ESA Science & Technology - No. 258 - Beginning of superior conjunction and planned science downlink shutdown

Cebreros ground station

All Cebreros ground station activities were nominal during this reporting period.

Between 18 March and 24 March, Cebreros pass durations were reduced to 3 hours to allow for station maintenance. This did not affect spacecraft operations as only brief passes were necessary during superior conjunction (16-24 March) to verify spacecraft safety.

High accuracy spacecraft ranging

The New Norcia and Cebreros ground stations were used for a Delta Differential One-way Ranging (delta-DOR, or DDOR) measurement at the start of the Cebreros communications pass on 5 March. The measurements slightly reduced the length of the Cebreros pass and hence the volume of data downloaded during the pass.

These high accuracy ranging measurements are carried out with the Venus Express spacecraft on a regular basis to support the accurate determination of the ephemeris for the planet Venus that is maintained by NASA's Solar System Dynamics Group.

For more information about DDOR, see the "Delta-DOR measurements".

Battery charge configuration changed

The mission's twenty-third eclipse season ended on 23 February. The battery's end-of-charge (EOC) state, which had been at 100% for the eclipse season, was subsequently lowered to 75% on 5 March.

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 5 March. This is a standard test that is repeated at intervals to determine the health of the battery. The battery 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.

Entry into superior conjunction and science downlink shutdown

The Sun-Earth-spacecraft alignment approached solar conjunction with an angle of 3.2 degrees on 16 March – the Sun was almost directly between Earth and the spacecraft. At this time, due to the distance between the spacecraft and Earth, the quality of the communications signal was such that it was acceptable (i.e., detectable against the background noise) only at very low data rates, too low for nominal command uplinks or science data downlinks. The flight control team therefore decided to shutdown Venus Express science operation downlinks in the period around superior conjunction.

Some science observations continued, but were limited by the fact that the data could not be downloaded. The fill state of the on-board memory was planned carefully, considering the limited storage space on board, to ensure that no data were overwritten by subsequent observations.  The on-board command buffer was loaded with future commands to be executed autonomously on 14 March, two days prior to the start of superior conjunction. This gave the team a day after the command upload to fix any unforeseen problems. However, everything worked well and conjunction operations were started at the beginning of the Cebreros ground station contact period on 16 March. The downlink telemetry bit rate was reduced to 298 bits per second, and the telecommand uplink bit rate was changed to 150 bits per second. 

From 25 March until the end of this reporting period, there were no communications or telemetry checks with the spacecraft as the Sun was directly in the line of sight from Earth. The antenna was pointed 10 degrees from Earth and the spacecraft was pointed away from its nominal Earth pointing to minimise unnecessary heating of the main antenna and feed horns electronics.  

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 on 30 March, Venus Express was at 258 million km from Earth. The one-way signal travel time was 859 seconds. The final oxidizer mass was 19.313 kg and the final fuel mass was 11.828 kg.

Scientific focus

This reporting period falls under the 90th Medium Term Plan (MTP), which covered the period from 3 to 30 March 2013. No eclipse, occultation, or drag experiment activities were scheduled during this reporting period, allowing the teams to schedule other types of observations, including stellar occultations, limb tracks, as well as wind tracking with the Venus Monitoring Camera (VMC) in the morning and evening sectors of the planet.

Venus Express entered superior conjunction on 16 March 2013, and remained in this configuration for the remainder of this reporting period.

As Venus is as far away as possible from Earth during superior conjunction, the data rate was at its lowest during this reporting period.

Due to the inclinations of the orbits of Venus and Earth with respect to the ecliptic, at times, during superior conjunction, Venus can be far enough above or below the Sun that communication with the spacecraft is possible. However, during this superior conjunction, no contact with the spacecraft was possible between 25 and 31 March, either for uplink or downlink, as the power output of the Sun was too strong and too close to the spacecraft line-of-sight to allow any signals from the spacecraft to be detected. The spacecraft was configured to be completely autonomous during this period.

Venus Express LTAN during 9 December 2012 to 30 March 2013. Credit: ESA

In addition, for a few days before and after this period, the line of sight between Earth and Venus was so close to the Sun that very little communication was possible. A clear communications signal in these circumstances would have required very high power, making only a very low data rate possible; not enough to downlink science data. It was possible to exchange only a small amount of data so that some spacecraft telemetry could be downlinked and a few commands uploaded. However, no science data were downlinked as per the operations plan for superior conjunction. This communication outage ran for 20 days starting on 17 March.

The limited science activities performed during this planning period were 'hot'; nadir observations, if carried out, exposed thermally sensitive faces of the spacecraft to solar illumination, limiting observation times. The Local Time at Ascending Node (LTAN) ranged from 14:42 hrs to 17:33 hrs.

For further information on LTAN, please see 'Terminator orbits'.

Change in Venus Express pericentre altitude during the period from 9 December 2012 to 30 March 2013. Credit: ESA	Venus Express coverage of Venus: 3 March to 30 March 2013. Credit: ESA

Payload activities

The science instruments performed nominally; they were not operated during the science operations shutdown discussed above.

Future milestones

• Restoration of communications with the spacecraft after superior conjunction
• End of superior conjunction and restart of normal science operations 
• Start of the fifteenth radio science occultation campaign

Legal disclaimer

This report is based on four ESOC mission operations reports, MOR 380 through MOR 383, as well as the MTP 90 Master Science Plan. Please see the copyright section in the Terms and Conditions for this site.