Hello, Mercury! BepiColombo speeds by

Section of moon-like cratered planet with components of BepiColombo spacecraft visible.
The joint European-Japanese BepiColombo mission captured this view of Mercury on October 1, 2021, as the spacecraft flew past the planet for a gravity assist maneuver. Click here for an annotated version of this image, which is via ESA.

Mercury! BepiColombo gets a gravity assist

The European-Japanese BepiColombo mission – launched in 2018 – successfully performed its first flyby of Mercury late in the day on October 1, 2021. BepiColombo is due to arrive in earnest, for an extended stay at Mercury, on December 5, 2025. But, as the innermost of our sun’s planets, Mercury is hard to get to. So BepiColombo’s journey to Mercury will require a flyby of Earth, two flybys of Venus, and a total of six flybys of Mercury. With each pass, the spacecraft will obtain a gravity assist. The gravity of these planets will alter the spacecraft’s course slightly, and give the craft the boost it’ll need to enter Mercury orbit. On its Mercury pass late yesterday, BepiColombo acquired the image above. The spacecraft passed only about 124 miles (199 km) from Mercury at its closest, at 23:34 UTC on October 1. ESA explained:

The region shown is part of Mercury’s northern hemisphere including Sihtu Planitia that has been flooded by lavas. A round area smoother and brighter than its surroundings characterizes the plains around the Calvino crater, which are called the Rudaki Plains.The 166-km-wide [103-mile-wide] Lermontov crater is also seen, which looks bright because it contains features unique to Mercury called ‘hollows’ where volatile elements are escaping to space. It also contains a vent where volcanic explosions have occurred. BepiColombo will study these types of features once in orbit around the planet.

ESA was very happy with this first flyby, calling it “flawless” and “radiant.”

Earth’s 3rd Mercury mission

Before BepiColombo, the first spacecraft to have visited Mercury was NASA’s Mariner 10. It swept past the planet three times in 1974 and ’75 (unfortunately with the same side of Mercury showing to the craft at each pass). Then there was the wonderful MESSENGER mission, which placed a craft in orbit around Mercury from 2011 to 2015. The fact is – due to Mercury’s closeness to the sun – the innermost planet is hard to visit. A spacecraft launched from Earth goes into an orbit around our sun. The trajectory of that solar orbit might take a craft past Mercury. But to enter into orbit around the planet directly is different. To do it, the craft has to brake constantly against the sun’s gravitational pull. Johannes Benkhoff, ESA’s Project Scientist for the BepiColombo mission, said;

There are two ways how to accomplish this braking. You either need a huge spacecraft that carries a lot of fuel, or you can use the gravity of other planets to slow you down along the way. To reach Mercury, you need to perform multiple such planetary flybys and so the journey takes a long time.

In BepiColombo’s case, it’s taking from 2018 to 2025.

Why go there at all? Because, well … Mercury! It’s sometimes called the most elusive planet because, to spot it in Earth’s sky, you’ve got to be looking at exactly the right place and time (later this month will be a good time to see Mercury in the east before sunrise from the Northern Hemisphere). A few decades ago, we didn’t know much about Mercury at all. It carried the romantic nickname the moon of the sun, because astronomers thought it always kept one face toward the sun, just as Earth’s moon keeps a single face toward Earth. Now we know that’s not true, although Mercury does spin very slowly. Its slow spin and fast speed in orbit (just 88 days to orbit the sun once) makes for a very long Mercurian day.

Mariner 10 and MESSENGER revealed a lot about Mercury. But there’s always more to learn. For example, despite temperatures on its surface rising to as high as 850 degrees Fahrenheit (450 degrees Celsius), there seems to be water ice on Mercury. The planet also appears to have a much too large inner core for its size and a surprising chemical composition.

Bottom line: Hello, Mercury! The BepiColombo mission successfully performed its first flyby of Mercury on October 1, 2021. It also sent back images. Flawless and radiant, ESA said.


Read more: Top 5 Mercury mysteries BepiColombo will solve

October 2, 2021

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Deborah Byrd

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