European scientists are preparing to monitor the neighbour from hell. Venus Express, a robot spacecraft little bigger than a fridge, is to be the first mission to the second rock from the sun in 15 years.

Venus is 4.6bn years old, of similar diameter and mass to the Earth, and made of the same rocks. It occupies the same neighbourhood and should be warm and welcoming, like Earth. But it is not.

Fred Taylor, of the University of Oxford, said: "It's very disturbing that we do not understand the climate on a planet so much like the Earth. It is telling us that we really don't understand the Earth. We have ended up with a lot of mysteries."

Venus Express will take off on October 26, from Baikonur in Kazakhstan, aboard a Russian Soyuz rocket. A second upper-stage Fregat rocket will push it on to a 162-day trajectory towards Venus. During April the spacecraft will be sent pirouetting as close as 155 miles around its partner, and then soaring 41,000 miles away. It will do this for 500 Earth days.

One day on Venus, which alone of all the planets rotates backwards, lasts 243 Earth days. The planet's air is thick, at 90 times the pressure at sea level on Earth, and there are dense clouds of sulphuric acid. Violent winds race around 50 times faster than the planet spins, and vast hurricanes hang over each pole. The surface is paved with basalt dating from planet-wide eruptions no more than 500m years ago.

There have been more than 30 missions to Venus but the last visitor to stay longer than a few hours was the US orbiter Magellan, in 1990. Scientists over the years have confirmed that the landscape has high domes and rolling plains, with what look like old ocean basins and riverbeds, but there is no liquid water.

The rock of Venus is hot enough to melt lead or tin, so hot that even the dark side of the planet glows faintly through the clouds. There is also a second reflected gleam from the planet's mountaintops, as if they were capped by a kind of metallic " snow". But right up till the arrival of the first Russian and US spacecraft in 1959 and 1962, scientists and fiction writers imagined Venus had tropical rainforest.

It is not known why there is a mix of sulphuric acid, water vapour and carbon dioxide - but the atmosphere amounts to a shining example of the greenhouse effect. After the moon, Venus is the brightest thing in the night sky, so bright it can be seen sometimes by daylight. But though 90% of the sun's radiation on Venus bounces back to space, 10% of it is trapped, which racks up the temperature to incinerating levels.

David Southwood, director of science for the European Space Agency, said: "For me Earth and Venus were born as alike as twins ... if you had seen the solar system half a billion years into its life you would have seen two planets form and you would not have known which would be the habitable one."