Venus Express – answering the riddle of our neighbour planet

© ESA

Venus Express is the first European mission to the planet and will be the first space probe to perform a global investigation of the Venusian atmosphere.

Unveiling the mysteries of the cloudy planet

Scientists are hoping that data collected by Venus Express will help them to make sense of this baffling planet.

Until advances in space exploration technology allowed probes to get close enough to the planet to start collecting real data, in the early 1960s, Venus was believed to be a sort of ‘sister planet’ to the Earth – both are of a comparable size, with similar densities and chemical compositions, and are close neighbours. There were even rather far-fetched notions of a moist tropical environment harbouring the possibility of some kind of life… However, the earliest research, penetrating the swath of dense clouds which surround the planet, quickly revealed that Venus is a world apart. It may in fact be the most inhospitable place in our solar system.

Venus is covered by a thick layer of sulphuric acid clouds, starting at about 50 kilometres above the surface, which in the upper part is whipped by winds of up to 400 kilometres an hour. Its atmosphere is composed of around 96% carbon dioxide, with the remainder mostly nitrogen. Below, on the surface, there is no evidence of water, a crushing atmospheric pressure 100 times that at sea level on Earth, and a roasting surface temperature of 750K (500°C), hot enough to melt lead. It could hardly be more different to the Earth. In addition, Venus rotates very slowly – a day on Venus corresponds to 243 Earth days – in the opposite direction to every other planet in the solar system except Uranus, and has a very weak magnetic field. Our neighbour planet clearly presents plenty of intriguing riddles to which Venus Express will be seeking to provide answers.

Lessons for Earth?

With the data obtained, scientists will be trying to make sense of the planet, its structure and dynamics, and looking for clues as to its evolution. A fascinating adventure in space, which could well have repercussions closer to home, as it is hoped that the data collected by Venus Express will also give clearer insights into our own planet’s long-term climatic processes. One particularly interesting area of investigation is the ‘runaway greenhouse effect’ of Venus’ climate – massive proportions of carbon dioxide and sulphur dioxide absorbing IR radiation, trapping solar energy and raising temperature – especially since it is possible that Venus once had oceans which would have been boiled away by this effect. The fact that Venus’ atmosphere seems to epitomise our worst fears of ‘environmental disaster’ is an issue of concern here on Earth, and one which is certainly worth examining more closely.

(c) EADS Astrium

A key feature of the Venus Express programme was the fullest re-use of expertise and technology inherited from ESA’s successful Mars Express and Rosetta missions (both primed by EADS Astrium), to maximise efficiency, reliability and value. Thus spacecraft design is the same as that for Mars Express, and six of the instruments are drawn from those missions. Nevertheless, certain aspects of the spacecraft, in particular the thermal aspects, had to be specifically tailored to the demands of the mission and its Venusian environment.

The solar array needs to withstand the extreme temperatures which will be encountered during the mission since Venus is the second closest planet to the Sun. Therefore, three quarters of the solar panels’ surface is covered in special mirrors, known as Optical Surface Reflectors, with the remainder consisting of Gallium-Arsenide cells which have a considerably broader operating temperature range (-167°C to +158°C) than the silicon ones used for the Mars Express solar array, and deliver power of 821 watts in the Earth’s orbit and 1,468 watts in the Venusian orbit at end of mission.

Whilst the chemical propulsion system on Venus Express is almost identical to the one used on Mars Express, the challenge here lay in adapting its performance to the mission. The sun's gravitational pull is around eight times stronger at Venus than at Mars, so the probe had to be provided with sufficient fuel to perform the crucial deceleration for orbit insertion. Venus Express is carrying about 20% more propellant than Mars Express – at 570kg this constitutes almost half the total spacecraft mass. Nearly three quarters of this propellant was used for this single 53-minute manoeuvre for Venus orbit capture. It was the longest ever performed in operation by this engine – but the team's confidence has been fully justified by spacecraft's successful entry into orbit around Venus!

Due to Venus’ orbit around the sun, when communicating data to Earth different sides of the spacecraft will face the sun at different times (unlike Mars Express where the same side of the spacecraft always faces the Sun), which posed a problem in maintaining the ‘cold face’ of the spacecraft in shadow. An additional high gain antenna, facing in the opposite direction to the primary antenna, was therefore added. Venus Express’ two antennas will be used interchangeably according to the spacecraft’s orientation towards the sun.