ESA Science & Technology - No. 220 - Atmospheric drag experiments

Atmospheric drag experiment campaign
In the middle of the reporting period the Venus Express ADE campaign #3b took place. It followed six weeks after the conclusion of the ADE campaign #3a (see status reports no. 218 and 219, linked from the right-hand menu, for more details).

Venus Express is in a 24-hour, highly eccentric, elliptical orbit that takes it out to about 66 000 km 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 km. After reaching this lowest value the orbit's pericentre altitude is raised again by mission operations to keep it in 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 set up to detect this drag by its integrated effect on the spacecraft's orbit. 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.

During the entire ADE campaign #3b, which ran from 11 to 16 April, the pericentre altitude was less than 180 km. The spacecraft's arcs around the pericentre were devoted to tracking-only passes of a few hours long, using the ESA New Norcia ground station. To minimize any disturbances to the orbit determination process, the daily reaction wheel off-loadings were scheduled outside these pericentre tracking passes; they were instead performed during the communication passes over the Cebreros ground station.

Solar array tilt experiments
An additional new method to detect the effect of Venus' upper atmosphere on the Venus Express spacecraft was demonstrated earlier this year (see status report no. 218). The method involves flying Venus Express with an asymmetrical tilt on the two solar array wings during pericentre passages. As the spacecraft passes through the very low-density upper atmosphere, the small forces on the two differently oriented wings result in a tiny torque on the spacecraft. This torque can be identified from the accumulated momentum on the reaction wheels, which need to counteract the atmospheric effects to maintain the spacecraft's attitude.

Six solar array tilt experiments have been performed in the reporting period: one on 3 April, and one each day from 12 to 16 April. During the experiments the orientation of the solar arrays was varied in steps, increasing the angle between the two wings, thereby increasing the amount of torque on the spacecraft (see also the related link in the right hand menu "Surfing an alien atmosphere").

Additional telemetry from the reaction wheels, at a higher sampling rate of 8 Hz, was acquired for analysis to look for the accumulated momentum and identify any signature of drag. During the solar array tilt experiments this data was collected for +/- 15 minutes around the pericentre passages. Up to 17 April the same type of data was also collected on the other days during the reporting period, but only for +/- 3 minutes around each pericentre.

Orbit Control Manoeuvre (OCM)
Following the conclusion of the ADE campaign #3b, and having nearly reached the currently set lower limit of 175 km for the pericentre altitude, an OCM was successfully performed on 17 April to raise again the pericentre altitude of the Venus Express orbit. The nominal magnitude of the OCM was 5.978 m/s. It was a prograde manoeuvre at apocentre, to raise the pericentre altitude by 96.65 km and increase the orbital period by 158 seconds.

DDOR
On 10 April delta differential one-way ranging (DDOR) was performed using the New Norcia and Cebreros ground stations to 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 highly accurately determine 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 is the transmitter and the two ground stations are New Norcia in western Australia and Cebreros in Avila, Spain.

VeRa gravity experiments
From 11 to 16 April, gravity experiments were performed daily using the radio links of the spacecraft's communications system. Open loop data were recorded during the New Norcia tracking passes around pericentre, in parallel to the ADE tracking data. The gravity experiments are set up to measure anomalies in Venus' gravitational field by analyzing the spacecraft's carrier signals received on ground, simultaneously in two radio bands. The detected anomalies provide insight into the properties of the crust and lithosphere of Venus.

Summary of main activities
During the reporting period mission operations have been conducted with support of the ESA Cebreros (CEB) ground station. Communication passes over Cebreros were performed daily. Additional tracking-only passes were performed with the New Norcia (NNO) ground station, as part of the ADE campaign #3b.

The table below shows a chronology of the main activities:

At the end of the reporting period (DOY 114) Venus Express was orbiting Venus at 223.2 million km from the Earth. The one-way signal travel time was 744 seconds.

Spacecraft

Attitude and Orbit Control System (AOCS)
The AOCS subsystem is in its nominal configuration and performing nominally. Operations continue with all four reaction wheels.

The friction levels and bearing temperatures of the reaction wheels remained stable throughout the reporting period. The friction on reaction wheel 2 continued to show small spikes, predominantly during the Earth pointing periods, but this behaviour has already been correlated to the higher bearing temperatures and has no impact on operations.

Reaction wheel 4 showed a slight increase in its friction level over a 5-day period up to 4 April, after which it dropped again to nominal levels. While this had no impact on the spacecraft pointing accuracy the event is under evaluation.

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
From 11 to 16 April (DOY 101 to 106) gravity experiments were performed daily. Open loop data were recorded during the New Norcia tracking passes around pericentre.

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

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

Future Milestones

• 30 April 2010: Start Earth occultation season #9
• 21 July 2010: Start of next quadrature phase (inbound)

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Legal disclaimer
This report is based on four ESOC mission operations reports, MOR #229 through #232. Please see the copyright section of the legal disclaimer (bottom of this page) for terms of use.