Atmospheric drag experiment campaigns
Venus Express is in a highly eccentric orbit that takes it out to about 66 000 km from the planet's surface when at apocentre, and as close as 250 km when at pericentre. This eccentric orbit is also polar, and takes the spacecraft directly above certain regions of Venus's upper atmosphere about which we have limited knowledge.
Scientists have devised Atmospheric Drag Experiment (ADE) campaigns to exploit the proximity to Venus's extreme upper atmosphere afforded by Venus Express's eccentric orbit. ADE campaigns have been carried out regularly with Venus Express since 2008.
ADEs use the entire spacecraft body and its attitude-sensing gyroscopes as an instrument to detect the drag of the atmosphere, hence determining its density by its integrated effect on the spacecraft's attitude and orbit position.
Remote sensing observations conducted routinely with Venus Express only target the lower atmospheric layers; the extreme upper layers of the atmosphere are inaccessible during routine observations. ADEs specifically target the planet's lesser-known extreme upper atmosphere — about 180 km above the surface — particularly in the polar regions. The pericentre of Venus Express's orbit is located almost precisely above the planet's North Pole. The drag measurements obtained with the ADE provide a direct estimate of the local atmospheric density in this region. A model of the Venusian atmosphere density is updated after each ADE.
Drop in pericentre altitude
Each ADE campaign lasts for a few consecutive days, and is scheduled around a season when the spacecraft's pericentre altitude decays slowly – by less than 1 km/day. These changes in the altitude of the pericentre are caused by a decrease in spacecraft speed at the apocentre; the spacecraft moves slowly in its orbit because of its great distance (66 000 km) from the planet.
At Venus' distance from the Sun, the constant pressure from the solar radiation on the spacecraft is noticeable. When the direction of the solar radiation directly opposes the direction of the spacecraft motion in orbit, the pressure on the spacecraft reduces the orbital speed; it is similar to riding a bicycle directly into a headwind. The reduced orbital speed further lowers the spacecraft altitude by the time it reaches the following orbital pericentre. To keep the altitude within the nominal range, regular firings of the spacecraft engine (Orbit Correction Manoeuvres) become necessary.
While in orbit over the terminator (the line dividing the night side from the day side) the solar pressure on the spacecraft is perpendicular to its orbit and has a relatively diminished effect on the spacecraft speed. As with the bicycle analogy, it is easier to pedal with the wind on your side than when it is blowing directly against your direction of travel. For the spacecraft, this means that the decay in the altitude of the pericentre is reduced during the periods when the spacecraft is around the terminator orbits; plots of the pericentre altitude versus time show a definite 'leveling out' or plateau for those periods.
|Change in Venus Express's pericentre altitude with time, starting from 13 November 2011 (orbit number 2032) and ending on 4 February 2012 (orbit 2115). Credit: ESA|
Other factors exist that continue to reduce the pericentre altitude but these affect the spacecraft orbit far more slowly. The usual engine firings that are performed to raise the pericentre altitude above the nominal 250 km limit are delayed, and the pericentre altitude is allowed to slowly and safely drop, and then lightly touch the very upper reaches of the Venus' atmosphere. The amount of drag experienced by the spacecraft during the ADE is significantly less than the drag that would be experienced in an aerobraking orbit change. The amount of drag that the spacecraft experiences is maintained at a level that is known to cause no problems to the spacecraft.
Pre-ADE torque experiments
|The torque experiment on Venus Express. Credit: ESA. Click here for more information on this video.|
A pre-ADE campaign is carried out prior to each ADE campaign. Density trends at very high altitudes are measured during pre-ADE campaigns to ensure that the prevalent atmospheric densities are within the expected range (as predicted by various atmospheric models). The safety of the spacecraft is paramount in every situation, and these early checks confirm that there are no surprises ahead as the pericentre altitude approaches the region where the data are expected to be most useful. Once the pericentre height drops below 190 km, high-rate reaction wheel telemetry is recorded with a sampling rate of 8 times per second in a window of ± 3 minutes around pericentre. The solar arrays are also rotated asymmetrically to increase the spacecraft cross-section and induce a slight roll in the spacecraft attitude. The reaction wheels respond to the very low roll rate by inducing a counter-roll, and the change in the wheels speeds are used to detect the pressure on the spacecraft due to the atmosphere. This is called a 'torque experiment'.
At the start of the ADE campaign, the solar arrays are rotated and positioned asymmetrically to create aerodynamic torque and induce roll – as done for the pre-ADE checks. However, the solar panel orientation is more extreme in order to generate more roll in the spacecraft attitude. As before, the reaction wheel telemetry is recorded at high sampling rates (8 times per second) but for an extended time window of ±15 minutes around pericentre passage.
In addition to the torque experiment, the ADE campaigns use ground stations to track the minute changes in the velocity of the spacecraft. These Doppler shifts can then be used to reconstruct the spacecraft orbit with high precision, and to disentangle the drag effect from gravitational effects and other forces. The spacecraft is pointed to Earth and the transponder is enabled with a constant, stable signal. Normally, telemetry is transmitted to Earth by modulating, or changing, this downlink signal. For the ADE however, no telemetry modulation is allowed during the dedicated tracking period reserved for the experiment. The variations in spacecraft velocity can be detected in the changes to the transponder signal measured using either the NASA Canberra or ESA New Norcia stations in Australia; both of which are in view of Venus Express during pericentre passage. These ground stations normally have two hours of visibility scheduled during the ADE passes.
The ADE campaigns require intense effort from the Flight Control and Flight Dynamics Teams. Even at high altitudes, the Flight Control Team closely monitors the spacecraft because of its unusual orientation during the pre-ADE and ADE campaigns. For each day of the campaigns, the Flight Dynamics Team calculates a unique escape manoeuvre to be used to immediately raise the spacecraft altitude in case it experiences problems and drops lower than was planned during the ADE pass. The Flight Dynamics Team also calculates the solar array rotation angles every day of the drag experiment to generate the necessary dynamic pressure while remaining within a safe range. Both the size of the rotation, and the relative aspect of each solar array to the incoming atmospheric flow, is determined daily on the basis of expected atmospheric density.