Eons ago, Venus may have been the gentle, tropical paradise that Earthlings once imagined. It was closer to the sun — but not too close. It was almost Earth-size — but not quite. And it had plenty of water, even oceans.

But that was then. Sometime in the distant past, the oceans started to heat up and then boiled away. The water vapour hung over the planet like a glove, trapping the heat below and creating a berserk greenhouse effect.

Today, Venus' atmosphere is 97 per cent carbon dioxide, and the planet is wreathed in clouds of sulphuric acid. The planet is apparently condemned to an eternal cycle of global warming, with surface temperatures that hover around 480C.

There are, perhaps, lessons to be learned here. "Venus is very unpleasant," says Hakan Svedhem of the European Space Agency. "We know the greenhouse effect on Earth is a very interesting topic. Maybe with Venus, we can better understand how our own atmosphere works."

On Tuesday, ESA's Venus Express, a honeycombed aluminum spacecraft carrying seven instruments and cloaked in a metallic gold polymer to fend off the heat, successfully completed a 51-minute rocket burn that put it into an elliptical polar orbit. For the next 500 days, with the possibility of extending for another 500, the spacecraft will probe mysteries that have confounded and fascinated scientists since exploration of the planet first began with NASA's Mariner 2 in 1962.

Chief among them is what happened to turn Venus into a child's vision of hell, with a superheated toxic soup of an atmosphere that is about 90 times denser at the surface than Earth's — about the same pressure as the ocean at a depth of nearly a kilometre.

NASA's Magellan mission, which ended in 1994, used radar to penetrate the cloud cover and map Venus' tortured surface, paved with lava flows and pocked with craters and volcanic mountain escarpments. Venus Express, by contrast, is "geared toward a very detailed study of the atmosphere," says ESA's Don McCoy, the project's manager.

There is a lot to understand. Measurements taken by early probes of Venus have made scientists all but certain that the planet once had extensive oceans that heated up and finally boiled off.

Quite probably the resulting cloud of water vapour provided the initial atmospheric blanket that turned the planet into a hothouse. "But where did (the water) go?" asks University of Michigan planetary scientist Stephen Bougher. "Nobody knows."

Heat could break the water into its constituent atoms, and the hydrogen could easily evaporate from the upper atmosphere and escape into space, but "something different" had to have happened to the heavier oxygen, Bougher says. One possibility is that a magnetic field induced by the solar wind may have swept charged oxygen particles away from the planet, he says.

Venus Express has an instrument that can measure atmospheric erosion and perhaps provide data that will help scientists reconstruct how Venus lost its oceans.

Over time, carbon dioxide replaced the water vapour, probably as a result of the erupting volcanoes that resurfaced Venus about 700 million years ago and spewed clouds of sulphurous gas into the atmosphere.

Venus Express will search for traces of sulphur dioxide, an indication that volcanoes have been recently active. Scientists may also be able to deduce whether volcanoes are still erupting by using the spacecraft's infrared camera to penetrate the atmosphere and take pictures of Venus's surface.

"By correlating these images with the Magellan data, we can tell whether vulcanism today is altering the landscape," says NASA scientist Adriana Ocampo, the agency's liaison with Venus Express. "This is a key question for Venus, and could be important in understanding climate change on Earth."

Another puzzle that has mystified scientists for decades is Venus's winds, which are negligible on the surface but reach speeds of about 350 km/h in the upper atmosphere, much faster than the planet rotates.

"There is no reason for the wind," says Svedhem, lead scientist with Venus Express. The spacecraft will measure wind speeds at various altitudes and correlate them with temperatures. The spacecraft will also gather data on the whirlpool-like atmospheric vortices at Venus's poles, another phenomenon that has no explanation.

"It's really embarrassing how little we know," Bougher says. "The cloud-top winds are so strong on a planet that rotates so slowly. Why?''

The $305 million Venus Express launched from Baikonur Cosmodrome, in Kazakhstan, on Nov. 9 last year. It was modelled on ESA's Mars Express, currently in orbit around Mars, and has some instruments that are identical to those on both the earlier spacecraft and on Rosetta, an ongoing ESA mission to a comet.

But unlike Mars Express, built to absorb warmth from a distant sun, Venus Express must shed heat. Besides its reflective coating, it has solar panels that are tiny, compared with those of Mars Express, and half the panels are mirrors. Getting electricity to operate the spacecraft, "is one of the easier aspects of the mission," McCoy says.

Venus Express weighs 1,200 kilograms fully fuelled and travelled about 400 million kilometres to reach Venus.

It was moving at a speed of 28,800 km/h relative to the planet but slowed by 15 per cent of its velocity so it could be captured into orbit. The initial ellipse extends from 248 kilometres above Venus' surface to 328,000 kilometres away, and the spacecraft will need a few more days to complete an orbit. McCoy said engineers will tighten the ellipse to a maximum distance of 65,000 kilometres with two more rocket burns and settle into a 24-hour orbit in about two weeks.

The Washington Post