by Paul Gilster | Jul 24, 2012 | Advanced Rocketry |
Many of the interstellar concepts I write about in these pages take on a life of their own. After the initial brainstorming, the idea gets widely enough disseminated that other scientists take it on, looking to modify and improve on the original concept. That’s been true in the case of solar sails and the more recently devised ‘lightsails,’ which use beamed energy from a laser or microwave source to drive the vehicle. We continue to study magnetic sails — ‘magsails’ — and various nuclear options like the inertial confinement fusion that powered Daedalus and perhaps Icarus. Sometimes insights arise when ideas are grafted onto each other to create a hybrid solution.
The idea I want to examine today, a hybrid design combining a Bussard-style interstellar ramjet with laser beaming — exemplifies this mix and match process. Working with Daniel Whitmire, A. A. Jackson, a frequent commenter and contributor here on Centauri Dreams, pondered the various issues the Bussard ramjet had run into, including the difficulty in lighting the proton/proton fusion reaction Bussard advocated early in the process. Writing at a time not long after he had finished up a PhD in relativistic physics (at the University of Texas), Jackson conceived the idea of beaming energy to the spacecraft and discovered that the method offered advantages over the baseline Bussard design. The laser-powered ramjet is a fascinating concept that has received less attention than it deserves.
Image: Physicist and interstellar theorist Al Jackson, originator of the laser-powered ramjet concept.
Bussard’s ramjet, you’ll recall, lit its fusion fires using reaction mass gathered from the interstellar medium by a huge magnetic ram scoop, which itself has proven problematic given the drag issues such a scoop introduces. The other way to power up a starship using an external source of energy is to beam a terrestrial or Solar System-based laser at the departing craft, which has deployed a lightsail to draw momentum from the incoming photons. Jackson and Whitmire found the latter method inefficient. Their solution was to beam the laser at a ramjet that would use reaction mass obtained from a Bussard-style magnetic ram scoop. The ramjet uses the laser beam as a source of energy but, unlike the sail, not as a source of momentum.
Running the numbers and assuming all photons transmitted by the laser will be absorbed by the ship, the authors discovered that the laser-powered ramjet (LPR) is superior to the baseline Bussard ramjet at low velocities, while superior to the laser-pushed sail at all velocities. The Bussard design becomes the most efficient of the three at velocities equal to and above about 0.14 c. The laser-powered ramjet, then, solves at least one of the Bussard vehicle’s problems, the fact that it has to get up to a significant percentage of lightspeed before lighting its fusion reaction. LPR propulsion could be used up to 0.14 c, with the vehicle switching over to full interstellar ramjet mode to achieve high efficiency at relativistic velocities.
The laser-powered ramjet offers other advantages as well. Think back to some of Robert Forward’s laser sail concepts and you’ll recall the problem of deceleration. With the sail powered by a laser beam from the Solar System, it’s possible to reach velocities high enough to take you to the nearest stars in a matter of decades rather than centuries. But how do you slow down once you arrive? Conceiving a manned mission to Epsilon Eridani, Forward came up with a ‘staged’ solution in which the sail separates upon arrival, with the large outer sail ring moving ahead of the vehicle and reflecting beamed laser energy to the now smaller inner sail, thus slowing it down. It would be so much easier if the beam worked in both directions!
But with the laser-powered ramjet, a round trip can be made using a single laser beam because the beam is being used as a source of energy rather than momentum. Jackson and Whitmire showed that the efficiency in the deceleration phase of the outbound journey as a function of velocity is the same as for the acceleration phase. And on the return trip, the energy utilisation efficiency is more favorable in both the acceleration and deceleration phases because the ship is traveling into the beam. In fact, the laser-powered ramjet is superior to both the laser sail and the Bussard ramjet even at high fractions of the speed of light when traveling into the laser beam.
Let’s go over that again: Jackson and Whitmire’s calculations focus on the energy utilisation efficiency parameter, showing that the laser-powered ramjet is superior to the laser sail at all velocities, whether the ship is receding from the beam or approaching (moving into the beam). The LPR is also superior to the Bussard ramjet at velocities less than about 0.14 c when receding from the beam, and superior to the Bussard design at all velocities when approaching. Add to this that the LPR concept requires no onboard proton-burning reactor — the authors assume the use of Whitmire’s ‘catalytic’ ramjet using the CNO (carbon-nitrogen-oxygen) cycle — and that the LPR’s power requirements are less than those of the laser sail.
Image: An interstellar ramjet at work. Credit: Adrian Mann.
You can imagine a future civilization taking advantage of these principles after establishing a presence around another star. A beaming system in the Solar System could eventually be complemented by one around the other star, so that no matter which way the ship was traveling, it would always be moving into the beam on a round-trip flight. I’ve sought in vain for an illustration of this concept in science fiction (and I keep wondering if Larry Niven hasn’t picked up on it somewhere), but I can find no example. It seems prime material for science fictional use.
Jackson and Whitmire looked at still another wrinkle on this notion in a paper the following year, about which more tomorrow. Today’s paper is “Laser Powered Interstellar Ramjet,” Journal of the British Interplanetary Society Vol. 30 (1977), pp. 223-226.
by Paul Gilster | Jul 23, 2012 | Advanced Rocketry |
I see that ‘Zarmina’ is back in the news. The informal designation refers to Gliese 581 g, an exoplanet candidate announced by the Lick-Carnegie team in an effort led by Steven Vogt (UC-Santa Cruz). First you see it, then you don’t — Gl 581 g has been controversial from the start, and is now the subject of a new analysis describing a 32-day orbit, a super-Earth in the habitable zone. More on the analysis later in the week, because my purpose today is to keep digging into the options for getting to a place like this once we’re sure it really does exist.
Gl 581 is just over 20 light years from the Sun in the constellation Libra, a red dwarf whose planetary system is one of the nearest yet detected. Among the options for propulsion in a future interstellar probe is Medusa, the brain-child of Los Alamos physicist (now retired) Johndale Solem. As examined here on Friday, Medusa is a nuclear-pulse system, like Orion in that it relies on the explosion of a series of atomic bombs to propel the vehicle. Where Medusa changes the equation is in its use of a gigantic sail, or ‘spinnaker,’ which replaces Orion’s massive pusher plate. The gossamer sail is connected to the payload by high-tensile strength cables.
Image: The Medusa hybrid design, combining nuclear pulse propulsion with a sail. (A) the payload capsule, (B) the winch mechanism, (C) the main tether cable, (D) riser tethers, and (E) the parachute mechanism. Credit: George William Herbert/Wikimedia Commons.
Because I’m just finishing up Kelvin Long’s Deep Space Propulsion (subtitled ‘A Roadmap to Interstellar Flight’), I was curious to see what Long had to say about Medusa, a concept that has received less attention than other nuclear methods like Orion or Daedalus’ inertial confinement fusion (ICF) design. Long wonders whether Medusa might be useful as a deceleration mechanism for a star probe:
As a probe approaches a target destination, the sail is unfurled rearwards with detonations causing a force in the negative thrust direction to the direction of motion. This may only require a small quantity of units to produce this result. Alternatively, ICF capsules could be ignited by laser beams rearwards of the vehicle, giving rise to the same effect, but on a more moderate level.
The immediate objection to this is the complexity of the Medusa mechanism, but Long anticipates this:
One of the biggest problems for the Medusa sail being used in a deceleration mode however is the reliability of it deploying in deep space after decades of being stored away. If the sail doesn’t deploy, then the probe will essentially be a flyby probe with limited observing time of the target star system and potentially left with a lot of unused units, which would have to be destroyed safely in a controlled way by the vehicle main computing system.
Make no mistake about it, Medusa is a complex system (see Friday’s post for a diagram of the propulsion cycle). Solem’s initial Los Alamos report refers to a canopy with a radius of 500 meters. The plan is to spin-deploy it and its 104 tethers, each made of high-strength polyethylene (aligned polyethylene) material, though Solem notes that superior materials will be available by the time the kind of manned missions he envisions come into play. Remember that he was thinking in terms of an interplanetary mission with Mars as the primary destination. Solem’s sketch, at the end of the Los Alamos report, gives you a feel for the idea, but translate this webwork of lines into fully 10000 tethers and ponder the issues such a contraption raises.
Image: Medusa and its tethers, as sketched by Johndale Solem. Credit: Los Alamos National Laboratory.
The problem of deployment and storage is enormous, and I can see why Long would wonder about re-deploying such a system after a lengthy interstellar transit, which is why he also considers it in terms of a first-stage booster, to be ejected after use. On the other hand, Solem’s calculations in the Los Alamos report and elsewhere showed that the canopy should be able to withstand the ignition of nuclear devices that would propel it. The tethers are another matter — as noted by ‘Eniac’ in the comments to Friday’s post, the tethers have to absorb the blast energy and are obviously crucial to the design. Like Robert Forward, Solem liked to think big, and in this case a gigantic canopy with extremely long tethers was expected to minimize radiation dangers for the crew and allow the tethers to survive the acceleration phase of the mission.
How fast could Medusa fly? The answer depends on the number of propulsion units detonated, with each detonation pulse adding to the velocity. Long also points out that the larger the canopy area, and the closer the canopy is to the detonation point, the higher the velocity. Using the values given in Solem’s 1993 paper in the Journal of the British Interplanetary Society, Long comes up with a specific impulse of 4100 kilometers per second, with an exhaust velocity in the region of 40 km/s.
Like Orion, Medusa runs into the problem of putting nuclear materials into space, one that Solem was all too familiar with. In the conclusion of the Los Alamos report, he has this to say:
We are currently prohibited by treaty from: (1) deploying weapons of mass destruction in space and (2) testing nuclear weapons in space. MEDUSA violates neither the letter nor the spirit of either prohibition, but it does use nuclear explosives. The radioactive debris from MEDUSA’s exhaust is so finely dispersed that it will be nearly undetectable. I assert that MEDUSA’S net environmental impact is less than NERVA; you have to do something with the spent reactor. I see no reason why nuclear explosive propulsion for interplanetary missions cannot be made politically acceptable. Perhaps we can be more creative and consider an international mission in which the nuclear explosives were jointly supplied by the superpowers. What a wonderful approach to nuclear disarmament and the enhancement of science for the benefit of all humanity!
Orion proponents make the same case, that their design would allow us to use up our nuclear stockpile for peaceful purposes while offering large vehicles for deep space exploration. It’s a utopian vision, and in both cases, it’s hard to see it happening anytime soon. Nuclear-pulse propulsion may make sense in space, but you first have to get the nuclear materials off the planet. The nuclear issue is one reason Friedwardt Winterberg, back in 1971, developed the idea of using intense, relativistic electron beams to ignite fusion, a method that captured the attention of the team developing Project Daedalus for the British Interplanetary Society. In such ways do the problems of one propulsion concept play into the revised thinking that fuels the next.
Citations for Solem’s Los Alamos report and his 1993 JBIS paper are at the end of last Friday’s post. I also want to mention two other papers by Solem: “The Moon and the Medusa: Use of Lunar Assets in Nuclear-Pulse Propelled Space Travel,” JBIS Vol. 53 (2000), pp. 362-370 and “Deflection and Disruption of Asteroids on Collision Course with Earth,” JBIS Vol. 53 (2000), pp. 180-196.
by Paul Gilster | Jul 20, 2012 | Sail Concepts |
Hybrid propulsion technologies have emerged naturally as we look at ways to reach the stars. They’re the result of trying to extract maximum performance from each option, and it sometimes turns out that putting two ideas together works better than either by itself. Next week we’ll be looking at one such concept, A. A. Jackson’s idea of combining the Bussard ramjet with laser beaming in ways that turn out to be surprisingly effective. Today I want to start the hybrid discussion – already about a week late because of competing news — by talking about Johndale Solem’s ‘Medusa,’ a combination of sail technologies with nuclear pulse propulsion.
Solem’s work evidently draws on the ideas of Ted Cotter at Los Alamos in the 1970s, which evolved into what George Dyson has described as a ‘rotating-cable pusher.’ Think back to the Orion concept, with its immense pusher-plate and shock absorbers that would withstand the explosion of nuclear devices behind the plate, propelling the vehicle forward while protecting the crew. What Cotter had in mind was doing away with the pusher-plate and instead having the ship, as it spun slowly around its axis, unreel steel cables that would radiate out from the vehicle, in Dyson’s words, ‘like the arms of a giant squid.’ With flattened plates at the end of each, the cables would absorb momentum from the explosions set off behind the vehicle.
I assume the squid reference came from George Dyson’s father, for Freeman Dyson is credited in his son’s book Project Orion: The True Story of the Atomic Spaceship as being the author of a 1958 memo, still classified, called ‘The Bolo and the Squid.’ That places early thinking on this highly modified concept all the way back in the days of active Orion research, which were complemented by a revival at Los Alamos in the early 1970s that resulted in Cotter’s work. It’s natural enough that Johndale Solem, himself working at Los Alamos, should have been the one to take the concept one step further with a design he called Medusa because it would mimic the motion of a jellyfish moving through the ocean as it moved through space. In a 1991 Los Alamos report, Solem wrote:
One can visualize the motion of this spacecraft by comparing it to a jellyfish. The repeated explosions will cause the canopy to pulsate, ripple, and throb. The tethers will be stretching and relaxing. The concept needed a name: its dynamics suggested Medusa.
Thus the scheme: Solem would likewise do away with the pusher plate of Orion, replacing it with a large sail deployed well ahead of the vehicle, with nuclear explosions to be detonated between the two so as to drive the sail and attached vehicle forward. You can see the basic idea in the illustration below. The Medusa idea evolves naturally from some of the problems inherent in the Orion design. No matter how large the pusher-plate, it could only receive a fraction of the momentum from the bomb blast debris, but even so, it had to be massive, and so did the shock absorbers that protected the crew. Solem realized that his sail could create a canopy that could intercept a much larger angle from the detonation point, and that the tethers could be made long and elastic enough to smooth out the acceleration experienced by the canopy.
Solem also considered a combination of tethers working with a servo winch in the space vehicle itself, a method with several advantages, as suggested in the same Los Alamos report:
When the explosive is detonated, a motorgenerator powered winch will pay out line to the spinnaker at a rate programmed to provide a constant acceleration of the space capsule. The motorgenerator will provide electrical power during this phase of the cycle, which will be conveniently stored. After the space capsule has reached the same speed as the spinnaker, the motorgenerator will draw in the line, again at a rate programmed to provide a constant acceleration of the space capsule. The acceleration during the draw-in phase will be less than during the pay-out phase, which will give a net electrical energy gain. The gain will provide power for ancillary equipment in the space capsule…
Image: Medusa in operation. Here we see the design 1) At the moment of bomb explosion; 2) As the explosion pulse reaches the parachute canopy; 3) Effect on the canopy, accelerating it away from the explosion, with the spacecraft playing out the main tether with its winch, braking as it extends, and accelerating the vehicle; 4) The tether being winched back in. Imagine all this in action and the jellyfish reference becomes clear. Credit: George William Herbert/Wikimedia.
Solem was keenly aware of the radiation problem posed by Orion, noting that Medusa would be assembled in space and probably launched from one of the Lagrange points, well out of the magnetosphere so that no charged particles would be trapped into Earth-bound trajectories. Interestingly, he thought of Medusa in terms of interplanetary rather than interstellar flight, noting that a major benefit of the proposal would be to reduce travel times that would lead to crew exposure to solar flare radiation and galactic cosmic rays. Such exposure led to proposals for massive shielding, whereas the swift Medusa would cut travel times by a factor of 5 to 10, with part of the shield being made up of the nuclear bombs that would be used as fuel. He even envisions astronauts using a crawl space inside the fuel as shelter during a solar storm.
Solem believed the best canopy material would be a high-strength aligned polyethylene of the kind that advances in materials technology should make available in the future. In the Los Alamos report he notes that:
We can reduce the mass of the canopy indefinitely by increasing its radius and the number of tethers. The tethers and the canopy material become progressively thinner. Mylar can be fabricated to a thickness of about ¼ mil, but other practical considerations, such as cost, will come into play long before the fabrication limit is reached. I will be conservative and say that we can spin-deploy a canopy 500 m in radius with 104 tethers.
More about Medusa, its possible interstellar applications, and hybrid mission designs on Monday. The Los Alamos report I refer to above is Solem’s “Some New Ideas for Nuclear Explosive Spacecraft Propulsion,” LA-12189-MS, October 1991 (available online). Solem also wrote up the Medusa concept in “Medusa: Nuclear Explosive Propulsion for Interplanetary Travel,” JBIS Vol. 46, No. 1 (1993), pp. 21-26. Two other JBIS papers also come into play for specific mission applications — I’ll give the citations for those next week.
by Paul Gilster | Jul 19, 2012 | Exoplanetary Science |
Finding new worlds with Kepler is an absorbing occupation, but the one thing missing from most exoplanet news is proximity. While we continue to search for planets around the Alpha Centauri stars, the closest candidate I know about is the gas giant thought to orbit Epsilon Eridani, some 10.5 light years out. If you’re looking for potential habitability, you have to extend all the way out to Gliese 581 (almost twice the distance), where planets are plentiful and there is at least the chance (GL 581d) that one skirts the edge of the habitable zone. There are probably many planets closer than 20 light years, but we don’t have the tools in space to find them easily.
Kepler, you’ll recall, studies a field of stars in Cygnus, Lyra and Draco, the goal being to develop a statistical approximation of the prevalence of Earth-sized planets in the galaxy. Looking out along the Orion arm, Kepler is watching stars anywhere from 600 to 3000 light years away. In fact, fewer than 1 percent of the stars Kepler sees are closer than 600 light years. We have missions on the drawing board like TESS — Transiting Exoplanet Survey Satellite — which are designed to study 2.5 million of the brightest stars in the sky, looking for nearby transiting planets, and the European Space Agency’s PLATO — PLAnetary Transits and Oscillations of stars — would likewise go to work on relatively bright stars near our Solar System.
Will these missions ever fly? While we wait for the inexorable review process to grind its way forward, it’s gratifying to see the discovery of another planet that’s not, in astronomical terms, all that far away. The candidate exoplanet, UCF-1.01, is 33 light years from us orbiting the red dwarf GJ 436, already known to be home to a hot Neptune designated GJ 436b. Working at the University of Central Florida, Kevin Stevenson and colleagues went to work on what appeared to be the signature of a small planet, going through observations from Spitzer as well as the Deep Impact spacecraft, the Very Large Telescope and the Canada-France-Hawaii Telescope on Mauna Kea. The find marks the first time Spitzer has been used in a transit discovery.
Image: GJ 436, a red dwarf in the constellation of Leo with a ‘hot Neptune’ and two new exoplanet candidates. Credit: ESO Online Digitized Sky Survey.
It’s an interesting little world, this UCF-1.01, with a diameter estimated at 8400 kilometers, or about two-thirds that of the Earth, quite a catch for Spitzer, which some project scientists believe may be able to discover exoplanets no larger than Mars. UCF-1.01 would orbit its star in a snappy 1.4 days, with surface temperatures reaching close to 600 degrees Celsius. Due to its size and proximity to the host star, the planet is unlikely to have retained its original atmosphere, though the paper does note that impacts or tidal heating could create a transient atmosphere.
Most likely, this is a dead, cratered world, conceivably a place covered in magma, according to Joseph Harrington (UCF), a co-author on the paper. If confirmed, the new world will be called GJ 436c, but the UCF team is not done yet. There is also evidence for a third world in this system, dubbed UCF-1.02. Confirming the new worlds will involve additional work. From the paper:
To definitively establish UCF-1.01 as a planet (to be called GJ 436c), we require only a few hours of additional observations, preferably from another telescope or at least at a different wavelength. Establishing UCF-1.02 as a planet (to be called GJ 436d) would likely require an extended observing campaign to constrain its period then successfully predict a transit.
We only have one Kepler world smaller than the two Spitzer candidates, though there will surely be more as the data analysis continues. Meanwhile, we’re still waiting for information about flight possibilities for PLATO, which will have a larger field of view than Kepler, while TESS is involved in a concept study that could lead to selection as a NASA Explorer-class mission. One way or another, through space-based resources or ground observation, the list of nearby exoplanets is going to grow, though we can only speculate on when — and where — we’ll find the first true Earth analog. Interesting, conceivably habitable M-dwarf planets are probably more common.
The paper is Stevenson et al., “Two nearby sub-Earth-sized exoplanet candidates in the GJ 436 system,” accepted for publication in The Astrophysical Journal (abstract).
by Paul Gilster | Jul 18, 2012 | Exotic Physics |
Anomalies are always fascinating because they cause us to re-examine our standard explanation for things. But in the case of the so-called ‘Pioneer anomaly,’ the Jet Propulsion Laboratory’s Slava Turyshev, working with a group of scientists led by JPL’s John Anderson, needed an explanation for practical reasons. The possibility that there was new physics to be detected had the scientists wondering about a deep space mission to investigate the matter, but missions are expensive and the case for a genuine Pioneer effect had to be strengthened or else put to rest.
All of this led Turyshev to begin a multi-year data-gathering mission of his own, scouring records related to Pioneer wherever they might be found to see if what was happening to the spacecraft could be explained. The effect was tiny enough that it was originally dismissed as the result of leftover propellant in the fuel lines, but that explanation wouldn’t wash. Something was causing the two Pioneers to decelerate back toward the Sun, a deceleration that was finally measured as being about 300 inches per day squared (0.9 nanometers per second squared).
I love Turyshev’s quote on the matter, as seen in this JPL news release:
“The effect is something like when you’re driving a car and the photons from your headlights are pushing you backward. It is very subtle.”
Subtle indeed, but combing through telemetry and Doppler data, the team made a number of memorable finds, starting with the discovery of dozens of boxes of magnetic tapes stored under a staircase at JPL itself. This one is a story I always cite when talking about the danger of data loss in a time of digital information. Fully 400 reels of magnetic tape were involved, carrying records from the 114 onboard sensors that charted the progress of each of the two missions. All this information had to be transferred to DVD, as did other data from floppy disks that had been preserved at NASA’s Ames Research Center by mission engineer Larry Kellogg.
Image: For old times’ sake (and because these guys are heroes of mine), great figures from the Pioneer era, here seen celebrating after what turned out to be one of the last contacts with Pioneer 10 in 2002 (the final contact was made in January of 2003). Left to right: Paul Travis, Pioneer senior flight controller; Larry Lasher, Pioneer project manager; Dave Lozier, Pioneer flight director and Larry Kellogg, project flight technician. Credit: NASA Ames.
We came just that close to losing the key Pioneer data altogether, a reminder of the need to back up information and convert it into new formats to ensure its preservation. The Pioneers were launched at a time when data were routinely saved on punch cards which were themselves then converted into different formats at JPL, and other information had to be tracked down at the National Space Science Data Center at NASA Goddard in Greenbelt, MD. All told, Turyshev and team collected about 43 gigabytes of data and – a close call indeed – one of the tape machines needed for replaying the magnetic tapes, an item about to be discarded.
In the salvaged files from the Pioneers we learn the secret of the anomaly: Heat from electrical instruments and the thermoelectric power supply produces the effect detected from Earth, or in the words of the recently published paper on this research, the anomaly is due to “…the recoil force associated with an anisotropic emission of thermal radiation off the vehicles.” Take account of the thermal recoil force and no anomalous acceleration remains. If this work stands up, the Pioneer anomaly, well worth investigating because it seemed to challenge our standard model of physics, can be explained in a way that is consistent with that model in every respect.
The paper is Turyshev et al., “Support for the Thermal Origin of the Pioneer Anomaly,” Physical Review Letters 108, 241101 (abstract). Credit for the Pioneer image at the top of the page: Don Davis/NASA.
