No. 224 - Start of quadrature operations
18 August 2010 17:42
Report for Period 18 July to 14 August 2010 (week 29 - week 32)
The reporting period, covering four weeks of Venus Express mission operations, was devoted to routine science activities. On 21 July the spacecraft entered a new quadrature phase that will last until 26 August. On 28 July a few anomalies were observed relating to the solid state mass memory (SSMM), which were solved by a warm restart of the SSMM. As part of the SSMM anomaly recovery a spacecraft safe mode was commanded on 30 July. After exit from the safe mode nominal operations were resumed. On 30 July another eclipse season started.
Inbound quadrature phase
The current quadrature phase is an inbound quadrature: it occurs before the inferior conjunction of Venus on 29 October 2010, and the distance between Venus and the Earth is decreasing.
During quadrature phases revised operating constraints on the VMC instrument lead to the necessity for changing the spacecraft attitude during Earth pointings to prevent unacceptable Sun illumination of the VMC. As part of the quadrature operations Venus Express therefore flies with a 10° tilt about its +X face during the Earth communication passes. To achieve and maintain this attitude, fake ephemerides on the positions of the spacecraft and the Earth are uploaded to Venus Express. The tilt, however, also leads to low-incident illumination on the spacecraft's +Y face. Because the +Y face carries several radiators, the spacecraft requires extended cool-down periods (about 17 hours) after the communication passes before resuming science observations. As a result, Cebreros passes are routinely skipped during the quadrature operations to allow more time for science observations. During this reporting period, ten passes were skipped. In addition, several passes were shortened to further increase the available time for science observations. With less Cebreros passes to upload commands, delayed telecommand (TC) files were used to store and call the commands on-board at a specific time.
Later in this quadrature phase, on 15 August 2010, a swap will be performed from high gain antenna 1 (HGA1) to HGA2 for use during Earth communication passes. A swap between the two high gain antennae is performed during each quadrature phase of the mission. This is to, during Earth pointings, avoid illumination of spacecraft faces that are not designed to cope with such exposure to the Sun. HGA1 can only be used for Earth communications during the periods when the Sun-spacecraft-Earth angle is smaller than 90°, while HGA2 is used when this angle is greater than 90° (see also "Earth Pointing Communication Phase" linked from the right-hand menu).
During the next quadrature phase, which starts at the end of December 2010 and is an outbound quadrature, a swap from HGA2 back to HGA1 will be made.
Two days before the start of the eclipse season, on 28 July, the batteries' end of charge (EoC) levels were restored to 100%. These had been set to 80% on 8 June (DOY 159), about a week after the previous eclipse season had ended, to extend the battery lifetime.
Summary of main activities
The table below shows a chronology of the main activities:
At the end of the reporting period (DOY 226) Venus Express was orbiting Venus at 108 million km from the Earth. The one-way signal travel time was 362 seconds.
All instruments were switched OFF on 30 July (DOY 211) as part of the spacecraft safe mode. They were switched ON again on 31 July (DOY 212) according to the science routine plan, after the Cebreros communication pass (except for MAG, which had been switched ON already during that pass).