On Friday night, October 1st, BepiColombo will finally reach Mercury – and shoot right past it.
BepiColombo, a European-Japanese collaboration, will ultimately whizz by Mercury five times before finally entering orbit on its sixth encounter more than four years from now. Friday’s encounter, at 23:34 UTC, will bring the spacecraft within 200 kilometers (120 miles) of the planet, giving a few of its instruments their first chance to study the innermost planet. More importantly, the close flyby will bend the path of BepiColombo’s orbit, setting up future close passes that will sequentially match the spacecraft’s orbit with Mercury’s.
BepiColombo is a stack of three spacecraft: a European-built Mercury Planetary Orbiter (MPO), a Japanese-built Mercury Magnetospheric Orbiter named “Mio,” and a European-built Mercury Transfer Module that propels them. The transfer module has huge solar arrays to power a solar-electric propulsion system, which is highly efficient but provides very low thrust. Once in Mercury orbit, the stack will separate; the transfer module will end its mission, and the two orbiters will go on to separate science orbits.
While in its cruise phase, BepiColombo has many limitations on the use of its science instruments. It can’t use any of them during the long periods of solar electric propulsion. It has to keep the spacecraft’s sunshield pointed toward the Sun, limiting how it can rotate to point its instruments. Many of the MPO’s instruments are actually facing Mio to protect them from damage during the long cruise, so they can’t see space at all right now.
But all the fields-and-particles instruments on Mio will be operating, studying Mercury’s magnetosphere for the first time since the MESSENGER mission ended in 2015. And the transfer module carries three cameras that monitor the condition of the solar panels; they’ll also glimpse Mercury as the spacecraft passes by.
Incoming, BepiColombo will see the planet’s nightside, while outbound, it will see a gibbous Mercury. Two of the three transfer module cameras will take photos for up to four hours after closest approach. The closest image will be taken from an altitude of about 1,000 kilometers. The European Space Agency plans to release the first images Saturday morning at 7:00 UTC.
The two cameras will be pointing in different directions: MCAM-2 toward Mercury’s northern hemisphere and MCAM-3 toward the south. Surface features should be visible in the images, and you can use the Quickmap tool from the MESSENGER mission to identify craters. Once BepiColombo is far enough away for the entire planet to fit in the field of view, the dramatic splash of Murasaki crater should be at the center of the disk, and another of Mercury’s rayed craters, Debussy, will be on the lower right side.
Just as on MESSENGER, mission science will begin with the data gathered during these flybys, sparse though it may be. MESSENGER was an important pathfinder for BepiColombo, and most of the questions that BepiColombo seeks to answer were raised during analysis of MESSENGER’s observations.
Why does Mercury have a magnetic field that is shifted dramatically north of the planet’s geographic center? What materials are trapped in the permanently shadowed regions of Mercury’s north polar craters? What are Mercury’s hollows? How does it have so much carbon in its crust? Why is its core so big? This flyby – and the four that come after it – will show us some of what BepiColombo can do, and tantalize us with the potential of future Mercury science.