ESA Science & Technology - No. 235 - Atmospheric Drag Campaign #5; Start of Radio Science Investigations Occultation Campaign #11

Atmospheric Drag Campaign experiment

The fifth Atmospheric Drag Campaign (ADE #5) was carried out during this reporting period; it started on 23 May 2011 and ran until 3 June 2011.

Venus Express is in a highly eccentric orbit that takes it out to about 66,000 kilometres from the planet when at apocentre, but much closer to the planet's surface when the spacecraft is at pericentre. The altitude at the pericentre passages naturally evolves to as low as 175 kilometres. When this happens the orbit's pericentre altitude is raised again by mission operations to keep it within the allowed range.

Around the lowest pericentre passages the spacecraft is expected to experience small levels of drag from the upper atmosphere of Venus. The Venus Express ADE campaigns are devised to detect this drag by its integrated effect on the spacecraft's attitude and orbit position. Using spacecraft tracking data, accurate orbit determinations are performed to keep track of any drag effect that shows up as a deviation from the expected orbit. The spacecraft attitude is maintained by reaction wheels; the changes in wheel speed that are needed to maintain the commanded attitude can be used to provide information on the density of the Venusian atmosphere. These measurements help to improve models of Venus' upper atmosphere, especially over the northern polar region.

Atmospheric Drag Experiments (ADE) have been performed on a regular basis since 2008. Over the course of this campaign (ADE #5) the pericentre altitude was approximately 165 kilometres above the Venusian surface. For the next ADE campaign (#6, in September 2011), the pericenter altitude will be higher, at about 175 kilometres.

For the current ADE campaign, the spacecraft's arcs around the pericentre were devoted to a four-hour-long tracking-only pass using either ESA's New Norcia ground station or the NASA Deep Space Network Canberra ground station, both located in Australia. In order to minimize as far as possible any disturbances to the orbit determination process, the daily reaction wheel off-loadings (WOLs) were performed during (rather than outside of [see note 1]) the Cebreros communications passes, at least one hour prior to the end of Earth communications through Cebreros. That provided at least one hour of tracking after the WOLs, allowing even the minor orbit perturbations from the WOLs to be determined and removed from the orbit data, resulting in improved accuracy of the ADE results.

As in the past, the ADE campaign was combined with torque experiments, where the spacecraft torque was recorded in order to determine the atmospheric drag experienced by the spacecraft; this provided data from which the atmospheric density could be determined.

VeRa Earth occultation campaign #11
The mission's eleventh Earth occultation campaign started on 14 June 2011 and will last until 10 July 2011. These campaigns take place when the Earth is occulted by the disc of Venus; this occurs around the pericentre passages as seen from the spacecraft. Under these conditions unique measurements can be made to investigate the fine structure of the neutral atmosphere and ionosphere of Venus.

The occultation measurements are part of the Venus Express Radio Science Investigations (RSI) performed under the Venus Radio Science experiment (VeRa). For these investigations the VeRa team makes use of the radio links of the spacecraft's communications system. The spacecraft's High Gain Antenna (HGA) is pointed toward the Earth before the approach to occultation. The on-board ultra-stable oscillator is used to generate a very stable signal which is sent to Earth over the HGA in X-band frequency. Just before and after the occultation the transmitted radio signal passes through the ionosphere and atmosphere of Venus. The effect that this has on the received signal gives valuable information on the characteristics of the atmosphere (such as temperature, density and pressure) as a function of altitude.

RSI occultation measurements were performed around pericentre on five orbits in the reporting period. Two ground stations were used for receiving the spacecraft's radio signals on ground: the ESA deep space antenna at New Norcia (NNO) and the NASA Deep Space Network ground station at Canberra (CAN).

VIRTIS cooler anomaly
The VIRTIS-H cooler is out of service following a malfunction on 14 June 2011. Following the successful cooling down of VIRTIS-H on that day, there were a large number of cooler failure events (H_Cool_Steady_State_Failure). The conclusion of a review board convened to address this matter is that the cooler failed, in the same way that the VIRTIS-M instrument cooler failed some years before. Operations can still be performed, but not at low temperatures. This means that infrared data will no longer be available.

Summary of main activities
During the reporting period, routine mission operations were conducted using the ESA Cebreros (CEB) ground station. A pre-ADE tracking experiment was performed around pericentre on 22 May 2011. Twelve ADE tracking experiments were performed using the New Norcia (NNO) and NASA Deep Space Network Canberra (CAN) ground stations (six per station). An Orbital Correction Manoeuvre (OCM) was performed on 5 June 2011. Five RSI Occultation Measurements were performed using the New Norcia (one pass) and Canberra (four passes) ground stations.

At the end of the reporting period on 18 June 2011, Venus Express was 245.7 million kilometres from Earth. The one-way signal travel time was 819.12 seconds.

At the end of the reporting period, the final oxidizer mass was estimated to be 32.494 kilograms, and the final fuel mass estimate was 20.243 kilograms.

Payload Activities

Future Milestones

Occultation season #11 ends on 10 July 2011. Superior solar conjunction will start on 6 August 2011.

Notes
1. Changes to the Venus Express spacecraft attitude are achieved by adjusting the speed of three reaction wheels onboard the spacecraft. Attitude changes are seen as a drift of stars in the field-of-view of the active star tracker. The wheel speeds are changed to maintain the attitude as detected in the star tracker. If this process were to continue indefinitely, the wheel speeds would eventually be changed so much that they would be outside of their nominal speed ranges. For this reason, the momentum that has 'built up' in the wheels is dumped every day. This is done by using propellant thrusters to hold the spacecraft steady while the reaction wheel speeds are changed back to the middle of their operating band. Normally these 'momentum dumps' are carried out in every orbit, outside of the Cebreros communications period and within six hours of apocentre, to minimize orbit disturbances from the thruster burns while allowing flexibility in scheduling science operations and not reducing the communications time that is available for downloading data.

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Legal disclaimer
This report is based on four ESOC mission operations reports, MOR #289 through MOR #292. Please see the copyright section of the legal disclaimer (bottom of this page) for terms of use.