Venus Express reaches lowest pericentre altitude
14 October 2008
The series of orbit correction manoeuvres designed to lower the pericentre altitude of the Venus Express orbit have been successfully completed, with the spacecraft reaching the lowest pericentre altitude since the start of the mission.Over a period of about four weeks, beginning on Sunday 13 July, the pericentre of the orbit has been permanently lowered from its former range of 250-400 km to 185-300 km. At the start of this period, the pericentre altitude was 360 km. The lowest pericentre altitude of 185 km was reached on 4 August 2008.
All the pericentre lowering orbit control manoeuvres (OCMs) were executed at apocentre. To maintain the same orbital period of nearly 24 hours, four apocentre raising OCMs were also performed with the spacecraft at pericentre. These occurred on the orbit prior to each pericentre lowering OCM. The tables below list the results of the eight OCMs as derived from orbit determinations after each manoeuvre.
| Pericentre Lowering Manoeuvres | |||||
| OCM | Date | Nominal alt. change (km) |
Target altitude (km) |
Nominal delta-V (mms-1) |
Over-/under- performance (mms-1) |
| #1 | 14.07.2008 | - 137.1 | 220 | 8411.7 |
- 23.5 (0.28%) |
| #2 | 21.07.2008 | - 22.7 | 200 | 1413.6 |
- 1.9 (0.13%) |
| #3 | 28.07.2008 | - 11.0 | 190 | 705.8 |
+ 2.5 (0.36%) |
| #4 | 04.08.2008 | - 5.5 | 185 | 321.8 |
- 1.52 (0.47%) |
| Apocentre Raising Manoeuvres | ||||
| OCM | Date | Nominal alt. change (km) |
Nominal delta-V (mms-1) |
Over-/under- performance (mms-1) |
| #1 | 13.07.2008 | + 176.3 | 949.7 | - 0.05 (0.005%) |
| #2 | 20.07.2008 | + 18.1 | 96.6 | + 0.51 (0.52%) |
| #3 | 27.07.2008 | + 10.0 | 53.6 | - 0.35 (0.65%) |
| #4 | 03.08.2008 | + 7.5 | 41.0 | - 0.51 (1.26%) |
More details on the OCMs are available in the status report listed below and in the right-hand menu. The four reports (no. 140 / no. 143) each cover a set of one apocentre raising manoeuvre and one pericentre lowering manoeuvre:
OCMs #1: Status report no.140
OCMs #2: Status report no.141
OCMs #3: Status report no.142
OCMs #4: Status report no.143
Atmospheric drag experiment
With the lowering of the spacecraft's pericentre altitude, Venus Express was expected to experience the influence of the upper atmosphere around the pericentre passages. This effect has indeed been observed, with the first clear indications of atmospheric drag (albeit at a very low level) occurring when the Venus Express spacecraft was at an altitude of around 190 km.
The effect was derived from the observed changes in the spacecraft's orbit as determined from tracking data. The tracking data has been collected regularly with ground stations on Earth over the period 30 July to 25 August 2008 and subsequently evaluated by the flight dynamics team. The detailed analysis of the atmospheric drag data is ongoing and a summary of the scientific information derived from these observations will be reported on later as well.
