ESA Science & Technology - No. 218 - Atmospheric drag experiments

Atmospheric drag experiment campaign
Venus Express is in a highly eccentric 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 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. These measurements help to improve models of Venus' upper atmosphere, especially over the polar regions.

The first two ADE campaigns were performed from 18 to 22 August 2008 (ADE campaign #1, see status report no. 145) and 12 to 17 October 2009 (ADE campaign #2, see status report no. 205). The current ADE campaign (#3a) started on 22 February 2010 and ends the first day of the next reporting period, on 28 February 2010. Over the course of this campaign the pericentre altitude gradually decreases from 187 km to 182 km. It will then continue to decrease and during the next ADE campaign (#3b), in April 2010, the pericentre altitude will be below 180 km.

For the current ADE campaign, the spacecraft's arcs around the pericentre were devoted to a four-hour long tracking-only pass using either the NASA Canberra 70m ground station or ESA's New Norcia ground station. In order to minimize as far as possible any disturbances to the orbit determination process, the daily reaction wheel off-loadings were scheduled outside the pericentre tracking passes, and instead performed during the Cebreros passes.

Solar array tilting
During the previous ADE campaign in October 2009 it was noted that apart from their effect on the Venus Express orbit, the aerodynamic forces experienced by the spacecraft around pericentre could also be detected by analysing the accumulated momentum on the reaction wheels.

This has led to the idea of having Venus Express fly with an asymmetric solar array orientation during the pericentre passages, where one of the two arrays is kept pointing at the Sun, while the other is tilted away from the Sun. This asymmetric configuration would increase the aerodynamic torque on the spacecraft, resulting in more accumulated momentum on the reaction wheels (as they compensate for this torque) and thereby improving the chance to detect it.

On 15 February (DoY 046) and 25 February (DoY 056) two solar array tilting tests were conducted, which included the spacecraft flying with a 90° tilt of one of the arrays for several minutes. The tests were successful and conclude the validation of the solar array tilting operations, in preparation for the mission's next ADE campaign (#3b) which is planned in April 2010. In addition, a clear signature of low levels of atmospheric drag was detected.

Orbital correction manoeuvre
A routine orbital correction manoeuvre (OCM) was successfully performed on 21 February (DoY 052) around pericentre. The manoeuvre constituted a phase correction, intended to keep the phasing of Venus Express in its orbit in sync with mission operations from ground. It was a retrograde manoeuvre, with a nominal magnitude (delta-V) of 295.2 mms^-1, to lower the apocentre altitude by 56 km and decrease the orbital period by 92 seconds.
The calibration within the orbit determination showed a slight over-performance of 1.3% (+3.8 mms^-1) with a 1-sigma uncertainty of 0.2%. The apocentre altitude was decreased by 0.74 km more than planned and the orbital period was decreased by 1.2 seconds more than planned.

Summary of main activities
During the reporting period mission operations have been conducted with support of the ESA Cebreros ground station. Communication passes were performed daily from 31 January to 27 February. As part of the ADE campaign several additional tracking-only passes were performed daily from 22 February onward, using the ESA New Norcia (NNO) ground station and the NASA Canberra (CAN) ground station.

The table below shows a chronology of the main activities:

At the end of the last Cebreros pass in the reporting period (DOY 058) Venus Express was orbiting Venus at 249.9 million km from the Earth. The one-way signal travel time was 833 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 of the reaction wheels remained stable throughout the reporting period. The friction on reaction wheel 2 continued to show 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.

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
VIRTIS-M was operated in the visual channels only.

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

Future Milestones

• 28 February 2010: end of ADE campaign #3a
• April 2010: ADE campaign #3b

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Legal disclaimer
This report is based on four ESOC mission operations reports, MOR #221 through #224. Please see the copyright section of the legal disclaimer (bottom of this page) for terms of use.