Scientists were counting down Tuesday for the blast off of a Russian booster rocket carrying a European-built probe aimed at exploring the hot, dense atmosphere around Venus.

The launch of the European Space Agency's Venus Express probe is planned for 10:33 p.m. Tuesday EST (0333 GMT Wednesday) at the Russian-leased Baikonur cosmodrome in Kazakhstan.

Venus Express was originally scheduled to go up on Oct. 26, but had to be postponed after checks revealed a problems with the thermal insulation in the upper stage of the Soyuz-Fregat rocket.

Once it separates from the rocket, scientists at ESA's mission control in Darmstadt, south of Frankfurt, plan to make contact with the probe.

"We've just had the pre-launch briefing and everything is in the green zone," said ESA spokeswoman Jocelyne Landeau. "Everything is ready for go."

The mission, Europe's first to Venus, is aimed at exploring the hot and dense atmosphere around the planet, concentrating on its greenhouse effect and the permanent hurricane force winds that constantly encircle it.

In addition, instruments on the probe will try to find an indication whether Venus' many volcanoes are active, and look at how a planet so similar to Earth could have evolved so differently.

"Venus is still a big mystery," Gerhard Schwehm , head of the planetary missions at ESA said Monday.

Not only is Venus the nearest planet to Earth within the solar system, but the two share roughly the same mass and density. Both have inner cores of rock and are believed to have been formed at roughly the same time.

But despite these similarities, the two have vastly different atmospheres, with Venus' composed almost entirely of carbon dioxide with very little water vapor. It is also the hottest surface of all the planets and lacks atmospheric pressure.

The probe, which cost about euro220 million, or $260 million, will take 163 days, or roughly five months, to reach Venus.

As a sister probe to ESA's Mars Express, launched in 2003, Venus Express shares many instruments with that, as well as the agency's Rosetta probe , launched last year and bound for a comet.

It is expected to begin its experiments in early June.

The last mission to Venus was Magellan, launched by NASA in 1989. It completed more than 15,000 orbits around the planet between 1990 and 1994.

Using radar, Magellan was able to map virtually all the surface of Venus, revealing towering volcanoes, gigantic rifts and crisp-edged craters.