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Visualisation of Venus Express during the aerobraking manoeuvre into the atmosphere of Venus. ESA–C. Carreau

Venus Express is ready to dive into a hostile atmosphere

The Venus Express spacecraft has spent eight productive years orbiting the planet Venus and is now ready to take the plunge.

Its orbit is slowly being lowered and from Wednesday it will repeatedly dive in and out of the planet’s dense and harsh atmosphere, travelling deeper than it ever has before.

It’s not known whether the spacecraft will survive the soaring temperatures or if its electrical components will be able to handle the strain.

But whatever the outcome, valuable data will be obtained by the spacecraft’s engineers to help inform future planetary missions, along with a rare opportunity to investigate a previously unexplored territory of Venus.

Time to take a chance

Venus Express is the European Space Agency’s first mission to Venus and is currently the only spacecraft studying that planet, the second from our sun.

It entered orbit in April 2006 for a mission that was due to last 500 days, or just over two Venus days as measured by the planet’s very slow rotation.

After all this time – and a number of successful mission extensions – the spacecraft’s fuel gauge is now close to empty.

So ESA has seized the opportunity to push the spacecraft into new territory.

It will be sent to plough through the atmosphere of Venus to demonstrate the techniques of aerobraking – using a planet’s atmosphere to slow down a spacecraft.

It’s a useful manoeuvre for getting into orbit around a planet without heavy reliance on rocket engines, which would require packing lots of fuel on board and increases the power needed to launch off Earth.

Swinging high and swinging low

It takes 24 hours for Venus Express to complete one highly elliptical orbit. The spacecraft soars 66,000km above the planet’s south pole before swinging around to skim the upper atmosphere just a few hundred kilometres above the north pole.

Bird’s eye view from Venus Express as it buzzes Venus every 24 hours.

After completing 500 days in orbit, Venus Express had mapped the entire planet twice over. The second pass filled in important data gaps missed the first time around and showed how things had changed from one long Venus day to the next.

During this period, the spacecraft maintained a height between 250km to 400km above the north pole. To open up new science opportunities and extend the work of the spacecraft beyond its primary mission, its orbit was cautiously lowered to between 185km and 300km, in the first atmospheric drag experiment (ADE).

This placed Venus Express in reach of the upper atmosphere of Venus. And on August 4, 2008, the spacecraft did indeed feel the effects and experienced a low level of atmospheric drag for the first time.

Pedalling against sunlight

Since then, over a dozen ADEs have been carried out, each one following a similar pattern.

Venus Express is low on fuel because it routinely needs a boost to maintain its orbit. If left alone, the orbit falls into a natural decay which lowers the spacecraft towards the north pole by as much as 4km in each successive orbit.

A number of factors are involved but the main effect is due to solar radiation.

Sunlight is known to exert pressure – it’s the reason why comet tails always point away from the Sun. So when Venus Express travels towards the incoming solar radiation its orbital speed is reduced, just like trying to ride a bicycle into a headwind. At such times, the spacecraft’s orbit drops most quickly towards the north pole.

Venus Express undergoes two ADEs: at the end of August 2012 it swept 170km above the north pole then, after receiving an orbital boost, its orbit decayed once more reaching 165km. ESA

But at other times, Venus Express travels across the solar radiation, or in other words our cyclist pedals against a crosswind. This is relatively easier to manage. The spacecraft doesn’t slow down by as much and its descent towards the north pole is briefly stabilised.

When this occurs, it’s the perfect time to hold off any orbital boosts and allow the spacecraft to slowly and carefully dip into the atmosphere.

The spacecraft is the experiment

The ADEs were perfect dress rehearsals for the major event occurring this week.

During each one, Venus Express dipped into the atmosphere for just six minutes of its 24-hour orbit, and would repeat this for around ten days. On three occasions it dropped as low as 165km above the north pole, reaching into the upper ionosphere.

Venus Express does not include an instrument capable of measuring the density of the atmosphere. Instead it uses its own reaction to the atmospheric drag it encounters.

Venus Express measures the density of the atmosphere by doing the twist.

During the ADEs, the spacecraft’s two solar panels are oriented asymmetrically, so that one faces the incoming stream of atmosphere, while the other is turned by 90 degrees and presents itself edge on.

As it’s buffeted by the weak atmosphere, the spacecraft experiences a torque. Stabilising reaction wheels work to compensate and maintain the spacecraft’s orientation. Just how much adjustment is needed translates into quite a precise estimate of the atmosphere’s density.

Furthermore, the spacecraft’s trajectory is also meticulously tracked so that any changes brought about by atmospheric drag can also be determined and turned into a measure of density.

Australia listens in

In all previous ADEs, the safety of Venus Express has been paramount and the spacecraft was closely monitored by the two ground stations in the right location to track it at those critical moments: ESA’s New Norcia station in Western Australia and NASA’s Deep Space Network in Canberra.

During an ADE, calculations would be made daily to determine the unique escape manoeuvre that would get Venus Express out of a bind and immediately raise its orbit. Venus Express was not allowed to experience levels of atmospheric drag that might comprise its safety.

But that’s about to change – it’s now or never for Venus Express.

Cloudy with a chance of annihilation

Science operations concluded on May 16 and since then the orbit of Venus Express has been left to decay and should stabilise at around 130km by Wednesday, June 18.

If it survives it will be boosted back to an altitude of 475km in mid-July. But that will be it for fuel reserves and a final orbit decay will be inevitable later this year to close out the mission.

The orbit of Venus Express is coasting towards the Venusian north pole. ESA

Interestingly, the spacecraft quickly encountered a denser atmosphere than measured on any ADE in the last three years. While nowhere near critical, the spacecraft was given a final boost to place it on a proper descent deep into the atmosphere.

For the first time, ESA’s Space Weather Coordination Centre, which provides forecasts for Earth and its flotilla of satellites based on solar activity, is now providing space weather updates pertaining to Venus.

Everything is being done to give Venus Express the best chance possible.

Space weather report from June 5, tracking the daily forecast for Venus. ESA

To boldly go

No spacecraft has been in orbit around Venus for as long as Venus Express and in that time it has made some great discoveries.

It found support for fresh volcanic activity, discovered water escaping from Venus’ atmosphere and uncovered evidence for Venusian lightning

It has recorded changes in extreme weather along with tracking cloud patterns, which may well inform climate change scenarios for Earth.

Let’s hope that the spacecraft’s high-gain antenna survives the dive through the atmosphere intact so that wonderful new data has a chance of making it back to Earth.

Updates on the adventures of Venus Express will be posted on ESA’s Rocket Science blog and via twitter @esaoperations.

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