Mapping Out Interstellar Clouds

Although I’ve written on a number of occasions about the project called Interstellar Probe, the effort to create what we might call a next-generation Voyager equipped to study space beyond the heliosphere, it’s always been in terms of looking back toward the Solar System. What is the shape of the heliosphere once we see it from outside, and how does it interact with the local interstellar medium? The Voyagers have given us priceless clues, but they were never designed for this environment and in any case will soon exhaust their energies.

Pontus Brandt (JHU/APL), who is project scientist for the Interstellar Probe effort, takes us beyond these heliosphere-centric ideas as he talks to Richard Stone in a fine article about the mission called The Long Shot that ran recently in Science. Because when you launch something moving faster than Solar System escape velocity, you just keep going, and while 1000 AU is often cited as a target for this mission, it’s really only a milestone marker telling us how long we’d like the spacecraft to fly with all its equipment functioning and robust. Beyond the heliosphere, though, we’re looking at interstellar clouds we know fairly little about, and in the long-term view, future interstellar missions will have to know this terrain.

When stars are born, clouds of gas and dust that were not incorporated into the final stellar system remain. Moving on an orbit around the Milky Way that takes some 230 million years to complete, the Solar System encounters these clouds, one of which is the Local Interstellar Cloud, although as Brandt told Richard Stone in the Science article, we really know so little about the cloud environment that our conception is on the order of a child’s sketch. According to the sketch, the Sun has been in the Local Interstellar Cloud for thousands of years (Brandt cites 60) but we’re on its boundaries and appear to be approaching the edge of the so-called G Cloud now. I should add that, as we’ll see in a moment, there are scientists who disagree.

Image: Our solar journey through space is carrying us through a cluster of very-low-density interstellar clouds. Right now the Sun is inside of a cloud (Local cloud) that is so tenuous that the interstellar gas detected by the IBEX (Interstellar Boundary Explorer mission) is as sparse as a handful of air stretched over a column that is hundreds of light-years long. These clouds are identified by their motions, indicated in this graphic with blue arrows. Credit: NASA/Goddard/Adler/U. Chicago/Wesleyan.

What happens when, in perhaps as little as two millennia, we make this crossing? It would be useful to know more about the heliosphere to answer the question. And we also need to know more about the temperature and density of a cloud like this, because the heliosphere itself seems malleable, capable of being deformed by the medium through which it moves. Compress the heliosphere and there are implications for life on Earth, for we are protected from dangerous cosmic rays – at least a high percentage of them – by its protective embrace. It would be good to know just how far the heliosphere can be compressed. All the way down to Earth’s orbit?

Here I want to quote from the article:

There’s evidence of such an event around the time early hominids were just beginning to pick up stone tools, and Brandt muses on a possible connection. “Let that creep up your spine for a moment,” he says. In recent years, scientists have discovered iron-60 isotopes in ocean crust samples dating from 2 million to 3 million years ago. Iron-60 is not found naturally on Earth: It’s forged in the cores of large stars. So, either a nearby supernova blasted the heliosphere with the iron dust, or the heliosphere drifted through a dense cloud laden with iron-60 from a previous supernova. Either way, Brandt says, “The heliosphere was way in, and we had a full blast of galactic cosmic rays and interstellar matter for a long, long time.” To look for relics of other such events, IP could use plasma wave antennas to essentially take the temperature of nearby electrons. Hot regions might mark the blast paths of material from past supernovae.

And here’s an interesting factoid, which I’m pulling from the Harvard & Smithsonian Center for Astrophysics: About half of the interstellar gas, almost entirely hydrogen and helium, is spread through 98% of the space between stars, hot but extremely low in density. The other half of the interstellar gas is compressed into 2% of the volume, and we observe it as interstellar clouds, the densest of which are molecular clouds, primarily formed of molecular hydrogen though including carbon monoxide and some organic compounds, with higher concentrations of dust than in the rest of the ISM.

We know about the interstellar medium both by astronomical measurements and spacecraft within the medium – our Voyagers again – that move amidst neutral gas and dust grains, some of which penetrate the heliosphere and can also be measured by spacecraft like New Horizons. Clearly, a craft designed from scratch to make these measurements outside the heliosphere would free us from the uncertainties of astronomical observation looking through the heliosphere to the medium outside.

The Local Interstellar Cloud moves toward us from the direction of Scorpius and Centaurus. All of this movement through clouds and voids is a reminder that the Sun orbits the galaxy and moves through different environments all the time. JPL notes that interstellar densities ranging from 10-5 to 105 atoms/cm3 can be observed near our system in the Milky Way.

Thus the interest in what happens next, as Brandt notes. For one thing, our future hopes for interstellar exploration focus particularly on the nearest stars, and Alpha Centauri is within the G-cloud the Sun now moves toward. We rely on hydrodynamic simulations to estimate the effects of the Solar System moving into a cloud of denser material. A spacecraft like Interstellar Probe could be launched to move ‘upstream’ of the Sun’s motion, essentially exploring the future environment through which we will pass. We’ll someday send much faster exploratory missions to sample the G Cloud in situ.

The Interstellar Probe concept study goes to the National Academies of Sciences, Engineering, and Medicine, which essentially prioritizes where we are going in space exploration over ten year periods. We won’t know how the panel enjoined with making these decisions will come down until 2024, and remember that competing ideas involving space beyond the heliosphere are out there, the most visible of which is the Solar Gravitational Lens (SGL) mission now in advanced study at the Jet Propulsion Laboratory. Be aware as well of a Chinese effort known as Interstellar Express.

One way or another, and this is true with or without the endorsement of a Decadal study, we will get spacecraft fully designed for the interstellar environment out beyond the heliosphere. I make no predictions on timing other than to say that the earliest we might expect a launch of this kind of mission is in the 2030s, and who knows what other factors may come into play if none of the current studies is funded? Nonetheless, the long-term picture I embrace makes robotic exploration beyond our Solar System inevitable whether time to launch is 10 or 100 or 1000 years from now. I think we’re wired to do it.

Space missions always bring surprises, and it’s only fair to note that the model of the Sun’s nearby cloud environment has been challenged in recent work. What alternate outcomes might an Interstellar Probe mission alert us to? Here is a snip from an interesting 2014 paper by Cécile Gry and Edward Jenkins proposing a model that is:

…fundamentally different from previous models (e.g., Lallement et al. 1986; Frisch et al. 2002, RL08) where the LISM is constituted of a collection of small clouds or cloudlets that are presented as separate entities moving as rigid bodies at different velocities in slightly different directions. In particular, in our picture, the LIC, the G cloud, and other distinct clouds of the RL08 model, are unified in a single local cloud.

And this, of course, would become apparent to any future mission pushing upstream from the heliosphere. We may have to send such a mission to make this call.

Back to the present, though. Pontus Brandt now takes his heliophysics expertise into an ongoing mission, with a new role as part of the New Horizons science team at JHU/APL. This is clearly a good fit, given that this spacecraft is already out there, operating more than 50 AU from the Sun with a suite of plasma and dust instruments that is exploring the dust and charged particles in the full flow of the solar wind. We have only one operating spacecraft in the Kuiper Belt, and this is it, as Brandt notes:

“New Horizons remains a pathfinder on a historic journey, and since we’re equipped with instrumentation not flown on Voyager, we will be able to answer some of the big questions about what upholds our vast heliosphere as it plows through the interstellar medium. Leaving the foreground ‘haze’ of the solar system’s dust and gas, New Horizons is also in a position to make some game-changing discoveries that not only give us glimpses into our changing local interstellar medium, but also discoveries on cosmological scales.”

Image: Pictured in the New Horizons mission operations center at the Johns Hopkins Applied Physics Laboratory, Pontus Brandt brings a new kind of expertise to the New Horizons science leadership team — heliophysics, where the team expects to make breakthroughs that no other mission can, with new capabilities never before available so far from the Sun. Credit: Johns Hopkins APL/Craig Weiman.

Brandt’s work at New Horizons is a reminder that the deeper we push into the Solar System, the more we also explore basic interactions between our star and an interstellar medium we are learning how to map. All of this couples with a continuing effort from Earth to locate future flyby targets for close observation. Going interstellar demands knowing where we’re coming from as much as knowing where we’re heading.

And once we do get to the point of sending a spacecraft all the way to another star? Let me quote something Ian Crawford said on this topic in a paper some years back:

If the Sun does lie within the LIC, then a mission to ? 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…, the trajectory to ? 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.

The paper on a new model for nearby interstellar clouds that I mentioned above is Gry & Jenkins, “The interstellar cloud surrounding the Sun: a new perspective,” Astronomy & Astrophysics 567 (2014), A58 (abstract). For further background on the interstellar medium, see Ian Crawford’s “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). The definitive book on the matter is Bruce Draine’s Physics of the Interstellar and Intergalactic Medium (Princeton University Press, 2011).

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Mapping the Boundary of the Heliosphere

Between the Solar System and interstellar space is a boundary layer called the heliosheath. Or maybe I should define this boundary as being between the inner, planetary part of the Solar System and interstellar space. After all, we consider the Oort Cloud as part of our own system, yet it begins much further out. Both Voyagers have crossed the region where the Sun’s heliosphere ends and interstellar space begins, while they won’t reach the Oort, by some estimates, for another 300 years.

The broader region is called the heliopause, a place where the outflowing solar wind of protons, electrons and alpha particles (two protons and two neutrons tightly bound) encounters what we can call the interstellar wind, itself pushing up against the heliosphere and confining the solar wind-dominated region to a bubble. We now learn that this boundary region has been mapped, showing interactions at the interface.

A paper describing this feat has now appeared, with Dan Reisenfeld (Los Alamos National Laboratory) as lead author. Says Reisenfeld:

“Physics models have theorized this boundary for years. But this is the first time we’ve actually been able to measure it and make a three-dimensional map of it.”

Image: A diagram of our heliosphere. For the first time, scientists have mapped the heliopause, which is the boundary between the heliosphere (brown) and interstellar space (dark blue). Credit: NASA/IBEX/Adler Planetarium.

Riesenfeld and team used data from IBEX, the Interstellar Boundary Explorer satellite, which orbits the Earth but detects energetic neutral atoms (ENAs) from the zone where solar wind particles collide with those of the interstellar wind. Reisenfeld likens the process to bats using sonar, with IBEX using the solar wind as the outgoing signal and mapping the return signal, which varies depending on the intensity of the solar wind striking the heliosheath. Changes in the ENA count trigger the IBEX detectors.

“The solar wind ‘signal’ sent out by the Sun varies in strength, forming a unique pattern,” adds Reisenfeld. “IBEX will see that same pattern in the returning ENA signal, two to six years later, depending on ENA energy and the direction IBEX is looking through the heliosphere. This time difference is how we found the distance to the ENA-source region in a particular direction.”

The IBEX data cover a complete solar cycle from 2009 through 2019. We learn that the minimum distance from the Sun to the heliopause is about 120 AU in the direction facing the interstellar wind, while in the opposite direction, we see a tail that extends to at least 350 AU, which the paper notes is the distance limit of the measurement technique. The asymmetric shape is striking. From the paper’s abstract:

As each point in the sky is sampled once every 6 months, this gives us a time series of 22 points macropixel–1 on which to time-correlate. Consistent with prior studies and heliospheric models, we find that the shortest distance to the heliopause, dHP, is slightly south of the nose direction (dHP ~ 110–120 au), with a flaring toward the flanks and poles (dHP ~ 160–180 au).

Animation: The first three-dimensional map of the boundary between our solar system and interstellar space—a region known as the heliopause. Credit: Reisenfeld et al

The data make it clear that interactions between the solar wind and the interstellar medium occur over distances much larger than the size of the Solar System. It’s also clear that because the solar wind is not steady, the shape of the heliosphere is ever changing. A ‘gust’ of solar wind causes the heliosphere to inflate, with surges of neutral particles along its outer boundary, while lower levels of solar wind cause a contraction that is detected as a concurrent diminution in the number of neutral particles.

IBEX has been a remarkably successful mission, with a whole solar cycle of observations now under its belt. As we assimilate its data, we can look forward to IMAP — the Interstellar Mapping and Acceleration Probe, which is scheduled to launch in late 2024 and should enable scientists to extend the solid work IBEX has begun.

The paper is Reisenfeld et al., “A Three-dimensional Map of the Heliosphere from IBEX,” Astrophysical Journal Supplement Series Vol. 254, No. 2 (2021) Abstract. The paper is part of a trio of contributions entitled A Full Solar Cycle of Interstellar Boundary Explorer (IBEX) Observations, available here.

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Voyager: A Persistent Clue to the Density of the Interstellar Medium

What are the long-lasting waves detected by Voyager 1? Our first working interstellar probe — admittedly never designed for that task — is operating beyond the heliosphere, which it exited back in 2012. A paper just published in Nature Astronomy explores what’s going in interstellar space just beyond, but still affected by, the heliosphere’s passage through the Local Interstellar Medium (LISM).

We have a lot to learn out here, for even as we exit the heliosphere, the picture is complex. The so-called Local Bubble is a low-density region of hot plasma in the interstellar medium, the environment of radiation and matter — gas and dust — that exists between the stars. Within this ‘bubble’ exists the Local Interstellar Cloud (LIC), about 30 light years across, with a slightly higher hydrogen density flowing from the direction of Scorpius and Centaurus. The Sun seems to be within the LIC near its boundary with the G-cloud complex, where the Alpha Centauri stars reside.

Image: Map of the local galactic neighborhood showing the Sun located near the edge of our local interstellar cloud (LIC). Alpha-Centauri is located just over 4 light-years away in the neighboring G-cloud complex. Outside these clouds, the density may be lower than 0.001 atoms/cc. Our Sun and the LIC have a relative velocity of 26 km/sec. Credit: JPL.

But if the interstellar medium is a sparse collection of widely spaced particles and radiation, it proves to be anything but quiet. We learn this from Voyager 1’s Plasma Wave Subsystem, which involves two antennae extending 30 meters from the spacecraft (see image below). What the PWS can pick up are clues to the density of the medium that show up in the form of waves. Some are produced by the rotation of the galaxy; others by supernova explosions, with smaller effects from the Sun’s own activity.

Vibrations of the ionized gas — plasma — in the interstellar medium have been detectable since late 2012 by Voyager 1 in the form of ‘whistles’ that show up only occasionally, but offer ways to study the density of the medium. The new work in Nature Astronomy, led by Stella Koch Ocker (Cornell University), sets about finding a more consistent measure of interstellar medium density in the Voyager data.

Image: An illustration of NASA’s Voyager spacecraft showing the antennas used by the Plasma Wave Subsystem and other instruments. Credit: NASA/JPL-Caltech.

A weak signal appearing at the same time as a ‘whistle’ in the 2017 Voyager data seems to have been the key finding. Ocker describes it as “very weak but persistent plasma waves in the very local interstellar medium.” When whistles appear in the data, the tone of this plasma wave emission rises and falls with them. Adds Ocker:

“It’s virtually a single tone. And over time, we do hear it change – but the way the frequency moves around tells us how the density is changing. This is really exciting, because we are able to regularly sample the density over a very long stretch of space, the longest stretch of space that we have so far. This provides us with the most complete map of the density and the interstellar medium as seen by Voyager.”

So we have an extremely useful instrument, Voyager 1’s Plasma Wave Subsystem, continuing to return data with increasing distance from the Sun. Analyzing the data over time, we learn that the electron density around the spacecraft began rising in 2013, just after its exit from the heliosphere, and reached current levels in 2015. These levels, which persist to the end of 2020 through the dataset, show a 40-fold increase in electron density. Up next for Ocker and team is the development of a physical model of the plasma wave emission that will offer insights into its proper interpretation.

As we begin to think seriously about interstellar probes in this century, it’s striking how much we have to learn about the medium through which they will pass. Voyager 1 is helping us learn about conditions immediately outside the heliosphere. A probe sent to Alpha Centauri will need to cross the boundary between the Local Interstellar Cloud and the G-cloud, a region we have yet to penetrate. The nature of and variation within the interstellar medium will require continuing work with our admittedly sparse data.

The paper is Ocker et al., “Persistent plasma waves in interstellar space detected by Voyager 1,” Nature Astronomy 10 May 2021. Abstract / Preprint.

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‘Oumuamua: A Hydrogen Iceberg?

Studies of interstellar interloper ‘Oumuamua move at lightning pace, to judge from a recent exchange on hydrogen ice. A study by Greg Laughlin and Darryl Seligman (both at Yale) just published in June, has now met a response from Thiem Hoang (Korea University of Science and Technology, Daejeon) and Harvard’s Avi Loeb. The issue is significant because if, as Laughlin and Seligman argued, ‘Oumuamua were made of hydrogen ice, then the outgassing that drove its slight acceleration would not have been detectable. At least one mystery solved.

Or was it? One reason the 0.2km radius object didn’t fit the description of a comet was that there was no explanation for its tiny change in velocity. Hoang and Loeb have examined the hydrogen ice concept and found it wanting. Says Hoang:

“The proposal by Seligman and Laughlin appeared promising because it might explain the extreme elongated shape of ‘Oumuamua as well as the non-gravitational acceleration. However, their theory is based on an assumption that H2 ice could form in dense molecular clouds. If this is true, H2 ice objects could be abundant in the universe, and thus would have far-reaching implications. H2 ice was also proposed to explain dark matter, a mystery of modern astrophysics.”

Which sounds interesting in itself because anything bearing on dark matter is worth a look, given our frustration at understanding the matter behind the hypothesis. Laughlin and Seligman had suggested a giant molecular cloud [GMC] as the origin for ‘Oumuamua, but Hoang and Loeb argue that the earlier paper, while considering the destruction of H2 ice in the interstellar medium through evaporation, did not take into account the improbability of such ices forming within a GMC, or the effects of that environment upon their later growth. From the paper:

Assuming that H2 objects could be formed in GMCs by some mechanisms (Füglistaler & Pfenniger 2016; Füglistaler & Pfenniger 2018; Seligman & Laughlin 2020), we quantify their destruction and determine the minimum size of an H2 object that can reach the solar system. We assume that the H2 objects are ejected from GMCs into the ISM by some dynamical mechanism such as tidal disruption of bigger objects or collisions (see Raymond et al. 2018; Rice & Laughlin 2019).

Image: An artist’s rendering of ‘Oumuamua, a visitor from outside the Solar System. Credit: The international Gemini Observatory/NOIRLab/NSF/AURA artwork by J. Pollard.

Hoang and Loeb’s calculations show that H2 icebergs are unlikely to grow to large size because of collisional heating not just from dust but from gas within the birth cloud, meaning that ‘Oumuamua likely wasn’t a hydrogen iceberg (and, along the way, taking out the ancillary proposition that dark matter may be accounted for by H2 snowballs). Micron sized grains in regions where the density of gas is high will cause the hydrogen on the grains to sublimate.

Assuming that H2 objects could somehow form in the densest regions of GMCs, we found that sublimation by collisional heating inside the GMC would destroy the objects before their escape into the ISM [interstellar medium]. We also studied various destruction mechanisms of H2 ice in the ISM. In particular, we found that H2 objects are heated by the average interstellar radiation, so that they cannot survive beyond a sublimation time of tsub ~ 10 Myr for R = 300 m (see Figure 1). Only H2 objects larger than 5 km could survive.

While giant molecular clouds like (GMC) W51, one of the closest to Earth at roughly 17,000 light years, could be a point of origin for the object, the authors argue that even this close GMC is simply too far away. Moreover, it might be hard for a hydrogen iceberg even to exit the giant molecular cloud in the first place. Collisional heating within a GMC would destroy objects like this through thermal sublimation long before they reached a distant stellar system. The paper finds that objects below 200 meters in radius would be destroyed within the parent GMC.

We also have to find a way to get ‘Oumuamua all the way from its birthplace to our Solar System. What Hoang and Loeb point out is that an iceberg made of hydrogen would be unlikely to survive an interstellar journey that would probably take hundreds of millions of years. An object like this is going to begin to evaporate. Their paper goes to work out the survivability of H2 ice from interstellar radiation given thermal sublimation and photodesorption along the way.

Many other factors come into play that cause problems for a hydrogen iceberg. It has to stand up to cosmic rays as well as impacts with matter in the interstellar medium. And we have to throw in what can happen when an object like ‘Oumuamua enters the Solar System, where solar radiation becomes an issue. The calculations presented here show the significance of thermal sublimation due to starlight, while going beyond this to reveal the effects of cosmic rays and impacts with interstellar matter, which turn out to be less significant.

Image: This is Figure 1 from the paper. Caption: Comparison of various destruction timescales (slanted colored lines) as a function of the object radius (in meters) to the travel time from a GMC at a distance of 5.2 kpc, assuming a characteristic speed of 30 km s?1 (horizontal black line). Credit: Hoang & Loeb.

The authors calculate a minimum radius of H2 objects in the range of 5 kilometers for survival in a journey that would have to take in formation in giant molecular clouds and movement through the interstellar medium. 10 million years would wreak havoc on an object the size of ‘Oumuamua.

We’re in a period of energetic debate, a time when the unresolved questions about ‘Oumuamua remain in play. It seems clear that we need a larger population of interstellar objects to place the current work in context, and Loeb has pointed out that we won’t have long to wait:

“If ‘Oumuamua is a member of a population of similar objects on random trajectories, then the Vera C. Rubin Observatory (VRO), which is scheduled to have its first light next year, should detect roughly one ‘Oumuamua-like object per month. We will all wait with anticipation to see what it will find.”

The paper is Hoang & Loeb, “Destruction of Molecular Hydrogen Ice and Implications for 1I/2017 U1 (‘Oumuamua),” Astrophysical Journal Letters Vol. 899, No. 2 (17 August 2020). (Abstract). The Seligman & Laughlin study that argued for “‘Oumuamua as a hydrogen iceberg is “Evidence that 1I/2017 U1 (‘Oumuamua) was Composed of Molecular Hydrogen Ice,” Astrophysical Journal Letters Vol. 896, No. 1 (9 June 2020). Abstract.

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Collisions in the Interstellar Medium

Memories play tricks on us all, but trying to recall where I saw a particular image of a laser lightsail is driving me to distraction. The image showed a huge sail at the end of its journey, docked to some sort of space platform, and what defined it were the tears and holes in the giant, shredded structure. It presupposed long passage through an interstellar medium packed with hazards, and although I assumed I would have seen it on the cover of some science fiction magazine, I spent an hour yesterday scanning covers on Phil Stephensen-Payne’s wonderful Galactic Central site, but all to no avail.

The image must have run inside a magazine, then, but if so, I’m at a loss to identify it other than to say it would have appeared about twenty years ago. I had hoped to reproduce it this morning because our talk about starship shielding necessarily brought up the question of whether an enormous lightsail — some of these are conceived as being hundreds of kilometers in diameter — wouldn’t be impractical in denser areas of the galaxy. And that brought to mind a 1986 exchange between the British astronomer Ian Crawford (Birkbeck College, London) and Robert Forward, the physicist who did so much to awaken us to the possibilities of interstellar flight.

This morning I’m about eight miles away from the library where I can find back issues of the Journal of the British Interplanetary Society, but Gregory Matloff and Eugene Mallove wrote about this correspondence in The Starflight Handbook, which I do have right here in front of me. Forward’s position was that a laser lightsail would be so thin that dust grains would pass right through it without depositing a great deal of their kinetic energy as heat. So maybe the shredded lightsail isn’t a necessary outcome of a beamed sail mission. From The Starflight Handbook:

During a 10-ly journey at 0.2 c, only 1/500 of the area of a 0.0160 micron (160 Å or angstrom) thick light sail will be lost. However, Forward and we agree that a great deal of theoretical and experimental work on interstellar erosion must still occur before we can set off for the stars free of bad dreams.

Dust Grains Between the Stars

I’ve been thinking about Ian Crawford partially because of his recent paper on manned spaceflight and its virtues, but also because of another exchange he had about two years ago, this one with Jean Schneider (Paris Observatory), who had been examining our response to the detection of biosignatures on exoplanets, and in passing discussed how difficult it would be to get a probe to an exoplanet to investigate them. Schneider was worried about the interstellar medium too, and went to work on the possibilities assuming a spacecraft velocity of 30 percent of the speed of light. Moving at that pace, Schneider calculated that a 100-?m interstellar grain would have the same kinetic energy as a 100-ton body moving at 100 kilometers per hour.

The Schneider/Crawford exchange is up on the arXiv site (references below), and you can read about it in Interstellar Flight: The Case for a Probe as well as Interstellar Flight and Long-Term Optimism, the two articles I wrote about it back in 2010. It was Crawford’s position that interstellar dust grains could indeed present a hazard that will need to be factored into the design of the vehicle, but Crawford found several mitigating factors including speed, pointing out that 30 percent of lightspeed was a overly ambitious target, and certainly a more problematic one, owing to the scaling of kinetic energy with the square of the velocity.

Crawford finds that the situation at 0.1 c is considerably better. From the paper:

The issue of shielding an interstellar space probe from interstellar dust grains was considered in detail in the context of the Daedalus study by Martin (1978). Martin adopted beryllium as a potential shielding material, owing to its low density and relatively high speci?c heat capacity, although doubtless other materials could be considered. Following Martin’s (1978) analysis, but adopting an interstellar dust density of 6.2 x 10-24 kg m-3 (i.e., that determined by Landgraf et al., 2000), we ?nd that erosion by interstellar dust at a velocity of 0.1 c would be expected to erode of the order of 5 kg m– 2 of shielding material over a 6-light-year ?ight.

We clearly need shielding, then, and that adds to the mass of the interstellar probe, but Crawford does not find the problem insurmountable. Of course, what we have yet to learn is the true size distribution of dust particles in the nearby interstellar medium, which is one reason we need a mission like Innovative Interstellar Explorer, to make such measurements in situ. Crawford works out the spatial density of 100-?m grains at about 4 x 10-17 m-3 based on work by Markus Landgraf (Johnson Space Center) and colleagues in 2000. Here we find just how much work lies ahead:

…over the 6 light-year (5.7 x 1016 m) ?ight considered by Martin (1978), we might expect of the order of two impacts per square meter with such large particles, and the injunction by Schneider et al. (2010) may, after all, appear pertinent. On the other hand, it is far from clear that it is valid to extrapolate the distribution to such large masses, not least because of the dif?culty of reconciling the presence of such large solid particles in the LIC with constraints imposed by the cosmic abundances of the elements (as also noted by Landgraf et al., 2000, and Draine, 2009). Clearly, more work needs to be done to better determine the upper limit to size distribution of interstellar dust grains in the local interstellar medium.

More work indeed, and Schneider, in a response to Crawford’s own response to his earlier paper, notes that the matter comes down to what we are willing to live with in terms of probabilities:

The question is what probability of collision is acceptable. If a collision is lethal, this probability must be extremely close to zero for a several hundred billion € mission.

Searching for Solutions

We’re not yet able to make the definitive call on just how many large interstellar grains our probe may run into in the local interstellar medium, but Crawford thinks the problem can be addressed through the detection of incoming large grains and the use of either laser or electromagnetic means to destroy or deflect them before they impact the spacecraft. Again I turn back to Alan Bond’s idea of a dust cloud ejected from the vehicle and preceding it along its course. Remember that Bond was working with the Daedalus concept of an initial, multi-year period of acceleration followed by decades of coasting to reach Barnard’s Star. A dust cloud like this could destroy larger interstellar grains before they ever reached the main vehicle. Adds Crawford:

This concept was developed for Daedalus in the context of protecting the vehicle in the denser interplanetary environment of a target star system, but it would work just as well for the interstellar phase of the mission should further research identify the need for such protection.

A mature space exploration infrastructure here in our own Solar System is probably the prerequisite for the kind of interstellar probe Crawford is talking about, and he notes the value of building that infrastructure not only in terms of creating the technologies we’ll need to get to the stars, but also in terms of making possible the search for life not only on Mars but further out in the system. How long it takes us to build this framework plays directly to Schneider’s point that:

It is presumptuous to predict exactly what will happen after one century and into the future, but it is more than likely that development of the capacity to observe the morphology of meter-sized organisms on exoplanets will take several centuries, at least in the framework of present and forseeable physical concepts. Another optimistic possibility would be that, in a nearer future, we will detect pictures of extraterrestrials with a good resolution in SETI signals. The debate must still go on.

The initial paper by Jean Schneider is “The Far Future of Exoplanet Direct Characterization,” Astrobiology Vol. 10, Issue 1 (22 March, 2010), available as a preprint. Ian Crawford responded in “A Comment on ‘The Far Future of Exoplanet Direct Characterization’ — the Case for Interstellar Space Probes,” Astrobiology Vol. 10, Number 8 (2010), pp. 853-856 (preprint). Schneider’s follow-up response to Crawford is “Reply to « A Comment on ”The Far Future of Exoplanet Direct Characterization” – the Case for Interstellar Space Probes » by I. Crawford,” Astrobiology Volume 10, No. 8 (2010). I don’t have the page numbers on the latter but the preprint is available.

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Lasers: Protecting the Starship

Interesting new ideas about asteroid deflection are coming out of the University of Strathclyde (Glasgow), involving the use of lasers in coordinated satellite swarms to change an asteroid’s trajectory. This is useful work in its own right, but I also want to mention it in terms of a broader topic we often return to: How to deal with the harmful effects of dust and interstellar gas on a fast-moving starship. That’s a discussion that has played out many a time over the past eight years in these pages, but it’s as lively a topic as ever, and one on which we’re going to need a lot more information before true interstellar missions can take place.

Lasers and the Asteroid

But let’s set the stage at Strathclyde for a moment. The idea here is to send small satellites capable of formation flying with the asteroid, all of them firing their lasers at close range. The university’s Massimiliano Vasile, who is leading this work, says that the challenge of lasers in space is to combine high power, high efficiency and high beam quality simultaneously. He adds:

“The additional problem with asteroid deflection is that when the laser begins to break down the surface of the object, the plume of gas and debris impinges the spacecraft and contaminates the laser. However, our laboratory tests have proven that the level of contamination is less than expected and the laser could continue to function for longer than anticipated.”

Vasile believes using a flotilla of small but agile spacecraft, each with a highly efficient laser, is more feasible than trying to deflect an asteroid with a single, large spacecraft carrying a much larger laser. One benefit is that the system is scalable — add as many spacecraft as needed for the job at hand. The other is that you have the redundancy afforded by multiple laser platforms. The Strathclyde work is also investigating whether a similar system could be used to remove space debris by de-orbiting problematic objects to avoid potential collisions.

Erosion Shields on the Starship?

If lasers can be used to alter an asteroid’s trajectory, we need to consider their uses in clearing out the space ahead of futuristic space probes. That the interstellar medium itself was going to be a problem became apparent as researchers began to study starship deceleration concepts in the early 1970s. Get your vehicle moving in the range of 0.3 c and any grain of carbonaceous dust a tenth of a micron in diameter it encounters carries a relative kinetic energy of 37,500,000 GeV, according to Dana Andrews (Andrews Space) in a 2004 paper. How that kinetic energy is dealt with is clearly a major issue.

By the late 1970s, aluminum and then graphite had been considered as possible erosion shields, with the preference going to graphite, but in 1978 Anthony Martin reviewed the literature and suggested a beryllium payload shield be deployed on the Project Daedalus probe, which would be moving at .12 c. It would be quite a large object, some 9 millimeters thick and 32 meters in radius, and even it didn’t completely solve the problem, for Daedalus would, upon arrival, be moving into a still denser gas and dust environment around Barnard’s Star. Daedalus designer Alan Bond suggested additional shielding in the form of a cloud of dust deployed from the probe, which would vaporize larger particles before they could damage the vehicle.

Image: Diagram of the Project Daedalus probe, developed by the British Interplanetary Society in the 1970s. Note the beryllium shield at upper left. Credit: Adrian Mann.

Clearing Out the Path

We’re still not through, though. What about particles larger than dust grains, up to hailstones in size? We are now talking about collisions that would be catastrophic, and must turn from passive to active measures to tackle the problem. Gregory Matloff and Eugene Mallove have suggested using a light or X-ray laser or a neutral particle beam firing ahead of the ship to deflect any objects detected in its forward-pointing radar. The Project Icarus team has looked at creating a bumper out of graphene, as discussed in this blog entry, and coupling it with a laser defense:

What I’m interested in for shielding is making a large, low-mass “bumper” which cosmic sand-grains run into before hitting the craft. After passing through several layers of graphene the offending mass is totally ionized and forms a high-energy spray of particles, but particles that can now be deflected by the vehicle’s cosmic-ray defences (akin to the mag-sail, but smaller with a higher current) and safely diverted away from sensitive parts.

The notion seems an adaptation of Conley Powell’s 1975 work on shields that move ahead of the ship, trapping ionized material on impact within a magnetic field. The earlier Daedalus researchers found that Powell’s ideas resulted in less erosion than other methods then being studied. This is an interesting shield, one placed perhaps 100 kilometers ahead of the spacecraft. Moreover, it is not passive but can signal the vehicle when grains have passed through it without being ionized:

This causes a signal to be sent back to the vehicle which then activates its final layer of defence, high-powered lasers. In microseconds the lasers either utterly ionize the target or give it a sideways nudge via ablation – blowing it violently to the side via a blast of plasma. Such an active tracking bumper would need to be further away than 100 km to give the laser defence time to react, though 1/600th of a second can be a lot of computer cycles for a fast artificial intelligence. The lasers might use advanced metamaterials to focus the beam onto a speck at ~100 km, without needing to physically turn the laser itself in such a split-second. Highly directional, high-powered microwave phased arrays exist which already do so purely electronically and an optical phased-array isn’t a stretch beyond current technology.

All of which takes me back to the University of Strathclyde work on laser deflection, and makes me wonder whether laser technologies first deployed against asteroids in our Solar System may one day be used to protect our interstellar voyagers.

Anthony Martin’s paper is “Bombardment by Interstellar Material and Its Effects on the Vehicle,” Project Daedalus Final Report (Journal of the British Interplanetary Society, 1978): S116–S121. Alan Bond discusses in-system shielding in “Project Daedalus: Target System Encounter Protection,” S123–S125 in the same publication. The Dana Andrews paper is “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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