Report - School of Physics

accepted that we are now peering into a much more distant and uncertain future, we can examine
some of the ideas which are being discussed.
Life Finder studies (Woolf et al., 2001) have been used to evaluate the requirements for Planet
Imager which, they consider, would require some 50–100 Life Finder telescopes used together in
an interferometric array. Their conclusions were that ‘the scientific benefit from this monstrously
difficult task does not seem commensurate with the difficulty’. This echoes the conclusions of Bender
& Stebbins (1996) who undertook a partial design of a separated spacecraft interferometer which
could achieve visible light images with 10 × 10 resolution elements across an Earth-like planet at
10 pc. This called for 15–25 telescopes of 10-m aperture, spread over 200 km baselines. Reaching
100×100 resolution elements would require 150–200 spacecraft distributed over 2000 km baselines,
and an observation time of 10 years per planet. These authors noted that the resources they
identified would dwarf those of the Apollo Program or the Space Station, concluding that it was
‘difficult to see how such a program could be justified’. The effects of planetary rotation on the time
variability of the spectral features observed by an imager, complicates the imaging task although
may be tractable, while more erratic time variability (climatic, cloud coverage, etc.) will greatly
exacerbate any imaging attempts.
Hypertelescopes/OVLA:
Parallel to the Planet Imager studies, in Europe the LISE group (Laboratoire d’Interférométrie
Stellaire et Exo-planétaire) carries out research in the area of high-resolution astronomical imaging,
including imaging extra-solar planets. The group is studying several complementary projects for
‘hypertelescopes’ on Earth and in space (Riaud et al., 2002; Gillet et al., 2003). The steps needed
to reach this goal are set out as requiring: (1) a hypertelescope on Earth – the OVLA (Optical
Very Large Array); (2) a 100-m precursor geostationary version in space; (3) a km-scale version
in a higher orbital location; (4) a 100 km version, including dozens of mirrors of typically 3 m
aperture. Labeyrie et al. proposed the mission ‘Epicurus’, an extra-solar Earth imager, to ESA in
1999 in response to the F2/F3 call for mission proposals.
Their basic ‘hypertelescope’ design involves a dilute array of smaller apertures (an imaging interferometer)
having a ‘densified’ exit pupil, meaning that the exit pupil has sub-pupils having a larger
relative size than the corresponding sub-apertures in the entrance pupil (see Fig. 1 of Pedretti
et al. (2000)). Their applicability extends to observing methods highly sensitive to the exit pupil
shape, such as phase-mask coronography.
In the most recent published studies (Riaud et al., 2002) the hypertelescope is combined with
such a coronograph to yield attenuations at levels of 10 −8 . Simulations of 37 telescopes of 60 cm
aperture distributed over a baseline of 80 m in the infrared, observing the 389 Hipparcos M5–F0
stars out to 25 pc (with simulated contributions from zodiacal and exo-zodiacal background) yields
10-hour snapshot images in which an Earth-like planet is potentially detectable around 73% of
the stars. Gains of a factor 20–30 with respect to a simple Bracewell nulling interferometer are
reported.
In space, the plans call for a flotilla of dozens or hundreds of small elements, deployed in the
form of a large dilute mosaic mirror. Pointing is achievable by globally rotating the array, which
is slowly steerable with small solar sails attached to each element. A ‘moth-eye’ version allows
full sky coverage with fixed elements, using several moving focal stations (Labeyrie, 1999b,a).
The geostationary precursor hypertelescope could be a version of TPF. An exo-Earth discoverer
would require a 100–1000 m hypertelescope with coronograph, while an exo-Earth imager would
require a 150 km hypertelescope with coronograph. In the approach of Labeyrie (1999b) a 30-min
exposure using a hypertelescope comprising 150 3-m diameter mirrors in space with separations
up to 150 km, would be sufficient to detect ‘green’ spots similar to the Earth’s Amazon basin on
a planet at 10 light-years, although these vegetation features are more prominent in the infrared
(Arnold et al., 2002).
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