ESA Science & Technology - No. 231 - End of Outbound Quadrature Operations; Radio Science Investigations – Continuation of Occultation Campaign #10

The spacecraft has been operating in outbound quadrature since 30 December 2010 and exited quadrature operations on 6 February 2011. Quadrature phases are periods of about five weeks during which the Sun-spacecraft-Earth angle is between 75° and 95°. They occur twice in every synodic period [see note 1] of Venus (584 days, or about 19 months), with an inbound quadrature phase ending 9 weeks before an inferior conjunction [see note 2] of Venus and an outbound quadrature phase starting 9 weeks after an inferior conjunction. This outbound quadrature phase started nine weeks after the inferior conjunction that occurred on 30 October 2010.

VeRa Earth occultation campaign #10

The mission's tenth Earth occultation campaign continued during this reporting period. These campaigns take place when the Earth is occulted by the disk of Venus; this occurs around the pericentre passages as seen from the spacecraft. Under these conditions unique measurements can be made to investigate the fine structure of the neutral atmosphere and ionosphere of Venus.

The occultation measurements are part of the Venus Express Radio Science Investigations (RSI) performed under the Venus Radio Science experiment (VeRa). For these investigations the VeRa team makes use of the radio links of the spacecraft's communications system. The spacecraft's High Gain Antenna (HGA) is pointed toward the Earth before the approach to occultation. The on-board ultra-stable oscillator is used to generate a very stable signal which is sent to Earth over the HGA simultaneously in the S-band and X-band. Just before and after the occultation the transmitted radio signal passes through the ionosphere and atmosphere of Venus. The effect that this has on the received signal gives valuable information on the characteristics of the atmosphere (such as temperature, density and pressure) as a function of altitude.

RSI occultation measurements were performed around pericentre on nine orbits in the reporting period. Two ground stations were used for receiving the spacecraft's radio signals on ground: the ESA deep space antenna at New Norcia (NNO) and the NASA Deep Space Network ground station at Canberra (CAN).

Summary of main activities

During the reporting period, routine mission operations were conducted using the ESA Cebreros ground station (CEB). Delta Differential One-way Ranging (DDOR) was performed on 20 February 2011.

Four Cebreros communications passes were skipped during this reporting period. As noted in previous reports (see for example report No. 230), there are only two faces of the spacecraft which are allowed to be illuminated by the Sun continually: the +X and +Z faces; after 'hot’ observations, the skipped passes allow the spacecraft to cool in the warmer 'tilt’ configuration that is necessary during the quadrature period. (During the quadrature phase revised operating constraints on the Venus Monitoring Camera require the spacecraft attitude to be adjusted, or 'tilted', to prevent unacceptable illumination of the camera.)

RSI measurements were performed by observing the occultation of signals from the Venus Express spacecraft as it passed through orbital pericentre. A total of nine observations were performed, using the NASA Deep Space Network Canberra (CAN) and ESA New Norcia (NNO) ground stations. RSI Occultation Campaign #10 will last until 23 March 2011. The first RSI 'Occultation egress only’ measurement was carried out using the New Norcia ground station.

The battery charging set points were changed during this period. During the eclipse seasons, when daily solar eclipses require the use of the batteries to supply power to the spacecraft, the batteries are recharged to 100%. This ensures that if a spacecraft anomaly occurs at the point when the batteries are most discharged, there is still sufficient power to keep the spacecraft operating for hours if a safe mode or other non-nominal activity occurs. Outside of the eclipse period, the worst-case power situation is not as constraining and, to preserve the battery life, lower battery set points are used. This means that the charging is not at 100% at all times, but sufficient power is available in an emergency and the battery life is extended.

At the end of the reporting period on 26 February 2011, Venus Express was 155.2 million kilometres from Earth. The one-way signal travel time was 517.44 seconds.

At the end of the reporting period, the final oxidizer mass was estimated to be 34.578 kilograms, and the final fuel mass estimate was 21.589 kilograms.

Payload Activities

Future Milestones

Atmospheric Drag Experiment (ADE) #5 will start on 23 May 2011. Superior solar conjunction will start on 6 August 2011.

Notes

1. The synodic period is the time between two successive identical configurations as seen from the Earth.
2. A conjunction is the alignment of two Solar System bodies (in this case Venus and the Sun) so that they have the same longitude as seen from Earth. An inferior conjunction occurs when Venus is between the Sun and the Earth; superior conjunction refers to when Venus is on the opposite side of the Sun from Earth.

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This report is based on four ESOC mission operations reports, MOR #273 through MOR #276. Please see the copyright section of the legal disclaimer (bottom of this page) for terms of use.