DESCRIPTION |
Instrument Overview
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From [SEIFF&KNIGHT1992]:
The ASI experiment will operate over an altitude range of about
870 km in two measurement modes, one to accommodate conditions of
high-speed entry at low ambient density; the other, the very
different conditions of parachute descent. The entry mode begins
at a nominal ambient density threshold of 10^-11 kg m^-3
(presently believed to be about 750 km altitude above the 1 bar
level) where the Probe deceleration is expected to be about 15
micro g. From measurements of Probe deceleration under the
action of atmospheric drag, atmospheric densities will be
derived. The density profile is integrated above a given
altitude to define the pressure at that level, and the
temperature profile is then obtained through the equation of
state, given the variation of atmospheric mean molecular weight
with altitude. This mode of operation is continued until the
Probe deploys its parachute, nominally at the 100 mb level (47 km
above the 1 bar level).
Then, during the subsequent parachute descent of the Probe to the
nominal end-of-mission depth at 16 bars, the thermal structure of
the atmosphere is defined by measurements of temperature,
pressure, and acceleration in descent mode. Current estimates
are that the mission could extend to below 20 bars, perhaps as
deep as 25 bars.
At entry, the Probe is effectively a 48 km s^-1 meteor, enveloped
by a bow-shock wave and a thin shock layer of ionized,
luminescent gases at extreme temperature (~15000 K at peak).
Under these conditions, measurements of the ambient atmosphere by
means of conventional low-density sensors are clearly infeasible;
it is futile to extend sensors into the shock layer or through
the bow-shock wave, because they quickly burn away and, outside
the probe bow wave, develop shock layers of their own.
Measurements of atmospheric density by way of Probe
decelerations, however, provides a direct means of sensing the
atmosphere.
After the parachute is deployed, the heat shield is jettisoned
and temperature and pressure sensors are exposed to the ambient
atmosphere. Acceleration measurements are continued, but at much
less frequent intervals, and in the absence of large vertical
winds, continue to define atmospheric density. From the three
measured state variables, temperature, pressure, and density, the
atmospheric mean molecular weight may be determined. Other
instruments on the Probe, including the mass spectrometer and the
helium abundance detector, will probably determine the molecular
weight more accurately, however. In these circumstances, the
accelerometer data may be used to define the magnitude of large
vertical winds in waves or gusts.
Altitudes relative to any convenient reference level (e.g., the 1
bar level) are defined by the temperature and pressure data
integrated in the equation of hydrostatic equilibrium. These
data will be used to establish the altitudes of measurement for
use by all Probe experiments. They also define the rate of
descent, which is necessary to the analysis of Doppler wind
experiment. In addition, atmospheric turbulence, radius to the
center of the planet, and Probe angle of attack are sensed or
derived.
The nominal end of mission, at 16 bars, is at an altitude of
about -120 km below the 1 bar level. The vertical measurement
range of about 870 km will be covered in a little over one hour -
the initial 700 km in less than 4 min. The time in descent on
the parachute at velocities from the order of 100 to 30 m s^-1
could be up to 75 min.
The instrument electronics perform a number of essential
functions. They receive and execute commands from the Probe
systems and thereby set the experiment mode (calibrate, entry, or
descent), control the measurement sequences, select sensor
ranges, collect data from the three sensor sets, amplify signals,
perform A/D conversions, do some onboard data processing, and
condition instrument power. The microprocessor controlled
electronics were designed and built by Martin Marietta Aerospace,
in Denver, Colorado.
This is a fourth generation entry probe atmosphere structure
instrument. Its predecessors were used on the four probes of the
Pioneer Venus Mission, on the Viking Mission to Mars, and on the
Planetary Atmosphere Experiments Test (PAET) entry probe into the
Earth's atmosphere. Best features of earlier instruments were
retained and some new capabilities have been added.
Scientific Objectives
=====================
(1) To accurately define the state properties as a function of
altitude below the 100 mb level to ~20 bars. (They have never
been measured below the 1 bar level.)
(2) To define the currently highly uncertain state properties of
the upper atmosphere.
(3) To measure the stability of the atmosphere, and identify
convective layers and stable layers, where they exist.
(4) To detect cloud levels from changes in lapse rate at their
boundaries.
(5) To define state properties within the clouds, and thus provide
supplementary information on cloud composition.
(6) To search for and characterize wave structures in the
atmosphere.
(7) To search for and measure intensity and scale of turbulence in
the atmosphere.
(8) To measure vertical flow velocities above a threshold of about
0.3 m s^-1.
(9) To establish an altitude scale for use in correlating all
Probe experiment data.
(10) To define the probe vertical velocity, necessary to the
analysis of the Doppler wind experiment.
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