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-μm 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 speciﬁc 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 ﬁnd 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 ﬂight.
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-μm 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) ﬂight 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 difﬁculty 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.