Surfing an alien atmosphere

Eight years after arrival at our nearest planetary neighbour and now low on fuel, ESA’s hugely successful Venus Express formally completes its routine scientific mission this week. But the spacecraft’s knowledge gathering is by no means over.

There’s a nice report in the main ESA website today highlighting mission end and profiling the upcoming ‘aerobraking’ campaign (see ‘Venus Express gets ready to take the plunge‘).

What’s ‘aerobraking’, you ask?

In a nutshell, aerobraking means lowering the spacecraft so that for part of each orbit it dips down very low and skims through the very upper-most reaches of the Venusian atmosphere. By doing so, the spacecraft ‘feels’ drag due to the gases at those altitudes, which can provide valuable data on the properties of the atmosphere and how the spacecraft and its components respond to such a hostile environment.

Think of it as a way to use the spacecraft to wring out every last possible drop of benefit from the mission: science gains additional information on the enigmatic planet’s atmosphere, while the mission operations teams gain significant experience in flight techniques that will be valuable for future missions.

To find out more about the operational aspects of the campaign, I spoke with Adam Williams, the deputy Spacecraft Operations Manager at ESOC.

“We’ve been working for several months to design and test a special low-altitude operational configuration for VEX, called ‘Aerobraking mode’, and we’re ready to begin,” he says.

The aerobraking campaign proper runs from 18 June to 11 July; the mission control team will start the so-called ‘walk-in’ phase right after the end of nominal science operations, i.e. starting next week. During the preparatory walk-in phase, the pericentre of the VEX orbit (the point of closest passage over the surface of Venus) will be steadily lowered, from the current 197 km to about 130 km.

“For us, 190 km is the lowest ‘routine’ altitude; anything lower than that requires special and very careful flight control,” says Adam.

This chart shows the planned evolution of the Venus Express pericentre (point of closest approach to surface) altitude during the aerobraking campaign. Altitude shown in km. Credit: ESA

He stresses that it’s very possible that the remaining propellant will be exhausted during aerobraking, or that the spacecraft will not survive these risky operations, which involve extremes of temperature, and severe power constraints.

If the spacecraft is still healthy after 11 July, its orbit will be raised again and limited operations and ‘bonus’ science will continue for several more months until it inevitably does run out of propellant by year’s end.

Why wouldn’t it survive?

Well, if it runs out of propellant, there will be no way to raise its pericenter altitude, and on each orbit, currently every 24 hours, the effects of solar gravity would pull it down lower and lower, until it would burn up (which is how the mission will end in any event).

Or it may suffer damage to its High Gain Antenna, which is the only means of communicating with the spacecraft at the distance it is from Earth at the moment (around 164 million km). Or the power subsystem may be affected by the intense use made of the batteries during the aerobraking operations.

The value, though, is clear: the campaign provides an excellent opportunity to develop and practice sophisticated automated flight control techniques and learn how to programme ‘low’ altitude operations – which will be invaluable for the preparation of future planetary missions.

The aerobraking orbits will take place with the spacecraft oriented in a special aerobraking attitude, to ensure dynamic stability as it passes through the atmosphere. This means the antenna is pointed away from Earth, so no communications will be possible until after each low-level pass. Two hours after each pass (this period will change as aerobraking progresses), the team at ESOC will download telemetry data via ESA’s 35m Cebreros ground station in Spain, which will tell how the pass went and whether there have been any effects on – or damage to – the spacecraft.

“Our first orbit in aerobraking mode orbit will take place next Tuesday, and we’re looking forward to an exciting ride,” says Adam.