ESA Science & Technology - No. 234 - End of Eclipse Season; Preparations for Atmospheric Drag Campaign #5

End of eclipse season

The solar eclipse season ended on 25 April 2011. During eclipse seasons, as Venus Express passes through the shadow of Venus during parts of its 24-hour orbit around the planet, the thermal and power requirements lead to increased demand on the spacecraft's batteries.

On 27 April 2011, a battery deep discharge test was executed, combined with new end-of-charge settings. The deep discharge test allows the rate of battery discharge to be plotted and compared to the prediction from the battery models. The model parameters can then be updated to accurately reflect the battery performance.

On the same day, images of the asteroids Ceres and Egeria were acquired with Star Tracker (STR) B. This was done to evaluate a new type of activity with the two star trackers on board Venus Express. These are low-resolution cameras, one primary and one backup, which are on the same face of the spacecraft but pointing in slightly different directions. They are capable of detecting only very bright objects, which are normally stars. The detected objects are compared to a database of stars, the stars in the camera field-of-view (FOV) are identified, and this allows high accuracy measurement of the spacecraft attitude. Since only one of the star trackers is used at a time, the redundant camera can be used to detect objects in the field-of-view. When the spacecraft is Earth-pointing in a known attitude, the sky in the predicted field-of-view of a star tracker camera can be calculated. A database of asteroid orbits can be used to predict if any asteroids would pass through the redundant star tracker field-of-view, and if they would be big enough (and therefore bright enough) to be detected by the star tracker camera. It was determined that two objects would be bright enough to be detected in the redundant star tracker field-of-view.

The experiment was conducted on 27 April 2011 and the objects were found in the field-of-view as predicted. More tests will be conducted in the future, to confirm these results. If the results are satisfactory, the redundant star tracker may be useful in detecting asteroids and comets. Observations of known objects can help improve the accuracy of the object's orbital parameters, and therefore not only its current position but also its future orbital positions.

Atmospheric Drag Campaign
Preparations for Atmospheric Drag Campaign (ADC) #5 are on-going; the ADE #5 readiness review was held on 12 May 2011. The campaign will last from 23 May until 3 June 2011. Tracking experiments around pericentre were performed on 27 April 2011 and 4, 8, 12, 15 and 18 May 2011. From 5 May 2011 onwards, the reaction wheel telemetry was sampled 8 times per second (8 Hertz, or 8 Hz). This very high sampling rate during all pericentre crossings provides telemetry from the reaction wheels showing how their speeds are changed to adjust the spacecraft attitude, in response to the atmospheric influence on the spacecraft. That telemetry data can then be used to determine the atmospheric density.

The pre-ADE tests showed an atmospheric density of twice the expected value. This led to a change on the solar array rotation angle to keep the maximum torques and heat flux within limits.

Summary of main activities

During the reporting period, routine mission operations were conducted using the ESA Cebreros (CEB) ground station.

Bad weather at the Cebreros ground station caused data losses on the downlink during the communications pass on 24 April 2011. The science data were downloaded during subsequent communications passes.

Delta Differential One-way Ranging (DDOR) was performed on 20 May 2011 using the ESA New Norcia (NNO) ground station.

One of the main computer servers (called VELTA) for Venus Express mission operations at the European Spaceflight Operations Centre (ESOC) experienced a hardware failure. The backup computer, VELTB, was used while the problem was investigated and resolved. An upgrade (or 'patch') to the VELTA and VELTB software was made.

On predictable dates, the solid state mass memory (SSSM) stops accepting data for 15 minutes. Since this feature of the SSMM was detected in pre-launch testing and is predictable, it presents no problems. Operations of the instrument which move data into the memory are halted for a few minutes, and then resumed. This occurred again on 07 May 2011.

At the end of the reporting period on 21 May 2011, Venus Express was 229.7 million kilometres from Earth. The one-way signal travel time was 765.6 seconds.

At the end of the reporting period, the final oxidizer mass was estimated to be 33.840 kilograms, and the final fuel mass estimate was 21.106 kilograms.

Payload Activities

Future Milestones

Superior solar conjunction will start on 6 August 2011.

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Legal disclaimer
This report is based on four ESOC mission operations reports, MOR #285 through MOR #288. Please see the copyright section of the legal disclaimer (bottom of this page) for terms of use.