After an eight-year journey, a spacecraft carrying European and Japanese probes began the monthslong, high-risk approach to Mercury on Thursday to study the sun-scorched planet.

The BepiColombo mission set off from Earth in 2018 on a winding path to study the smallest and least understood planet in our solar system, with the aim of releasing the two probes into its orbit.

On Thursday, it began its arrival phase, described by the European Space Agency (ESA) as its "most operationally challenging planetary arrival sequences ever attempted."

Mercury's relatively tiny mass—it is only slightly bigger than the moon—means its gravitational pull is extremely weak compared to the sun, making it complex for spacecraft to approach the planet without ending up incinerated or lost in space.

"It's a very ambitious mission," Santa Martinez, the mission manager at ESA, told a news conference ahead of its arrival.

The spacecraft had to travel more than 10 billion kilometers (6.2 billion miles), executing a series of nine flybys to speed up, slow down and perfect its trajectory.

The two probes, which are attached to each other, finally separated Thursday from the transfer module that had propelled them—a crucial first step in the final approach.

It was the equivalent of "launching a new spacecraft. Only this spacecraft happens to be around a different planet," explained Ignacio Tanco, ESA's head of inner solar system mission operations, before the separation began.

All went to plan
On Thursday, Mercury was some 63 million km (39 million miles) from the sun and 200 million km (124 million miles) from Earth.

"At these distances ... any real-time operation becomes impossible," Tanco said, explaining that there was about a 30-minute delay between checks being performed and commands being executed from the ground to the craft.

It means the spacecraft had to check and execute the separation autonomously—a "tricky business," Tanco said.

But all went to plan...