Issues with the High Definition Space Telescope (HDST) ExoEarth Biosignature Case: A Critique of the 2015 AURA Report "From Cosmic Birth to Living Earths: the future of UVOIR Astronomy"
"From Cosmic Birth to Living Earths" advocates a 12-meter optical/near-IR space telescope for launch ~2035. The goal that sets this large size is the detection of biosignatures from Earth-like planets in their habitable zones around G-stars. The discovery of a single instance of life elsewhere in the universe would be a profound event for humanity. But not at any cost. At 8-9B USD this High Definition Space Telescope (HDST) would take all the NASA astrophysics budget for nearly 20 years, unless new funds are found. For a generation NASA could build no "Greater Observatories" matching JWST in the rest of the spectrum. This opportunity cost prompted me to study the driving exobiosphere detection case for HDST. I find that: (1) the focus on G-stars is not well justified; (2) only G-stars require the use of direct imaging; (3) in the chosen 0.5 - 2.5 micron band, the available biosignatures are ambiguous and a larger sample does not help; (4) the expected number of exobiospheres is 1, with a 5% chance of zero; (5) the accessible sample size is too small to show that exobiospheres are rare; (6) a sufficiently large sample would require a much larger telescope; (7) the great progress in M-star planet spectroscopy - both now and with new techniques, instruments and telescopes already planned - means that a biosignature will likely be found before HDST could complete its search in ~2045. For all these reasons I regretfully conclude that HDST, while commendably ambitious, is not the right choice for NASA Astrophysics at this time. The first exobiosphere discovery is likely to be such a major event that scientific and public pressure will produce new funding across a range of disciplines, not just astrophysics, to study the nature of Life in the Universe. Then will be the time when a broader science community can advocate for a mission that will make definitive exobiosphere measurements.