A European and Japanese spacecraft is set to begin an epic five billion mile journey to the planet Mercury tomorrow.

The BepiColombo spacecraft is due to be launched from the European space port at Kourou in French Guiana, at around 2.45am.

The spacecraft will take seven years to reach the planet closest to the sun.

In 2025 it will place two probes, one European and the other Japanese, in orbit around Mercury, the least explored planet in the solar system.

A number of Irish companies have been involved in the development of the spacecraft over the past decade.

Scientists hope the €1.5bn mission will unravel some of Mercury's mysteries, such as the reason for its oversized iron core, its spectacular volcanic vents, and hints of water ice in shadowy parts of the scorching hot planet.

The answers they get will shed new light on the origins and evolution of the solar system.

A key feature of BepiColombo is that it is the first interplanetary mission to employ advanced electric ion propulsion technology.

Four "Star Trek-style impulse engines", two firing at a time, will emit beams of electrically charged, or "ionised", xenon gas.

They will be used not to accelerate the craft but to act as a brake against the sun's enormous gravity.

A complex series of fly-bys past the Earth, Venus, and Mercury will also help to reduce BepiColombo's velocity by 7km/s.

At top speed after launch, the spacecraft will be moving at 60km/s.

An Ariane 5, ESA's most powerful rocket, will blast BepiColombo onto an "escape trajectory" that will free it from the shackles of Earth's gravity immediately.

One of the biggest challenges for mission planners was ensuring the spacecraft could withstand searing temperatures of more than 350C so close to the sun.

Protective measures include a heat shield, novel ceramic and titanium insulation, ammonia-filled "heat pipes", and in the case of the Japanese orbiter, "roast-on-a-spit" spinning.

A suite of 11 instruments on the MPO will map the surface of Mercury and probe its chemical composition for up to two years.

Meanwhile, the Japanese space agency Jaxa's Mercury Magnetospheric Orbiter will focus on the planet's unusual magnetic field.

Only two spacecraft have previously visited Mercury.

NASA's Mariner 10 flew past the planet three times in 1974-75 and the American space agency's Messenger probe orbited Mercury from 2011 to 2015.

BepiColombo was named after the late Giuseppe "Bepi" Colombo, an Italian scientist and engineer who played a leading role in the 1974 Mariner 10 mission.

Speaking ahead of the launch, ESA director general Professor Jan Woerner, said: "For me, science is the basis of humankind because it teaches curiosity.

"It is fulfilling this quest for information, and at the end of the day we don't know what we will find. This is really an impressive scientific mission.

"We are doing science, we are doing technology, we are doing international co-operation, and we are doing something for society."

Asked if ESA was competing with NASA, he said: "There is no competition, at least for me.

"Competition is a driver for sure, but co-operation is an enabler. I think the race in space is over."

Among those involved in the development of the BepiColombo spacecraft was the Dublin office of Nammo, a Norwegian company involved in producing space rocket propulsion components.

Staff in Ireland worked on the project management of the delivery of the thruster for BepiColombo in 2010.

They were also responsible for the manufacture of a number of small components in the thruster valve assembly and for carrying out testing of parts.

A second Irish company, CAPTEC, has also been involved in the BepiColombo project since 2010.

It worked on the central database, integrating the data used by all the software components of the spacecraft. 

The Malahide-based firm also helped develop the software for the new Electrical Propulsion System - an ion drive that will propel the craft between planets, replacing the traditional chemical rocket.

This will be the first time such a drive has been used for an interplanetary mission.

CAPTEC engineers are travelling to the European Space Agency's control centre at Darmstadt, Germany, today to attend the launch tomorrow morning.

Additional reporting Will Goodbody