ESA Science & Technology - No. 224 - Start of quadrature operations

Inbound quadrature phase
Quadrature phases are periods of about 5 weeks during which the Sun-spacecraft-Earth angle is between 75° and 95°. They occur twice in every synodical period of Venus (584 days, or about 19 months), with an inbound quadrature phase ending 9 weeks before a Venus inferior conjunction and an outbound quadrature phase starting 9 weeks after the inferior conjunction.

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.

Eclipse season
On 30 July (DOY 211) another eclipse season started, which will last until 13 September 2010 (DOY 256). During eclipse seasons, as Venus Express passes through the shadow of Venus on parts of its 24-hour orbits around the planet, the thermal and power requirements lead to increased demand on the spacecraft's batteries.

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
During the reporting period mission operations have been conducted with the support of the ESA Cebreros ground station. The communication passes over Cebreros were performed daily up to the start of the quadrature operations. After that, Cebreros passes were regularly skipped and several communication passes were shortened to increase the available time for science observations. Two additional passes were performed with the New Norcia ground station, for checking the spacecraft status, for commanding a safe mode and performing the subsequent recovery to normal operations after the SSMM anomaly.

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.

Payload Activities

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).

ASPERA
The instrument was regularly operated as part of the routine plan.

MAG
The instrument was regularly operated as part of the routine plan.

PFS
The instrument was not operated during the reporting period.

SPICAV
The instrument was regularly operated as part of the routine plan.

VMC
The instrument was regularly operated as part of the routine plan.

VeRa
No radio science observations were made during the reporting period.

VIRTIS
VIRTIS-M was operated in the visual channels only.

Future Milestones

• 15 August 2010: Swap from HGA1 to HGA2
• 26 August 2010: End of inbound quadrature phase
• 13 September 2010: End of eclipse season
• October 2010: Atmospheric drag experiment campaign #4
• 29 October 2010: Venus at inferior conjunction
• 31 December 2010: Start of outbound quadrature phase

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Legal disclaimer
This report is based on four ESOC mission operations reports, MOR #245 through #248. Please see the copyright section of the legal disclaimer (bottom of this page) for terms of use.