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SYFY WIRE Bad Astronomy

Watch Earth zoom by as a spacecraft passes us on its way (eventually) to Mercury

By Phil Plait
A view of Earth from BepiColombo as it swung by our home planet on 10 April, 2020 on its way to Mercury. Credit: ESA/BepiColombo/MTM, CC BY-SA 3.0 IGO

BepiColombo is a joint European-Japanese mission currently on its way to the planet Mercury. It will map the surface and magnetosphere of the small world, and also make measurements that will characterize its interior, including the weirdly oversized core of the planet. It's actually three spacecraft in one: a pair of orbiters that carry the scientific detectors, and a transfer vehicle that supports the orbiters on their way to Mercury and will place them into their orbits around it.

But first BepiColombo has to get there, and that's quite a journey — it will do this via a series of gravity assists by other planets, passing close to them and giving them a little bit of its orbital energy, which will drop the spacecraft closer to the Sun. This will take a total of seven years (launch was in 2018, arrival in 2025), and will include two Venus flybys and six of Mercury itself to get itself into the proper trajectory for orbital insertion.

It also has to do a single flyby of Earth to drop it toward Venus, and that was accomplished on April 10, 2020. Last Friday night!

BepiColombo passed just 12,700 kilometers above Earth's surface, practically skimming our atmosphere, in order to maximize orbital energy transfer. It came in over eastern Asia, traveling west over Africa before heading back into deep space. And for much of its approach and recession it snapped images on its cameras, which have been assembled into a jaw-dropping animation of the flyby.


The first part shows the Earth approaching as BepiColombo was heading toward us. Features on our planet are difficult to make out, but the swirling clouds are obvious. That sequence was taken with Monitoring Camera 3; you can see the spacecraft’s high-gain antenna at the top of the animation.

The next bit is a little different: As is passed Earth, BepiColombo traveled into Earth's shadow, so the Sun was blocked, effectively eclipsing it. Seen from the ground by a telescope run by the ESA's Near-Earth Object Coordination Centre in Chile, the spacecraft appears as a dot and the stars streaked, since the telescope tracked on BepiColombo's motion. You can see it fade as it gets eclipsed.

A view of Earth from BepiColombo as it began its flyby on 10 April, 2020 on its way to Mercury. Credit: ESA/BepiColombo/MTM, CC BY-SA 3.0 IGO

The third sequence made me gasp out loud. This was taken by Monitoring Camera 2, and you can see the medium-gain antenna at the top, the magnetometer boom (a long rod with sensitive magnetic field detectors in it) extending down from the upper right corner, and what I think is a bit of a solar panel on the lower right (obviously, solar power is a good idea near Mercury, which is 1/3rd the distance from the Sun the Earth is).

But oh my, that planet! The brightness fluctuates a bit between images, but you can see our fair Earth swinging past, the terminator marking the division between day and night. The angle is a bit odd (north on Earth is to the lower left), so I annotated a single image (including some of the spacecraft itself visible) to help (note that I rotated the image to put the continents in a more familiar orientation):

An image of Earth during the 10 April 2020 flyby of BepiColombo (in the foreground, left), with features noted (right). Credit: ESA/BepiColombo/MTM, CC BY-SA 3.0 IGO and Phil Plait

And the the fourth, final part? That shows the rear view as BepiColombo leaves Earth behind. You can see our planet shrink as the spacecraft flies on... then the Moon joins the view from the right, above the solar panel! Holy wow. Those two objects are thousand of kilometers across and nearly 400,000 km apart, but they seem to visibly dwindle as BepiColombo's journey continues, plunging down toward the inner reaches of our solar system.

I've seen footage and images like this so many times, but I will readily admit that it makes my heart swell every time I see a new one. There is something poetic, almost magical, about not just seeing Earth from space, but seeing recognizable features, especially from a mission on its way to an entirely other planet — two, including Venus. Speaking of which, the first flyby of our evil twin is on October 15, 2020, and that will be something to see.

BepiColombo’s journey to Mercury

A second pass of Venus in 2021 then leads to the series of them of Mercury, and oh my, I can't wait. It's been many years since the MESSENGER mission ended with it crashing onto the surface of Mercury after returning devastatingly beautiful and scientific important images and data of the solar system’s smallest planet. It's hard to get to, needing such a huge change in orbital energy to drop so close to the Sun, and then of course dealing with the fierce light and heat. That’s why we haven’t sent many spacecraft there… so it makes any mission that much sweeter.

Until the Venus encounter, I'll leave you with that one last image: BepiColombo bidding farewell to the Earth and Moon. Goodbye, little spacecraft. Enjoy your time in the Sun.

The Earth and Moon seen by BepiColombo as it leaves our planet on its way to Mercury via Venus. Credit: ESA/BepiColombo/MTM, CC BY-SA 3.0 IGO and Phil Plait