Going Interstellar (24 page)

Icarus
: A Lesson from Mythology

Icarus was a character from ancient Greek mythology. In an attempt to escape the labyrinth prison of King Minos, his father Daedalus fashioned a pair of wings made of feathers and wax for both himself and his son. Icarus, so the story goes, flew too close to the Sun and the wax on his wings melted. He fell into the sea and died after having touched the sky.

To paraphrase Sir Arthur Eddington from
Stars and Atoms
, the standard interpretation of the Icarus myth is that he was a man performing a stunt who met his ill-fated doom due to his antics. However, an alternative interpretation of the myth is that he is the man who illuminated a serious constructional deficiency in the flying machines of his era. Perhaps there is also a lesson for science here. A more cautious Daedalus applies his theories only where he feels confident they will succeed; but by his overindulgent caution, their veiled faults remain undiscovered. Conversely, Icarus will drive his theories to the threshold of collapse, and we may at least hope to learn from his flight how to construct a better machine.

Purpose and Ambitions of Project
Icarus

Project
Icarus
, as the successor to Project
Daedalus
, is a theoretical engineering design study for an unmanned interstellar craft. Its overall purpose can be summarized as follows:

• To design a credible interstellar probe that embodies the essential concepts for a successful mission in the coming centuries.

• To allow a direct technology comparison with
Daedalus
and provide an assessment of the maturity of fusion-based space propulsion for future precursor missions.

• To generate greater interest in the real term prospects for interstellar precursor missions that are based on credible science.

• To motivate a new generation of scientists to be interested in designing space missions that go beyond our solar system.

Using these four purposes as a guide, the collective scope of the project is codified in the
Icarus
Terms of Reference:

1. To design an unmanned probe that is capable of delivering useful scientific data about the target star, associated planetary bodies, solar environment and the interstellar medium.

2. The spacecraft must use current or near-future technology and be designed to be launched as soon as is credibly determined.

3. The spacecraft must reach its stellar destination within as short a time as possible, not exceeding a century and ideally much sooner.

4. The spacecraft must be designed to visit any one of a variety of target stars.

5. The spacecraft propulsion must be mainly fusion based (e.g.
Daedalus
).

6. The spacecraft mission must be designed so as to allow some deceleration for increased encounter time at the destination.

One of the main differences between
Daedalus
and
Icarus
is the requirement that there be some deceleration at the target system.
Daedalus
had a cruise velocity 12% of light speed, and would have raced through the target system within days. It would have been in close proximity to any planet for only a matter of seconds. This short encounter time would severely restrict the scientific return from the mission, and so
Icarus
is committed to address the issue of deceleration.

Parallel Objectives

Project
Icarus
is clearly a highly scientific endeavor whose success will be measured by the credibility and quality of the work that is created. Despite these academic ambitions, there are additional motives behind the project that are worthy of further examination.

One such motive is to use
Icarus
as a vehicle for training a new generation of interstellar engineers. The field of interstellar propulsion is sprinkled with luminaries whose names have become synonymous with the field of interstellar propulsion. These visionaries include VIPs such as Robert Bussard, Bob Forward, Greg Matloff, Robert Frisbee and Alan Bond, whose names will be immediately recognizable to interstellar aficionados. To maintain the healthy vision of a future where interstellar travel is possible, a new generation of capable enthusiasts is required. Project
Icarus
was designed with this specific motive in mind, and a quick glance at the
Icarus
designers reveals an average age close to thirty. Thus, one hope is that, upon completion of the project, an adept team of competent interstellar engineers will have been created, and that this team will continue to kindle the dream of interstellar flight for a few more decades until, presumably, they too become grey and find their own enthusiastic replacements.

Another parallel objective of Project
Icarus
is to evolve the possibility of interstellar flight from being merely feasible to actually being practical. The
Daedalus
design demonstrated that, with sufficient determination, a craft could be built using known principles of physics that could reach another star system in approximately fifty years. However, some critical components to
Daedalus
may lead a conservative spectator to believe that
Daedalus
was too ambitious.

Two impractical aspects of the original
Daedalus
spring to mind. The first is a feature of the propulsion system which necessitates the firing of marble-sized pellets, consisting of mainly deuterium and Helium-3, into a reaction chamber at a rate of 250 pellets per second. These pellets would then be ignited by high powered relativistic electron beams in a process known as inertial confinement fusion (ICF). Though considered a credible way to liberate energy from the fusion fuel, the fusion ignition rate of 250 hertz is difficult to be taken seriously given that the National Ignition Facility (NIF), a large U.S. fusion ignition project located at the Lawrence Livermore Laboratory, will likely accomplish only one such event per day under ideal conditions! Thus, an improvement on this rate by a factor of approximately twenty-one million would be required to achieve
Daedalus
fusion pellet ignition rates. While certainly not impossible, this pellet frequency requires a rather vast improvement on current technology. However, it’s important to recognize that the NIF is a physics demonstrator, and that it is not designed to be optimized for rapid ignition rates.

The second feature of
Daedalus
that appears improbable is the choice of fuel. As mentioned earlier, Helium-3 is a critical component and incredibly rare on Earth. Jupiter would have to be mined by sophisticated orbiting balloons and its Helium-3 ultimately transported to the
Daedalus
. Again, while certainly not impossible, such planetary mining operations would very likely imply that a massive space-based infrastructure should already be in place. The original
Daedalus
team acknowledged that a culture with this capability would likely be centuries ahead of our own. Estimates for the total costs of building a
Daedalus
class spacecraft lie in the ten to one hundred trillion dollar range. With the current NASA budget for 2011 lying close to eighteen billion dollars, increasing it a thousand times is not impossible, given sufficient ambition, but viewed from today’s geopolitical landscape, it is highly unlikely.

A full and systematic treatise regarding the additional and subtle impracticalities of the
Daedalus
design are beyond the scope of this essay. Suffice to say that most scientists of today would consider the design to be overly ambitious; rather than marvel at its audacity, those same scientists would likely dismiss
Daedalus
as unrealistic. For this reason, one parallel objective of
Project Icarus
is to create a credible engineering design that is feasible with minimal extrapolations of current technology. One way of accomplishing this is through incremental improvements of the relevant Technology Readiness Levels (TRL).

The TRL scale is used to gauge the relative maturity of a concept. The scale has nine levels, with TRL 1 being the lowest level of technological maturity, and TRL 9 the highest. For example, the definition of TRL 1 is: “Basic principles observed and reported: Transition from scientific research to applied research. Essential characteristics and behaviors of systems and architectures. Descriptive tools are mathematical formulations or algorithms.”

Contrast this with the other end of the scale, TRL 9, which is defined as: “Actual system ’mission proven’ through successful mission operations (ground or space): Fully integrated with operational hardware/software systems. Actual system has been thoroughly demonstrated and tested in its operational environment. All documentation completed. Successful operational experience. Sustaining engineering support in place.”

The nine levels are a convenient way to assess the maturity of a technology.

Many features of the
Daedalus
design lie in the TRL 1 to TRL 3 range, indicating a low level of maturity. One measure of success for Project
Icarus
will be the evolution of critical interstellar technologies to a higher TRL level, since this assists in the promotion of the design credibility. For this reason, TRL evolution and comparison to
Daedalus
for key research areas is a valuable objective for Project
Icarus
.

One final, and particularly interesting, parallel motive is to use the project as an experiment in the efficacy of volunteer researchers collaborating in a purely virtual capacity. In many ways, Project
Icarus
may represent a new way for scientific research to be conducted. The
Icarus
team currently consists of twenty-nine team members, located across six different countries. The researchers are not paid for their efforts and are primarily motivated by a passion for the field. Interaction between team members is mostly conducted by email. However, a private internet forum also exists where the team can engage in extended discussions on a variety of topics. Internet telephony is also utilized, on occasion, as the need for (virtual) face-to-face communication arises. This mainly electronic team is, in itself, an interesting experiment in the virtualization of scientific collaboration and, should the outcome be successful,
Project Icarus
could serve as a prototype for future scientific and engineering endeavors.

Nuclear Fusion—A Propulsion Scheme for the Future

One of the Terms of Reference of Project
Icarus
is that the propulsion system must be ‘mainly fusion-based,’ which is currently considered TRL 2. This mandate to use fusion was based on the fact that
Daedalus
was itself fusion-powered. As the successor to
Daedalus
it seemed appropriate for
Icarus
to utilize this same energy source so as to maintain continuity with the original project. Alternatives to this form of propulsion do exist, and popular non-fusion options include solar sailing and even antimatter. The fusion decision was made early on and met with no objections from any team members.

A more comprehensive discussion of the physics of thermonuclear fusion may be found in Dr. Gregory Matloff’s companion essay in this book. Briefly summarized, fusion is a process whereby two atoms are provided with sufficient kinetic energy to merge and create a larger atom and some by-products. Energy is created in the form of electromagnetic radiation and the vast amounts of kinetic energy contained in the new products that are formed from the reaction.

To give some perspective, fusion processes liberate approximately one million times more energy than even the most powerful chemical reactions. Imagine, for a moment, a hypothetical car of the future, where just one gram of fusion fuel could, in theory, power the vehicle for its entire lifetime. This is, of course, a huge oversimplification, and probably not feasible based on the mechanical architecture that would be necessary to harness the fusion energy, but it emphasizes the point quite nicely. Indeed, fusion processes are what have powered our own star, the Sun, for about five billion years, and will continue to do so for five billion years more.

Fusion has been understood since the early twentieth century, and efforts to harness the energy have been ongoing for most of the latter half of the twentieth century. To date, the only effective utilization of fusion energy has been in rapid and uncontrollable thermonuclear bombs, generally referred to as H-bombs. However, the controlled release of energy in power stations has not yet reached a sustainable break-even which is a situation where more energy is released than is actually put in to create the reaction in the first place. Despite this contemporary lack of success, many believe it is simply a matter of time until the technology is perfected. Indeed, progress in experimental fusion reactors has been consistent for a number of decades.

Inertial Confinement Fusion (ICF) is considered a promising approach to fusion propulsion. In ICF the charged reaction products themselves are turned directly into thrust via magnetic nozzles. This process leads to far fewer thermodynamic losses and enables much of the fusion energy to be channeled to create thrust for the spacecraft. The
Daedalus
spacecraft was to be powered by ICF.

The Project
Icarus
group has identified no less than seventeen unique approaches to nuclear fusion, including plasma jet driven magneto-inertial confinement fusion, z-pinch fusion, antimatter catalyzed fusion and electrostatic inertial confinement fusion. At the current phase in the project, no one method has yet shown to be a favorable fusion technique that would prove ideal for an interstellar mission. However, research continues, and in the future a candidate will be selected.