Centauri Dreams reader Brian Koester passed along a link to a provocative video last month that spurs thoughts about the nature of interstellar probes. The video is a TED talk delivered by Paul Rothemund in 2007. For those not familiar with it, TED stands for Technology, Entertainment, Design, a conference that began in 1984 and now brings together interesting scientific figures whose challenge is to give the best talk they can on their specialty within the span of eighteen minutes.
I’ve been pondering Rothemund’s talk for some time. You can call this Caltech bioengineer a ‘DNA origamist,’ meaning that he is exploring ways to fold DNA into shapes and patterns. As becomes clear in his presentation, folding DNA into ‘smiley’ faces or maps has a certain wow factor, but once you get past the initial wonder of working at this level, you begin to appreciate how research in DNA nanotechnology points toward self-assembling devices that can be built at the micro-scale.
Molecular Computing to the Stars
And now we’re off to the races, for as Koester noted in his email to me, a small interstellar probe could theoretically create a molecular computer which could then, upon arrival, create electronic equipment of the sort needed for observations. Think of a probe that gets around the payload mass problem by using molecular processes to create cameras and imaging systems not by mechanical nanotech but by inherently biological methods.
A Von Neumann self-replicating probe comes to mind, but we may not have to go to that level in our earliest iterations. The biggest challenge to our interstellar ambitions is propulsion, with the need to push a payload sufficient to conduct a science mission to speeds up to an appreciable percentage of lightspeed. The more we reduce payload size, the more feasible some missions become — Koester was motivated to write by considering ‘Sundiver’ mission strategies coupled with microwave beaming.
The question becomes whether molecular computing can proceed to develop the needed instrumentation largely by tapping resources in the destination system, a process John Von Neumann called ‘interstellar in-situ resource utilization.’ The more in-system resources we can tap (in the destination system, that is), the lighter our initial payload has to be, and yes, that raises countless issues about targeting the mission and the flexibility of the design once arrived to conduct the needed harvesting.
What an interesting concept. It’s fascinating to see how far the notion of self-replication has taken us since Robert Freitas produced a self-replicating interstellar probe based on the original Project Daedalus design. That one, called REPRO, would mine the atmosphere of Jupiter for helium-3, just like Daedalus, and would use inertial confinement fusion for propulsion. But REPRO would carry a so-called SEED payload that, upon arrival on the moon of a gas giant, would produce an automated factory that would turn out a new REPRO every five hundred years.
But REPRO would have been massive (each SEED payload would weigh in at close to five hundred tons), with all the challenges that added to the propulsion question. Freitas later turned to nanotech ideas in advocating a probe more or less the size of a sewing needle, with a millimeter-wide body and enough nanotechnology onboard to activate assemblers on the surface of whatever object it happened to find in the destination system.
Now we’re looking at a biological variant of this concept that could, if extended, be turned to self-replication. Rothemund says that he wants to write molecular programs that can build technology. A probe built along these lines could use local materials to create the kind of macro-scale objects needed to form a research station around another star, the kind of equipment we once envisioned boosting all the light years to our target. How much simpler if we can build the needed tools when we arrive?
A Long Leap for DNA Origami
Caltech’s Erik Winfree, who works with Rothemund, gave New Scientist a recent update on where the work stands:
Although the team has so far used the technique to build simple pipes… much more is possible, Winfree says. “Metaphorically, this is similar to how genetic programs within cells direct the growth of an organism.”
Winfree and Rothemund speculate that the technique could provide a way to assemble molecular components into useful structures such as tiny electric circuits. It is also possible to use the self-assembling DNA structures to perform computational tasks, adds Winfree.
“It is very powerful for information processing,” he says. “It’s what’s known as a Universal Turing Machine, which means it can carry out any information processing task.”
Can the method ultimately be extended to serve our interstellar purpose? You can get an overview of Paul Rothemund’s work from the video I linked to above, and also in his paper “Folding DNA to create nanoscale shapes and patterns,” Nature Vol 440 (16 March 2006), pp. 297-302 (available online). Publications from Caltech’s DNA and Natural Algorithms Group are available here.
Finally, on the question of assembly and replication, see Barish et al., “An information-bearing seed for nucleating algorithmic self-assembly,” Proceedings of the National Academy of Sciences Vol. 106, No. 15, pp. 6054-6059 (available online).