  "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.
