Only one American probe has so far approached Mercury, the closest planet to the Sun, marked with high temperatures and strong radiation. The dense, rocky planet could hold the key to understanding how the solar system was formed.

The mission was endorsed as "appropriate" on Wednesday by a subgroup of specialists at the Space Activities Commission, said an official of the education and science ministry which oversees Japan's space programme.

"The commission is expected to give formal approval in mid or late July," said another official at the ministry's space policy planning division.

"It will then apply for budgetary allocation in the year to March 2005 to finance research as well as production of test models," he added.

The Japan-Europe project will be the first full-scale exploration of Mercury and will use a surface lander and two orbiting units, the officials said.

The Japanese and European space agencies plan to use Russian Soyuz rockets to send the probes between late 2010 and early 2011. The probes are expected to reach Mercury in 2014.

The US probe Mariner-10 orbited the planet in 1974 and 1975 and the US National Aeronautics and Space Administration (NASA) plans to send another orbiter, Messenger, in March 2004. Messenger is expected to reach Mercury in five years.

The officials said the Japanese government was expected to spend as much as 13.5 billion yen (114.4 million dollars) on the entire project, dubbed "BepiColombo" after the Italian physicist who pioneered studies on Mercury.

The European Space Agency is expected to pay around 60 billion yen, the officials said.

Japan will provide an orbiter to study the magnetosphere field of Mercury. The two other probes will be produced by the European agency.

Mercury will be the third planet after Mars and Venus to be targeted by Japan's planetary exploration projects.

In 1985, Japan sent a probe Sakigake (Forerunner) to observe the magnetic field of the Halley Comet within a range of 700 to 150,000 kilometers (438 to 93,750 miles).

In 1998, the country launched Nozomi (Hope) on a mission to Mars. But due to technical glitches the probe will only reach Mars by the end of this year, four years behind schedule.

Japan also has a project to send an orbiter to Venus but its details are yet to be worked out.

Mercury remains an enigma some 30 years after the Mariner-10 mission. The probe could only image about half of the planet's surface during its three flybys before it lost contact with the Earth.

SPACE.WIRE