The impetus for this study dates back to a 2006 workshop held at Princeton University. As with prior interstellar sessions, various propulsion and mission concepts were presented as if to advocate their selection [9]. In the subsequent discussions, it was agreed that it was not possible to pick a winner. Each concept used different assumptions, plus the performance predictions are unproven. Rather than attempt far-future decisions, the discussion turned to identifying the most critical make-break questions for each approach, and of those, which can be affordably researched next. Pursuit of these questions shaped the goals and strategies of the nonprofit Tau Zero Foundation that was incorporated that same year [10].
Interest in interstellar flight is increasing. The continuing discovery of exoplanets, the announcements of privately funded mission plans, and the inkling that faster-than-light flight is now at least theoretically possible, provokes more interest. The following paragraphs describe some of the more significant provocations, in chronological order.
Theories for faster-than-light (FTL) flight are now part of the scientific literature. The first traversable wormhole article was published in 1988 [11], the first warp drive paper in 1994 [12], and the first scholarly book compiling these challenges along with other breakthrough propulsion pursuits was published in 2009 [13]. Even the hint that FTL might someday be possible, makes interstellar flight more attractive.
In 2014, the first potentially habitable Earth-size exoplanet (Kepler-186f, ≈ 500 ly) was confirmed by the Keck and Gemini Observatories. Now it is certain that potentially habitable planets exist, perhaps even Earth-like planets [14].
In April 2016, an ambitious plan for an interstellar mission was announced, with an offering of $100 Million for its initial research from Russian billionaire, Yuri Milner. The project, called "Breakthrough StarShot," is based on using a powerful laser array to push a small light-sail up to 20% lightspeed to reach Alpha Centauri within a 22 year flight time [15]. This project continues to receive significant media attention, further amplifying public interest in interstellar flight.
And in August of 2016, a potentially habitable exoplanet, Proxima b, was discovered orbiting our nearest neighboring star, Proxima Centauri, at only 4.2 ly distant [16]. Though subsequent analysis casts doubt that the planet could support human life, due to the intense radiation from its sun, the fact that our nearest neighboring star hosts an exoplanet that is roughly similar to Earth's size and temperature spurs further interest in the search for habitable worlds.
Congressional interest followed. An 18 May 2017 report for the fiscal 2017 Appropriations Committee included the following instructions: "The Committee encourages NASA to study and develop propulsion concepts that could enable an interstellar scientific probe with the capability of achieving a cruise velocity of 0.1c. These efforts shall be
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Millis 2018 Grant NNX17AE81G_for_CR.docx centered on enabling such a mission to Alpha Centauri, which can be launched by the one-hundredth anniversary, 2069, of the Apollo 11 moon landing." [17].