No. 232 - Start of Eclipse Season; Completion of Radio Science Investigations Occultation Campaign #10
12 Apr 2011 16:12
Report for the period 27 February to 26 March 2011 (week 9 - week 12)
The reporting period, covering four weeks of Venus Express mission operations, was devoted to routine science activities. Twelve occultations were observed for Radio Science Investigations (RSI) campaign #10.
An eclipse season started on 13 March 2011; in preparation for this, the end-of-charge setting for the battery was returned to 100% on 11 March 2011. During eclipse seasons, Venus Express passes through the shadow of Venus during parts of its orbit around the planet; the resulting thermal control and power requirements lead to increased demands being placed on the spacecraft’s batteries.
VeRa Earth occultation campaign #10
The mission's tenth Earth occultation campaign was completed during this reporting period. These campaigns take place when the Earth is occulted by the disk 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 simultaneously in the S-band and X-band. 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 twelve 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).
Summary of main activities
During the reporting period, routine mission operations were conducted using the ESA Cebreros ground station (CEB). Delta Differential One-way Ranging (DDOR) was performed on 20 March 2011, using the New Norcia (NNO) ground station. RSI measurements were performed by observing the occultation of signals from the Venus Express spacecraft as it passed through orbital pericentre. A total of twelve observations were performed, using the NASA Deep Space Network Canberra (CAN) and ESA New Norcia ground stations. RSI Occultation Campaign #10 ended on 23 March 2011.
Bad weather at the Cebreros ground station caused the data downlink on 4 March 2011 to be interrupted. Some Venus Monitoring Camera (VMC) data were lost; all other science data were downloaded during subsequent communications passes.
Venus Express has two star trackers, which are used by the spacecraft control system to maintain knowledge of the spacecraft attitude in space. On 07 March 2011, there was an interruption of communications between the main star tracker A and the spacecraft control software. This is not common, but has occurred in the past. The power to the star tracker was cycled (off, then on), the software re-started correctly and the communications were restored. The cause of the communication problem is not known, but the brief communication breaks are not considered serious.
At the end of the reporting period on 26 March 2011, Venus Express was 183.4 million kilometres from Earth. The one-way signal travel time was 611 seconds.
At the end of the reporting period, the final oxidizer mass was estimated to be 34.496 kilograms, and the final fuel mass estimate was 21.537 kilograms.
Atmospheric Drag Experiment (ADE) #5 will start on 23 May 2011. Superior solar conjunction will start on 6 August 2011.