The Ariane 5 vehicle on the launch pad in Kourou, French Guiana, with its BepiColombo payload.
In a statement after the launch, ArianeGroup said the satellite had successfully escaped Earth's gravity field and was beginning its long journey where it will reach speeds of up to 40,000 kilometers an hour.
Following a seven-year voyage to reach Mercury's orbit, the spacecraft stack will separate.
A complex series of fly-bys past the Earth, Venus, and Mercury will also help to reduce BepiColombo's velocity by 7km/s.
BepiColombo is a joint project of the Japanese aerospace exploration Agency (JAXA) and European space Agency (ESA).
Scientists hope the mission will tell them more about how the solar system was made and give them a better idea about the potential for life on exoplanets, which are planets found orbiting other stars.
BepiColombo, named after renowned Italian scientist Giuseppe "Bepi" Colombo, is taking a convoluted route to Mercury, flying past the Earth once, Venus twice and Mercury six times before releasing its orbiters to study the latter planet in more detail.
With the journey totalling some 8.5 billion kilometres - the equivalent of travelling from the Earth to Neptune and back again - Bepi will actually travel 38 times the maximum distance between the Earth and Mercury. This complicated trajectory is a effect of Mercury's high orbital speed and proximity to the sun; Mercury-bound spacecraft have to thread a needle to reach the planet and avoid getting sucked in by our star's powerful gravity.
Solving the solar system: Many of the Mercury's features don't match those of our other planetary neighbors. That orbiter carries instruments that are primarily dedicated to the study of Mercury's magnetic field.
A seven-year, eight-million-kilometer voyage expected to shed light on the mysteries of Mercury, the solar system's least-understood planet, is almost ready for takeoff.
Like the Parker Solar Probe now en route to the Sun, BepiColombo features newly developed insulation to protect itself from solar radiation.
"We want to characterize the surface, what material is on the surface, we'd like to measure temperatures, we'd like to see the interaction with the solar wind, which is special on Mercury because it is so close to the sun", he said.
Mark McCaughrean of the European Space Agency says that "no one country can build these missions" on its own and that "critical" collaborations are being broken up by Brexit.
To generate the 10,000 volts of electricity needed to power the ion engines, BepiColombo has a pair of giant solar panels spanning 30 metres that unfold after launch. Both will go into different orbits around the planet.
"This wider array of instruments will enable us to not only make new discoveries, but also review Messenger's data", said Dominique Delcourt, a member of the team running instruments on the Mercury Magnetospheric Orbiter.
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