ESA Science & Technology - No. 248 - Continuation of the Earth occultation season 13, start of eclipse season 21, inferior conjunction and Venus transit

Cebreros ground station
Operations with ESA's Cebreros ground station were nominal during the reporting period. The average downlink data volume per orbit was 4990 Mbits.

DDOR high-accuracy spacecraft ranging
ESA's Cebreros and New Norcia ground stations were used for a Delta Differential One-way Ranging (delta-DOR, or DDOR) measurement on 7 June 2012. These 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" link in the right-hand column.

Time correlation
A regular time correlation was performed on 9 June 2012, after the communications pass with the Cebreros ground station. The new time correlation was applied from then onwards. As with almost all spacecraft, the Venus Express on-board 'clock' is actually a count-up timer. On a regular basis, this on-board timer is compared to atomic clock time on Earth and the 'time' represented by the count-up timer value is synchronized with the more accurate atomic clocks on Earth.

Orbit control manoeuvres
During the reporting period, one scheduled orbital correction manoeuvre was carried out to maintain the parameters of the Venus Express orbit. The manoeuvre was performed smoothly and successfully around the pericentre on 3 June 2012.

Battery charge reconfiguration
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 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 at those times, as no provision is needed for eclipses. Lowering the battery's end-of-charge state prolongs its life.

On 6 June 2012, the mission's twenty-first solar eclipse season started (see below). Prior to that, the spacecraft was to be left in autonomous mode for three days due to a communications outage caused by the inferior conjunction (see below). Therefore the battery state of charge was raised to 100% on 31 May 2012, where it will remain until the end of the eclipse season in July 2012.

Change in Venus Express pericentre altitude (28 April to 18 August 2012). Credit: ESA

Continuation of the thirteenth Earth occultation season
The thirteenth Earth occultation season, which started on 27 March 2012, continued during this reporting period. It is the mission's longest occultation season and will end on 6 October 2012.

During occultation seasons, the geometry places Venus between the spacecraft and Earth for part of each orbit, around the pericentre. As Venus Express moves along its orbit, the Earth appears to pass behind Venus and then to re-emerge a few minutes later. This geometry can be exploited by the Venus Radio Science experiment (VeRa), a radio science investigation (RSI), to study the planet's atmosphere.

Shortly before Venus Express reaches pericentre, the spacecraft starts broadcasting a very stable radio signal to Earth. The signal must be far more accurate than the usual signal during a communications pass, hence it is generated using a special on-board ultra stable oscillator. As the Earth begins to move behind Venus, the radio signal passes through the Venusian atmosphere where it is refracted and then received on Earth. The refracted signal can be measured and processed to yield details about the atmosphere that cannot be obtained in other ways.

One very important result is a profile of the atmospheric temperature at different heights. As each measurement is also taken at slightly different planetary latitudes, the individual temperature profiles can be combined to obtain a picture of atmospheric temperatures around the high latitudes of Venus, both on the dayside and night-side.

The radio signal was broadcast with the spacecraft's smaller high gain antenna, HGA-2. It was received on Earth with the New Norcia (NNO) antenna in western Australia. The NNO ground station is used for the Venus Express radio science investigations because the spacecraft's orbital period is 24 hours, and the pericentre always occurs when the NNO station has the best angle to view Venus.

Start of the twenty-first eclipse season
The twenty-first eclipse (solar occultation) season started on 6 June 2012. During eclipse seasons, as viewed from the spacecraft, Venus occults the Sun for part of each orbit. On 6 June, during the Venus transit (see below), and in the following orbit on 7 June, eclipse ingress and egress observations were made with the Solar Occultation at Infrared (SOIR) channel of the SPICAV spectrometer. The SOIR channel is specially designed to observe the Sun through the Venusian atmosphere to measure atmospheric absorption; these pericentre eclipse observations had priority around the Venus transit period. It was hoped that ground-based measurements of the solar absorption taken from Earth could be compared with the SOIR measurements taken in orbit around Venus.

After the transit period, SOIR solar occultation observations were taken in roughly half the orbits. These observations can only be done during eclipse seasons, and are given a high priority in those periods. For further information, please see 'Science observations with Venus Express during an eclipse' – linked from the right-hand menu.

Venus transit of the Sun
During the night of 5/6 June 2012, a rare celestial event took place: Venus appeared to slowly cross the solar disc as seen from Earth, when Venus passed between the Earth and the Sun during inferior conjunction. From the vantage point of Venus Express, in orbit around the planet, this transit period was not unusual in any way. However, to support some Earth-based observations of the Venus transit, two special operations of the SPICAV/SOIR spectrograph instrument were scheduled on 6 and 7 June: an ultraviolet scan of the Sun's surface was performed by the SPICAV spectrometer, and solar eclipse ingress and egress observations were made with the SOIR channel of the SPICAV instrument.

Inferior conjunction
A Venus inferior conjunction occurred on 6 June 2012. Around Venus conjunctions, when the angle on the sky between the Sun and Venus comes within three degrees as seen from the Earth, the enormous energy output of the Sun interferes with the reception of the Venus Express communications signal.

Due to the serious reduction in the signal-to-noise ratio of the communications link, the spacecraft's uplink and downlink communications were suspended for few days around the inferior conjunction, from 4 to 7 June.

The science operations could continue during this period by using an extended upload of instrument commands, and holding the data on-board until the downlink communications resumed.

Between 4 and 8 June inclusive, the spacecraft had to operate within special constraints:

• The default spacecraft pointing during ascending branch periods with no science observations is Earth-pointing. During the time with no Earth communications, the spacecraft was pointed instead to a cold inertial position. The fixed spacecraft attitude kept any thermally sensitive spacecraft faces from being exposed to the Sun.
• For the three conjunction days of 4, 5 and 6 June, no Earth communications were allowed because of the low signal-to-noise ratio on the communications link and to thermally protect the spacecraft.
• On 7 June, there was limited downlink due to better, but still poor, signal quality. Housekeeping telemetry was transmitted, but no science data.
• On 8 June, the spacecraft still had excess heat that needed to be dissipated. The transmission of science data resumed.

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 23 June 2012, Venus Express was 50 million kilometres from Earth. The one-way signal travel time was 166 seconds. The final oxidizer mass was 25.937 kg and the final fuel mass was 16.056 kg.

Scientific focus
This reporting period falls under medium term plan (MTP) #80, which covers the period 27 May through 23 June 2012. This MTP included an inferior conjunction, at which a transit of Venus occurred: the moment of 'greatest transit', when Venus appears closest to the centre of the Sun, occuring at around 01:29 UT on 6 June). A long Earth occultation season continued throughout the whole of this MTP. A solar eclipse season started on 6 June.

Venus Express coverage of Venus (27 May through 23 June 2012). Credit: ESA

The MTP was considered 'hot', as pointing down at the planet nadir would expose thermally sensitive faces to the Sun, thus limiting observation times. The local time at ascending node changed from about 09:00 in the first orbit to about 12:00 (noon) in the last orbit.

Change in Venus Express's local time at ascending node (28 April - 18 August 2012). Credit: ESA

Payload activities
 

Future milestones

• Continuation of the thirteenth Earth occultation season
• End of the twenty-first solar eclipse season
• Introduction of a leap second on 30 June 2012

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Legal disclaimer
This report is based on four ESOC mission operations reports, MOR #340 through MOR #343, as well as the MTP080 Master Science Plan. Please see the copyright section of the legal disclaimer (bottom of this page) for terms of use.