ESA Science & Technology - No. 254 - End of the twenty-second eclipse season, start of the tenth atmospheric drag experiment campaign, routine star tracker calibration, and straylight calibration for Comet Machholz observations

Cebreros ground station
All activities at the Cebreros ground station were nominal during this period. The spacecraft telemetry data rate was medium, 114 kbps, at the beginning of the reporting period. It decreased to 91 kbps on 19 November and then to 76 kbps beginning 6 December as Earth and Venus moved further apart.

The Cebreros communications passes were interrupted from 4 to 8 December to allow for Wheel Off Loading (WOL) for the tenth atmospheric drag experiment campaign.

End of the twenty-second eclipse season
The twenty-second eclipse (solar occultation) season, which started on 19 September 2012, concluded on 19 November.

During eclipse seasons, as viewed from the spacecraft, Venus occults the Sun for part of each orbit. Eclipse ingress and egress observations were performed in almost every orbit of the eclipse season, which coincided with the first third of this reporting period (11 to 19 November), with the Solar Occultation at Infrared (SOIR) channel of the SPICAV (Spectroscopy for Investigation of Characteristics of the Atmosphere of Venus) instrument. The SOIR channel is specially designed to observe the Sun through the Venusian atmosphere to measure atmospheric absorption. SOIR solar occultation observations can only be acquired during eclipse seasons, therefore they have a very high scientific priority in these periods.

For further information, please see 'Science observations with Venus Express during an eclipse' - linked from the right-hand menu.

Battery charge reconfiguration and deep discharge test
The mission's twenty-second eclipse season ended on 19 November. The battery's end-of-charge (EOC) state, which had been at 100% for the eclipse season, was subsequently lowered to 24.4V, or 80%.

During solar eclipse seasons, the spacecraft is in complete darkness during a portion of each orbit when Venus blocks out the Sun. At this time, the spacecraft bus and payload are powered by the on-board battery, which must then be recharged again to 100% prior to the next eclipse. The battery (composed of three individual battery packs) is sized so that at the end of its life, the spacecraft can go through the longest eclipse and still retain enough stored power for the spacecraft to successfully complete a transition to 'safe mode' in the event of a spacecraft system anomaly.

Outside eclipse seasons, sufficient battery power must be maintained at all times for a transition to safe mode. However, a lower (than 100%) battery energy level (or end-of-charge state) can be maintained, as no provision is needed for eclipses. Lowering the battery's end-of-charge state prolongs its life.

In addition to the change in battery charge state, a battery deep discharge test was performed on 21 November. This is a standard test that is repeated at intervals to determine the health of the battery. The batter is discharged very deeply, and the battery voltage is compared to the predicted values. The model describing the battery discharge is updated if necessary. In this case, the battery health was nominal.

Tenth Atmospheric Drag Experiment campaign
The tenth Atmospheric Drag Experiment (ADE) campaign started on 13 November and continued until the end of this reporting period. Science observations were carried out in two parts: pre-ADE (beginning 15 November) and ADE observations (beginning 4 December). 

The pre-ADE passes started at a pericentre altitude of 200 kilometres. Five pre-ADE passes were performed during this planning period, on 15, 18, 21, 24 and 28 November.

The ADE passes started at 166 kilometres on 4 December. The spacecraft completed five drag passes during the last five orbits of this planning period (4 to 8 December). Of the five drag passes in this period, three were performed in conjunction with the NASA Deep Space Network (DSN) 70 metre antenna in Canberra, Australia, and three were carried out in conjunction with the ESA Deep Space Antenna (DSA) 35 metre dish at New Norcia, Australia. The lowest altitude reached was 165 km.

Change in Venus Express pericentre altitude during the period from 19 August to 8 December 2012. Credit: ESA

For more information on Atmospheric Drag Experiment campaigns, see the link in the right-hand menu.

Attitude calibration with redundant star tracker
The Venus Express spacecraft has two star trackers: star tracker A is used for routine operations, while the redundant unit, star tracker B, is on standby in case of emergencies. The star trackers, which are essentially low-resolution telescopes, are the primary means of determining the spacecraft attitude. The star trackers capture images on a solid state imaging array, which are then compared against a database of stars using the star tracker's own computer. This information is used for fine determination of the spacecraft pointing.

One way to maintain the pointing accuracy is by verifying, after an observation is completed, that the spacecraft was pointing exactly where it was supposed to. After data are obtained for each orbit, the flight dynamics team analyses the pointing, to compare the planned pointing (derived on the basis of existing attitude reconstruction algorithms) with the actual pointing (derived from spacecraft telemetry); the data are then corrected or 'validated' for any differences found. The final determination of the actual pointing is called the 'reconstructed' attitude data.

Occasionally, for additional accuracy, the attitude reconstruction algorithms themselves are validated and calibrated. One such validation procedure was carried out with the redundant star tracker on 13 November. The star tracker acquired an image of a star whose location is well known, and used this as a reference from which the spacecraft attitude was derived to a high accuracy.  This attitude information will be used by the flight dynamics team to validate the attitude reconstruction algorithms.

Orbit correction manoeuvres
No orbit correction manoeuvres were planned during this period. 

Straylight calibration for Comet Machholz observations
Comet Machholz observations were performed on 6, 8, 21, 22 and 23 July 2012 (see also status report #250, linked in the right-hand menu). Unfortunately, strong straylight from the Sun was observed in the SPICAV spectrometer data. By definition, straylight originates from a source other than the intended object. It typically enters the instrument field of view at a high angle, and reflects off the sides of the optical chamber into the light detectors.

To improve the comet observation spectra, two observations were performed in this planning period, on 14 and 15 November. As Comet Machholz is no longer in view, SPICAV was pointed at the same angle (as the original observation) to the Sun, in order to try and capture the same level of scattered straylight. These newly obtained straylight spectra will be used to calibrate the original observations of the comet, hence improving the images that were obtained.

Summary of main activities
The table below shows a chronology of the main spacecraft bus activities in the reporting period: 

At the end of the reporting period, Venus Express was at 217.7 million kilometres from Earth. The one-way signal travel time was 725 seconds. The final oxidizer mass was 23.588 kg and the final fuel mass was 14.581 kg.

Scientific focus
This reporting period falls under the 86th Medium Term Plan (MTP), which covered the period from 11 November to 8 December 2012. The telemetry data rate was medium, starting at 114 kbps and finishing at 76 kbps as Earth and Venus moved further apart.

The local time at ascending node changed from 02:40 hrs to 05:40 hrs during this reporting period. This meant that the season was 'cold', so that pointing the +Z instrument face of the spacecraft at nadir did not expose any thermally sensitive faces of the spacecraft to the Sun.

Venus Express LTAN (19 August to 8 December 2012). Credit: ESA

The MTP included pre-ADE passes, followed by the beginning of the tenth ADE campaign on 4 December. Dual S- and X- band tracking was performed during each orbit from 4 to 15 December for 4 hours around pericentre.

The first third of this reporting period coincided with the twenty-second eclipse season. Priority was thus given to solar occultations in eclipse until the season ended on 19 November.

Venus Express coverage of Venus (11 November to 8 December 2012). Credit: ESA

The VMC is able to obtain infrared images of the surface at a low resolution during eclipse seasons, so observations of the surface with the Venus Monitoring Camera (VMC) were possible during this reporting period. However, no specific ground targets were identified as scientifically interesting in this particular reporting period, and so no imaging of the surface was carried out.

Payload activities

No unusual or anomalous activities occurred with the Venus Express science instruments during this 28-day period.

 
Future milestones

• Continuation and end of observations for the tenth Atmospheric Drag Experiment campaign
• Triple planning: early delivery of three weeks of commands to allow for Christmas holidays
• Start of the fourteenth occultation season

 
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Legal disclaimer
This report is based on four ESOC mission operations reports, MOR #364 through MOR #367, as well as the MTP86 Master Science Plan. Please see the copyright section of the legal disclaimer (bottom of this page) for terms of use.