8 years, 10 billion km, one shot: The spacecraft racing toward Mercury faces its trickiest hour yet

A spacecraft carrying European and Japanese probes will complete an eight-year journey to arrive at Mercury on Thursday, though it still faces high-risk manoeuvres to study the Sun-scorched planet, AFP reported.

The BepiColombo mission, which set off from Earth in 2018 on a winding path to study the smallest and least understood planet in the Solar System, must now begin a months-long arrival phase that the European Space Agency (ESA) has described as one of the most operationally challenging planetary arrival sequences ever attempted.

"It's a very ambitious mission. Mercury is a challenging destination... because of the extreme environment so close to the Sun," Santa Martinez, the mission manager at ESA, said at a press conference ahead of the arrival.


Mercury's relatively tiny mass, only slightly larger than the Moon, means its gravitational pull is extremely weak compared to the Sun, making it complex for spacecraft to approach the planet without being incinerated or lost in space. The spacecraft had to travel more than 10 billion km, executing a series of nine flybys to speed up, slow down and perfect its trajectory.

The two probes, currently attached to each other, will separate on Thursday at 1200 GMT from the transfer module that has propelled them. "It's going to be equivalent to launching a new spacecraft. Only this spacecraft happens to be around a different planet," said Ignacio Tanco, ESA's head of inner solar system mission operations.

From April, ESA's Mercury Planetary Orbiter (MPO) will begin studying the planet closest to the Sun with unprecedented precision, passing as close as 480 km from its surface, while the Japanese Mercury Magnetospheric Orbiter (Mio) will study its space environment and magnetic field from an elliptical orbit extending as far as 11,000 km away.

On Thursday, Mercury will be around 63 million km from the Sun and 200 million km from Earth. At that distance, real-time operation is impossible, with about a 30-minute delay between checks performed on the ground and commands reaching the spacecraft, according to AFP. This means the spacecraft must check and execute the separation autonomously.

"It's a tricky business," Tanco said, adding that if all goes to plan, spring mechanisms will release the probes, pushing them away from the transfer module at a few centimetres per second, without any explosion or blast. Many components, including thermal controls, propulsion and altitude systems, are being tested for the first time in this environment.

"So, what can go wrong? It's probably easier to list the things that cannot go wrong, which are very few," Tanco said. "But of course, we are ready, we are flexible, and we need to be prepared for the unexpected."

The mission has already faced hurdles, including a 2024 thruster glitch that caused the spacecraft to fly by closer than planned, delaying its overall trajectory. The next critical phase will arrive in December, when the two probes separate into their respective orbits.

Mercury remains the least studied of the four rocky, innermost planets in the Solar System, which also include Venus, Earth and Mars. NASA's Mariner 10 captured the first close images of its lunar-like surface in 1974, and the MESSENGER probe orbited the planet in 2011, though much remains unknown about its magnetic field, iron core and surface hollows.

With sixteen instruments and a budget of 1.7 billion euros, BepiColombo "is going to bring to humanity and to the scientific community unprecedented views of Mercury... and it's going to open a new chapter in our understanding of the solar system," Martinez said.

(With inputs from AFP)