ESA Science & Technology - No. 226 - Mission operations update

The mission's next atmospheric drag experiment (ADE) campaign (#4) starts on 14 October and will be covered in the next reporting period. During that campaign the Venus Express spacecraft will be used again to probe the density of the upper atmosphere of Venus by flying through it with an asymmetric configuration of the solar panels.  The resulting small torque on the spacecraft around the pericentre passes will be determined, and from it the atmospheric density can be derived.

Determining the torque is done by analysing the telemetry from the reaction wheels, which accumulate momentum as they continuously counteract the torque to maintain the spacecraft's attitude.  Starting already on 20 September (DOY 263), additional telemetry from the reaction wheels, at a higher sampling rate of 8 Hz, is acquired daily during several minutes around each pericentre passage.

This method to derive the atmospheric density was introduced earlier this year and successfully tested and employed during previous ADE campaigns.  See the status reports no. 218 and no. 220 for more details.

Pre-ADE experiments
During the reporting period the pericentre altitude of Venus Express has been steadily decreasing, from about 185 km down to about 167 km.  Throughout the ADE campaign later this month, the pericentre altitude will be about 165 km, which is the lowest altitude reached by Venus Express to date. In preparation of the campaign, three pre-ADE experiments were performed during the reporting period on 14 and 22 September and 2 October.  The experiments were performed around the pericentre, flying with an asymmetric solar wing configuration.  The three pericentre passes were on the dayside of Venus and at an altitude of 186.4 km, 178.2 km and 169.9 km, respectively.  A review of the three experiments was held on 6 October to identify possible changes to the forthcoming ADE campaign.

Delta differential one-way ranging
Delta differential one-way ranging (DDOR) was performed twice in the reporting period, on 12 September and 4 October, using two ground stations to simultaneously track the spacecraft.  These measurements are performed regularly to support the accurate determination of the ephemeris for the planet Venus that is maintained by NASA's Solar System Dynamics Group.

The DDOR measurement principle uses a technique to determine - with high accuracy - the position of a transmitter in space and relies on observing the time delay between reception of the signal from the transmitter by two ground stations at different positions on Earth.  In this case, the Venus Express spacecraft was the transmitter and the two ground stations were New Norcia in western Australia and Cebreros in Avila, Spain.

Orbit change manoeuvres
Two orbit change manoeuvres (OCMs) were performed on 12 September and 3 October.  Both were retrograde manoeuvres performed at pericentre, to lower the apocentre altitude, which is about 66 000 km.  For the first OCM, on 12 September, the manoeuvre had a nominal magnitude of 0.539 metres per seconds, and was carried out to lower the apocentre altitude by 101 kilometres and decrease the orbital period by 165 seconds.

The calibration within the orbit determination showed a slight over-performance of 0.52% (+2.8 millimetres per second) with a 1-sigma uncertainty of 0.02% (0.1 millimetres per second).  The apocentre altitude was decreased by 0.46 kilometres more than planned and the orbital period was decreased by 0.75 seconds more than planned.

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.  Several passes were shortened.  In addition, the ESA New Norcia ground station was used for the DDOR measurements on Day-of-Year (DOY) 255 and 277.  A tracking-only pass was performed with the 15-m dish at the ESA Perth ground station on DOY 265, as an experiment for possible future utilisation during aerobraking. Bad weather at Cebreros on DOY 267 caused some data gaps.  The data were copied and stored on board for later transmission.  They were successfully downlinked on DOY 282.  At the start of the pass on DOY 282 the science data downlink was affected again by bad weather, but all data packets were retrieved within the pass.

The table below shows a chronology of the main activities:

At the end of the last Cebreros pass in the reporting period (DOY 282) Venus Express was orbiting Venus at 48.8 million km from the Earth. The one-way signal travel time was 162 seconds.

At the end of this reporting period, the final oxidizer mass was estimated to be 36.899 kilograms, and the final fuel mass estimate was 23.092 kilograms.

Payload Activities

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.

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

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

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

Future Milestones

• 14 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 #253 through #256. Please see the copyright section of the legal disclaimer (bottom of this page) for terms of use.