Science during aerobraking

Today’s post highlights the continuation of science data gathering during the demanding aerobraking campaign, which formally gets underway today. This was contributed by Donald Merritt, the Venus Express Science Ground Segment Manager. Donald works in the VSOC – the Venus Express Science Operations Centre – at ESAC, Spain.

Performing aerobraking with the Venus Express spacecraft puts a great deal of stress on the spacecraft, as well as on the flight control team operating the spacecraft from the European Spacecraft Operations Centre (ESOC) in Darmstadt, Germany. Because of the enormous workload on the team during this very unusual activity, the original plan was to turn off the science payload during aerobraking. Payload operations can be time consuming: instrument commands must be checked on the ground, loaded into the ground system, uplinked to the spacecraft and checked after execution – and then the data must be distributed. And this would need to be done at a time when the spacecraft was being operated in a truly novel way with a real level of danger for the spacecraft.

Payload commanding is coordinated and checked by the Venus Express Science Operations Centre (VSOC), located at ESA’s ESAC Establishment, near Madrid, Spain, and the VSOC team understands the many constraints imposed by spacecraft operations. However, given the unique science information that might be obtained during the Venus Express aerobraking period, the flight control team at ESOC agreed to support the operation of a minimum set of instruments during this very busy – but potentially valuable – campaign.

The Venus Express spacecraft has six active science instruments; VSOC worked with the flight control team to determine what science was possible, given the details of the aerobraking operations. Like most spacecraft, there is a radio science experiment, which employs the Venus Express high-gain antenna and sensitive electronics in using radio waves for scientific investigations. However, radio science operations would not be possible because of the required orientation of the spacecraft. So, scratch that idea.

Three other instruments also require specific spacecraft pointings to be useful. The VIRTIS imaging spectrometer, Venus Monitoring Camera and the SPICAV spectrometer must be pointed at the desired target in order to record data. And since they are all mounted on one face of the spacecraft, that face would have to be kept pointing toward the target.

However, during the aerobraking phase, there is no way to maintain this. The pointing of Venus Express during the active aerobraking phase of each orbit, as we descend into the upper atmosphere, must be in a very specific orientation and this precludes directing the instruments toward the target. And during the remainder of each orbit, the configuration of the spacecraft would not allow the instruments to be pointed at any useful targets.

Finally, the instrument planning is generally done months in advance, and there was too much uncertainty about the state of the spacecraft that far into the future. So, scratch use of those instruments, too.

That left what are known as the in situ experiments, which do not require any special pointing and are used to make measurements of the environment directly around the spacecraft.

Venus Express has two of these: the Magnetometer, used to measure the magnetic field through which the spacecraft is passing; and the Aspera-4 plasma and particle detector. Operations for these looked possible.

But this meant that the VSOC and the instrument teams [Each instrument has its own separate team of scientists, based at the institutes that have provided the individual instruments, who support its operations – Ed.] would have to figure out a way to take useful science data in such a way as to minimize interference with the aerobraking operations. And both instrument teams have done just that.

Aspera

The Aspera instrument is normally operated near pericentre (point of lowest passage above the surface during any particular orbit), and then turned off. And it is again operated near the apocentre (highest point) of the Venus Express orbit, at around 66 000 kilometres from the planet’s surface. The instrument planning had to be done months in advance, with no clear idea of what the orbit period would be at any given time nor what other activities might be under way due to the aerobraking.

VSOC proposed that the instrument be operated in blocks of slightly less than four hours; those blocks would be distributed continuously around the orbit during the advance planning. Then if any block of operations was in any way a problem, either for the instrument or for the flight control team, the blocks could be removed completely. With this modular approach, the orbits would be covered by blocks, and any block could be removed at any time, for any reason – which would leave as many blocks as possible in order to acquire the most science data possible. After iterations with the Aspera team, this was agreed and is being implemented.

Magnetometer

The ‘Mag’ normally runs continuously. VSOC and the magnetometer team thought it would be possible to also run it this way during aerobraking. There would be no need for commanding; Mag would just keep running as normal. We would just need to operate it in such a way that the time to download the data would not put a burden on spacecraft operations, as was also being done with the Aspera instrument.

Months in advance of the start of aerobraking, these plans were put into effect and the instrument commands were created, tested and provided to the flight control team.
But this is science, and there are always surprises.

By its very nature, the Venus Express team is working in unknown territory here. And we have recently learned that we had to change some of the instrument operations. Because the solar panels must be oriented for maximum drag during the descent into the atmosphere in each orbit, the panels are not oriented to directly face the Sun. Depending on the angle of the sunlight falling on the solar panels, this has the possibility of reducing the energy output from the panels and puts a big strain on the power system of the spacecraft. As the aerobraking began [The aerobraking passage as part of the ‘walk-in’ phase began on 20 May – Ed.], the response of the power system to the aerobraking was still uncertain. Therefore, everyone agreed to turn off the Aspera and Mag instruments for the first week or so of the aerobraking period until the situation could be better understood.

After those first few orbits, it was determined that there is sufficient power to operate the instruments, and their operations were initiated. But that this will probably change in the future, as the angles between the spacecraft and the Sun change as Venus rotates around the Sun in its orbit.

VSOC also needed to modify the Mag commanding, too. As with the Aspera operations, the Mag operations have been broken into discrete blocks, but only two: those that take place during that part of the orbit when the spacecraft is configured for aerobraking and passes through the atmosphere, and those that take place during the rest of each orbit. In this way, when the geometries change and there is not enough power from the solar panels to operate during the active aerobraking phase, the magnetometer operations at that time can be easily removed in order to increase the power margin on the spacecraft.
Before Venus Express, only one other spacecraft has ever performed aerobraking at Venus. NASA’s Magellan spacecraft did it many years ago, but it had no instruments on board that could take science data during the passage through the atmosphere. Thanks to the hard work of the Venus Express spacecraft and instrument teams, we will get the first such science data during this unprecedented opportunity.

Just as they have done for eight years, the Venus Express flight control team has done whatever they could in order to maximize the science return from the mission.

There’s no way to know for sure what we’ll see. But with science, that is always the case. And that is what makes it so interesting, and so fun.

Editor’s note: The magnetic field of Mars was detected for the first time in 1997 when NASA’s Mars Global Surveyor spacecraft conducted aerobraking at the Red Planet.