Voyager has traveled farther than any other spacecraft to date, yet it has traveled less than one-thenth of one percent of the distance to Proxima Centauri (142 AU by 2018, 0.053% of 270,000 AU). It took Voyager over four decades to reach that distance and would take another 80-thousand years to span the distance equal to reaching the Centauri stars (velocity ~ 0.00006 c). The farthest conceivable missions based on contemporary technology could only reach less than a half-percent of the way to Proxima Centauri in 50 years (1000 AU, 0.37% of 270,000 AU) [32].
In addition to the challenge of raw distance, there is the challenge of how much distance there is between interesting destinations. The logarithmic scales used in figure 2, and similar charts from other studies, can be misleading when trying to grasp the true scale of interstellar destinations. When plotted on logarithmic scales, it appears that destinations of interest are spread evenly. When listed linearly however, the vast gaps between points of interest become more apparent. If a chart encompassing the distance to our nearest star spanned the full height of this report's margins (270,000 AU ≈ 23 cm, 9"), then the maximum distance achievable within a 50 year flight time using existing technology would only barely distinguishable from the chart's origin (1000 AU ≈ 1 mm ≈ 1/32").

[Table: Table 1 lists interstellar destinations by distance along with other factors that come into play in later parts of this report. Note in particular the column "ESI," which stands for "Earth Similarity Index," from the "Planetary Habitability Laboratory" at the University of Puerto Rico at Arecibo. An ESI of 1.00 would be fully Earth-like. The other key destinations are as follows. The edge of our solar system can be defined as beyond around 200 AU. The next destination of interest past that point is 550 AU, where the gravitational lensing effect of our sun can be used to magnify images of whatever is on the opposite side of the sun [36]. It has been proposed that this solar gravitational lens has the magnification to be able to image an exoplanet with enough resolution to distinguish land features [37].] Beyond that point, the next targetable object is almost 500 times farther away, specifically the Centauri star systems (270,000 AU, 4.2 ly). In the vast void between those points of interest there exist only sparse densities of comets and asteroids; the Hills cloud (2,000 AU), Oort cloud (10,000 AU), and the G-cloud (41,000 AU) [38]. These features are difficult to discern using Earth-based astronomy, but probes passing through them could make direct in-situ measurements of the fields and particles.

Once past the Centauri systems, there are already eight potentially habitable planets detected within 41 ly, half of which are within 22 ly. Based on the astronomical data available before 2010, Claudio Maccone created statistical estimates for how far away the nearest Earth-like planet might be, and how far away the closest extraterrestrial civilizations might be. The distance estimates to the closest Earth-like planet span roughly 50 ly to 100 ly. The distance estimates to the nearest possible civilization are beyond 2000 ly [39]. Though these are rough statistical estimates, they at least help convey the scale of the challenge.
Another aspect of distance is time—how far can be reached within human timescales and the ultimate longevity of spacecraft. A common mission duration cited for interstellar concepts is 50 years, perhaps based on a long career span where the people who launched the mission will still be able to witness its findings. There has not been a study to assess the limits of human patience for interstellar missions.
There has also not been a study to estimate the longest possible operating duration for a space probe, yet 200 years is a fair starting estimate. Thus, even at lightspeed, the farthest a probe could reach into our galaxy is 200 ly.
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Millis 2018 Grant NNX17AE81G_for_CR.docx Kepler 62, 422

Without faster-than-light flight,
Kepler 1229
Kepler 186
this zone will remain
inaccessible.
Trappist-1
Gliese 667
Gliese 191
Proxima b
Sun Gravitational Lens edge
Probable spacecraft operational limit
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