Science observations with Venus Express during an eclipse
Venus Express travels in an inertially fixed orbit: rather than being fixed with reference to the centre of the planet, the orbital orientation is fixed with reference to the background stars, in other words, the spacecraft remains 'pointed' in the same direction relative to the stars. To an external observer, this gives the appearance that Venus rotates within the orbit of the spacecraft. In this configuration, the spacecraft covers all planetocentric longitudes within one Venus sidereal day (243 Earth days).
At certain orbital inclinations, the spacecraft goes through the shadow of the planet and experiences an eclipse. The Sun-Venus-spacecraft orbit geometry is such that these eclipses occur approximately every three months, and the duration of each of these 'seasons' is about one month.
For Solar System bodies, 'North' is taken to be the same direction as that for Earth. The point of intersection where the satellite crosses the planet's equator and travels North is known as the orbit's ascending node. Conversely, the point where the satellite crosses the equator and travels towards the planet's southern hemisphere is called the descending node. The half of the orbit on which the spacecraft is moving South to North is called the ascending branch, and has the ascending node where the spacecraft orbit crosses the equator. The half of the orbit where the spacecraft moves North to South is known as the descending branch, and has the descending node where the spacecraft orbit crosses the equator.
Eclipses can occur in either the ascending or descending branch. As the spacecraft enters and leaves the eclipse, the spacecraft's instruments can observe sunlight passing through the atmosphere of the planet. This makes for a unique science opportunity: by studying the sunlight transmitted through the Venusian atmosphere during the eclipse, it is possible to study the composition and structure of the atmosphere itself.
These eclipse observations are performed by the Spectroscopy for Investigation of Characteristics of the Atmosphere of Venus (SPICAV) experiment using its very high spectral resolution channel known as Solar Occultation at Infrared (SOIR). Each orbit during an eclipse season contains two eclipses: the ingress and the egress from the dark eclipse period (or umbra). This gives the SOIR channel two observation opportunities, and SOIR will normally observe both. The spacecraft keeps a fixed inertial orientation in space, and the SPICAV/SOIR channel instrument is pointed to the Sun during the entire eclipse period from ingress to egress.
SPICAV/SOIR performs high-resolution observations of the transmitted solar spectrum while the spacecraft records the altitude, latitude and longitude of the observed regions. The solar spectrum has been thoroughly studied and is well understood; this existing knowledge can be used to interpret data obtained during observations and determine which wavelengths are absorbed by the atmosphere. This reveals details of Venus's atmospheric chemistry including its molecular constituents at different heights and the ambient temperature and pressure at the observed regions.
Because these eclipses occur over the course of several orbits, the solar spectrum is observed after it passes through the atmosphere at a different latitude and longitude each time. The data from these observations are compiled to create a comprehensive model of the upper and middle atmosphere of Venus's northern hemisphere.
This technique requires that the spacecraft pass into the shadow of the eclipse. Therefore, eclipse observations are only possible during the part of the orbit when the spacecraft is close to the planet.
Venus Express's orbit is highly eccentric. At apocentre, the spacecraft is about 66,000 km from the Venus, too far to pass into the eclipse's umbra. Due to this, Venus Express sees no eclipses in the lower latitudes of Venus and eclipse observations are not possible in this region. At pericentre, the spacecraft is as close as 250 km from the planet; this proximity brings it into the planet's shadow and makes eclipse observations possible. The resulting observations and atmospheric data are only available for the northern latitudes of the planet.