“It seems that destiny has taken a hand.” Thus Humphrey Bogart, in a pivotal scene from the iconic 1942 film Casablanca. In Bogart’s case, destiny had to do with the sudden arrival of Claude Rains and the gendarmerie at Rick’s Café Américain, with profound implications for his relationship with Ilsa. In my case, fate was more jejune, involving the failure of my PC’s power supply just as I was asking myself whether it was now time for my August vacation. The power supply left little doubt. Surely a sign from the cosmos that after all the recent work reconfiguring the site’s software, I should take some time off?
That’s how I plan to interpret it, in any case. In the meantime, I’ll get the PC problem resolved. As to the still developing work on the site, a couple of things to note:
1) I am all too aware that the mobile experience is problematic, depending on what phone you use. I find this bewildering, as many people see the site correctly on their phones, whereas people like me see a very skinny column of text with huge side margins. So that is right up there on the list, and I plan to work on it during my time off. Since I can’t travel anywhere right now, I will be staying put. I will be on the site each day, moderating comments and also trying to work out glitches. But I will not be posting new material under my byline for approximately two weeks.
2) Speaking of glitches, this one came out of nowhere. I’ve learned that older posts have comment sections that are not formatting correctly. This too needs to be fixed, and part of my time off will be spent in the quest for answers on that matter.
Otherwise, in the next couple of weeks, I plan to watch old movies and read the novels now at the top of my fiction reading stack. These include books by Richard Ford, Alastair Reynolds, Graham Greene and Emily St. John Mandel, although I have to finish up Alan Furst’s wonderful The World at Night before proceeding to the first of these.
One last thing: Every now and then I get a message from someone who has had trouble trying to leave a comment with the new interface. Here’s the method: To comment, what you need to do is click on the title of the post, which will open the same post with the comment section in place at the end of the text.
As I say, keep the comments coming, as I’ll be here to put them through. Thanks to all for your patience and suggestions re the site changes. More to come.
Those of you who follow Centauri Dreams through email probably received an inadvertent test post this morning. My apologies. The post was triggered by work on the site’s internals and was generated automatically by the email software module. Work on the site continues, but I think the email issue is fixed, so I anticipate no more of these. Thanks for your patience.
Centauri Dreams began as a website back in August of 2004. I’m startled to realize, looking through the stats that my site’s software provides, that in the subsequent nineteen years, there have been 4,659 posts, along with close to 100,000 comments. The irony is that I started the site simply as a research venue for myself, thinking to keep up with the latest news by building a collection of articles and scientific papers. It took about a year before I even switched on the comments function.
One of the benefits of publishing for such a length of time is perspective, as the interstellar research scene has grown and changed over the past two decades. But one thing I didn’t do is keep up with the software. Always focused on content, I’ve kept writing but have let too many generations of internal programming stay mired in older iterations. The dangers of this are obvious. A site with obsolete internals is all too open to hacking. And now, completely normal upgrades to some of the site’s functionality threaten to break some of the older software. Something has to be done.
What’s now happening is a thorough re-doing of the internals of Centauri Dreams, one that will solve the immediate problems and allow upgrades to some of the external programs I use. The most obvious change to readers will be the site theme, although things should remain pretty familiar. I want Centauri Dreams to continue with its basic layout, and that means no advertising, no pop-up windows, no annoyances to distract from the text. Behind the scenes, the site will be rendered more secure and also more efficient, with less chance of an errant move on my part bringing things down.
Please bear with me as the work proceeds. The new look comes with significantly tightened security. Work behind the scenes will continue on a number of issues I want to resolve. I’ll tweak the look and feel around the edges, but let’s get through the transition first. This should be done within the next day or two. If any late-arriving comments get lost along the way, I’ll get those restored as soon as I can. Anticipating problems – and they always turn up, no matter what – should help to deflect them.
The preliminary program for the Interstellar Research Group’s 8th Interstellar Symposium in Montreal is now available. For those of you heading to the event, I want to add that the early bird registration period for attending at a discount is May 31. Registration fees go up after that date. Registering at the conference hotel can be handled here. Registration before the 31st is recommended to get a room within the block reserved for IRG.
I was saddened to learn of the recent death of James Early, author of a key paper on interstellar sail missions and a frequent attendee at IRG events (or TVIW, as the organization was known when I first met him). Jim passed away on April 28 in Saint George, UT at the age of 80, a well-liked figure in the interstellar community and a fine scientist. I wish I had known him better. I ran into him for the first time in a slightly awkward way, which Jim, ever the gentleman, quickly made light of. What happened was this. In 2012 I was researching damage that an interstellar sail mission might experience in the boost phase of its journey. Somewhere I had seen what I recall as a color image in a magazine (OMNI?) showing a battered, torn sail docked in what looked to be a repair facility at the end of an interstellar crossing. It raised the obvious question: If we did get a sail up to, say, 5% of the speed of light, wouldn’t even the tiniest particles along the way create significant damage to the structure? The image was telling and to this day I haven’t found its source. I think of the image as ‘lightsail on arrival,’ and if this triggers a memory with anyone, please let me know. Anyway, although our paths crossed at the first 100 Year Starship symposium in Orlando in 2011, I didn’t know Jim’s work and didn’t realize he had analyzed the sail damage question extensively. When I wrote about the matter on Centauri Dreams a year later, he popped up in the comments:
I presented a very low mass solution to the dust problem at the 100 Year Starship Symposium in a talk titled “Dust Grain Damage to Interstellar Vehicles and Lightsails”. An earlier published paper contains most of the important physics: Early, J.T., and London, R.A., “Dust Grain Damage to Interstellar Laser-Pushed Lightsail”, Journal of Spacecraft and Rockets, July-Aug. 2000, Vol. 37, No. 4, pp. 526-531.
I was caught by surprise by the reference. How did I miss it? Researching my 2005 Centauri Dreams book, I had been through the literature backwards and forwards, and JSR was one of the journals I combed for deep space papers. Later, at a TVIW meeting in Oak Ridge, we talked, had dinner and Jim kidded me about my research methods. As I saw it, his paper was a major contribution, and I should have known about it. Yesterday I asked Andrew Higgins (McGill University) about the paper and he had this to say in an email:
Jim Early’s paper (written with Richard London in 1999) on dust grain impacts addressed one of the bogeys of interstellar flight: The dust grain impact problem when traveling at relativistic speeds. Their analysis showed—counterintuitively—that the damage caused by a dust grain on an interstellar lightsail actually decreases as the sail exceeds a few percent of the speed of light. While the grain turns into an expanding fireball of plasma as it passes through the sail, the amount of thermal radiation deposited on the sail decreases as the fireball is receding more quickly from the sail. This was a welcome result suggesting sails might survive the interstellar transit, and their study remains the seminal work on dust grain interactions with thin structures at relativistic speeds.
Image: Dinner after the first day’s last plenary session in Oak Ridge in 2014. That’s Jim Benford at far left, then James Early, Sandy Montgomery and Michael Lynch. The family has set up a website honoring Jim and offering photos and an obituary. He got his bachelor’s degree in Aeronautics at MIT, following it with a master’s degree in mechanical engineering at Caltech, and a PhD in aeronautics and physics at Stanford University. He was involved with development activities for the Delta launch vehicle while obtaining his bachelor’s degree by working at NASA Goddard Space Flight Center in the summers and then at McDonnell-Douglas after finishing his master’s degree. He joined Lockheed and Hughes aircraft for a time before finally ending up at the Lawrence Livermore National Laboratory working on laser physics until he retired.
Sail in Flight
So let’s look at Jim’s paper on sails, a subject he continued to work on for the next two decades. Although Robert Forward came up with sail ideas that pushed as high as 30 percent of the speed of light (and in the case of Starwisp, even higher), Jim and his co-author Richard London chose 0.1 c for cruise velocity in their paper, which provides technical challenges aplenty but at least diminishes the enormous energy costs of still faster missions, and certainly mitigates the problem of damage from dust and gas along the way. Depending on the methods used, the sail as analyzed in this paper may take a tenth of a light year to get up to cruise velocity. It’s worth mentioning that the sail does not have to remain deployed during cruise itself, but deceleration at the target star, depending on the methods used, may demand redeployment. Breakthrough Starshot envisions stowing the sail in cruise after its sudden acceleration to 20 percent of c. Early and London use beryllium sails as their reference point, these being the best characterized design at this stage of sail study, and assume a sail 20 nm thick. In terms of the interstellar medium the sail will encounter, the authors say this:
Local interstellar dust properties can be estimated from dust impact rates on spacecraft in the outer solar system and by dust interaction with starlight. The mean particle masses seen by the Galileo and Ulysses spacecraft were 2×10-12 and 1×10-12g, respectively. A 10-12g dust grain has a diameter of approximately 1 µm. The median grain size is smaller because the mean is dominated by larger grains. The Ulysses saw a mass density of 7.5×10-27g cm-3. A sail accelerating over a distance of 0.1 light years would encounter 700 dust grains/cm2 at this density. The surface of any vehicle that traveled 10 light years would encounter 700 dust grains/mm2. If a significant fraction of the particle energy is deposited in the impacted surface in either case, the result would be catastrophic.
The question then becomes, just how much of the particle’s energy will be deposited on the sail? The unknowns are all too obvious, but the paper adds that neither of the Voyagers saw dust grains larger than 1 ?m at distances beyond 50 AU, while a 1999 study on interstellar dust grain distributions found a flat distribution from 10-14 to 10-12 g with some grains as large as 10-11 g. Noting that a 10-12 dust grain has a diameter of about 1-?m, the authors use a 1-?m diameter grain for their impact calculations. The results are intriguing because they show little damage to the sail. Catastrophe averted:
At the high velocities of interstellar travel, dust grains and atoms of interstellar gas will pass through thin foils with very little loss of energy. There will be negligible damage from collisions between the nuclei of atoms. In the case of dust particles, most of the damage will be due to heating of the electrons in the thin foil. Even this damage will be limited to an area approximately the size of the dust particle due to the extremely fast, one-dimensional ambipolar diffusion explosion of the heated section of the foil. The total fraction of the sail surface damaged by dust collisions will be negligible.
The torn and battered lightsail in its dock, as seen in my remembered illustration, may then be unlikely, depending on cruise speed and, of course, on the local medium it passes through. Sail materials also turn out to offer excellent shielding for the critical payload behind the sail:
Interstellar vehicles require protection from impacts by dust and interstellar gas on the deep structures of the vehicle. The deployment of a thin foil in front of the vehicle provides a low mass, effective system for conversion of dust grains or neutral gas atoms into free electrons and ions. These charged particles can then be easily deflected away from the vehicle with electrostatic shields.
And because the topic has come up in a number of past discussions here, let me add this bit about interstellar gas and its effects on the lightsail:
The mass density of interstellar gas is approximately one hundred times that of interstellar dust particles though this ratio varies significantly in different regions of space. The impact of this gas on interstellar vehicles can cause local material damage and generate more penetrating photon radiation. If this gas is ionized, it can be easily deflected before it strikes the vehicle’s surface. Any neutral atom striking even the thin foil discussed in this paper will pass through the foil and emerge as an ion and free electron. Electrostatic or magnetic shields can then deflect these charged particles away from the vehicle.
Consequences for Sail Design
All of these findings have a bearing on the kind of sail we use. The thin beryllium sail appears effective as a shield for the payload, with a high melting point and, the authors conclude, the ability to be increased in thickness if necessary without increasing the area damaged by dust grains. Ultra-thin foils of tantalum or niobium offer higher temperature possibilities, allowing us to increase the laser power applied to the sail and thus the acceleration. But Early and London believe that the higher atomic mass of these sails would make them more susceptible to damage. Even so, “…the level of damage to thin laser lightsails appears to be quite small; therefore the design of these sails should not be strongly influenced by dust collision concerns.” Dielectric sails would be more problematic, suffering more damage from heated dust grains because of their greater thickness, and the authors argue that these sail materials need to be subjected to a more complete analysis of the blast wave dynamics they will experience. All in all, though, velocities of 0.1 c yield little damage to a thin beryllium sail, and thin shields of similar materials can ionize dust as well as neutral interstellar gas atoms, allowing the ions to be deflected and the interstellar vehicle protected. These are encouraging results, but the size of the problem is daunting, and given the apparent cost of the classically conceived interstellar probe, the prospect of impact damage calls for continued analysis of the medium through which the probe would pass. This is one of the advantages of sending not one large craft but a multitude of smaller ‘chipsat’ style vehicles in the Breakthrough Starshot model. Send enough of these and you can afford to lose a certain percentage along the way. I can only wish I could sit down with Jim Early again to kick around chipsat concepts, but what a fine memorial to know that your paper continues to influence evolving interstellar ideas. The paper is Early, J.T., and London, R.A., “Dust Grain Damage to Interstellar Laser-Pushed Lightsail,” Journal of Spacecraft and Rockets, July-Aug. 2000, Vol. 37, No. 4, pp. 526-531.
I’m going to need to take some time off, a decision prompted by responsibilities outside the interstellar community that have grown to the point where I lack the time to maintain a consistent schedule on the site. I’ll keep moderating comments as usual, and I have some first-rate essays coming up from other authors, but my own writing is going to have to be sporadic for the time being.
Long-term, I plan to keep Centauri Dreams active for a long time, so bear with me. As soon as I can do it, I will get back to a more consistent schedule. For now, though, expect a slower pace of new posts from me.