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Practical is a relative term depending on one's priorities. To some this means choosing the most feasible approach, regardless of how long it takes to reach another star system. To others, star ships are not practical unless they are generation ships, and to still others, star flight won't be practical until it is as simple as envisioned in science fiction. |
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From
the
seemingly simple to the seemingly impossible; there are many ways
to contemplate interstellar flight. The first column refers to
spacecraft already in flight, such as the Voyager-1 spacecraft launched
in 1977. Although these are the deepest human objects in space, they
are not 'practical' as interstellar craft unless one has the patience
to wait 80,000 years to reach the destination. The
"Most Affordable" column refers to such concepts as space sails or
electric propulsion whose design and
construction could begin today. Presumably, these would cost the least,
but their performance would be less than desired by many advocates.
Actual minimum cost versus performance assessments have
not yet been calculated. Further
advancing technology will increase performance
but will most likely also increase costs. This column
is a contentious category since many individual researchers have their
favorite approaches that they think are better than all the other
ideas. The difficulty is that it is very hard to determine if
those performance predictions are
accurate since the technology has not yet been built. To
get more accurate performance predictions (but at the loss of
guaranteeing feasibility), one can consider technology that performs at
the limits of its basic physical principles. In such cases, the
predictions are not based on technology data-sheets, but rather on
basic physics equations (rocket equation, kinetic energy, etc.) that
can calculate the minimum amount of propellant required, or the minimum
beamed energy required, etc. Although no technology has ever
achieved 100% performance compared to its physics, such calculations
are useful to determine the upper limit of what is possible based on
the physics that we know. They also help identify which
technology options might be best to pursue - those whose improvement
would have the broadest overall effects. And
finally, there is the category of "Undiscovered
Physics."
This refers to notions like space drives and warp drives that require
further advances in physics before their designs can even be
contemplated. Research into such possibilities has been appearing
in
reputable journals in growing numbers since around 1994, with some
landmark publications as early as the mid 1960s. This
topic has matured to where the next-step investigative questions can be
pursued, but it is still too soon to know if such notions will
eventually become possible or remain forever impossible. If such
breakthroughs can be achieved, the limits of even the most ideal
conceivable technology will be surpassed. Depending
on an individual's priorities, each of these categories have
advocates. Rather than pick one, the Tau Zero Foundation
encourages progress across all these fronts, but with the emphasis on
reliable conclusions, in affordable, incremental steps. Examples of
various approaches are pictured next. |
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INCESSANT OBSOLESCENCE POSTULATEAs much as continuing advances in science and technology will make it easier to launch an interstellar mission, these advances also create a quandary, the "incessant obsolescence postulate." This term, coined by Marc Millis in 1999 and first published in 2004 [Gilster 2004: p.157], refers to the quandary that no matter when an interstellar probe is launched, a more modern probe launched later will reach the destination sooner. This quandary has also been called the "incentive trap" [Kennedy 2006], "catch me if you can" (Robert Forward [citation?]), and "Zeno's paradox in reverse," (a term possibly originating from David Brin during the 1994 workshop; "Interstellar Robotic Probes – Are we ready?" [Belbruno 1996]). In the original Zeno's paradox, the challenge is that a destination will never be reached if each step only halves the distance in-between. The incessant obsolescence postulate is only a postulate, not a theorem nor even a principle. It is presented here not as an immutable constraint, but as one of the impediments for planning interstellar missions. Although this postulate will remain true in the near term, it will eventually expire, as has been shown by Kennedy [2006] and others [e.g. Nordley]. When considering the nonlinear nature of both relativistic motion and technological advancements, there will be a point where an optimum launch opportunity occurs. Waiting longer does not get you to the destination sooner. In addition to the eventual expiration of incessant obsolescence, there are other conditions that could also make this postulate collapse, such as:
Another aspect that affects the influential power of incessant obsolescence, is why an interstellar mission is attempted. Incessant obsolescence is only a barrier to progress if the sole motivation is getting there first. If the motivation is, instead, to learn from each step, then this postulate becomes irrelevant. Bottom
line to all this: The incessant obsolescence postulate, if taken too
seriously, is an impediment to progress. To progress, it is better to
focus on what can be learned from attempting interstellar flight - even
if it's just deeper study into the issues and options. Again,
that is why the motto of this Foundation is "ad astra incrementis." |
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WHY ITS SO HARDSpace is Big. Travel takes Time Interstellar space is staggeringly vast. Quoting from the Hitchhiker's Guide to the Galaxy [Adams] – a fictional work that got this detail exactly right: "Space is big. Really big. You just won't believe how vastly, hugely, mind bogglingly big it is. I mean you may think it’s a long way down the road to the chemist [pharmacy], but that’s just peanuts to space." In reality, interstellar space is so huge that it makes light-speed seem slow. Our nearest neighboring stars are 4.3 light years (ly) away. That means, at light-speed ("in a flash!"), it would take 4.3 years to get there. That's longer than it takes most teenagers to get through high school. And that is just interstellar flight. I don't think we will be contemplating Intergalactic flight any time soon. Hazards and Reliability And if that were not challenging enough, space is not a hospitable environment for humans and their inventions. Beyond the protective magnetic field of the Earth, space contains radiation that can destroy life and fry sensitive electronics. The hazards vary in intensity and abundance and short excursions beyond the protective bounds of Earth can be safely done (well, sort of). But on the time scales of interstellar flight, the vehicle must be designed to endure the hazards and protect its payload for the full duration of the mission. Right now, just in terms of equipment reliability, it is quite conceivable to build devices that can operate for decades. For conceivable interstellar flight, we need to increase that by a factor of 10 to 1000. Think of it: An interstellar probe, based on conceivable propulsion, would have to operate flawlessly on time scales roughly the same as the history of humanity. Imagine how cool it would be if all the appliances in your home were that durable! Or your car! To put this more painfully into perspective, consider that the Pyramids - something built of simple stone - are themselves already eroding. Enough "Fuel" for the Journey The term "fuel" is used here for familiarity rather than because it is the accurate way to convey the needs of spaceflight. If you drive a car you should be very aware that you need fuel to go anywhere. That basic notion also pertains to spaceflight, but there are additional details. "Fuel," in the strictest sense, is an energy source – a chemical energy source. When that expensive gasoline fuel reacts with free ($0) oxygen in the air, they burn and expand and push pistons which then turn a crankshaft which then turns the wheels that push against the road to move your car. So, in short, the fuel is the energy source for the car. But there is another important piece of that story that typically goes unnoticed – the road. The road plays an important part in moving your car. Imagine your car floating in the air, hung under a balloon. You press on the gas pedal, hoping to move forward, but your wheels just spin. The view might be great from up there under that balloon, but your car's wheels aren't connected to anything. You just drift. In order for you to move in one direction, another mass needs to be moved in the other. Newton's laws are all over this situation, and if you've already learned them well, this will be obvious. Back on the ground, the road is your connection to that other mass - the entire Earth. When your wheels turn, they push the entire Earth one way (which actually moves ever so slightly) propelling you in the other direction. Some of you are wondering about this because you can't fathom how you could move the Earth. Well, the amount of push is the same on your car and on the Earth - action and reaction – but the Earth is so much more massive than your car. The ratio of how much the Earth moves compared to how much your car moves is the same as the ratio of the mass of your car (tiny) to the mass of the Earth (gargantuan). In space there are no roads. There is no air either, so aircraft propellers are useless. Spacecraft have to bring all there reaction mass with them - that is the stuff they push against. The proper term for this onboard reaction mass is "propellant." All rockets, even electric ion thrusters, need propellant. For chemical rockets, their propellant also serves double duty as their energy source. There is a fuel propellant and an oxidizer propellant, that react and dramatically expand out the rocket nozzle in one direction, thrusting the rocket in the other. For short journeys, the amount of propellant required is quite achievable, but for interstellar flight it is an entirely different situation. |
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| Tau Zero Foundation is a 501(c)(3) public
charity Your donations are tax deductible |
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Graphic + layout design : AntiGravite / Szames for the Tau Zero Foundation