“The most erroneous stories are those we think we know best – and therefore never scrutinize or question.”

~ Stephen Jay Gould


Depending on one’s perception and motives, there is more than one ‘ideal’ starship. So far, many of these underlying perspectives have not been explicitly stated and, thus, will impede fair comparisons and next-step decisions.

To improve future decisions, the span of motives and perceptions is explicitly examined here.


As 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: no matter when an interstellar probe is launched, a subsequent probe will reach the destination sooner and with more modern equipment. This 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 to interstellar missions.

Although this incessant obsolescence postulate appears valid, it will eventually expire. Due to the combination of the nonlinear nature of both advancement trends and relativistic spaceflight, there will be a point when waiting longer does not get you to the destination sooner. ("wait equation" ~ Kennedy).

In addition to the eventual expiration of incessant obsolescence, there are other conditions that will make this postulate collapse, such as:

  • Multiple missions and destinations
  • Trip time becomes irrelevant:
    • Colony ships without destinations
    • Human lifespan increases dramatically
    • Societal attention span increases dramatically
  • Significantly closer destinations become available (reduces trip time)
  • The pace of technological development or energy production dramatically slows (societal retardation)
  • Propulsion physics breakthroughs are achieved (reducing trip times)
  • Motivations other than first-to-destination drive mission planning.


The most significant factor that negates the incessant obsolescence postulate is motivation. The incessant obsolescence postulate is only a limitation when the motivation is to reach the destination first – as if interstellar flight were a race. Instead, for example, if the motivation is to depart the Earth the earliest, then the incessant obsolescence postulate is irrelevant. If the goal is to stimulate technological developments, then launching soon and frequently is more prudent. If the purpose is to provide humanity with an alternate survival strategy, then the focus would be toward building a colony ship, where flight time is irrelevant.

To reflect the consequences of differing motivations, the table below is offered. These are listed explicitly to help readers assess if their own perceptions are being affected by implicit motivations.

Comparing Interstellar Motivations & Actions



Being first: conquest of space

Stymied by the incessant obsolescence postulate

Human survival beyond the habitability of Earth

Priority on self-sustaining, multi-generation colonies beyond Earth

Human expansion: finding and settling other habitable worlds

Look first for nearest habitable worlds

Develop all the technologies needed to send a colony there

Nearer-term gains, seeking investment returns within tolerable durations

Pursue relevant science & technology, with an eye toward harvesting benefits during that progress.

Intellectual stimulation & progress in science, technology, and education

Pursue discretionary interests with discretionary resources.

Implies that a multiplicity of interests and approaches exist.


In addition to the influence of motivations, there is also an effect from perceptions of practicality – perceptions which can be implicit and thus unknowingly affect decisions. Practical is a relative term depending on one's priorities and biases. To some "practical" means choosing the most affordable 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.

This table presents how these differing priorities map to different levels of technological ambition. From this it is hoped that the motivations to pursue specific technological ambitions become more explicit. Note: the values within this table are provisional estimates for illustrative comparisons, rather than verified values.


Differing Perceptions of Practicality versus Ambitions of Technological Development


Feasibility Today

Trip Time



Available technology


10's of millennia

103 kg

$ billions

Advanced technology

(e.g. vex = 0.03c)



104 –106 kg

$10's of billions

Ultimate technology

(e.g. vex = 0.3c)



107 kg


New propulsion physics



107 kg +


*Faster than light

Consider, for example, if economy and feasibility were the driving factors. Although this would make it practical to launch something soon, only a tiny probe could be sent and it would take dozens of millennia just to reach our nearest neighboring star – certainly longer than the spacecraft's warranty period. Conversely, when considering star-flight within a human lifetime, seeking breakthroughs in physics would seem practical.  

Content Contributors

  • Marc Millis

The Tau Zero Foundation is a 501(c)3 non-profit organization dedicated to pioneering advancements toward interstellar flight.