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.


Starship Surfing: Ride the Bow Shock

We’ve been looking at slowing down a starship, pondering ways the interstellar medium itself might be of use, and seeing how the stellar wind produced by the destination star could slow a magsail. A large solar sail could use stellar photons, but the advantage of the magsail is that it’s going to be effective at a greater distance, and we can also consider other trajectory-bending effects like the Lorentz turning studied by Robert Forward and P.C. Norem. But if you take a look at the relevant papers on magsails and other uses of the medium, you’ll find that they all assume the interstellar medium is more or less uniform. We know, of course, that it is not.

For one thing, the Sun itself seems to be near the boundary of the Local Interstellar Cloud, and there are a number of such clouds within about 5 parsecs of the Solar System. In fact, we’re not exactly sure whether the Sun is just outside the LIC or barely within it. In any case, as Ian Crawford has pointed out, Centauri A and B appear to be outside of the LIC in the direction of the G cloud, yet another denser region of the local interstellar medium. Although Robert Bussard assumed densities of about 1 hydrogen atom per cubic centimeter for his ramjet, a starship between denser clouds may encounter far less, perhaps 0.01 hydrogen atoms in the same volume.

The other wildcard is the fact that leaving and approaching a stellar system, we encounter the kind of interesting effects shown in the image below. This is anything but a uniform interstellar background. The bubble created by the solar wind is called the heliosphere, at the outer boundary of which is the heliopause (here the solar wind is balanced by inward pressure from the interstellar medium), and as you can see in the diagram, the bow shock forms on the outer edge as the star moves through the ionized gases of the medium. Still within the heliosphere is the region called the termination shock, where the speed of the solar wind is abruptly reduced — between the termination shock and the heliopause is the area known as the heliosheath.

Image: The complicated interactions between the Sun and the local interstellar medium. Credit: NASA/JPL.

Physicist and writer Gregory Benford calls the bow shock, that bumper of plasma and higher density gas that forms 100-200 AU from the star, “the obvious place to decelerate.” Obvious it may be, but I haven’t encountered the idea in the literature before, and it’s an ingenious enough notion that I suspect we’ll be seeing a paper or two on the matter before long. The suddenly higher density and plasma content available here should allow interesting maneuverability along the lines of the Lorentz force turning that Forward and Norem studied for course correction and round-trip missions. The bow shock should also offer prime ground for deceleration.

We have early data on the termination shock from Voyager 2, which crossed it at 84 AU back in 2007, while Voyager 1 entered the heliosheath at 94 AU in 2004, and Benford figures the plasma density increase at the bow shock should be one to two orders of magnitude above the interstellar density, and that means one or two orders of magnitude more deceleration. I want to quote him on this from a recent email:

My main point is that these are 3D structures, so a starship could navigate through them using the Forward I x B torque model which steers without decelerating. Each of the bow shock, heliopause and termination shocks are surfaces one can sail on and in, maximizing the deceleration.

So here is the method for the star sailors of the far future:

I imagine that any trial of a starship in, say 100 years, will begin with expeditions into the several hundred AU shock environment, have a look at distant iceteroids and maybe dwarf stars. Then turn back and try to decelerate using magsail skills on the shock surfaces available. (I surf, and this is like inverse surfing, using natural wave phenomena to slow.) Develop the tech and skills to sail the interstellar seas!

As a starship approaches a star, sensing the shock structures will be like having a good eye for the tides, currents and reefs of a harbor.

Image: Spitzer image and artists conception of the bow shock around R Hya. Credit: NASA/JPL, Toshiya Ueta.

Now we can look at certain astronomical images in a new light, as witness the Spitzer imagery and subsequent artist’s concept above. This is the star R. Hydrae in infrared, showing the bow shock about as well defined as I have ever seen it. Approaching a star using these decelereation methods would involve a long period of braking moving through and along the bow shock, heliopause and termination shock, staying within the high density plasma to take advantage of the increased densities there with the starship’s magsail fully deployed. After the long spiral into the inner system, continued magsail braking or perhaps inner system braking using a solar sail would allow the vehicle to maneuver and explore the new solar system.


Braking Against a Stellar Wind

This morning I want to pick up on the ‘problem of arrival’ theme I began writing about on Friday, and we’ll look at interstellar deceleration issues a good bit this week. But I can’t let Monday start without reference to the Icarus results from Gran Sasso that finds neutrinos traveling at precisely the speed of light. All of this adds credence to the growing belief that the earlier Opera experiment was compromised by equipment problems. The news is all over the place (you might begin with this BBC account) and while we’ll keep an eye on it, I don’t plan to spend much time this week on neutrinos. We still have much to get done on the subject of slowing down.

Magsails and Local Resources

When you begin to unlock the deceleration issue, the options quickly multiply, and you find yourself looking into areas that weren’t remotely the subject of your earlier research. As we saw on Friday, the concept of magnetic sails grew organically out of Robert Bussard’s idea of an interstellar ramjet. Bussard didn’t want to slow down — he wanted to go very fast indeed. Read the comments on that post and you’ll find Al Jackson’s entertaining reminiscences of a dinner with Bussard (Tau Zero author Poul Anderson was present too), and a reminder that the scientist always claimed to have come upon the ramjet idea because of an encounter with Mexican food. The usual story has it that it was a burrito which, bitten down upon, suddenly opened for Bussard the splendors of matter being forced into a cylinder at high speeds.

Or maybe he was eating huevos rancheros — the story seems to have varied a bit over the years. Whatever the case, the idea of scooping up interstellar hydrogen and fusing it turned into a 1960 paper for Acta Astronautica and, along the way, into a critique by Robert Zubrin and Dana Andrews that showed just how much drag an electromagnetic scoop could generate. Andrews was working for Boeing at the time, and had grown interested in using Bussard concepts right here in the Solar System, thinking that a big enough scoop could gather hydrogen for use in an ion engine that could be powered up by an onboard nuclear reactor. A self-fueling ion drive might not be adaptable for interstellar missions, but for interplanetary work it seemed worth a look.

But the numbers were intractable. The magnetic scoop Andrews hoped to deploy created more drag than the ion engines produced thrust. The two researchers quickly found that the scoop’s best function was as a magnetic sail, and their work on the idea appeared in the literature in the early 1990s. In his 1999 book Entering Space, Zubrin recalls that the time was right for the magsail given that Paul Chu (University of Houston) had just invented the first high-temperature superconductors, which a magsail could theoretically use to create the magnetic field that would allow it to ride on the solar wind. Practical high-temperature superconducting wire born out of this work might one day allow magsails to achieve higher thrust-to-weight ratios than solar sails.

Magsails have clear propulsion implications, but Zubrin states the obvious about their most effective uses:

…the most interesting and important thing about the magsail is not what it can do to speed up a spacecraft — what’s important is its capability for slowing one down. The magsail is the ideal interstellar mission brake! No matter how fast a spaceship is going, all it has to do to stop is deploy and turn on a magsail, and the drag generated against the interstellar plasma will do the rest. Just as in the case of a parachute deployed by a drag racer, the faster the ship is going, the more ‘wind’ is felt, and the better it works.

Which takes us to the idea of using in-situ resources to tackle the deceleration problem. If your goal is to launch a starship that can decelerate in the destination system to explore it, the magsail lets you do the job without carrying the deceleration fuel aboard the vehicle. Play around with the numbers long enough and you’ll see what a huge boost this would be, for otherwise you’re carrying all the fuel needed to slow down a starship (moving, perhaps, at .10 c!), and that means you’ve got to get all of that fuel up to cruise in the first place. The idea of creating drag against the interstellar medium and a destination stellar wind thus has a powerful appeal.

Rise of the Superconductor

When Bussard studied how his ramjet could operate in a region of interstellar space where the density of hydrogen was roughly 1 hydrogen atom per cubic centimeter, he saw that he would need a collecting area of 10,000 square kilometers. This is so vast that even if it were made of 0.1-centimeter mylar, a physical scoop would weigh something on the order of 250,000 tons. But a much smaller collector generating a magnetic field seems practical given the advances in superconducting alluded to above, with a loop of superconducting wire deployed from the spacecraft, the current applied to it cycling continuously to generate the magnetic field. Here’s how Zubrin and Andrews described it in a paper based on their presentation at the 1990 Vision-21 symposium at NASA’s Lewis Research Center (now Glenn Research Center):

The magnetic sail, or Magsail, is a device which can be used to accelerate or decelerate a spacecraft by using a magnetic field to accelerate/deflect the plasma naturally found in the solar wind and interstellar medium. Its principle of operation is as follows: A loop of superconducting cable hundreds of kilometers in diameter is stored on a drum attached to a payload spacecraft. When the time comes for operation the cable is played out into space and a current is initiated in the loop. This current once initiated, will be maintained indefinitely in the superconductor without further power. The magnetic field created by the current will impart a hoop stress to the loop aiding the deployment and eventually forcing it to a rigid circular shape.

Image: A space probe surrounded by a magnetic sail. Early work on these concepts has taken place at the University of Washington under Robert Winglee, with reports available at NASA’s Institute for Advanced Concepts site. Credit: NASA/University of Washington.

Thus the hybrid concept Andrews and Zubrin came up with in the Vision-21 work, extending ideas they had first presented in a 1988 paper: Use laser beaming technology to push a sail to interstellar cruise speeds, then deploy a magsail upon arrival to reduce deceleration time. The authors looked at the numbers and worked out 0.8 years for acceleration, 17.4 years of coasting at almost half the speed of light, and 18.8 years for deceleration. This gets you about 10 light years out in around 37 years, a mind-bending pace that uses a huge sail and some generous assumptions about laser power that we’ll look at tomorrow. For there are other ways to use lasers, even for deceleration, and other ways, too, to exploit the local interstellar medium.

Zubrin and Andrews’ paper from Vision-21 is “Use of Magnetic Sails for Advanced Exploration Missions,” in the proceedings of Vision-21: Space Travel for the Next Millennium” (NASA Conference Publication 10059. The citation for their 1988 work is given in yesterday’s Centauri Dreams post.


IBEX: The Heliosphere in Motion

The beauty of having spacecraft that far outlive their expected lives is that they can corroborate and supplement data coming in from much newer missions. That’s the case with our Voyager spacecraft as they continue their progress at system’s edge. The Voyagers will be moving outside the heliopause in not so many years, and when they do, they will tell us much about the behavior of charged particles in the interstellar medium. This will bulk up incoming results from IBEX, the Interstellar Boundary Explorer spacecraft, as it studies the neutral particles that routinely penetrate the heliosphere. Our knowledge of true interstellar space is growing.

It’s at the heliopause that we see the boundary between the area defined by the solar wind flowing outward from the Sun and the interstellar medium that surrounds it. Racing outward at an average of 440 kilometers per second, the solar wind is pushing into a region of dust and ionized gas, inflating the bubble we know as the heliosphere. The entire heliosphere incorporates the Sun and all the planets. Because interstellar neutral atoms do not interact with magnetic fields, they move through the heliopause readily, while charged particles do not. Hence the utility of the Voyagers, which will now move into a region from which we’ve had no prior data return.

Image: An artist’s rendition of a portion of our heliosphere, with the solar wind streaming out past the planets and forming a boundary as it interacts with the material between the stars. Credit: Adler Planetarium/IBEX team.

IBEX is a long way from the heliopause, orbiting the Earth with an apogee of 322,000 kilometers and a perigee of 16,000 kilometers, but its instruments are designed to map the distant interactions between the solar wind and ionized interstellar material, creating a map of the boundary. In addition, its low-energy energetic neutral atom camera is measuring inflowing interstellar neutral particles of the kind first measured by the Ulysses spacecraft. The IBEX measurements, recently discussed in a set of new papers, are helping us map the distribution of elements like hydrogen, helium, neon and oxygen as they enter the heliosphere from the local interstellar medium. By contrast, Ulysses was only able to detect neutral interstellar helium.

One thing we’re learning is the shape of the heliosphere. Interstellar neutral atoms enter the heliosphere at a speed of roughly 84,000 kilometers per hour. That’s a good deal slower than what Ulysses found, and IBEX sees the neutral particles entering the heliosphere from a somewhat different direction. “With this lower speed, the external magnetic forces cause the heliosphere to become more squished and misshapen,” says David McComas (SwRI), IBEX principal investigator. “Rather than being shaped like a bullet moving through the air, the heliosphere becomes flattened, more like a beach ball being squeezed when someone sits on it.”

IBEX is also showing us that our Sun is located in a region of space where dust and gas are extremely thinly dispersed, a situation different from the relative abundances of certain elements during the period when the Sun was formed. The neon to oxygen ratio in the Sun offers a glimpse of what the abundances of those elements were in that early era. IBEX, by measuring oxygen and neon from the interstellar medium, is finding less oxygen than expected, an indication of possible changes in the medium during the Sun’s lifetime. Alternatively, it is possible that the oxygen is still present but locked up in ice grains in the local interstellar material. McComas again:

“Our solar system is different than the space right outside it, suggesting two possibilities. Either the solar system evolved in a separate, more oxygen-rich part of the galaxy than where we currently reside, or a great deal of critical, life-giving oxygen lies trapped in interstellar dust grains or ices, unable to move freely throughout space.”

Interstellar space is far from empty, and IBEX is showing us what our early interstellar precursor missions may encounter. The Sun is evidently close to the boundary of a local cloud of gas and dust, one of many such clouds in our local galactic neighborhood. Ulysses results had indicated the Sun was placed between the ‘Local Cloud’ and the ‘G-Cloud,’ close to the boundary of the Local Cloud. The new IBEX work challenges this assumption, finding that the heliosphere is still fully within the Local Cloud. But we’re moving quickly (within a few thousand years at most) to leave the Local Cloud and move into what McComas calls ‘a much different galactic environment.’ IBEX and the Voyagers, then, are filling in our knowledge not only of the heliopause but the regions that surround our system as we move through the galaxy.

Image: The 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 that is so tenuous that the interstellar gas detected by IBEX 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. Credit: NASA/Goddard/Adler/U. Chicago/Wesleyan.

Make no mistake about the importance of the heliosphere — it is an important zone of protection against dangerous cosmic radiation, and thus has implications for the evolution of life, given that varying levels of radiation create genetic mutations and possible extinctions. How the makeup of the interstellar medium affects the heliosphere is thus a major issue, and changes as we move outside the Local Cloud could affect the heliosphere in ways we don’t yet understand. We’ll also use IBEX data to inform our studies of the analogous ‘astrospheres’ surrounding other stars.

Coda: Does any of this business about moving out of a local cloud and into more open interstellar space evoke for anyone else the same pleasing memories of Poul Anderson’s Brain Wave (1954)? Just checking.

The papers on the recent IBEX findings appeared in a Special Supplements issue of the Astrophysical Journal called “Interstellar Boundary Explorer (IBEX): Direct Sampling of the Interstellar Medium,” which appeared on January 31.


Voyager: Solar Wind Velocity Zero

When Voyager 2 was passing Neptune back in 1989, I stuck a video tape in the VCR and recorded the coverage — two video tapes, actually, because I wasn’t sure how much coverage there was going to be, and I didn’t want to miss anything. That meant getting up in the middle of the night to change tapes, but I figured the loss of sleep was worth it. Going back to those tapes today, I’m still struck by the same sense of awe that both the Voyagers were simply going to continue, that although the media spoke as if their journeys were over after their encounters at Titan and Neptune respectively, they still had years of power left and would continue talking to us deep into the 21st Century.

Image: Voyager 1 looks back to capture six planetary portraits. These six narrow-angle color images were made from the first ever “portrait” of the solar system taken by Voyager 1, which was more than 4 billion miles from Earth and about 32 degrees above the ecliptic. The spacecraft acquired a total of 60 frames for a mosaic of the solar system which shows six of the planets. Credit: Voyager 1 team/NASA.

The spacecraft nearing Neptune on my tapes was roughly 30 AU from the Sun. As of this morning, twenty-one years later, Voyager 2 is 94.03 AU out, and Voyager 1 (which left the ecliptic to make a flyby of Titan in 1980) is at 115.53 AU. The distances play with the imagination and offer a useful perspective. Voyager 1 is sixteen light hours from Earth — to be reasonably precise, 16 hours, 7 minutes as of 1202 UTC on the 14th. We’ve never sent anything sixteen light hours out before, but even at that distance, we’re only now penetrating the edge of the Solar System.

I have more to say about the size and scale of things, but I’m going to hold most of that for tomorrow in the form of a discussion of an interesting new book. For today, let’s talk about what Voyager 1 has found at 17.4 billion kilometers from the Sun. Out there, at the edge of the heliosphere, the spacecraft has moved into a region where the velocity of hot ionized gas — plasma — from the Sun has slowed to zero. With no outward movement of the solar wind, Voyager is seeing signs that the pressure of the ‘interstellar wind’ has turned the Sun’s wind sideways.

Voyager 1, then, is getting close to interstellar space, a crossing that will be marked by a sudden drop in the density of hot solar wind particles and an increase in the density of cold particles. The velocity of the solar wind has slowed at a rate of about 20 kilometers per second every year since August of 2007, at which point it was moving outward at some 60 kilometers per second. Readings over the last few months show an outward speed of zero since June. Rob Decker (JHU/APL) is a Voyager Low-Energy Charged Particle Instrument co-investigator, working with the instrument that provides the data fueling these results. Says Decker:

“When I realized that we were getting solid zeroes, I was amazed. Here was Voyager, a spacecraft that has been a workhorse for 33 years, showing us something completely new again.”

The thinking among Voyager scientists is that Voyager 1 is still within the heliosheath, the outer shell of the heliosphere (the bubble formed by the solar wind as it fills nearby space). After reaching the termination shock, the solar wind slows down and heats up in the heliosheath. Current models of the heliosphere’s structure, as discussed at the ongoing American Geophysical Union meeting in San Francisco, will be tuned up by the new data, allowing us a better estimate of Voyager 1’s entry into true interstellar space, now thought to be about four years away.

“In science, there is nothing like a reality check to shake things up, and Voyager 1 provided that with hard facts,” said Tom Krimigis, principal investigator on the Low- Energy Charged Particle Instrument, who is based at the Applied Physics Laboratory and the Academy of Athens, Greece. “Once again, we face the predicament of redoing our models.”

As we tune those models, we might ponder, along with the distance of the Voyager duo, their speed. Voyager 1 is the faster craft because of gravity assists at Saturn and Titan. It’s moving at about 17 kilometers per second, with Voyager 2 at 15 kilometers per second. Put that into interstellar terms and the scale of things again seizes the imagination. Launched in 1977, the Voyagers have had thirty-three years to get to where they are today. If Voyager 1 were pointed at Alpha Centauri, it would face a journey of 41.5 trillion kilometers. At 17 kilometers per second, that works out to 76,476 years.


Dust and Fast Missions

The recent debate between Jean Schneider (Paris Observatory) and Ian Crawford (University of London) is the sort of dialogue I’d like to see more of in public forums. When I began researching Centauri Dreams (the book) back in 2002, I was deeply surprised by the sheer energy flowing into interstellar flight research. True, it lacked focus and tended to be done by researchers in their spare time, as opposed to being funded by universities or government agencies, but I had not realized that the topic itself was under such serious investigation by so many scientists.

All those fascinating concepts, from laser sails to fusion runways, were the catalyst for this site, where keeping an eye on the ongoing discussion is the order of the day. In an era of short-term thinking and instant gratification through one gadget or another, taking a longer look at the human enterprise and where it is going is an imperative. One way to do that is to consider whether our species has a future in deep space, and just what the shape of that future might be. Discussions like Schneider’s and Crawford’s look long-term, at what we might one day accomplish with our technologies, and whether or not interstellar missions really are feasible.

We can surely say this much: Nothing in physics rules out interstellar flight, even though to accomplish it in the relatively near-term would require extremely long mission times for robotic vehicles whose expense would dwarf their potential utility even if we had the patience to wait out their journey. Let’s hope that by continued research we can learn to do better, and that means taking existing concepts and reworking them in light of new technologies to see what the possibilities are. All of which is fascinating stuff, and necessary as foundation-building even if we are, as seems most likely, one or more centuries away from the launch of any such mission.

Measurements of the Medium

First steps need to be taken in any enterprise. On that score, I remind you of the interstellar dust issue that Jean Schneider first raised in his original story in Astrobiology. We’re worried about a fast-moving vehicle (Schneider talks about 0.3c, though he and Crawford later drop the number to 0.1c) and the prospects of its encountering dust grains that could produce lethal damage. We’ll eventually need precursor missions to the edge of the Solar System and beyond to understand how the interstellar medium differs from interplanetary space in terms of dust.

In the afterword to his novel Flying to Valhalla, Charles Pellegrino makes a vivid case for potential disaster:

“Flying through space at significant fractions of lightspeed is like looking down the barrel of a super particle collider. Even an isolated proton has a sting, and grains of sand begin to look like torpedoes.”

Much data gathering is ahead, but the process has already begun. For now, we have New Horizons on its way to Pluto/Charon and the Kuiper Belt, taking useful readings through the mission’s dust counter, which is, by a happy choice, named after Venetia Burney, the English girl who named Pluto (Michael Byers’ account of the Pluto naming discussion in his novel Percival’s Planet is wonderful). New Horizons is now just beyond the orbit of Uranus and, as I write this morning, is 2 hours and 32 minutes light time (just over 18 AU, or 2,738,437,000 kilometers) from the Earth.

Image: An artist’s view of New Horizons approaching the Pluto/Charon flyby. Credit: SwRI.

Dust detecting instruments that have measured dust beyond the orbit of Jupiter have been rare, beginning with those aboard the Pioneer 10 and Pioneer 11 spacecraft, which were followed up by the dust analyzer aboard Cassini (more about this, and Voyager 2’s contribution , in a moment). The New Horizons dust counter is fairly straightforward, a thin plastic film on a honeycombed aluminum structure about the size of a cake pan mounted on the outside of the spacecraft. Each dust particle that strikes the detector sets off a unique signal, allowing its mass to be inferred. The fact that this is a student-built project (though with NASA engineering standards and professionally built flight instruments) brings home the excitement of this deep space mission in a way that, let’s hope, will galvanize future scientists and engineers.

Voyager 2 was also useful in dust measurements, though in a more indirect way. While the Pioneers carried dust counters, Voyager 2 measured the effects of space-borne dust by its effect on the spacecraft’s plasma wave instrument. The latter was designed to measure charged particles inside the magnetic field of gas giant planets, but usefully enough, it also registered a hit when the spacecraft encountered dust, picking up the plasma the vaporized particle created. Cassini carried a Cosmic Dust Analyzer to measure interplanetary dust grains beginning with its gravity assist at Venus in 1999 and lasting until arrival at Saturn in 2004.

It will be fascinating to see how New Horizons’ dust data compare with earlier missions (thus far what is being seen is in agreement with data from the Galileo and Ulysses missions in Jupiter space). What we learn about dust in the Kuiper Belt will be entirely new information as New Horizons speeds through this vast region, giving us clues about what an interstellar precursor mission might one day encounter. Some scientists, Schneider among them, believe dust could be a showstopper for probes moving at 10 percent or more of the speed of light. If that proves to be the case, we’re in for serious rethinking of interstellar mission concepts.