparison, Jupiter’s wind speed clocks in at around 370 kilometers per hour. According to Allers, the difference is in agreement with theory and simulations of brown dwarf wind speeds.
This initial measurement may alert us to a technique that can measure winds not only on other brown dwarfs but on some giant exoplanets, although as co-author Peter K.G. Williams notes, the magnetic fields of such worlds are weaker than those of brown dwarfs, so the radio measurements would be conducted at lower frequencies than those used in the 2MASS J10475385+2124234 work. Williams (Center for Astrophysics | Harvard & Smithsonian) led the radio astronomical observations at the VLA.
“For the first time ever, we measured the speed of the winds of a brown dwarf — too big to be a planet, too small to be a star,” Williams adds. “The results rule out a few unusual models and prove that this new technique works and can be applied to more objects… This new technique opens the way to better understanding the behavior of atmospheres that are unlike anything found in our solar system.”
Image: Brown dwarf, left, and Jupiter, right. Artist’s conception of brown dwarf illustrates magnetic field and atmosphere’s top, which were observed at different wavelengths to determine wind speeds. Credit: Bill Saxton, NRAO/AUI/NSF.
The paper is Allers et al., “A Measurement of the Wind Speed on a Brown Dwarf,” Science Vol. 368, Issue 6487 (10 April 2020). Abstract.
by Paul Gilster | Apr 8, 2020 | Outer Solar System |
Given their distance from the Sun, the gas and ice giants of our Solar System should not have upper atmospheres as hot as they are. Clearly, something is happening at the planets themselves to account for the warmth, and new analysis of Cassini data, just published in Nature Astronomy, lays out the case for auroral activity at the north and south poles of Saturn as the explanation there. The paper offers the results of the most complete mapping yet made of the temperature and density of a gas giant planet’s upper atmosphere.
Tommi Koskinen, a co-author on the paper describing these findings is a member of Cassini’s Ultraviolet Imaging Spectrograph (UVIS) team:
“The results are vital to our general understanding of planetary upper atmospheres and are an important part of Cassini’s legacy. They help address the question of why the uppermost part of the atmosphere is so hot while the rest of the atmosphere – due to the large distance from the Sun – is cold.”
Image: This false-color composite image, constructed from data obtained by NASA’s Cassini spacecraft, shows the glow of auroras streaking out about 1,000 kilometers (600 miles) from the cloud tops of Saturn’s south polar region. It is among the first images released from a study that identifies images showing auroral emissions out of the entire catalogue of images taken by Cassini’s visual and infrared mapping spectrometer. Credit: NASA/JPL/ASI/University of Arizona/University of Leicester.
The process at Saturn is much the same as what drives Earth’s auroral activity. Plasma from the solar wind moves along magnetic field lines in the upper atmosphere, where electrons collide with atmospheric atoms and molecules, exciting them to higher energy levels that radiate light at various colors and wavelengths. The differences in atmospheric composition mean that at Saturn, the light we see comes from emissions of molecular and atomic hydrogen.
Like Voyager, the trove of Cassini data keeps churning up new insights. In this case, we’re dealing with data collected during the spacecraft’s Grand Finale, the 22 breathtakingly tight orbits made before the craft was flown into Saturn’s atmosphere to protect potentially life-bearing moons like Enceladus. Cassini had already orbited Saturn for more than 13 years and was at the end of its fuel supply, but those final orbits were exceedingly productive.
Targeting bright stars in the constellations of Orion and Canis Major, Cassini spent six weeks observing the stars rising and setting behind the planet, allowing scientists to measure the changes in starlight passing through the atmosphere. The temperature map used in the new paper relies on density measurements that provided the information needed to calculate atmospheric temperatures. Temperatures were found to peak near the auroras, giving credence to the idea that auroral electric currents are the heat source for the upper atmosphere.
Image; The aurora at Saturn’s southern pole is visible in these false-color images, gathered by Cassini’s ultraviolet imaging spectrograph (UVIS) on June 21, 2005. Blue represents aurora emissions from hydrogen gas excited by electron bombardment, while red-orange represents reflected sunlight. The images show that the aurora lights at the polar regions respond rapidly to changes in the solar wind. Previous images have been taken closer to the equator, making it difficult to see the polar regions. Changes in the emissions inside the Saturn south-pole aurora are visible by comparing the two images, taken about one hour apart. The brightest spot in the left aurora fades, and a bright spot appears in the middle of the aurora in the second image. Credit: NASA/JPL/University of Colorado.
The rate of density decrease in the atmosphere depends on temperature, and the combination of temperature and density measurements helped the researchers determine the speed of the winds at altitude. Thus we get a glimpse of the zone where space weather interacts directly with a planet, showing how a global wind system distributes energy that is initially accumulated near the poles and then flows on into the equatorial regions. We wind up with the observed temperatures at altitude that are twice what we would expect from solar heating alone.
The paper is Brown et al., “A pole-to-pole pressure-temperature map of Saturn’s thermosphere from Cassini Grand Finale data,” Nature Astronomy 6 April 2020 (abstract).
by Paul Gilster | Apr 7, 2020 | Exoplanetary Science |
We looked recently at Voyager 2’s flyby of Uranus, via a new paper that examined the craft’s magnetometer data to draw out information about the planet’s magnetic environment. Science fiction author Stanley Weinbaum, author of the highly influential “A Martian Odyssey” in 1935, christened Uranus ‘The Planet of Doubt’ in a short story of the same name. Weinbaum couldn’t have known about the world’s magnetic field axis, which we’ve learned is tilted 60 degrees away from its spin axis. The latter itself is 98 degrees off its orbital plane. Doubtful planet indeed.
Here we have a world that is spinning on its side, one that demands answers as to how it got that way. A giant impact at some point in its history is a natural assumption, but how do we explain the fact that the Uranian moons as well as the planet’s ring system all show the same 98 degree orbital tilt as their parent? Back in 2011, a team led by Alessandro Morbidelli (Observatoire de la Cote d’Azur) ran a variety of simulations to test impact scenarios and discovered that a sufficiently early impact could have reformed the entire protoplanetary disk, leading to its 27 moons being in the position we see today. See the Centauri Dreams post A New Slant on the Planet of Doubt for more on the Morbidelli et al. paper.
Image: Uranus is uniquely tipped over among the planets in our Solar System. Uranus’ moons and rings are also orientated this way, suggesting they formed during a cataclysmic impact which tipped it over early in its history. Credit: Lawrence Sromovsky, University of Wisconsin-Madison/W.W. Keck Observatory/NASA.
Now we have another look at the problem, this from a team led by Shigeru Ida (Earth-Life Science Institute, Tokyo Institute of Technology). The key to this new paper is the understanding that while impacts would have been more common in the early Solar System than now, the outer planets would have experienced impacts that were different in result than those in the inner system. A rocky Mars-sized object might smack into a rocky Earth to create our Moon, but collisions between two icy objects have different results in the outer Solar System.
Out here, we’re dealing with planets with an abundance of volatiles, elements that would be gases or liquids in the warmer regions of the inner system, but are frozen at large distances from the Sun. The temperature needed to vaporize water ice is low, and the team assumes, reasonably, that both Uranus and its impactor were dominated by ices. Thus an impact early in the formation history of Uranus would have vaporized this ice, with leftover materials remaining gaseous and becoming incorporated primarily into the forming planet. Ida’s computer modeling shows that such impacts produce not one or two large moons but a number of small ones.
The inclination of both ring and moon system at Uranus makes it clear that the impact was early and formative for the entire Uranian system. Bear in mind that the ratio of the planet’s mass to its moons is larger than the ratio of Earth’s mass to its Moon by a factor of more than one hundred. Working with substantial water vapor mass loss, the researchers’ simulation reproduces the observed mass-orbit configuration of the Uranian satellites by incorporating the predicted distribution of ices as they re-condense. The results parallel the system we see today, indicating it is the result of the evolution of this volatile-laden impact-generated disk.
The authors contrast this with the giant impact model for Earth’s Moon, arguing that about half of the solid or liquid disk created by the strike was integrated into the Moon. The difference is the high condensation temperature at Earth’s orbital distances, meaning that the rocky and liquid material of the Earth-Moon impact would have solidified quickly, allowing the Moon to collect a significant amount of the debris created by the collision due to its gravity shortly after impact.
The work may have applications in other stellar systems. From the paper:
We have shown that the current Uranian major satellites are beautifully reproduced by the derived analytical formulas based on viscous spreading and cooling of the disk generated by an impact that is constrained by the spin period and the tilted spin, independent of details of the initial disk parameters. Although we have focused on Uranus, the model here provides a general scenario for satellite formation around ice giants with the scaling by the mass and the physical radius of a central planet, which is totally different from satellite formation scenarios around terrestrial planets and gas giants. It could also be applied for the inner region of Neptune’s satellite system, where we can neglect the effect of Triton that may have been captured. Observations suggest that many of [the] discovered super-Earths in exoplanetary systems may consist of abundant water ice, even in close-in (warm) orbits. The model here may also give a lot of insights into possible icy satellites of super-Earths.
The paper is Shigeru Ida et al., “Uranian satellite formation by evolution of a water vapour disk generated by a giant impact,” Nature Astronomy 30 March 2020 (abstract / preprint).
by Paul Gilster | Apr 3, 2020 | Advanced Rocketry |
The interstellar ramjet conceived by Robert Bussard may have launched more physics careers than any other propulsion concept. Numerous scientists over the years have told me how captivated they were with Poul Anderson’s treatment of the idea in his novel Tau Zero. Al Jackson takes a look at Bussard’s concept in today’s essay, referencing its subsequent treatment in the literature and adding a few anecdotes about Bussard himself. The original paper was submitted on February 1, 1960 to Astronautica Acta, then edited by Theodore von Kármán (a ‘tough judge,’ Al notes) and published later that spring. Although the ramjet faces numerous engineering issues, its ability to resolve the mass-ratio problem in interstellar flight makes it certain to receive continued scrutiny.
by A. A. Jackson
Writers of science fiction prose noticed the difference between interplanetary flight and interstellar flight earlier than anyone. Various fictional methods of faster-than-light (FTL) were invented in the 1930s, John Campbell even inventing the term ‘warp drive’. Asimov’s Galactic Empire is only facilitated by FTL ‘jump-drives’. Slower than light interstellar travel made an appearance in Goddard and Tsiolkovsky’s writings in the form of ‘generation ships’, usually called ‘worldships’ now.
As far as I know, the first engineer to look at the very basic physics — quantitative calculations — of relativistic interstellar flight was Robert Esnault-Pelterie; he made relativistic calculations before 1920 that were published in his book L’Astronautique (1930). The first derivation of the relativistic rocket equation occurs in Esnault-Pelterie’s writings. This was long before Ackeret (J. Ackeret, “Zur Theorie der Raketen,” Helvetica Physica Acta 19, p.103, 1946). The classical mass ratio rocket equation of Tsiolkovsky showed the difficulty of space travel. The relativistic rocket equation showed that interstellar flight was even more difficult.
Eugen Sänger, who had been interested in interstellar flight in the 1930s, addressed the interstellar mass ratio problem in 1953 with a paper on photon rockets, “Zur Theorie der Photonenraketen” (Vortrag auf dem 4. Internationalen Astronautischen Kongreß in Zürich 1953). Sänger, more than almost anyone before him, studied the hard physics of antimatter rockets and relativistic rocket mechanics. Using the most energetic energy source, antimatter, would require tons of it in a conventional rocket. There was sore need of a better method.
Bussard
Robert W Bussard was a rangy man who looked like he walked the halls of power. I had dinner with him at a San Francisco section of the American Institute of Aeronautics and Astronautics meeting in 1979. We had invited Poul Anderson, author of Tau Zero; Anderson and Bussard had never met. Over dinner Bussard told me he started working on nuclear propulsion at Los Alamos in 1955, and that he and R. DeLauer wrote the first monograph on atomic powered rockets in 1959 [1]. He also said he had been looking at work at Lawrence Radiation Laboratory in 1959.
Bussard told me he had always been interested in interstellar flight. One day at breakfast at Los Alamos he got a tortilla rolled up with scrambled egg in it. That cylinder made him think of a fusion ram starship! I have to wonder if that story is true, for had he been looking at Livermore’s lab papers he probably saw Project Pluto, the nuclear powered atmospheric ramjet.
Bussard sat down in 1959 and wrote the paper “Galactic matter and interstellar flight,” published in Astronautica Acta in 1960. This paper is thoroughly technical; Bussard summarizes Ackeret, Sänger and Les Shepherd’s studies of interstellar flight [2]. Sänger had shown that even using antimatter one still had a mass ratio problem with a conventional rocket. Bussard then presents an amazing new concept that solved the mass ratio problem [3]. He notes that one can scoop interstellar hydrogen and fuse it to produce a propulsion system.
The treatment is rigorously special relativistic; using conservation of energy and momentum he derives the equations of motion of an interstellar ramjet. He accounts for the energy production and propulsion efficiency of the vehicle in general terms. He uses the most energetic fusion mechanism, the proton-proton fusion reaction which converts .0071 of the rest mass of collected protons to energy. Bussard derives the property that the ramjet will need to be boosted to an initial speed.
Image: Robert Bussard in 1959 with his Astronutica Acta issue.
Bussard discusses the engineering physics problems; the difficulty of using the p-p chain is enormous. He notes that interstellar hydrogen can be unevenly distributed, there being rich and rarefied regions. He gives a simplified model for scooping and sometimes it is missed that he mentions magnetic fields as a ‘collector’. Bussard also notes both radiation losses and radiation hazards during the operation of the ramjet.
Sagan
The Bussard Ramjet got a boost in 1963 when Carl Sagan noted that there was a solution to the mass ratio problem for interstellar flight [4]. Sagan summarized this paper in Intelligent Life in the Universe in 1966 [5], probably the best popularization of the Ramjet. Sagan also noted that ships accelerating at one gravity could circumnavigate the universe, ship proper time, in about 50 years. He references Sänger in the paper version [4] and the calculation of the mechanics of a 1g starship. As far as I know, the 1957 paper of Sänger [6] is the first exposition of a constant acceleration starship and the consequences of time dilation when extreme interstellar distances are traveled. Bussard mentioned, very briefly, a magnetic field as a scoop, but Sagan describes such a collector in a more elaborated though qualitative way.
Fishback
John Ford Fishback published his MIT bachelor’s thesis in Astronautica Acta in 1969 [7]; this was supervised by Philip Morrison. Morrison and Cocconi were the fathers of radio SETI. Morrison seems to have taken an interest in Sagan’s mention of Bussard’s ramjet — I’m not sure if it was Morrison or Fishback who suggested the study. The paper is a remarkable marshalling of electrodynamics, charged particle motion, plasma physics, the physics of materials and special relativity.
Fishback constructs a model for the magnetic scoop field taking into account the fraction of hydrogen ingested and reflected. Using conservation laws, he derives the most detailed equations of motion accounting for mass and radiation losses that had been published anywhere. In the scooping process, Fishback examines the statistical distribution of gas in the galaxy and derives a relativistic expression for ship proper acceleration with ‘drag’. An important consequence, expressed for the first time, is the mechanical stress on the scoop field magnets. He derived an upper limit on the maximum Lorentz factor that can be obtained as a ramjet accelerates at 1 g for a long time due to stress on the source of the scoop field.
[For more on Fishback, see Al’s John Ford Fishback and the Leonora Christine from 2016, with further thoughts by Greg Benford.]
Image: John Ford Fishback in 1967 and first page of his paper in Astronautica Acta. Sadly, Fishback would take his own life in 1970 at the age of 23.
Martin
In 1971 [8] and 1973 [9] Tony Martin reviewed Fishback’s paper, making useful clarifying observations. Martin provides details of calculation that Fishback leaves to the reader on the relation of the fraction of particles that are magnetically confined to the reactor intake as a function of the confining field and the starship’s speed. In his second paper, Martin corrects a numerical error by Fishback showing that the cutoff speed due to the stress properties of the magnetic source is 10 times larger than was calculated. Martin also gives a nice calculation of the size of the magnetic scoop field. Fishback and Martin’s papers account for the ‘drag’ due to reflected particles; this result seems unknown to later critics of the ramjet.
Whitmire
I met Dan Whitmire in 1973, when we were both working on doctorates in physics at the University of Texas at Austin. Dan and I were talking about interstellar flight one day and I showed him Bussard’s paper. Dan was in the nuclear physics group at Texas and took an immediate interest in the problem with proton-proton fusion as had been pointed out by Bussard and Martin. Then he came up with an ingenious solution: Carry carbon on board the starship and use it as a catalyst to implement the CNO fusion cycle [10]. The CNO process is 1018 times faster than the PP chain at the fusion reactor temperatures under consideration. This reduces the fusion reactor size to 10s (and more) of meters in dimension. Since carbon cycles in the process, in theory one would only need to carry a small amount; however it is not clear how under dynamic conditions one would recover all the catalysis needed.
Later Developments
The above are the core studies of the interstellar ramjet. Hybrid methods occurred to several researchers. Alan Bond [11] proposed a vehicle that carried a separate energy source yet scooped-up interstellar hydrogen not as fuel but simply as reaction mass, this is known as the augmented interstellar ramjet. Conley Powell [12] presented a refined analysis of this system. The author [13] presented a study using antimatter added to the scooped reaction mass for propulsion as an augmented method. Relevant to the augmented ramjet is antimatter combined with matter for propulsion as studied by Forward and Kammash [14, 15].
T. A. Heppenheimer published a paper in the Journal of the British Interplanetary Society [16] noting the problems with the p-p chain for fusion without citing Dan Whitmire’s solution. Heppenheimer notes radiation losses but does not cite Whitmire and Fishback, who addressed the problems of bremsstrahlung and synchrotron radiation in the reactor and the scoop field.
Matloff and Fennelly [17] have interesting papers on charged particle scooping with superconducting coils. Cassenti looked at several modifications and aspects of the ramjet [18].
Recently Semay and Silvestre-Brac [21, 22] re-derived the equations of motion of the interstellar ramjet, first done by Bussard and Fishback. They find some new extensions with solutions of the relativistic equations for distance and time.
Dan Whitmire and the author [23] removed the fusion reactor by taking the energy source out of the ship and placing it in the Solar System. If one scoops hydrogen but energizes it with a laser system it is possible to make a ramjet that is smaller and less massive. Such a system probably has a limited range similar to laser pushed sails.
An excellent survey of interstellar ramjets and hybrid ram systems can be found in the books by Mallove and Matloff [24] and a recent monograph by Matloff [25], see these books and the references listed in them. See also Ian Crawford’s paper [26].
The Interstellar Ramjet in Science Fiction
It seems the Bussard Ramjet first appeared in a Larry Niven short story called “The Warriors” (1966). Later Niven used the Ramjet in his other fiction, inventing, I think, the term Ram Scoop. However I think the best known use of the Ramjet is Poul Anderson’s Tau Zero [26]. The core story in Tau Zero is not the Interstellar Ramjet but the constant acceleration circumnavigate-the-universe calculation first done by Eugen Sänger.
My guess is that Anderson only saw Carl Sagan’s exposition on this in Intelligent Life in the Universe. The Greek letter ‘Tau’ was introduced by Hermann Minkowski in 1908; it is the time measured by the travelers in the starship Leonora Christine, while the time measured by people back on earth is t. Special relativistic time dilation leads to (ship time)/(Earth Time) going to almost zero. Accelerate at one g for 50 years and one covers a distance of about 93 billion light years that is roughly the size of the universe.
Image: What would become Tau Zero first appeared in shortened form as “To Outlive Eternity” in the pages of Galaxy in June, 1967.
The Bussard Ramjet Leonora Christine sets out for Beta Virginis, approximately 36 light years away. A mid-trip mishap robs the ship of its ability to slow down. Repairs are impossible unless they shut down the ramjet, but if the crew did that, they would instantly be exposed to lethal radiation. There’s no choice but to keep accelerating and hope that the ship will eventually encounter a region in the intergalactic depths with a sufficiently hard vacuum so that the ramjet could be safely shut down. They do find such a region and repair the ship.
Anderson then introduces the mother of all twists. The Leonora Christine has accelerated for so long that the crew discover relative to the universe a cosmological amount of time has elapsed. The universe is not ‘open’ but fits the re-collapse model, it is going for the big crunch. I know of no other science fiction novel with more extreme problem solving that this hard SF story.
Anderson’s cosmology for Tau Zero seems to come totally from George Gamow [28]. Gamow and his students did pioneering work on early time cosmology, an elaboration of earlier work done by Georges Lemaître. When Poul Anderson wrote the novel, he may have been aware that Big Bang cosmology had evolved beyond Gamow’s models …. However, having his starship eventually orbit the ‘Cosmic Egg’ or Ylem was a solution to the crew’s problem. Alas, even in Gamow’s cosmology the ‘Ylem’ is the universe, so no way to ‘orbit’ it. Poetic license for the sake of a Ripping Yarn! (An intersecting exercise is to see what the trajectory of the Leonora Christine‘s plot problem is in current accelerating universe cosmology.)
After Niven and Anderson, the Bussard Ramjet became common currency in science fiction, although it has faded somewhat in recent times. Recently a fusion ramjet, SunSeeker, appears as an integral part of the Bowl of Heaven series by Greg Benford and Larry Niven [29].
Final Thoughts
There seems to be a thread of pessimism about the Bussard Ramjet centered around drag on the ramjet due to interaction with the scoop field. This is an issue that Fishback deals with in his analysis; he shows one cannot just use a dipole magnetic field. A more complex collector field is needed. Fishback and Martin do show there is a fundamental physics limitation. Even using the strongest material theoretically possible, there is an upper limit to a mission Lorentz factor, probably equal to 10,000. Above this one will bust the scoop coil due to magnetic stress. The cosmological peril of the Leonora Christine depicted in Tau Zero is not physically possible.
The main show stopper for the ramjet is the engineering. There is no way with foreseeable technology to build all the components of an interstellar ram scoop starship. Several aspects should be revisited. (1) The source of the magnetic scoop field, Fishback [7] derived one, Cassenti elaborated another [20]; (2) the fusion reactor — the aneutronic fusion concept is direct conversion of fusion to energy [30]; (3) hybrid systems, especially laser-boosted ramjets.
Since basic physics does not rule a ramjet out, it is possible that an advanced civilization might build one. Freeman Dyson [31] pointed out many times that what we could not do might be done by some advanced civilization as long as the fundamental physics allows it. An interesting consequence of this is that interstellar ramjets may have been built and might have observable properties. Doppler-boosted waste heat from such ships might be observable. Plowing into HII regions in the galaxy, a starship’s magnetic scoop field might produce a bow-shock which could be observable. Isolated objects in this galaxy with Lorentz factors in the thousands would be unusual and if they are accelerating even more unusual.
The idea of picking up your fuel along the way in your journey across interstellar space may be the optimal solution to the mass ratio problem in interstellar flight. The interstellar ramjet warrants more technical study.
Appendix
Because Robert Bussard sketched a ramjet with a physical ‘funnel’ …all the many illustrations I have seen since seem to have some kind of ‘cow catcher’ on the front. Though it is reasonable that such a structure is the source of an electromagnetic device, I think it more likely that the ‘scoop’ field will be produced by a magnetic configuration that directs the incoming stream into the mouth of the reactor without any extra funnel-like forward structure. Here is a rough schematic done for me by artist Doug Potter. There is a ‘bulb’ representing the magnetic source field (maybe the parabolic magnetic field calculated by Fishback), a reactor section and an exhaust. Not a very elegant representation of the ramjet but a suggested configuration.
References
1. Bussard, R. W., and R. D. DeLauer. Nuclear Rocket Propulsion, McGraw-Hill, New York, 1958
2. L. R. Shepherd, “Interstellar Flight,” Journal of the British Interplanetary Society, 11, 4, July 1952
3. R.W. Bussard, “Galactic matter and interstellar flight,” Astronautica Acta 6 (1960) 179-195
4. C. Sagan, “Direct contact among galactic civilizations by relativistic interstellar spaceflight,” Planet. Space Sci. 11 (1963) 485-498
5. Sagan, Carl; Shklovskii, I. S. (1966). Intelligent Life in the Universe. Random House
6. Sänger, E., “Zur Flugmechanik der Photonenraketen.” Astronautica Acta 3 (1957), S. 89-99
7. Fishback J F, “Relativistic interstellar spaceflight,” Astronautica Acta 15 25-35, 1969
8. Anthony R. Martin; “Structural limitations on interstellar spaceflight,” Astronautica Acta, 16, 353-357 , 1971
9. Anthony R. Martin; “Magnetic intake limitations on interstellar ramjets,” Astronautica Acta 18, 1-10 , 1973
10. Whitmire, Daniel P., “Relativistic Spaceflight and the Catalytic Nuclear Ramjet” Acta Astronautica 2 (5-6): 497-509, 1975
11. Bond, Alan, “An Analysis of the Potential Performance of the Ram Augmented Interstellar Rocket,” Journal of the British Interplanetary Society, Vol. 27, p.674,1974
12. Powell, Conley, “Flight Dynamics of the Ram-Augmented Interstellar Rocket,” Journal of the British Interplanetary Society, Vol. 28, p.553, 1975
13. Jackson, A. A., “Some Considerations on the Antimatter and Fusion Ram Augmented Interstellar Rocket,” Journal of the British Interplanetary Society, v33, 117, 1980.
14. R.L. Forward, “Antimatter Propulsion”, Journal of the British Interplanetary Society, 35, pp. 391-395, 1982
15. Kammash, T., and Galbraith, D. L., “Antimatter-Driven-Fusion Propulsion for Solar System Exploration,” Journal of Propulsion and Power, Vol. 8, No. 3, 1992, pp. 644 – 649
16. Heppenheimer, T.A. (1978). “On the Infeasibility of Interstellar Ramjets”. Journal of the British Interplanetary Society 31: 222
17. Matloff, G.L., and A.J. Fennelly, “A Superconducting Ion Scoop and Its Application to Interstellar Flight”, Journal of the British Interplanetary Society, Vol. 27, pp. 663-673, 1974
18. Matloff, G.L., and A.J. Fennelly, “Interstellar Applications and Limitations of Several Electrostatic/Electromagnetic Ion Collection Techniques”, Journal of the British Interplanetary Society, Vol. 30, pp. 213-222, 1980
19. Matloff, G.L., and A.J. Fennelly , B. N , “Design Considerations for the Interstellar Ramjet,” 44th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit, 2008
20. Cassenti, B. N , “The Interstellar Ramjet,” 40th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, 2004
21. Claude Semay and Bernard Silvestre-Brac, “The equation of motion of an interstellar Bussard ramjet,” European Journal of Physics 26(1):75, 2004
22. Claude Semay and Bernard Silvestre-Brac, “Equation of motion of an interstellar Bussard ramjet with radiation loss,” Acta Astronautica 61(10):817-822, 2007
23. Whitmire, D. and Jackson, A, “Laser Powered Interstellar Ramjet,” Journal of the British Interplanetary Society Vol. 30pp. 223-226, 1977
24. Mallove, E. F., and G.L. Matloff, The Starflight Handbook, Wiley, New York, 1989
25. Matloff, G., Deep-Space Probes, Praxis Publishing, Chichester, UK, 2000
26. Ian A Crawford, “Direct Exoplanet Investigation Using Interstellar Space Probes.” In Handbook of Exoplanets Springer 2017
27. Anderson, Poul. Tau Zero. New York: Lancer Books (1970)
28. George Gamow, The Creation of the Universe (1952)
29. Benford, G. and Niven, L., Bowl of Heaven series, Macmillan.
30. Benford, G., Private communication.
31. Dyson, F. J., “The search for extraterrestrial technology,” in Marshak, R.E. (ed), Perspectives in Modern Physics, Interscience Publishers, New York, pp. 641-655
by Paul Gilster | Apr 2, 2020 | Exoplanetary Science |
The Kepler mission gave us, along with plenty of exoplanetary scenarios, a statistical look at a particular patch of sky, one containing parts of Lyra, Cygnus and Draco. Some of the stars within that field were close (Gliese 1245 is just 15 light years out), but the intention was never to home in on nearby systems. Most of the Kepler stars ranged from 600 to 3,000 light years away. Instead, Kepler would produce an overview of planets around different stellar types, including some in the habitable zone of their stars.
As with all such observations, we’re limited by the methods chosen, which in Kepler’s case involved transits of the host star. TESS, the Transiting Exoplanet Survey Satellite, likewise uses the transit method, though with particular reference to broad sky coverage and close, bright stars. We can deploy the widely anticipated James Webb Space Telescope, to be launched next year, to follow up interesting finds, but let’s also consider how useful the Wide Field Infrared Survey Telescope (WFIRST) is going to be, for this instrument brings a new population of planets into the mix.
Not that WFIRST won’t be able to spot transits as well, but the real interest here is microlensing, a phenomenon a good deal less common than transits, but one holding the promise of finding planets far more distant than Kepler. Looking toward the passage of a star and planetary system in front of a background star, astronomers can catch the lensing effect produced by the warping of spacetime. That quick brightening can contain within it the signature of one or more planets.
As you would imagine, finding these occultations is tricky, for they don’t occur very often. David Bennett is head of the gravitational microlensing group at NASA’s Goddard Space Flight Center. Kepler monitored more than 150,000 stars in its primary mission, but WFIRST will have a lot more to play with, says Bennett :
“Microlensing signals from small planets are rare and brief, but they’re stronger than the signals from other methods. Since it’s a one-in-a-million event, the key to WFIRST finding low-mass planets is to search hundreds of millions of stars.”
Image: WFIRST will make its microlensing observations in the direction of the center of the Milky Way galaxy. The higher density of stars will yield more exoplanet detections. Credit: NASA’s Goddard Space Flight Center/CI Lab.
Slated for launch in the mid-2020s, WFIRST will round out what we’ve learned from previous missions and different exoplanet detection methods. Radial velocity is sensitive to planets close to the host star and, with ever greater spectroscopic precision, can tease out information about smaller worlds further out. Transits are excellent for finding small worlds in tight orbits. What microlensing brings to the table are planets of all sizes — and perhaps even large moons — orbiting at a wide range of distances from the host, as far out as Uranus and Neptune and potentially much farther.
Here the bias, if we want to call it that, is toward planets from the habitable zone outward, fully complementing our other methods, which function so much better in inner systems. We have no idea how common ice giants are, but WFIRST should help us build the census. We go from Kepler’s 115 square degree field of view of stars typically within 1,000 light years to a 3 square degree field that, because it’s toward galactic center, will track 200 million stars. Their average distance will be in the range of 10,000 light years, far beyond the reach of other methods.
Microlensing has produced its share of planets — 86 so far — but bear in mind that the observations that have turned them up have been in visible light, which means that looking toward the center of the galaxy, shrouded in dust, has not been feasible. That’s beginning to change with the United Kingdom Infrared Telescope (UKIRT), which from its vantage in Hawaii has begun mapping the region. Data from UKIRT will help determine the WFIRST microlensing observation strategy.
Savannah Jacklin (Vanderbilt University) has led studies using UKIRT data:
“Our current survey with UKIRT is laying the groundwork so that WFIRST can implement the first space-based dedicated microlensing survey. Previous exoplanet missions expanded our knowledge of planetary systems, and WFIRST will move us a giant step closer to truly understanding how planets – particularly those within the habitable zones of their host stars – form and evolve.”
But WFIRST microlensing goes beyond exoplanet discovery to take in everything from black holes to neutron stars, brown dwarfs and ‘rogue’ planets that have been ejected from their planetary systems. TESS is currently tracking 200,000 stars over the entire sky. The infrared studies of WFIRST will dramatically add to what Kepler and TESS have given us, using machine learning tools now being refined by UKIRT to comb through the data. An overview of planetary populations at all distances from the host star, and with a target field containing hundreds of millions of stars, is the much desired result.
