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Venus Express Earth occultation seasons

Earth occultation seasons occur regularly in the Venus Express mission. For short periods of time during such seasons, the spacecraft's radio signal can pass through Venus' atmosphere before it is received on Earth. These periods are exploited for unique radio science measurements of Venus' atmosphere.

Venus Express is in a highly elliptical polar orbit around Venus, with the pericentre over the planet's north polar region. The spacecraft’s orbital plane is inertially fixed relative to the background stars, rather than to the planet. This means the orbit's orientation remains the same relative to the sky, independent of Venus' motion around the Sun or the planet's axial spin.

As the planets revolve around the Sun, there are regular seasons when the line of sight from Earth to Venus is within a narrow enough range from Venus Express' orbital plane for Earth occultations to occur. During these seasons, the spacecraft passes behind Venus as seen from Earth, once every orbit. From the spacecraft's point of view the Earth then disappears behind Venus (the Earth is occulted) and re-emerges a few minutes later. These Earth occultations always occur in the part of the spacecraft's orbit where Venus Express is closest to the planet, i.e. around pericentre. 

Radio science measurements

The Earth occultation geometry can be exploited by the Venus Radio Science experiment (VeRa), a radio science investigation (RSI), to study Venus' atmosphere. VeRa uses the spacecraft's communication system to broadcast a very stable radio signal to Earth, where it is recorded. The measurements are started shortly before Venus Express reaches pericentre. As the spacecraft begins to move behind Venus as seen from Earth, the radio signal passes through a progressively longer swathe of Venus' atmosphere; this affects the signal and eventually it is blocked by the planet.

Analysis of these radio signals results in a profile of the atmospheric temperature at different heights. Because each occultation measurement is also taken at slightly different planetary latitudes, the individual temperature profiles can be combined into a broader picture of atmospheric temperatures around the high northern latitudes of Venus, both on the dayside and night-side of the planet. Only the northern latitudes are covered, since the occultation measurements can only be taken when the spacecraft is close to the orbit's pericentre, which lies over the planet's north polar region.

Temperature profiles can also be determined via completely different means using the SPICAV (Spectroscopy for Investigation of Characteristics of the Atmosphere of Venus) instrument, and its Solar Occultation at Infrared (SOIR) channel. These measurements provide important cross-correlations of the VeRa results.

The radio signal used for the Earth occultation measurements must be far more accurate than the usual radio signal during a communications pass, hence it is generated using the VeRa's ultra stable oscillator.

The radio signal is broadcast with either of the spacecraft's two high gain antennae, HGA-1 or HGA-2, depending on which antenna is in use during the current mission phase. It is received on Earth with the New Norcia (NNO) antenna in western Australia. ESA's NNO ground station is used for the Venus Express radio science investigations because the spacecraft's orbital period is 24 hours, and the pericentre always occurs when the NNO station has the best angle to view Venus.

One consequence of repeatedly performing the radio science experiment at pericentre is that these observations block other Venus Express instruments from performing observations near pericentre. That is because the spacecraft's attitude for Earth occultation measurements is such that the high gain antenna is pointing to Earth, and the other remote sensing instruments cannot observe anything scientifically interesting in their fields-of-view.


Last Update: 22 November 2012

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