Image caption Akatsuki will try to find active volcanoes on Venus

Japan has had to postpone the launch of its Akatsuki mission to Venus.

Poor weather at the Tanegashima spaceport early on Tuesday morning (local time) prevented the probe from leaving Earth on its H-IIA rocket.

Mission controllers have a window that stretches into early June to get Akatsuki away and on to its six-month cruise to the inner world.

Having studied the forecasts, they will now target a launch at 0658 on Friday local time (2158 GMT Thursday).

The H-IIA vehicle will also deploy a number of piggy-back experiments, including a small satellite to practise the technique of sailing on sunlight.

Once Akatsuki gets to Venus, it will study the planet's atmosphere and hunt for signs of lightning and active volcanoes.

It will be joining a probe from the European Space Agency that arrived at the planet in 2006.

Planetary comparisons

Venus is almost identical in size to our planet, and is thought to have a similar composition. But there the resemblance ends.

A dense, largely carbon dioxide, atmosphere acts as a blanket, trapping incoming solar radiation to heat the planet's surface to an average temperature of 460C (860F).

Surface pressure is about 90 times that on Earth. Several Soviet probes sent to Venus in the 1960s were crushed as they approached the surface.

By studying this hostile world, scientists hope to understand better how a warming future on our own planet might evolve.

"Although Venus is believed to have formed under similar conditions to Earth, it is a completely different world from our planet with extremely high temperatures due to the greenhouse effect of carbon dioxide and a super rotating atmosphere blanketed by thick clouds of sulphuric acid," explained Takeshi Imamura, Akatsuki's project scientist.

"Using [Akatsuki] to investigate the atmosphere of Venus and comparing it with that of Earth, we hope to learn more about the factors determining planetary environments."

Volcano hunt

The thick Venusian atmosphere is opaque to instruments that operate at visible wavelengths and so the Japanese probe carries five cameras that are sensitive in the infrared and ultraviolet parts of the electromagnetic spectrum.

This instrument suite will enable scientists to investigate the clouds layer by layer.

AKATSUKI ('DAWN') VENUS ORBITER Will study atmosphere and surface

Size: 1.0m by 1.4m by 1.4m

Mass at lift-off: About 500kg

5 cameras; 1 radio experiment

Designed for 4.5-year life

Will sit in 300km by 80,000km orbit

The Japanese team wants to get a better understanding of why Venusian weather systems moves so swiftly.

"On Venus, a high-speed wind called super-rotation is blowing all over the planet, in the direction of planetary rotation, with a velocity reaching 400km per hour at an altitude of around 60km from the surface," explained Dr Imamura.

"This wind blows 60 times faster than the planet's rotation, which is very slow (one full rotation takes 243 Earth days). Akatsuki will investigate why this mysterious phenomenon occurs. Another objective is to study the formation of the thick sulphuric acid clouds that envelop Venus, and to detect lightning on the planet."

Infrared sensitivity can also be used to study surface composition.

Akatsuki will use this capability to try to find active volcanoes.

Europe's Venus Express probe recently found lava flows that could have been younger than 250,000 years old.

Solar 'yacht'

Akatsuki's H-IIA rocket carries with it five much smaller satellites, some weighing just a few kilos.

A lot of interest has centred on a solar sail project called Ikaros (Interplanetary Kite-craft Accelerated by Radiation Of the Sun).

Image caption An artist's impression of how Ikaros will look in space

This 320kg, 1.8m-wide, disc-shaped spacecraft will deploy an ultra-thin, ultra-light, membrane.

The pressure of sunlight falling on this structure should drive the disc out to Venus behind Akatsuki.

This type of "solar sailing" technique has long been touted as a means of moving spacecraft around the Solar System, or even just helping conventional satellites to maintain their orbits more efficiently.

The large sail (14m along the square) will also incorporate solar cells to generate power.

The mission team will be watching to see if Ikaros produces a measurable acceleration, and how well its systems are able to steer the craft towards Venus.

Jonathan.Amos-INTERNET@bbc.co.uk