Into the Interstellar Void

by Paul Gilster | Sep 1, 2010 | Interstellar Medium | 24 comments

We often think about interstellar probes only in the context of what they find at the end of their journeys — astrobiologically interesting planets seem to be the whole story. But not so fast. As Ian Crawford (University of London) notes in a recent paper, there are quite a few fascinating — and indeed critical — things we need to learn about interstellar space itself, in this case what is known as the Local Interstellar Medium (LISM). Crawford, who has been analyzing these matters for the Project Icarus team, notes how much we’ve learned about the LISM since the Daedalus days.

The new paper grows out of Crawford’s presentation at the British Interplanetary Society symposium ‘Project Daedalus – Three Decades On,’ which was held last September in London. It reflects his thinking on interstellar probes in relation to planetary and stellar studies and astrobiology as well as the nature of the medium through which the probe will fly. But today I want to focus on the LISM because what we might erroneously consider to be relatively empty space turns out to be challenging indeed.

Not So Empty Space

After all, at the kinds of speeds we’re discussing for an interstellar probe, somewhere in the range of 10 percent of lightspeed at minimum (Daedalus aimed at 12 percent), what seems empty void can be a minefield. Consider just a few of the issues Crawford raises here:

Direct measurements of the interstellar dust density, and the size distribution of dust particles, will be especially important because sub-micron sized dust particles will erode exposed surfaces (see the analysis performed for Daedalus by Martin [16]), and larger particles (i.e. larger than a few tens of microns) may, if present in the LISM, cause a catastrophic failure of the vehicle. Determining some of the other properties of the LISM will also be important for longer term planning of other interstellar propulsion concepts – for example, determining the LISM hydrogen density, and its ionisation state, will be important for assessing the future practicality of interstellar ramjets…

We’d better, in other words, get up to speed about the interstellar medium before we launch any true interstellar craft, and that means a series of early missions of a precursor nature that can tell us what to expect. Crawford notes that the Sun is currently located close to the boundary of a small low-density interstellar cloud known as the Local Interstellar Cloud (LIC), one of several such clouds within several parsecs of our Solar System — indeed, one study identifies seven such clouds within 5 parsecs of Sol. These are found within the Local Bubble, about which this:

These [clouds] are immersed in the very empty (n_H ~ 0.005 cm^-3) and probably hot (T ~ 10⁶ K) Local Bubble (LB) in the interstellar medium, which extends for about 60-100 parsecs from the Sun in the galactic plane before denser interstellar clouds are encountered (at high galactic latitudes the LB appears to be open, forming a chimney-like structure in the interstellar medium which extends into the galactic halo…)

We can learn a lot about these matters, including the properties of the Local Interstellar Cloud, by spectroscopic studies of interstellar absorption lines toward nearby stars. Investigations into our Solar System’s interactions with the heliosphere are also useful, but we’ll need to augment these with direct measurements of the interstellar materials just beyond the heliopause, and that means developing the capability to get space probes to distances up to 1000 AU or more.

A Rationale for Probing the Void

Beyond that, and having taken these findings in account in its design, a true interstellar probe would be a priceless tool for measuring everything from dust density and composition to the interstellar magnetic field strength enroute. Crawford points out that a spacecraft moving at 0.1c could do daily measurements 17 AU apart (roughly half the radius of the Solar System) which would offer unprecedented knowledge of the structure of the Local Interstellar Medium.

Centauri A and B evidently lie beyond the Local Interstellar Cloud, although the line of sight from Earth is dominated by another interstellar cloud known as the G cloud. An interstellar mission to this system would tell us whether the Sun is actually embedded in the Local Interstellar Cloud or just outside it in the region where the LIC is interacting with the G cloud. Conceivably a Centauri mission would thus get to sample two different interstellar clouds, along with the low density material between them. As Crawford writes, we would receive a windfall of data:

If the Sun does lie within the LIC, then a mission to α Cen would sample the outer layers of the LIC, an interval of low density LB material, the edge of the G cloud, and the deep interior of the G cloud. This would sample one of the most diverse ranges of interstellar conditions of any mission to another star located with 5 pc of the Sun, as most other potential targets lie within the LIC… Even if the Sun lies just outside the LIC (as argued by Redfield and Linsky [8]), the trajectory to α Cen would still permit detailed observations of the boundary of the G cloud (and its possible interaction with the LIC), and determine how its properties change with increasing depth into the cloud from the boundary.

Shielding Against the Medium

Needless to say, such a pathfinder mission would help in the planning of all follow-up interstellar missions. At 10 percent of lightspeed, a probe would have to be shielded against damage from high speed collisions with dust in the interstellar medium, a topic both the Daedalus and Icarus designers have had to take into consideration. It will be fascinating to see how the shielding options will be modified between Daedalus (1970s) and today’s Icarus design.

As I mentioned above, Crawford’s paper delves deeply into the rationale for an interstellar mission, going beyond the interstellar medium question to address planetary and stellar studies and astrobiology. It’s a fascinating rationale for undertaking these studies and continuing to advocate precursor missions. And note this final caveat re Alpha Centauri as a destination:

The relative proximity of α Cen, together with its interesting interstellar sightline and the presence of stars of three different spectral types, makes it an attractive target for humanity’s first interstellar mission. However, as the bulk of the scientific benefits of interstellar spaceflight pertain to planetary science and astrobiology, a final prioritization must await future developments in the detection of planetary systems around the nearest stars. Fortunately, expected advances in astronomical instrumentation over the next century should ensure that a comprehensive list of prioritized targets is available well before rapid interstellar travel is technically feasible.

The paper is Crawford, “The Astronomical, Astrobiological and Planetary Science Case for Interstellar Spaceflight,” published in the Journal of the British Interplanetary Society Vol. 62 (2009), pp. 415-421 (preprint).

IBEX: From System’s Edge to Nearby Space

by Paul Gilster | Aug 17, 2010 | Interstellar Medium | 1 comment

When the Project Daedalus team went to work to design a starship back in the 1970s, they contemplated using the atmosphere of Jupiter as their source for helium-3, an isotope needed in vast quantity for Daedalus’ fusion engines. More recently, though, attention has turned to the lunar surface as a possible source. Now the IBEX spacecraft, normally charged with studying the interactions between the heliosphere and what lies beyond, has been used to examine a useful recycling process as particles hit the Moon, pushed there by the Sun’s 450 kilometer per second solar wind.

A Glow of Energetic Neutral Atoms

The process is straightforward — lacking a magnetosphere, the Moon takes the full force of the solar wind, absorbing most of its particles into lunar dust. But the IBEX team, led by David McComas (Southwest Research Institute), has been able to show that about ten percent of the solar wind particles escape back to space in the form of energetic neutral atoms, or ENA’s, detectable by the spacecraft. IBEX, traveling in an eight-day orbit around the Earth, sees this ‘glow’ in its ENA detectors.

What IBEX detects are enough solar wind particles bouncing off the lunar surface as ENA’s to account for about 150 tons of hydrogen atoms per year, the rest remaining behind, some doubtless in the form of surface helium-3, whose measurement will one day help us calculate how useful a source it may become. But IBEX (Interstellar Boundary Explorer) is primarily focused on a much more distant venue. Its real mission is to see what happens to those same solar wind protons when they encounter interstellar atoms at the edge of the heliosphere.

ENA’s from Deep Space

Energetic neutral atoms are created at system’s edge when solar wind protons draw electrons from interstellar atoms, making them electrically neutral and thus no longer controlled by magnetic fields. Those ENAs that bounce back in the direction of the Earth can be recorded by IBEX, which studies a section of the sky about seven degrees across, scanning overlapping strips that complete a 360-degree map of the sky every six months.

The IBEX surprise, announced last October, was the discovery that the expected variations in emissions from the interstellar boundary were not evident. Instead, IBEX found what McComas at the time called “a very narrow ribbon that is two to three times brighter than anything else in the sky.” It’s chastening to remember that the Voyager spacecraft totally missed this feature because, unlike their point source measurements, IBEX can use its detectors to build up a complete map.

Charting Earth’s Magnetopause

The spacecraft has also been used to observe Earth’s magnetosphere from the outside, using the same ENA detection methods to see the interactions between the solar wind and the magnetic bubble surrounding our planet. Here the parallel between the heliosphere and the magnetosphere is interesting. The magnetosphere protects the Earth’s surface, causing the solar wind to pile up along its outer boundary (the magnetopause) before being diverted to the side. The heliosphere’s interstellar boundary, in a similar way, protects the Solar System from the worst effects of galactic cosmic ray radiation.

Image: IBEX found that Energetic Neutral Atoms, or ENAs, are coming from a region just outside Earth’s magnetopause where nearly stationary protons from the solar wind interact with the tenuous cloud of hydrogen atoms in Earth’s exosphere. Credit: NASA/Goddard Space Flight Center.

The IBEX team worked closely with the European Space Agency’s Cluster 3 spacecraft in observations made in March and April of last year. The new maps thus created show the teardrop shape of the magnetopause as solar wind protons pull electrons from hydrogen atoms in the Earth’s outer atmosphere. This region, the outer exosphere, is now shown to be tenuous indeed, with about eight hydrogen atoms per cubic centimeter. Thus ENAs helps us notch another needed measurement of a region that has been tricky to study.

The Wind and the Sail

You can see, too, that we’re gradually building up a picture of the solar wind that will help us analyze whether propulsion options like magsails have potential. Is the solar wind stable enough to allow accurate navigation by a magnetic sail-enabled spacecraft? Certainly the potential of hitching a 450-kilometer per second ride to the outer Solar System has appeal, and the deployment issues involved in large solar sails disappear with a magsail. But let’s see what IBEX and other missions can tell us about the solar wind as it reacts to the interstellar medium at system’s edge and plays against the magnetosphere closer to home.

You can read more about IBEX in this NASA mission page. For more on the solar wind’s interactions with the Earth’s magnetosphere, see Fuselier et al., “Energetic neutral atoms from the Earth’s subsolar magnetopause,” Geophysical Research Letters Vol. 37 (8 July 2010), L13101 (abstract). For the lunar observations, see McComas et al., “Lunar backscatter and neutralization of the solar wind: First observations of neutral atoms from the Moon,” Geophysical Research Letters Vol. 36 (2009), L12104 (abstract).

A Dusty Finish in Glasgow

by Paul Gilster | Apr 16, 2010 | Interstellar Medium | 11 comments

I’m always sorry when a good conference like the Royal Astronomical Society’s 2010 gathering ends, even if I’m attending it ‘virtually’ from the other side of an ocean. But virtuality has its advantages, as I’m reminded by several conference attendees who have struggled with Icelandic volcano ash when trying to book flights out of the UK. If I were with them in Glasgow, I’d praise my good fortune for extra time in Scotland and immediately take the train for Inverness, then on to Skye and the Inner Hebrides, where I’ve spent many good days and intend to spend many more.

Volcanic ash or no, it was a lively conference with tantalizing results on planetary system residues in white dwarfs and retrograde exoplanet orbits, and a number of other issues that can be found in the conference program. I’ll close our RAS coverage here with two items that deal with interstellar dust rather than the Earth-based dust and ash that closes airports. Red giants, the kind of star our Sun will eventually turn into in a later phase of its life, expel dust and gas that produces raw materials for a new generation of stars and planets. The outcome is often a beautiful nebula, but it can also be a dusty disk surrounding what the red giant finally becomes, a white dwarf.

Dusty Disks Around Aging Stars

The new work pegs disk formation around stars at various stages of their evolution, an outstanding question being how long these disks survive. The images Foteini Lykou (Jodrell Bank Centre for Astrophysics) showed at the RAS session were of disks caught early in their lives. M2-9 is a striking example, a nebula with symmetrical lobes of gas extending from it, with a binary star system at its heart. A red giant and a white dwarf are hidden by the disk, the dust originating in the red giant. Lykou believes the disk inside M2-9 is less than 2000 years old, a startling figure given the usual time-scales of astronomical observation.

Image: Top: Bipolar nebula M2-9 (credit: B. Balick/HST) with a reconstructed image of its dusty disc observed by VLTI. Bottom: Round nebula around Sakurai’s Object (credit: A. Zijlstra, University of Manchester) with its corresponding disc.

But the disk around Sakurai’s Object, a round nebula some 11,400 light years from Earth, is even younger. It’s composed of amorphous carbon (think coal or soot) and is growing rapidly. Says Lykou:

“The disc in Sakurai’s Object was created within the last 10 years, so we have the opportunity to study a newborn disc. It is expanding radially — and rapidly — in space. During our observation period in 2007, we saw the disc extend from 10 thousand million kilometers to 75 thousand million kilometers.”

We’re still speculating on what happens to these disks but current thinking is that interstellar radiation probably breaks the constituent dust grains down and thereby replenishes the interstellar medium with new materials. Observing objects like these calls for interferometry, combining the light caught by multiple instruments to sharpen the field of view. The astronomers here used the Very Large Telescope Interferometer at the European Southern Observatory in Chile, which combines four 8.2-meter telescopes and works in the infrared.

Explaining Interstellar Water

I’ll close our RAS coverage with a different kind of look at interstellar dust, one that helps to explain where water from the interstellar medium comes from. The problem is that while hydrogen atoms are extremely common in deep space, little gaseous molecular oxygen (O2) seems to be available and ozone has not been detected in these regions. Atomic oxygen (O) is plentiful, but gas phase reactions between hydrogen and atomic oxygen cannot account for the amount of water observed. Moreover, even the observed amounts of atomic oxygen show a shortage in star-forming regions when compared to the rest of interstellar space.

So while it’s one thing to say that Earth’s water was delivered by comets formed from interstellar material, the question of where that water came from in the first place has remained unanswered. And just where is the missing atomic oxygen?

Enter Victoria Frankland (Heriot-Watt University), whose team of researchers think they have found the answer in the form of the dust grains that make up approximately 1 percent of the interstellar medium. The dust, Frankland believes, provides the surface that allows the needed reactions to take place. Some molecules remain stuck to the surface so that an icy coat — mainly water ice — builds up over time. Says Frankland:

“These initial experiments are having some interesting results in that they are allowing us to look at how the ice coating develops on the dust particles. It appears that oxygen atoms may become trapped inside the icy mantles. We need to do more work, but it could be that our experiments might help solve the mystery of the missing atomic oxygen as well as where the water has come from.”

The image below is too beautiful not to run:

Image: Molecular cloud and star-forming region in the interstellar medium. Credit: NASA/JPL-Caltech/L. Allen (Harvard-Smithsonian CfA).

Interstellar dust is gorgeous to the eye when lit up in star-forming regions like this one, and we’ve often speculated here on the problem of such dust for fast moving spacecraft of the far future. Now we’re learning how dusty disks can form not just around young stars but around burnt out stars nearing the end of their fusion reactions. And we’re seeing that dust may play a role in the essential production of water, so necessary for the formation of life. How appropriate, then, that volcanic ash and dust should mark the end of the illuminating sessions in Glasgow, a reminder that scientific theory paints a real and sometimes all too frustrating world.

Mapping the Interstellar Medium

by Paul Gilster | Feb 10, 2010 | Interstellar Medium | 6 comments

We’ve long known that the spaces between the stars are not empty, but are pervaded by a highly dilute mix of gas and dust. Now we’re getting maps that show the presence of large cavities in this interstellar medium, created by supernova events as well as outflowing solar winds from clusters of hot, young stars. The Sun resides in the so-called Local Cavity, a low-density area of neutral gas that is about 80 parsecs in radius. The Local Cavity is, in turn, surrounded by a ‘wall’ of dense, neutral gas, with gaps in the wall — ‘interstellar tunnels’ — that are low-density pathways to surrounding cavities.

We study the interstellar medium by looking at the light produced by stars and using absorption line spectroscopy to see how that light is affected by gases between us and the stars in question. Johannes Hartmann’s classic study of the spectrum of Delta Orionis in 1904 was a huge advance, looking at absorption from the ‘K’ line of calcium and concluding that the gas was not present in the atmosphere of the star but within the matter in space along the line of sight to the star. Interstellar sodium was detected fifteen years later and the study of the interstellar medium went into higher gear, especially in the sightline toward Orion.

This Wikipedia article on the interstellar medium quotes Norwegian explorer and physicist Kristian Birkeland, who described the medium as understood in 1913:

“It seems to be a natural consequence of our points of view to assume that the whole of space is filled with electrons and flying electric ions of all kinds. We have assumed that each stellar system in evolutions throws off electric corpuscles into space. It does not seem unreasonable therefore to think that the greater part of the material masses in the universe is found, not in the solar systems or nebulae, but in ’empty’ space.”

Have a look at the image below, which draws this into perspective. It’s based on new data, gathered primarily at the European Southern Observatory in Chile, that has been folded into previously published results. A French-American team is behind the work, offering up a catalog of absorption measurements toward 1857 stars within 800 parsecs of the Sun. The image shows cold and neutral gas density within a distance of about 300 parsecs.

Image: Map of partially ionized interstellar gas within 300 parsecs around the Sun, as viewed in the Galactic plane. Triangles represent the sight-line positions of the stars used to produce the map. White to dark shading represents the low to high values of the gas density, and orange shading is for areas with no reliable measurement. The Local Cavity is shown as the white area of low density gas that surrounds the Sun at about 80 parsecs. Credit: B. Welsh/R. Lallement/S. Raimond/J.-L. Vergely.

We’re still early in the quest to understand the local interstellar medium, even though many surveys at various wavelengths have been completed. Knowing the chemical and physical characteristics of the medium will help us understand the evolution of stars as they exchange matter with the space around them. From a spaceflight perspective, probing beyond our own Solar System with future technologies will require understanding the spatial distribution and dynamics of the material we’re pushing into, much as early ocean voyagers had to acquire a working knowledge of wind patterns and ocean currents.

The Local Cavity within which our Sun resides is thought to have been created about 15 million years ago by supernova activity, but its history remains highly speculative. The paper is Welsh et al., “New 3D gas density maps of NaI and CaII interstellar absorption within 300 pc,” to be published in Astronomy & Astrophysics 510 (2010), A54 (abstract).

Cyclers: Transportation Network Among the Stars?

by Paul Gilster | Jan 1, 2010 | Interstellar Medium | 51 comments

Last July at the Aosta conference Greg Matloff presented a paper on using near-Earth objects for transportation. It’s an interesting concept (discussed here), one that takes advantage of the fact that there are a few such objects that pass close by the Earth and then go on to cross the orbit of Mars. Greg was able to show that it would be possible to exploit this trajectory to use the NEO as what Buzz Aldrin has called an ‘orbital cycler,’ hitching a ride at least one way and disembarking upon arrival.

Reducing Starship Mass

The idea is useful because space travel requires so much energy. Put all this in the interstellar context, as science fiction writer Karl Schroeder does in this interesting essay, and you realize that whether we’re talking about beamed sails or antimatter or nuclear fusion, most of the mass of the vehicle is involved with accelerating and decelerating it. Schroeder pondered the question of using the cycler idea on an interstellar level. All you decelerate at destination would be your payload, while the cycler vessel simply keeps in motion, available for re-use at a much lower cost.

Schroeder likens a cycler to a generation ship in that it is intended to be self sufficient, and imagines using magnetic or plasma sails and particle beam propulsion for acceleration of rendezvous craft and their deceleration upon arrival in the destination system. A cycler is:

…the way-station for travelers, who embark and disembark at the solar systems it passes. Since it supplies life support, passengers need only carry supplies necessary for them to make the rendezvous, which would probably take a few months’ time. Even more dramatically, a non-living cargo sent to rendezvous with a cycler can be very light. Instead of accelerating an entire starship, you’d only accelerate the cargo, plus a wire to form the magsail and some attitude jets to make the rendezvous and docking. In other words, a cycler rendezvous craft is almost all cargo.

Thrustless Turning Between the Stars

Read Schroeder’s novel Permanence (Tor, 2002), for a look at cyclers in the context of a vividly imagined future universe. Cyclers stay in motion, using a combination of Lorentz-force turning and, if manageable, gravitational slingshot to alter their trajectory to pass by a number of stars before returning to Earth to begin the same journey again. Each cycler, even at fifty percent of lightspeed, takes a long time to make the rounds, but a network of such cyclers could sustain communications and transport needs for colonists on planets around nearby stars.

The cycler becomes a way station for cargo or travelers who rendezvous with it, ride the cycler to destination, and then use a magsail and particle beam propulsion from the destination system to decelerate once they’ve left the cycler upon arrival. This presupposes, of course, the ability to build these resources in the destination system, which Schroeder imagines occurring through a series of cargo drops involving robotic and perhaps nanotech tools to create the needed infrastructure.

How does Lorentz-force turning work? Here’s Schroeder on the subject:

The key to making cyclers work is our ability to use the magnetic fields of the interstellar medium as a way of turning the craft. In Lorentz Force turning, you unreel several extremely long wires (tethers) and give them a high electric charge. Their interaction with the galactic magnetic field results in a slow, constant course correction for the ship. Over time, it can be enough to change the trajectory from one star to another… A Schroeder cycler would use this active interaction with the galactic field to change its course; hence it is using different principles than an Aldrin cycler, which relies on orbital mechanics and is essentially (and preferably) passive.

Is Lorentz-force turning sufficient to manage such a trajectory? It’s possible we might need to use forms of propulsion like ion engines or beamed energy from the systems the cycler passes through to help turn the vehicle, so part of the cargo sent to any cycler might include the necessary fuel. But the advantages of the cycler are still notable. In Permanence, Schroeder writes about the ‘lit’ stars like our Sun, contrasting them with the much harder to find brown dwarfs, noting that there may be more brown dwarfs than any other spectral type in the galaxy. Add brown dwarfs into a cycler network and the power question changes as we exploit their magnetic fields. Schroeder again:

…Jupiter and the sun both display prodigious magnetic fields. A brown dwarf could be expected to do the same. This means brown dwarfs can probably supply the kinds of energy required to launch starships; instead of using solar power, as we might do near Earth, at a brown dwarf we would directly generate electrical power by putting long wires (tethers, like the Lorentz Force cables) in orbit around the dwarf. A wire in a moving magnetic field produces electricity; in the kind of all-encompassing and intense field a dwarf might have, a lot of current would be produced; and if you orbited a million wires… again, things scale up nicely.

Growth of an Interstellar Network

What Schroeder imagines is a ring of connected colonies using a network of cyclers to promote commerce and trade, a network encompassing both ‘lit’ stars and brown dwarfs. A new solar system is ‘seeded’ with robots programmed to build a particle beam system to decelerate incoming traffic. While colonization time frames are still large, the development of regular transport into such a system would allow regular cycler visitation and cargo delivery. A human colony could first be established by settlers leaving a passing cycler on a magsail rendezvous ship, knowing they would be part of the growing interstellar network.

Read Permanence for a look at cyclers in action. Thrustless turning using the interstellar magnetic field has been discussed in the scientific literature. Both Robert Forward and P. C. Norem considered Lorentz-force turning of an electrostatically charged spacecraft, and Greg Matloff has studied an approach to electrodynamic thrustless turning involving a partially sheathed superconductor. For more on that one, see Matloff’s Deep Space Probes (Springer, 2005). The Forward paper is “Zero Thrust Velocity Vector Control for Interstellar Probes: Lorentz Force Navigation and Circling,” AIAA Journal 2 (1964), pp. 885-889.

The Norem paper, meanwhile, is “Interstellar Travel: A Round Trip Propulsion System with Relativistic Velocity Capabilities,” AAS paper 69-388 (June, 1969). Cyclers are a fascinating scenario for in-system travel, but driving even stripped-down cargo vessels to a rendezvous at half the speed of light, much less getting the cycler accelerated in the first place, remains a mammoth challenge. While there are no easy propulsion solutions, a far future society working with cycler principles could indeed create an interstellar network. All of which leaves those of us in the 21st Century to ponder the continuing conundrum: How do we push that much mass up to such speeds?