Here is another great post from Centauri Dreams, written by Andreas Hein. Good stuff.
2089, 5th April: A blurry image rushes over screens around the world. The image of a coastline, waves crashing into it, inviting for a nice evening walk at dawn. Nobody would have paid special attention, if it were not for one curious feature: Two suns were mounted in the sky, two bright, hellish eyes. The first man-made object had reached another star system.
Is it plausible to assume that we could send a probe to another star within our century? One major challenge is the amount of resources needed for such a mission. [1, 2]. Ships proposed in the past were mostly mammoths, weighing ten-thousands of tons: the fusion-propelled Daedalus probe with 54,000 tonnes and recently the Project Icarus Ghost Ship with over 100,000 tonnes. All these concepts are based on the rocket principle, which means that they have to take their propellant with them to accelerate. This results in a very large ship.
Another problem with fusion propulsion in particular is the problem of scalability. Most fusion propulsion systems get more efficient when they are scaled up. There is also a critical lower threshold for how small you can go. These factors lead to large amounts of needed propellant and large engines, for which you need a large space infrastructure. A Solar System-wide economy is probably needed, as the Project Daedalus report argues .
Image: The Project Icarus Ghost Ship: A colossal fusion-propelled interstellar probe
However, there is a different avenue for interstellar travel: going small. If you go small, you need less energy for accelerating the probe and thus less resources. Pioneers of small interstellar missions are Freeman Dyson with his Astrochicken; a living, one kilogram probe, bio-engineered for the space environment . Robert Forward proposed the Starwisp probe in 1985 . A large, ultra-thin sail which rides on a beam of microwaves. Furthermore, Frank Tipler and Ray Kurzweil describe how nano-scale probes could be used for transporting human consciousness to the stars [6, 7].
At the Initiative for Interstellar Studies (I4IS), we wanted to have a fresh look at small interstellar probes, laser sail probes in particular. The last concepts in this area have been developed years ago. How did the situation change in recent years? Are there new, possibly disruptive concepts on the horizon? We think there are. The basic idea is to develop an interstellar mission by combining the following technologies:
- Laser sail propulsion: The spacecraft rides on a laser beam, which is captured by an extremely thin sail .
- Small spacecraft technology: Highly miniaturized spacecraft components which are used in CubeSat missions
- Distributed spacecraft: To spread out the payload of a larger spacecraft over several spacecraft, thus, reducing the laser power requirements [9, 10]. The individual spacecraft would then rendezvous at the target star system and collaborate to fulfill their mission objectives. For example, one probe is mainly responsible for communication with the Solar System, another responsible for planetary exploration via distributed sensor networks (smart dust) .
- Magnetic sails: A thin superconducting ring’s magnetic field deflects the hydrogen in the interstellar medium and decelerates the spacecraft .
- Solar power satellites: The laser system shall use space infrastructure which is likely to exist in the next 50 years. Solar power satellites would be temporarily leased to provide the laser system with power to propel the spacecraft.
- Communication systems with external power supply: A critical factor for small interstellar missions is power supply for the communication system. As small spacecraft cannot provide enough power for communicating over these vast distances. Thus, power has to be supplied externally, either by using laser or microwave power from the Solar System during the trip and solar radiation within the target star system .
Image: Size comparison between an interplanetary solar sail and the Project Icarus Ghost Ship. Interstellar sail-based spacecraft would be much larger. (Courtesy: Adrian Mann and Kelvin Long)
Bringing all these technologies together, it is possible to imagine a mission which could be realized with technologies which are feasible in the next 10 years and could be in place in the next 50 years: A set of solar power satellites are leased for a couple of years for the mission. A laser system with a huge aperture has been put into a suitable orbit to propel the interstellar, as well as future planetary missions. Thus, the infrastructure can be reused for multiple purposes. The interstellar probes are launched one-by-one.
After decades, the probes start to decelerate by magnetic sails. Each spacecraft charges its sails differently. The first spacecraft decelerates slower than the follow-up probes. Ideally, the spacecraft then arrive at the target star system at the same point in time. Then, the probes start exploring the star system autonomously. They reason about exploration strategies, exchange and share data. Once a suitable exploration target has been chosen, dedicated probes descend to the planetary surface, spreading dust-sized sensor networks onto the pristine land. The data from the network is collected by other spacecraft and transferred back to the spacecraft acting as a communication hub. The hub, powered by the light from extrasolar light sends back the data to us. The result could be the scenario described at the beginning of this article.
Image: Artist’s impression of a laser sail probe with a chip-sized payload. (Courtesy: Adrian Mann)
Of course, one of the caveats of such a mission is its complexity. The spacecraft would have to rendezvous precisely over interstellar distances. Furthermore, there are several challenges with laser sail systems, which have been frequently addressed in the literature, for example beam collimation and control. Nevertheless, such a mission architecture has many advantages compared to existing ones: It could be realized by a space infrastructure we could imagine to exist in the next 50 years. The failure of one or more spacecraft would not be catastrophic, as redundancy could easily be built in by launching two or more identical spacecraft.
The elegance of this mission architecture is that all the infrastructure elements can also be used for other purposes. For example, a laser infrastructure could not only be used for an interstellar mission but interplanetary as well. Further applications include an asteroid defense system . The solar power satellites can be used for providing in-space infrastructure with power .
Image: Artist’s impression of a spacecraft swarm arriving at an exosolar system (Courtesy: Adrian Mann)
How about the feasibility of the individual technologies? Recent progress in various areas looks promising:
- The increased availability of highly sophisticated miniaturized commercial components: smart phones include many components which are needed for a space system, e.g. gyros for attitude determination, a communication system, and a microchip for data-handling. NASA has already flown a couple of “phone-sats”; Satellites which are based on a smart phone .
- Advances in distributed satellite networks: Although a single small satellite only has a limited capability, several satellites which cooperate can replace larger space systems. The concept of Federated Satellite Systems (FSS) is currently explored at the Massachusetts Institute of Technology as well as at the Skolkovo Institute of Technology in Russia . Satellites communicate opportunistically and share data and computing capacity. It is basically a cloud computing environment in space.
- Increased viability of solar sail missions. A number of recent missions are based on solar sail technology, e.g. the Japanese IKAROS probe, LightSail-1 of the Planetary Society, and NASA’s Sunjammer probe.
- Greg Matloff recently proposed use of Graphene as a material for solar sails . With an areal density of a fraction of a gram and high thermal resistance, this material would be truly disruptive. Currently existing materials have a much higher areal density; a number crucial for measuring the performance of solar sails.
- Material sciences has also advanced to a degree where Graphene layers only a few atoms thick can be manufactured . Thus, manufacturing a solar sail based on extremely thin layers of Graphene is not as far away as it seems.
- Small satellites with a mass of only a few kilograms are increasingly proposed for interplanetary missions. NASA has recently announced the Interplanetary CubeSat Challenge, where teams are invited to develop CubeSat missions to the Moon and even deeper into space (NASA) . Coming advances will thus stretch the capability of CubeSats beyond Low-Earth Orbit.
- Recent proposals for solar power satellites focus on providing space infrastructure with power instead of Earth infrastructure [18, 19]. The reason is quite simple: Solar power satellites are not competitive to most Earth-based alternatives but they are in space. A recent NASA concept by John Mankins proposed the use of a highly modular tulip-shaped space power satellite, supplying geostationary communication satellites with power.
- Large space laser systems have been proposed for asteroid defense 
In order to explore various mission architectures and encourage participation by a larger group of people, I4IS has recently announced the Project Dragonfly Competition in the context of the Alpha Centauri Prize . We hope that with the help of this competition, we can find unprecedented mission architectures of truly disruptive capability. Once this goal is accomplished, we can concentrate our efforts on developing individual technologies and test them in near-term missions.
If this all works out, this might be the first time in history that there is a realistic possibility to explore a near-by star system within the 21st or early 22nd century with “modest” resources.
I remember when the original Project Icarus study came out in the 1970s and I was absolutely enthralled with it.
At last, interstellar exploration could be possible, not fantasy.
Then the Icarus came out a couple of years ago. The ship was more advanced, but the size doubled. How is that possible in this age of miniaturization?
I think it’s because people love the idea of Battlestar Galactica or U.S.S. Enterprise sized interstellar craft.
You gotta have powerful engines and weapons to cope with angry aliens, right?
Andrea Hein is being smart and paying respect to Robert Foward and Freeman Dyson by writing this study with up to date ideas which encompasses Cube Sat tech and other commercial space company technologies.