A series of orbit control manoeuvres (OCMs) is underway to alter the orbit of the Venus Express spacecraft, with the goal of reducing the pericentre altitude to 185 km. These manoeuvres, which began on 13 July, provide new opportunities for scientific observations of regions which have not been probed by the spacecraft so far.

In April 2006, Venus Express successfully entered orbit around Venus. Several manoeuvres over the period 15 April - 6 May 2006 then lowered the spacecraft into its operational orbit: a 24-hour, elliptical, quasi-polar orbit, in which it has been since.

The nominal mission of 500 days was successfully completed by the summer of 2007. Venus Express is now in the middle of the extended period of operations that runs up to May 2009.

The highly eccentric orbit takes Venus Express out to 66 000 km from the planet when at apocentre. The pericentre altitude varies between 250 and 400 km due to natural perturbation of the orbit, mainly by the Sun's gravity. Regular corrections to compensate for this perturbation are performed to maintain the pericentre in the desired altitude range.

Over a period of about four weeks, beginning on Sunday 13 July, the pericentre of the orbit is being permanently lowered from its former range of 250-400 km to 185-300 km. The lowering will be done in four steps at one-week intervals.

Each step includes two manoeuvres: one at pericentre to raise the apocentre altitude, and one at apocentre during the next orbit to lower the pericentre altitude (for more details see below). The combined effect of each pair of manoeuvres will leave the orbital period largely unchanged.

At the start of these manoeuvres the pericentre altitude was ~360 km. Figure 2 shows the planned evolution of the pericentre altitude during the lowering activities. The inset shows the period around the time when the lowest altitude (185 km) will be reached. Also plotted is the expected evolution over the subsequent months, including the first quarter of 2009.

New Opportunities for Science
The lowering of the pericentre altitude greatly extends the science that can be done with the remote sensing instruments on-board Venus Express. The instruments that benefit the most from the closer proximity to Venus are the magnetometer (MAG) and the Analyser of Space Plasmas and Energetic Atoms (ASPERA). Specifically, after the completion of the manoeuvres:

+ energetic neutral and charged particles can be characterised over a wider range of altitudes and in a different environment than was possible up to now.

+ in-situ studies of the environment will be possible well into the ionosphere. (The base of the ionosphere is at an altitude of ~120 km and varies little with time. The top of the ionosphere is formed by the ionopause - the boundary at which the electron density drops significantly. The altitude of the ionopause does vary with time and lies in the range of about 225-400 km.)

+ a better characterisation of the magnetic field can be performed in the north-polar region (where the pericentre of the spacecraft's orbit lies). It will be possible to study the lower part of the externally induced magnetic field (caused by the passing solar wind carrying the interplanetary magnetic field that interacts with the ionosphere). In addition, at the lowest altitudes a search can be conducted for magnetic fields due to a weak dynamo or to other processes related to the interior of Venus.

+ improved characterisation of lightning events can be performed.

Drag experiment
As a result of lowering the spacecraft's altitude at pericentre, the spacecraft will start to experience drag from the upper atmosphere around the pericentre passages. Although very small, the integrated effect of this drag on the spacecraft orbit is expected to be clearly noticeable below an altitude of ~200 km.

The integrated effect of atmospheric drag can be derived from orbit determinations, based on spacecraft tracking data.

To perform these measurements a series of thirteen dedicated passes with the ESA New Norcia ground station and NASA DSN ground stations are scheduled in addition to the normal periods of spacecraft tracking and communication. These extra passes fall in the period between 30 July and 2 August and are performed around the pericentre passages of Venus Express.

At even lower altitudes than those achieved with this pericentre lowering campaign, it would be possible for the spacecraft's accelerometers to directly measure the effects of the atmospheric drag on the spacecraft. This would allow for a more detailed determination of the atmospheric density with altitude. Such operations, however, if performed, still lie beyond the current extended mission (ending May 2009).