The joint European and Japanese mission BepiColombo is on its way to Mercury but to get there it will have to pass by our planet again. This will happen on April 10 and lucky people in the southern latitudes will be able to catch a glimpse of the mission as it passes overhead, 12,700 kilometers (7,891 miles) from the ground.
The mission was launched in late 2018 and will finally reach the Solar System's innermost planet in 2025. It is made up of two satellites launched together that will study the magnetic field of Mercury as well as its interior structure and surface. It will fly by Earth on April 10 at 00.25 am ET (4.25 am UTC/5.25 am BST).
“This is the last time we will see BepiColombo from Earth,” said Joe Zender, BepiColombo Deputy Project Scientist at the European Space Agency (ESA), in a statement. “After that, it will head deeper into the inner Solar System.”
This flyby is not a PR stunt or a last wave to Earth, but is integral to the mission to give it a bit of a gravity assist. It might seem counterintuitive but going towards the center of the Solar System is not an easy task. You can’t just shoot something towards the Sun, and there is a simple reason why: Our planet is moving very fast around the Sun, so it doesn’t fall in. Everything launched from Earth would have that additional speed to deal with. A large amount of energy is required for BepiColombo to enter a stable orbit, or you have to be very clever with celestial mechanics.
So the mission will use Earth as a celestial brake. As it gets close, it will use our planet's gravitational pull to slow down and bend its trajectory towards the inner portions of the Solar System. This maneuver was first devised by Italian mathematician and engineer Giuseppe “Bepi” Colombo and first employed by NASA to send the Mariner 10 probe to Mercury in the 1970s.
The current flyby should go smoothly but there is a certain tension due to the precautions put in place by ESA to protect workers from catching SARS-CoV-2, the coronavirus responsible for COVID-19. The team is still hard at work but with minimal in-person interaction.
“The Earth swing-by is a phase where we need daily contact with the spacecraft,” said Elsa Montagnon, BepiColombo Spacecraft Operations Manager at ESA. “This is something that we cannot postpone. The spacecraft will swing by Earth independently in any case.”
The upcoming flyby is the first of nine gravity assist maneuvers that are necessary for BepiColombo to reach Mercury. On October 15, it will perform the first of two flybys at Venus, with the second in August 2021. The final six will be orbit-tightening maneuvers and will happen around Mercury from October 1, 2021.
BepiColombo should be visible from East to West on the morning of April 10. You can calculate the visibility and brightness from your location here. However, the Moon will be particularly bright around this time, waning from the brightest Supermoon of 2020 on April 7, which may impede views from Earth. On the plus side, Jupiter, Saturn, and Mars will all be aligned on April 10, and visible to the naked eye so even if you miss BepiColombo, you'll get a good view.