by Paul Gilster | Feb 9, 2009 | Interstellar Medium |
Almost four years ago I wrote a Centauri Dreams entry about Dana Andrews’ views on shielding an interstellar spaceship. The paper is so directly relevant to our recent discussion on the matter that I want to return to it here. Andrews (Andrews Space, Seattle) believes that speeds of 0.2 to 0.3 c are attainable using beamed momentum propulsion. That being the case, he turns in his “Things to Do While Coasting Through Interstellar Space” paper to questions of human survival.
Particles with a Punch
Collision with interstellar dust becomes a major issue when you’re traveling at speeds like these, a fact Andrews is quick to quantify. For a starship moving at 0.3 c, a typical grain of carbonaceous dust about a tenth of a micron in diameter should have a relative kinetic energy of 37,500,000 GeV. Our hypothetical star mission with human crew moving at a substantial fraction of light speed will run into about thirteen of these dust particles every second over every square meter of frontal area.
This gets interesting when put in the context of cosmic rays. Most galactic cosmic rays, which as Andrews notes are completely ionized atoms accelerated to extremely high energy states, have energies between 100 MeV and 10 GeV. You can see the overlap. Travel fast enough and even small grains of dust behave like energetic cosmic rays as our vessel encounters them. Clearly, dust between the stars is something we have to reckon with on any interstellar journey.
Absorbing Particles (and the Cost)
In my post on Friday, I looked at the Project Daedalus dust shield and its role in the journey to Barnard’s Star. A key question: Do we want to absorb cosmic rays and dust particles, or redirect them? The former can be envisioned in terms of a human crew surrounded by layers of supplies and equipment, with an outer shell for the spacecraft composed of 25 cm of multiple layers of polyamides, metal foils and polyethylenes to assist in radiation protection. The mass of structure and shielding obviously becomes a major factor. As Andrews writes:
The striking facts from this mass statement are the large masses associated with structure and shielding, and the small masses associated [with] things like food and water. This is because all waste and CO2 is recycled through growing plants and algae to provide clean air, food, and water.
The author is figuring that three percent of food intake would have to be derived from stored supplies to offer the necessary trace minerals and nutrients. He adds a 33 percent margin to that number and bumps the stored food percentage up to four percent, along with an extra year’s food supply as margin in case of intermittent problems with the life support system along the way.
As to protection from galactic cosmic rays (GCR), we can create a workable but quite bulky shielded environment using these methods:
Since 99% of the GCR is ionized hydrogen or helium, it’s obvious why hydrogen makes the best shielding (because like masses scatter better). Hence, plastics with high hydrogen content were selected for the hull and shielding materials. The total shield thickness for the hull alone is about 20 gm/cm2, which will cut the dose rate to about 25 rem/year. Assuming the sleeping quarters are 3 meters by 4 meters and 2.3 meter high we can shield the walls with approximately 30 gm/cm2 of water storage (top and bottom shielded by dry storage and machinery), which should reduce the annual dose to about 15 rem. That’s three times the recommended yearly dosage for radiation workers in the USA, but within (barely) the overall guidelines for astronauts.
A Magnetic Shielding Option
But there is another way to solve the problem. Magnetic shielding, using large current loops of superconducting wire to create a protective magnetic field, could reflect or deflect charged particles around the habitat. And the advantages of using the magnetic approach are considerable.
Get this: Although the mass of the magnetic shielding support structure is close to that of the hull shielding option we looked at above, the living quarters go from a space seven meters high and ten meters long (in three levels) to a habitat 200 meters in diameter with over 6000 cubic meters of usable volume. Foil bumpers are used to break down incoming dust into atoms and ionize the result, which can be then handled by the magnetic field. We arrive at a design like the one below:
Image: Magnetically shielded interstellar habitat (to scale). Credit: Dana Andrews.
Clearly we have a long way to go before building such a vehicle, considering our own halting attempts at sustainable life support (Andrews points to the problems of Biosphere 2, and to the challenging environment aboard the International Space Station). But there is nothing to prevent us from refining these technologies, while magnetic shielding and dust particle ionization is well within the realm of conventional physics. So there are ways around the dust and radiation problem if we do venture between the stars.
Just how a workable life support system could be maintained is the subject of an intriguing appendix. The 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.
by Paul Gilster | Feb 6, 2009 | Interstellar Medium |
It would be helpful if space were a bit more empty. A key problem facing an interstellar probe would be encounters with dust in the planetary system it leaves and, as it reaches cruising speed, dust impact in space between the stars. Although our Solar System seems to be in an unusually sparse pocket of space, the galaxy-wide distribution of hydrogen is roughly one atom per cubic centimeter. Dust — bits of carbon, ice, iron compounds, and silicates — is far rarer still, but enough of a factor to a ship moving at a significant fraction of the speed of light that the designers of the Project Daedalus craft built in a payload shield 32-meters in radius to protect their starship.
Then again, much depends on your location. Have a look at the image below. It’s an area called the Red Rectangle some 2300 light years from Earth in the constellation Monoceros. Although the center of the image seems to be a single star, it’s actually the double star system HD 44179. The Red Rectangle is a nebula, a cloud of gas and dust that shows what can happen when we move into areas of intense dust concentration. You can imagine that movement through an environment like this one would demand serious shielding requirements for any craft on a mission of interstellar exploration. And although we can avoid nebulae, any interstellar flyby probe has to reckon on the gas and dust within its destination system as it screams through to study the inner planets.
Obviously, we’d like to know more about dust, and that’s a problem. Donald York (University of Chicago) states the matter baldly, saying of interstellar dust “We not only do not know what the stuff is, but we do not know where it is made or how it gets into space.” York and collaborators have been studying the Red Rectangle looking for clues, and they’ve turned up a workable hypothesis.
Image: A Hubble Space Telescope image of the Red Rectangle. What appears to be the central star is actually a pair of closely orbiting stars. Particle outflow from the stars interacts with a surrounding disk of dust, possibly accounting for the X shape. This image spans approximately a third of a light year at the distance of the Red Rectangle. Credit: Credit: NASA; ESA; Hans Van Winckel (Catholic University of Leuven, Belgium); and Martin Cohen (University of California, Berkeley).
One of the stars at the heart of the Red Rectangle nebula has burned through its initial hydrogen, collapsing upon itself until it could generate the heat to burn helium. Such stars go through a period of transition that, in a matter of tens of thousands of years, causes the star to lose an outer layer of its atmosphere. The thinking is that dust forms in this cooling layer and is then pushed out from the star by radiation pressure, along with large amounts of gas. The larger star in the Red Rectangle is too hot to concentrate dust in its atmosphere, but double-star systems like this one often show a disk of material forming around the second star, creating a jet that blows dust out into the interstellar medium. Here’s Adolf Witt (University of Toledo) with more detail:
“Our observations have shown that it is most likely the gravitational or tidal interaction between our Red Rectangle giant star and a close sun-like companion star that causes material to leave the envelope of the giant. The heavy elements like iron, nickel, silicon, calcium and carbon condense out into solid grains, which we see as interstellar dust, once they leave the system.”
Take the Red Rectangle process and make it ubiquitous and you’ve located one source for the dust that is such a factor in interstellar space. Back to Daedalus, which was designed to move at 12 percent of lightspeed for the fifty year journey to Barnard’s Star. Along with its beryllium shield for the cruise phase, Daedalus would have needed additional protection for the stellar encounter, which designer Alan Bond suggested could take the form of a cloud of dust deployed from the main vehicle, heating and vaporizing any larger particles before they could damage the payload. And because Daedalus would deploy smaller probes within the system, each would need a cloud of its own.
Gregory Matloff and Eugene Mallove once suggested that a starship could use, in addition to a shield, a high-powered beamed energy device to destroy or deflect any larger objects in its path. So the options for interstellar protection are slowly being placed on the table. But first we have to learn more about the nature of the problem, which means studies like these that tell us how dust forms in the first place. The paper is Witt et al., “The Red Rectangle: Its Shaping Mechanism and its Source of Ultraviolet Photons,” accepted for publication in the Astrophysical Journal and available online. A University of Chicago news release is available.
by Paul Gilster | Jan 14, 2009 | Interstellar Medium, Sail Concepts |
Riding the solar wind with some kind of magnetic sail is one path into the outer Solar System, but before we can develop an operational technology around the idea, we have to learn much more about how the solar wind works. This stream of charged particles flows outward from the Sun at great speed — up to well over 400 kilometers per second — creating the ‘bubble’ in the interstellar medium known as the heliosphere, within which our Solar System exists. Understanding how that wind interacts with the true interstellar space that lies beyond will give us a better idea of its properties and those of the boundary region at system’s edge.
Image: The Solar System in context, placed within the heliosphere created by the solar wind. Credit: Southwest Research Institute.
IBEX (Interstellar Boundary Explorer) is a space mission that may tell us more as it examines the edge of the heliosphere. Tuned up after two months of commissioning, the spacecraft is now gathering data, mapping the interactions between the solar wind and the interstellar medium by measuring energetic neutral atoms from the Solar System’s edge. Thus we complement and extend the early heliosphere data provided by the Voyagers and investigate how charged solar wind particles behave in this critical area, through which any mission into the interstellar void will have to pass. The data thus far are useful, but only a beginning:
“We are seeing fabulous initial results from IBEX, but just as artisans use looms to build up colorful textiles by weaving one thread at a time, the IBEX sensors also need time — six months — to build up a complete map of the sky,” says Dr. David McComas, IBEX principal investigator and senior executive director of the Space Science and Engineering Division at Southwest Research Institute. “So far, the intricate pattern of this fascinating interaction is only just beginning to disclose itself to us.”
This is useful information not only in terms of understanding how our local neighborhood interacts with the galaxy around it, but also in analyzing how this distant region helps to shield the Earth from a great deal of cosmic ray radiation. Also useful would be an accurate picture of the shape of the heliosphere, and how it may be affected by magnetic fields in the interstellar medium. Given the target of its study, it’s interesting to reflect that IBEX is relatively close — in Earth orbit — although a very high one reaching five-sixths of the way to the Moon. Much of that orbit is outside Earth’s magnetosphere, critical for accurate observations.
by Paul Gilster | Jul 2, 2008 | Interstellar Medium |
My wife is the most gifted poet I know. I often marvel at her ability to see things with new eyes, to take experiences we have shared and look at them with such a fresh and uncluttered view that the events are transformed and new meaning extracted from them. All of which came to mind this morning in a far different context as I pondered how good science does much the same thing. A case in point in this ‘poetry of science’ is offered by a view of the edge of the Solar System made not with photons but with neutral atoms, in data gathered by the twin STEREO spacecraft. It’s a new kind of astronomy that draws on a different way of looking at the unexplored frontiers of the heliosphere.
Our Voyager spacecraft, of course, are in this region, so we’re getting new data all the time, but from an optical perspective, the outer heliosphere is invisible. This is where the solar wind — that stream of charged particles moving outward from the Sun — reaches the limits of the Sun’s influence, a place known as the heliopause. But we can get more specific still and talk about the heliosheath, a region filled with plasma on the borderline between the heliosphere and true interstellar space. Voyager 2 (having crossed the ‘termination shock,’ where the solar wind slows as it pushes against interstellar matter) is now in the heliosheath. And what the STEREO data are showing us is what happens to the energies dissipated in the area of that heliosheath.
But wait — aren’t the two STEREO spacecraft designed to study the Sun, or more specifically, solar storms as they move through space? True enough, but detectors on each have detected neutral atoms originating from the heliosheath region. What seems to be happening, according to a paper in Nature on this subject, is that ions heated up in the termination shock wound up losing their charge to cold atoms in the interstellar medium, thus beginning a flow back toward the Sun, which is what the STEREO spacecraft are detecting. Says Linghua Wang (UC Berkeley):
“We were surprised that these particle intensities didn’t depend on the magnetic field, which meant they must be neutral atoms… This is the first mapping of energetic neutral particles from beyond the heliosphere. These neutral atoms tell us about the hot ions in the heliosheath. The ions heated in the termination shock exchange charge with the cold, neutral atoms in the interstellar medium to become neutral, and then flow back in.”
Image (click to enlarge): STEREO detected energetic neutral atoms (ENAs) from the edge of the solar system, where the solar wind meets the interstellar medium. Hot ions in the heliosheath — the region between the termination shock and heliopause — are uniquely traced by ENAs and are more intense (indicated by color code) around the nose of the heliosphere, with an asymmetric double peak. The twin STEREO A and B spacecraft are shown in the sun-centered orbit they share with Earth. Last year, the Voyager 2 spacecraft passed into the heliosheath, joining Voyager 1. There, these interstellar explorers continue their journey into the farthest reaches of the heliosphere. Credit: University of California, Berkeley; L. Wang.
We’ll be watching with interest as the Interstellar Boundary Explorer (IBEX) prepares for launch later this year. The mission is designed to map lower-energy ions in the heliosheath using neutral atoms, extending our understanding of how the termination shock is put together. That’s useful information as we probe the effects of our star’s motion through the local interstellar medium, a region we will someday explore with tools specifically designed (unlike Voyager’s) for the purpose of analyzing interstellar space.
Pushing out twice the distance of Pluto, the heliosphere is the realm through which all human spacecraft have thus far moved. Innovative Interstellar Explorer or another such mission will one day establish a dedicated observatory outside it as we learn more about the conditions through which future interstellar missions may fly. The STEREO results are a useful step. The spacecraft are closer to home, but they’re showing us things we’ve never seen, and there’s a bit of poetry in that as well.
A number of papers (and an impressive cover) are devoted to the latest studies of the outer edges of the heliosphere in Nature. The paper discussed here is Richardson et al., “Domination of heliosheath pressure by shock-accelerated pickup ions from observations of neutral atoms, ” Nature 454 (3 July 2008), pp. 81-83 (abstract). Check this editor’s summary for all other related references.
by Paul Gilster | Feb 28, 2008 | Interstellar Medium |
We think about our interstellar neighborhood in terms of stars, like Alpha Centauri and Tau Ceti, but the medium through which our relative systems move is itself a dynamic and interesting place. The Sun is currently passing through a shell of material known as the Local Interstellar Cloud. And that cloud is, in turn, located at the edge of a vast region known as the Local Bubble, scoured of material by supernova explosions in the nearby Scorpius-Centaurus and Orion Association star-forming regions. Within the past 105 years, the Sun emerged from the interior of the Local Bubble; it now moves obliquely in the direction of the high-density molecular clouds of the Aquila Rift, a star-forming region that itself reminds us how energetic ’empty’ space really is.
If we’re ever going to send fast missions outside the Solar System, we’re going to need plenty of data about the materials through which our vehicles move, particular as velocities mount to the point where collision with even small particles can be devastating. As of now, we’re only piecing together a broad idea of the galactic environment within 500 parsecs of the Sun, and remain in need of more detailed, in situ results from regions much closer in. Fortunately, space-based data have begun to accumulate.
Image: An artist’s take on the Ulysses mission, now approaching the end of its life, but a workhorse that has taught us much about the interstellar medium. Credit: Jet Propulsion Laboratory.
Take the Ulysses mission, which Harald Krüger (Max-Planck-Institut für Sonnensystemforschung) and Eberhard Grün (Max-Planck-Institut für Kernphysik) examine in their recent paper. The scientists take a hard look at the Sun’s position and its relation to nearby interstellar dust, noting interesting clues like the Arecibo radar findings about micron-sized interstellar meteor particles, which seem to radiate from the direction of the Geminga pulsar. Are these evidence that the supernova responsible for the Geminga pulsar created the Local Bubble itself?
At this point, we simply don’t know, but space-based studies of these exotic materials should help to clarify things. First examined during the 1930s, when astronomical evidence of starlight scattering and weakening gave clear signs of its existence, interstellar dust became available for close study when spacecraft began flying dust detectors. It became clear thirty years ago that interstellar dust grains can cross the heliospheric boundary and make their way toward the Solar System. In the 1990s, Ulysses’ dust instrument identified the mass, speed and approach direction of impacting grains as they swept through the heliosphere.
Krüger and Grün write about using future Ulysses studies to learn more, but recent NASA news seems to rule that scenario out, as the spacecraft’s power systems are in a state of terminal decline. Future Ulysses studies would have been useful for two reasons: The spacecraft’s data would have covered the entire 22-year range of a solar cycle, offering a unique set of measurements. And further readings from the outer heliosphere might have made it possible to explain the origin of an odd thirty degree shift in the flow direction of the dust Ulysses monitored.
For data from the period 1996 to 2000 showed a specific impact direction from interstellar dust, which little prepared investigators for the following:
Six years later, when Ulysses was travelling through almost the same spatial region and had an almost identical detection geometry for interstellar grains, the situation was vastly different: ?rst, the range in approach directions of the grains was somewhat wider…; second, and more noticeable, in 2005/06 the approach direction of the majority of grains was shifted away from the helium ?ow direction. Preliminary analysis indicates that this shift is about 30? away from the ecliptic plane towards southern ecliptic latitudes… At the moment, we do not know whether it is a temporary shift limited to the time period stated above or whether it continues to the present time. Furthermore, the reason for this shift remains mysterious. Whether it is connected to a secondary stream of interstellar neutral atoms shifted from the main neutral gas ?ow… is presently unclear.
So even within the Local Interstellar Cloud, through which we now move, the concentration of dust is little understood. Properly interpreted, these dust grains may be able to tell us more about how heavy elements are moved about within the interstellar medium and how they are affected by the five distinct clouds of gaseous material within five parsecs of the Sun. Data from the recent Stardust mission should be of help, their interpretation aided by what Ulysses has found so far. What we lack, of course, are direct observations of interstellar dust outside the Solar System, something that future missions like Innovative Interstellar Explorer may remedy if funded for launch some time in the next decade.
The paper is Krüger and Grün, “Interstellar Dust Inside and Outside the Heliosphere,” submitted to Space Science Reviews and available online.
by Paul Gilster | Aug 20, 2007 | Interstellar Medium |
I don’t want today to pass without noting that it is the thirtieth anniversary of the launch of Voyager 2. Both Voyagers remain healthy, continuing studies of the solar wind, magnetic fields and energetic particles with their five functioning science instruments. As this JPL news release notes, the Voyagers run on less than 300 watts of power, which they tap from radioisotope thermoelectric generators. At 15.5 billion kilometers (Voyager 1) and 12.5 billion (Voyager 2), the vehicles are the farthest human-made objects, unable to use the power of distant Sol.
Image: Artist concept of the two Voyager spacecraft as they approach interstellar space. Image credit: NASA/JPL.
So our first mission into nearby interstellar space continues to go quite well, with both spacecraft reporting home despite one-way radio travel times of fourteen and twelve hours respectively. Voyager 1 seems to have encountered the heliosheath — where the solar wind slows as it encounters the thin gas between the stars — in late 2004. Voyager 2’s likely encounter with the heliosheath, which may begin later this year, should provide further information about this distant area. Larry Klaes published a nice overview of Voyager’s famous golden records today.
