An absorption nebula, or ‘dark’ nebula, is a dense cloud of interstellar dust that can block the visible light from objects within or behind it. Today’s image shows a striking cloud in the star-forming region Lupus 3, a dazzling view that simply demanded placement on Centauri Dreams. If you’re looking for an interstellar flight angle, think about the issue of shielding a relativistic starship in regions so dense with gas and dust, zones that can stretch for hundreds of light years. But I need no other angle here — the image is majestic in and of itself.
Image: A dark cloud of cosmic dust snakes across this spectacular wide field image, illuminated by the brilliant light of new stars. This dense cloud is a star-forming region called Lupus 3, where dazzlingly hot stars are born from collapsing masses of gas and dust. This image was created from images taken using the VLT Survey Telescope and the MPG/ESO 2.2-metre telescope and is the most detailed image taken so far of this region. Credit: ESO/R. Colombari.
Lupus 3 is found in Scorpius, about 600 light years from Earth, part of the larger complex called the Lupus Clouds. The cold dust absorbs and scatters light as it passes through the cloud, making objects within them viewable only in radio or infrared wavelengths. Frozen carbon monoxide and nitrogen are primary components blocking visible light, but molecular hydrogen, helium and a variety of more transparent ingredients are found within. Some of these absorption nebulae are familiar, great black swathes against background stars, like the Coalsack Nebula.
Lupus 3 is rich in T Tauri stars, variables that are pre-main-sequence stars in the process of contracting. These are young objects that have not yet begun hydrogen fusion at the core but are still powered by gravitational energy as their contraction takes them toward the main sequence. Note the two bright stars in the center of the above image. Growing hotter and brighter, their radiation and stellar winds would have swept out nearby gas and dust, allowing their emergence at visible wavelengths. Our Sun may have formed in a region much like this.
Image: This wide-field view shows Lupus 3, where new stars are forming along with clusters of brilliant stars that have already burst out of their dusty stellar nursery. It is likely that the Sun formed in a similar star formation region more than four billion years ago. This view was created from images forming part of the Digitized Sky Survey 2. Credit: ESO/Digitized Sky Survey 2. Acknowledgement: Davide De Martin.
Shielding a Starship
Let’s get back to interstellar flight issues for a moment. So much depends on where we are traveling when we think about dust mitigation. When the Daedalus designers worked on the first full specifications for a starship back in the 1970s, they included a payload shield some 32-meters in radius. Future starships moving at a high fraction of c would obviously do well to avoid dense nebulae, but we also have to think about shielding fast flyby probes as they enter destination systems, where gas and dust levels can reach dangerous levels.
The Daedalus beryllium shield was one option, and Gregory Matloff has suggested that a forward-firing laser could also be deployed to deflect larger particles along the route. The issue could be significant: Dana Andrews (Andrews Space) has calculated that for a starship moving at 0.3 c, a tenth of a micron grain of typical carbonaceous dust would have a relative kinetic energy of 37,500,000 GeV. We have much to learn about the true size distribution of dust particles in the nearby interstellar medium before embarking on interstellar missions, which is Bruce Draine, author of the definitive Physics of the Interstellar and Intergalactic Medium, is serving as a consultant with the Breakthrough Starshot effort.
For more on star formation in regions like this, see Comerón, “The Lupus Clouds,” in Handbook of Star Forming Regions Vol. II, Astronomical Society of the Pacific (2008), available here in full text. Dana Andrews’ paper is Andrews, “Things to Do While Coasting Through Interstellar Space,” AIAA-2004-3706, 40th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Fort Lauderdale, Florida, July 11-14, 2004.
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Maybe we should send off an ‘instellar dust mission’ or two before really getting going?
If mass-production of laser-pushed lightsail WaferSat starprobes (see: http://arxiv.org/pdf/1710.10732.pdf , http://www.deepspace.ucsb.edu/projects/starlight , and http://www.deepspace.ucsb.edu/wp-content/uploads/2016/10/Brashears_etal_2016_SPIE_WaferSatPrototype.pdf ) becomes practical, it would be prudent–and scientifically interesting in its own right–to dispatch some probes to high dust density, low dust density, and dust cloud regions of interstellar space. The same could also be done for areas of interstellar space containing low, high, and “cloud density” (neutral and excited-to-emission) areas of hydrogen and various chemical compounds, which often also contain dust. This could also help to provide “ground truth” data on the relative abundances and densities of the elements and compounds drifting between the stars that are mapped by radio telescopes.
Arthur C Clarke’s “The Songs of Distant Earth” has the starship Magellan shielded with water ice, the replenishment of which is the setup for the ship’s arrival at the planet Thalassa. The abundance of water everywhere makes this a suitable shield material, which can also be used as an emergency supply of water and oxygen for the crew.
In practice, it might make more sense to replenish the shield in the local Oort cloud unless shore leave is an important function. For crew in hibernation, or the transport of cargo, or even frozen biological tissues, a purely automated rebuilding of the shield with cometary ice might be the better way for interstellar travel, especially if the ice is also needed as a source of fusion fuel.
Mapping the density of dust in interstellar space might be an important function before interstellar flight becomes “common”, much as maps of shoals and reefs are important to terrestrial ocean-going ships. If the dust density in a solar system is where most of the damage occurs, then ships should steer clear of systems. For ships approaching target systems, they will likely have decelerated sufficiently to make this dust less of a problem.
Just a word of praise really. I frequent your site because I always learn much more than I could by wading through the heavy science journals. It is exciting to learn how the dust in Lupis 3 functions within the larger context of star formation and of course interstellar travel. The Comeron review is so readable, I think I will actually finish reading the whole thing! Bruce Draine’s e-book is within my budget but I would ask if you or anyone familiar with it would think a lay science geek would understand it?
Moreover, I have enjoyed your most recent series of posts very much indeed. Thanks for making it easy and thought provoking at the same time. I look forward to each new post, Sensi :)
Thanks for the kind words! I’ve only read parts of the Bruce Draine book when I needed some targeted research, but I think the thing to do, given the cost of some of these texts, is to try to locate a copy at a university library just to get a feel for the text. You will probably wind up with a copy, but I’d recommend checking it first just to be sure. I had the chance to talk to Dr. Draine at one of the Breakthrough committee meetings — he’s very straightforward and what I’ve seen of his prose is clear and concise.
Those interstellar dust clouds bring to mind three science fiction works, Fred Hoyle’s “The Black Cloud,” the 1973 animated “Star Trek” series episode “One of Our Planets is Missing,” and Poul Anderson’s “Tau Zero” (whose title is ‘organizationally familiar’ around these parts…). While these dust clouds present a problem, I wonder if they might also offer an opportunity:
In Arthur C. Clarke’s 1957 book “The Making of a Moon: The Story of the Earth Satellite Program,” he mentioned a variant of the ion engine that should produce greater thrust, because of the mass of its exhaust. Also, there is an older, grid-less ion thruster design that may work in that mode:
Clarke wrote about ion engines using either ionized inert gases (as current ones do), *or* electrostatically-charged *dust* as the propellant. Since dust (being a finely-divided solid) is much more massive (per volume of material) than any gas, it should provide more thrust and higher acceleration, *and* it could easily be sourced from any solid celestial body (and perhaps also from interstellar dust clouds via a ramscoop, using a foward-sweeping laser to ionize the dust particles)—even an icy one such as a comet, as they contain much dust—so that such a spaceship (or even a space probe) could easily refuel itself from such bodies. (If necessary for refueling from some bodies [or even the…comae? of comets], a set of grinding wheels, perhaps aboard a wheeled or “Phobos hopper”-type rover, could produce dust of the necessary fineness [mesh size] to make good ion thruster fuel.) Also:
Gerard O’Neill’s electrically-driven mass driver engines for space tugs and spaceships are conceptually the same, but his engines’ much larger pellets of compressed lunar or asteroidal regolith would create a meteoroid hazard, while (within a solar system) sunlight pressure, the Yarkovsky effect, and the YORP effect would tend to prevent fine ion engine “dust exhaust” from doing the same thing. in addition:
One engine that is rather like a dust-fueled ion thruster is the NanoFET (see: https://www.centauri-dreams.org/?p=8647 and http://www.google.com/search?q=nanofet+propulsion&cad=h ), whose reaction mass nanoparticles are comparable in size to dust particles. Regarding an older ion thruster design that might work with dust as fuel:
The May 1961 issue of “Mechanix Illustrated” magazine has a cover article (see: http://www.google.com/search?ei=9YBzWouDM5TYjwPcp5m4Cw&q=Mechanix+Illustrated+May+1961+issue&oq=Mechanix+Illustrated+May+1961+issue&gs_l=psy-ab.12..33i160k1.3310.19582.0.224188.8.131.52.184.108.40.206.2098.2j17.19.0….0…1c.1.64.psy-ab..0.31.1659…0j0i131k1j0i67k1j0i22i30k1j33i22i29i30k1j33i21k1.0._3KD3yH6XOU ) about a grid-less ion engine that Goodrich High-Voltage Astronautics built and successfully tested in a vacuum chamber. This ion engine can also be seen (in a vacuum chamber) on page 111 of the May 1961 issue of “Popular Mechanics” magazine (see: http://books.google.com/books?id=5N0DAAAAMBAJ&q=ion+engine#v=snippet&q=ion%20engine&f=false [clicking on the blue “Page 111” lettering, or on the ‘half-picture’ itself, fully opens that page]). Instead of grids, this ion engine—a cutaway model of which is shown in the “Mechanix Illustrated” article—had two hollow, frustum-shaped (‘tip-less cone’ shaped) electrodes whose center holes faced each other, with a flat disc-shaped electrode (which also had a center hole) between them. “Downstream” from these electrodes were two focusing electrodes, and also another electrode which injected electrons into the ion beam, so that the engine emitted an electrically-neutral exhaust beam. Now:
Since this engine had no inward-protruding parts or grid electrodes that the exhaust beam could strike and therefore erode over time (it could run on argon gas or liquid metal fuel [mercury and/or cesium]), electrostatically-charged dust should also be able to pass through an ion engine of this design without sandblasting its innards. Perhaps an ion engine of this type, using ionized interstellar cloud dust drawn into the vehicle via a ramscoop and a dust-ionizing laser, could be used to explore the interior regions of the clouds (to examine “proto-proto-stars and planetary systems”), to serve as a “booster” for returning home (or moving on to other stars), and/or–if operated in a “reverse-thrust mode”–to provide electrostatic (with ion thrust assist) braking upon arrival at the interstellar clouds?
Perhaps this is similar
also many papers on electrostatic propulsion in here:
>relative kinetic energy of 37,500,000 GeV
The Telescope Array project regularly observes high energy cosmic rays three orders of magnitude higher than this. (Not very many, but still …) The largest such observation, seen at the precursor facility, is another order of magnitude higher; it’s nicknamed the Oh-My-God particle.
This project uses the atmosphere as a scintillation medium, which is the relevance for interstellar travel. Any spacecraft, together with all its contents, is a scintillation medium (intended or not) for any particle it hits, from protons on up. How much of that energy is deposited depends on cross-section and spacecraft geometry, but at modest spacecraft sizes much of the kinetic energy on an incoming particle will leave as kinetic energy of daughter particles. How much of a practical radiation threat this presents need precise estimates.
I imagine that interstellar probes will use nuclear pulse propultion and combine an Orion pusher plate and a Medusa drive parachute. Best of both worlds!
Nothing vaporized interstellar obstacles like a .15 kiloton nuclear shaped charge,
and any pesky fragments that do make it through, won’t make much of a dent in an Orion drive pusher plate.
So, the combination of Medusa drive AND Orion pusher plate seems to solve the problem nicely.
I would think a combination of particle beam and laser system would deal with any dust. The particle beam has great kinetic energy to break up a dust grain and the laser would disperse it more once it has broken up move effectively. If we had a small open conduit from the main drive engine all the way out the front of the craft high energy particles and light (gamma/x-rays-UV) could be used to disperse dust particles ahead of the craft.
I’d like to complain about your choice of units! An energy of 37,500,000 GeV seems like an immense number, but if the converter is correct, this is only about 0.006 J.
Yes numbers get strange when we talk about propulsion.
The worst example is found on atomic rockets when one photon drive eat so much energy it is mindboggle but the actual force propelliong the rocket is negligible.
I’ve long thought the ideal approach for dealing with interstellar dust is to support a light sail directly in the path of the ship. Dust particles hitting it will be converted to charged particle showers, which could be diverted by a magnetic field. The further ahead of the ship, the more the particle shower will have diverged before reaching the magnetic shield, but the wider the sail has to be to deal with particles that have an unusually high speed perpendicular to the flight path. Even at low relativistic speeds, the sail would not have to be very much wider than the ship unless very distant indeed.
Each particle would blow a very tiny hole in the sail, which in low dust regions would be of little concern, but could be patched by pushing a small piece of sail up against it.
Such a shield would be ineffectual against particles larger than dust, but they’re very infrequent.
Is this describing a lightweight “Whipple Shield” for starships?
I had to look that up, but, yes, I guess it is. A laser levitated Whipple sheild. Much lighter than an iceberg…
The details on the shield can be found in papers by London and Early: JBIS, v68, pp205-210, July 2015 and J. of Spacecraft and Rockets, v37, pp526-531
Since the solar system moves with respect to the local interstellar medium, we actually can measure the local interstellar dust properties. Interstellar gas generally ionized and blocked from the heliosphere at the contact shock by the sun’s magnetic field. The larger dust particles, however, penetrate and drift through the solar system where they have been measured by several spacecraft. Almost all these particles are less than one micron, but there is not enough data to properly characterize the potential population of rare larger particles. This data is likely adequate for any initial interstellar probes.
Current interstellar large vehicle design concepts use light foil shields like those suggested by Brett that provide adequate protection without the mass penalties or older ice or heavy metal shield concepts. Alex raises the most likely concern which is dust adjacent to a star for a fly-by mission at high velocities. This potentially denser and larger dust is usually concentrated in the local ecliptic plane which only rarely will be the approach angle.
The very small Breakthrough Starshot vehicles may not be able to use these foil shields. Understanding the dust hazard and design responses for these vehicles remains an interesting challenge.