BepiColombo Is Finally Meeting Mercury For The First Time

Artist impression of BepiColombo flying by Mercury on 1 October 2021. Image Credit: ESA/ATG medialab

Today, BepiColombo is set to meet Mercury for the first time as it carries out the first of what will be six flybys on its mission to start studying the smallest planet in 2025. At 11:34 pm UTC (6:34 pm ET) on October 1, the mission will fly just 200 kilometers (124 miles) from the surface of the innermost Solar System world, taking its first images and science data of Mercury. 

BepiColombo, a collaboration between the European Space Agency (ESA) and the Japanese Space Agency (JAXA), comprises two orbiters that will separate into complementary orbits. The mission will understand Mercury like never before — and that starts tonight.  

“We’re really looking forward to seeing the first results from measurements taken so close to Mercury’s surface,” Johannes Benkhoff, ESA’s BepiColombo project scientist, said in a statement. “When I started working as project scientist on BepiColombo in January 2008, NASA’s Messenger mission had its first flyby at Mercury. Now it’s our turn. It’s a fantastic feeling!”

The recent flyby of Venus, used to slingshot the spacecraft towards the inner Solar System, actually allowed mission control to adjust the trajectory a little bit and they expect the actual altitude of the spacecraft over Mercury to be 198 kilometers (123 miles). Being able to have an uncertainty of just 2 kilometers (just over 1 mile) from 100 million kilometers away, is truly a testament to how incredible the mission team is.

It will be a very brief visit, however. BepiColombo is moving too fast to go into orbit, but will instead swing past the planet, with the next five scheduled passes set to use the planet's gravity to slow it down enough to finally get into orbit by December 2025.

The next flyby of Mercury will take place next June, then one a year later in 2023, two in 2024, then one in 2025 before the orbital insertion in December of that year. When Bepi finally gets there, it will have taken three years, two months, and multiple assisted gravity flybys. It's not easy to get into the inner Solar System. 

This appears counter-intuitive. The Sun is the biggest object around so it would seem if you dropped something in the Solar System, it would simply fall inwards. And that is true, as long as that object is not already moving around the Sun. When we shoot a rocket into space, that rocket also has the speed of the Earth going around our star. That speed makes sure that the Earth doesn’t roll in, but if you want to send your rocket closer to the Sun you've got to slow down the rocket a lot. In this case, enough to match the speed of Mercury.

Doing that with chemical propulsion is very expensive. However, using planets' gravitational pull as a celestial brake slows down and bends a craft's trajectory towards the inner portions of the Solar System. This maneuver was first devised by Italian mathematician and engineer Giuseppe “Bepi” Colombo and was first employed by NASA to send the Mariner 10 probe to Mercury in the 1970s.

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