Download Observing the surface of Venus from Earth's Stratosphere

Observing the surface of Venus from Earth’s stratosphere
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Jörn Helbert , Riccardo Nadalini
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( Institute for Planetary Research, DLR, Germany active space technologies, Berlin, Germany)
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Summary
The surface of Venus is hidden from “traditional”
remote-sensing multi-spectral instruments by a
thick cloud cover and an optical thick
atmosphere. In the near infrared around 1
micron there are however a few narrow windows
that allow to observe the surface of Venus. This
has been demonstrated by ground-based
telescopic observers. With VIRTIS instrument on
the ESA mission Venus Express such
observations could be performed for the first time
from an orbit around Venus. However VIRTIS
was never build for this task and there have
been a number of issues complicating the
observations.
Building on the experience and the lessons
learned from the VIRTIS instrument we are
therefore currently designing an instrument
specifically for the task of observing the surface
of Venus. In order to validate the design we plan
a demonstrator to be flown on a stratospheric
balloon. The balloon will take the instrument high
enough to effectively reduce the influence of
Earth’s atmosphere allowing high quality
observations of Venus. We will show here the
first design studies for the demonstrator.
time observing campaigns are basically
impossible.
A stratospheric balloon platform provides perfect
observing conditions and depending on the type
of balloon observing runs of up to 100 days are
possible. This makes a Venus observing
instrument not only a good demonstrator for
future orbital missions, but also a valuable
science instrument in it self.
Introduction
The CO2 dominated atmosphere of Venus is
partly transparent in a number of small spectral
windows between 0.8 and 1.31 microns. In these
windows thermal radiation from the surface
escapes and can provide information on surface
temperature and emissivity. The latter is an
indicator for composition and has been used
very successful in the VIRTIS data to identify
different surface types on Venus. Furthermore
the thermal data allows searching for the heat
signature of active volcanoes. This task requires
a good coverage – both temporal and spatial – of
the surface of Venus.
Challenges
The first challenge for the system will be to
initially locate Venus from a moving unstable
balloon platform. For this task we currently study
a system using a differential GPS system.
Once acquired a closed loop system will track on
the planet itself. For this purpose the field of view
of the system is approximately twice the
maximum apparent size of Venus on the sky.
The stabilisation is performed by a moving
platform for the telescope for the coarse pointing
and a tilt-pointing mirror for the fine pointing.
The challenges for thermal engineering will be
different but comparable to an orbiter instrument,
inline with the idea of the demonstrator. The
actual thermal environment on the balloon
platform is more benign than in the orbit around
Venus, however we are using of the shelf
components, which have a very restricted
temperature range. In order to avoid optical
distortion and deformation of the filters it is of
prime importance to keep the whole optical
system iso-thermal. While the orbital instrument
will address this with a integrated monoblock
structure for the demonstrator we focus on a
rigid thermally conductive support frame.
Why on a ballon?
Ground-based telescopic observation of Venus
allows using the spectral windows in the near
infrared. However the Earth atmosphere is in this
spectral region very variable and only partly
transparent. For this reason high altitudes are a
pre-requisite for good observing conditions. The
time allocation on large telescope facilities like
the NASA IRTF on Hawaii or the ESO
telescopes in Chile is highly competitive and long
The basic concept
The baseline approach for the demonstrator is to
address the same science questions as an
orbital instrument, while use mainly of the shelf
hardware. This includes the use of a standard
laboratory InGaAs NIR camera, a commercially
available filter wheel, and telescope. The frame
is designed from standard components with a
focus on rigidity. Position and orientation
information is provided by a differential GPS
system instead of a Sun or star tracker. Data is
stored onboard with an optional telemetry link to
monitor the instrument status.
The goal is to design the demonstrator with
enough flexibility to be used on different balloon
and potentially also other sub-orbital platforms.