Spacecraft bound for Mercury begins tricky arrival phase

A spacecraft carrying European and Japanese probes began its months-long, high-risk approach to Mercury on Thursday, after an eight-year journey from Earth. The BepiColombo mission aims to study the smallest and least understood planet in the Solar System. It will eventually release its two probes into orbit around Mercury.

What is the BepiColombo mission's Mercury arrival?

BepiColombo is a joint European Space Agency and Japanese mission that launched in 2018 to study Mercury up close. On Thursday it entered its arrival phase, which ESA describes as one of the most operationally challenging planetary arrival sequences ever attempted. The spacecraft travelled more than 10 billion kilometers, using nine flybys to adjust its trajectory before this stage.

Why is landing a spacecraft on Mercury so difficult?

Mercury is only slightly bigger than the Moon, so its gravitational pull is extremely weak compared with the Sun's. This makes it hard for a spacecraft to approach the planet without being incinerated or lost in space. Santa Martinez, the mission manager at ESA, called it a very ambitious mission during a press conference held ahead of the arrival.

The distances involved add another layer of difficulty. On Thursday, Mercury was about 63 million kilometers from the Sun and 200 million kilometers from Earth. Ignacio Tanco, ESA's head of inner solar system mission operations, said any real-time operation becomes impossible at these distances, since commands take roughly 30 minutes to reach the craft from the ground.

How did the two Mercury probes separate from their transfer module?

The two probes, attached to each other, separated on Thursday from the transfer module that had propelled them, marking a crucial first step in the final approach. Tanco compared the moment to launching a new spacecraft, except this one happens to be around a different planet. Because ground commands could not arrive in time, the spacecraft had to check and execute the separation autonomously, which Tanco called a tricky business.

The maneuver went according to plan. The spacecraft autonomously cut the four mechanical struts connecting the orbiters to the module, and spring mechanisms gently pushed them apart at a speed of 40 centimeters per second. The signal confirming the operation's success took nearly two hours to reach the control center in Darmstadt, Germany, arriving at 1353 GMT and prompting applause from the team. Elsa Montagnon, in charge of module operations at ESA, said the telemetry arrived exactly when expected and that the spacecraft completed its reconfiguration as planned.

What happens next for the BepiColombo probes?

The probes are due to enter Mercury's orbit on November 21. The next tricky phase comes on December 9 and 10, when the two probes separate into their own individual orbits around the planet. From April, ESA's Mercury Planetary Orbiter will begin studying Mercury with unprecedented precision, passing as close as 480 kilometers from its surface.

The Japanese Mercury Magnetospheric Orbiter, known as Mio, will focus on Mercury's space environment and magnetic field. It will follow an elliptical orbit that carries it as far as 11,000 kilometers from the planet. Martinez said the mission's 16 instruments and 1.7-billion-euro budget will bring unprecedented views of Mercury and open a new chapter in understanding the Solar System.

Which spacecraft have visited Mercury before BepiColombo?

Mercury is the least studied of the four rocky, innermost planets, which also include Venus, Earth and Mars. NASA's Mariner 10 was the first probe to capture a close picture of Mercury's lunar-looking surface, in 1974. The MESSENGER probe later orbited the planet in 2011, but many mysteries remain about its magnetic field, iron core and surface hollows, which BepiColombo now aims to help resolve.