ESA Science & Technology - No. 227 - Atmosphere Drag Experiment

The ADE #4 campaign started on 14 October 2011, collecting information about the Venusian atmosphere using tracking data and by measurement of the torque exerted on the spacecraft while the solar arrays were in a special configuration. The campaign ended on 25 October 2011.

Orbit Change Manoeuvres
Orbit Change Manoeuvres (OCM) were executed on 31 October 2011 and 1 November 2011. The OCM on 31 October was a retrograde manoeuvre at pericentre with a nominal Δv of 0.3415 metres per second, intended to lower the apocentre altitude by 64 kilometres and decrease the orbital period by 105 seconds.

The calibration within the orbit determination showed an underperformance of 1.00 % (-3.42 × 10^-3 metres per second) with a 1-sigma uncertainty of 0.06 % (1.9 × 10^-4 metres per second). The apocentre altitude was decreased by 640 metres less than originally planned and the orbital period was decreased by 1.05 seconds less than planned.

The OCM on 1 November was a prograde manoeuvre at apocentre with a nominal Δv of 10.6431 metres per second, intended to raise the pericentre altitude by 172 kilometres and increase the orbital period by 280 seconds.

The calibration within the orbit determination showed an underperformance of 0.21 % (-22.4 × 10^-3 metres per second) with a 1-sigma uncertainty of 0.03 % (3.7 × 10^-3 metres per second). The pericentre altitude was increased by 360 metres less than originally planned and the orbital period was increased by 0.6 seconds less than planned.

The combined result of the two manoeuvres was an overall increase in the orbital period that was 0.45 seconds less than planned.

Summary of main activities
During the reporting period, routine mission operations were conducted using the ESA Cebreros ground station (CEB). The Atmosphere Drag Experiment was performed using the ESA ground station at New Norcia (NNO) and the NASA Deep Space Network ground station at Canberra (CAN). The CEB communications pass scheduled for 12 October was skipped at the request of the Venus Express Science Operations Centre (VSOC) in order to allow the Venus Monitoring Camera (VMC) to perform a special observation that required the spacecraft to be pointed in a way that prevented communication with the ground station.

A number of Cebreros communications passes were interrupted for scheduled reaction wheel off-loading operations.

After the ADE passes, telemetry data from the reaction wheels were collected at a much higher rate than normal (RW 8Hz data) for six passes. These data, collected for three minutes on either side of pericentre (closest approach) passage, can be used by the flight dynamics team to detect atmospheric drag. This is done by studying the performance of the reaction wheels as the Attitude and Orbit Control System (AOCS) moves to compensate for the aerodynamic torque produced by the atmosphere on the spacecraft. Variations in the reaction wheel speeds change the angular momentum of the reaction wheels; those reaction wheel changes can be used to calculate the atmospheric density.

At the end of the reporting period, Venus Express was at 41.95 million kilometres from Earth. The one-way signal travel time was 139.8 seconds.

At the end of the reporting period, the final oxidizer mass was estimated to be 35.053 kilograms, and the final fuel mass estimate was 21.896 kilograms.

Payload Activities

Future Milestones

The next outbound quadrature phase will start in December 2010. Quadrature phases are periods of about five weeks during which the Sun-spacecraft-Earth angle is between 75° and 95°. They occur twice in every synodic period [see note 1] of Venus (584 days, or about 19 months), with an inbound quadrature phase ending 9 weeks before an inferior conjunction [see note 2] of Venus and an outbound quadrature phase starting 9 weeks after an inferior conjunction. This outbound quadrature phase will start nine weeks after the inferior conjunction that occurred on 30 October 2010.

Notes

1. The synodic period is the time between two successive identical configurations as seen from the Earth.
2. A conjunction is the alignment of two Solar System bodies (in this case Venus and the Sun) so that they have the same longitude as seen from Earth. An inferior conjunction occurs when Venus is between the Sun and the Earth; superior conjunction refers to when Venus is on the opposite side of the Sun from Earth.

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Legal disclaimer
This report is based on four ESOC mission operations reports, MOR #257 through MOR #260. Please see the copyright section of the legal disclaimer (bottom of this page) for terms of use.