The Italian-built San Marco 3 was a cooperative space effort between
the Italian Space Commission (CRS) and NASA. The primary objectives of the mission were
- to provide density, neutral composition, and temperature data describing the equatorial
upper atmosphere at altitudes of 200 km and above and
- to measure variations in these parameters as functions of solar and geomagnetic
activity.
A secondary objective was to determine the neutral density by using three independent
measuring techniques. The spacecraft was a 75-cm-diameter sphere. Four 48-cm antennas
protruded from the top of the sphere, for command and telemetry transmission. The
structure of the spacecraft formed an integral part of the drag balance experiment. A
light, external, outer shell was connected by a series of elastic arms to a heavier
internal framework. Thus, from changes in the flexible arms connecting the two structures,
atmospheric drag (and therefore density) was determined. Other onboard experiments
included an omegatron mass spectrometer that directly measured the density and temperature
of molecular nitrogen and a Neutral Atmospheric Composition Experiment (NACE) that
directly measured the density of the molecular nitrogen, molecular oxygen, atomic oxygen,
argon, and helium. Unlike the earlier San Marco spacecraft, San Marco 3 employed an
attitude control system, and a spin rate control system. In addition, solar panels were
mounted equatorially on the inner core. The satellite performed normally after launch
until vehicle reentry on 28 November 1971.
The Italian-built San Marco 4 spacecraft was part of a cooperative
space effort between the Italian Space Commission (CRS) and NASA. The scientific objective
of this flight was to measure the diurnal variations of the equatorial neutral
thermosphere density, composition, and temperature for correlation with simultaneous
Atmospheric Explorer C data. Studies of the physics and dynamics of the lower thermosphere
were included. The spacecraft carried
- a Neutral Atmosphere Composition Experiment (NACE) to determine upper atmospheric (160
km and above) concentrations of argon, helium, atomic oxygen and molecular oxygen and
nitrogen,
- a Neutral Atmospheric Temperature Experiment (NATE) to determine the temperature of
ambient molecular nitrogen and
- an accelerometer to measure atmospheric density near satellite perigee.
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