Mercury looms large in new spectacular images taken from close up by BepiColombo

July 17, 2023   Issue #591

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Over Your Head

There’s a lot of cosmos up there. Let’s take a look at it!

As I’ve mentioned a few times, I’m now writing quite a bit for Scientific American, a magazine I’ve loved for literally decades. I’m excited to let y’all know that we’re putting together a weekly column called “Over Your Head”, where I’ll be talking to you about things to see in the sky, including timely events (eclipses, bright comets, planetary conjunctions, meteor showers, and the like) and more evergreen topics dealing with phenomena that are fun to see and interesting to learn about.

The goal is to have articles that will go up every Friday, though there may be some adjustments to this as time goes on and as scheduling allows.

One went up last Friday in fact! And it’s about something you may have noticed yourself: The lovely Summer Triangle hanging over the eastern horizon. This is made of three of the brightest stars in the sky, Vega, Deneb, and Altair, and each of them has pretty cool characteristics I really enjoyed researching and describing.

I also tell a (true) story about seeing it when I was out walking the dogs. What I didn’t mention in the article is that as Buddy and Daisy were finishing up I was looking at Vega and thinking about how the Triangle would make a good article, when I noticed a bright satellite passing very close to it. And just as I was starting to smile to myself at the coincidence of seeing this just as I was considering making it into a piece for SciAm, a bright and very fast meteor burst into view passing right next to Vega and the satellite! That made me laugh out loud.

I always tell people to look up, because there’s a lot going on over your head. It’s such good advice that I follow it, too.

This issue of the Bad Astronomy Newsletter is free to the public, so if you like it and think someone else will too, go ahead and share it! Just click the button below and add in some friends. Easy peasy chicken dinner.

Pic o’ the Letter

A cool or lovely or mind-bending astronomical image/video with a description so you can grok it

On June19, 2023, the joint European-Japanese planetary mission BepiColombo passed just 236 kilometers above the surface of Mercury, the third time it’s flown past the wee planet in preparation for entering orbit around it in 2025.

And the images it took were <chef’s kiss>.

The first image was taken from a distance of 2,536 kilometers, and shows the battered and cratered surface in some detail. One interesting feature is Beagle Rupes, a system of cliffs over 600 km long, seen as a “sigma” shape to the upper right, splitting the elongated crater Sveinsdóttir. As it cooled after it formed (or possibly after a giant impact stripped a lot of its crust away), Mercury’s interior contracted, and the cooler, more solid crust cracked and wrinkled as it shrank too, created immense cliff systems like Beagle Rupes.

The second image was taken from about 4,000 km away and shows more of the surface to the east of the first image. The third was from 11,780 km away as BepiColombo moved on. In each you can see part of the spacecraft body on the left as well as the main antenna that will send the data taken back to Earth.

BepiColombo was launched in late 2018, but getting to Mercury isn’t easy. The Earth moves around the Sun at about 30 kilometers per second, and you have to negate some of that velocity if you want your spacecraft to drop toward the Sun. But Mercury orbits at a whopping 47 km/s, so you have to speed the craft up to catch it! To accomplish all this the engineers designed a trajectory that went around the Sun once so that BepiColombo passed Earth, doing it in such a way to drop it toward the Sun (it had to pass Venus twice as well to drop down even more).

The images have been put together into a video, too:

[Credit: Image: ESA/BepiColombo/MTM, CC BY-SA 3.0 IGO; Music composed by ILĀ.]

But then, once it got close enough, it had to do the reverse trick using Mercury for a gravity assist to speed it up so it can catch up. But Mercury is a small, low-mass planet, with weak gravity, so the spacecraft has to do six gravity assists with Mercury! Only then will it be able to approach it relatively slowly enough to enter orbit.

That’ll happen in December 2025. I can’t wait! Mercury is a fascinating world, and a difficult one to study from Earth because its smaller orbit means it never gets very far from the Sun in our sky, so you can only observe it in twilight. It’s very dense, almost as dense as Earth, and we think the core of the planet is very large compared to its overall size. Did an ancient giant impact scrape off a large part of its lightweight crust, making its average density higher?

BepiColombo’s mission is to “study and understand the composition, geophysics, atmosphere, magnetosphere and history of Mercury”, a grand set of goals that will be helped by the most sophisticated machine ever sent to the planet. And that’s saying a lot, given how amazing the NASA MESSENGER spacecraft was. We learned a huge amount from that mission, so I’m really looking forward to seeing what BepiColombo will show us.

Et alia

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