I hear you like space: ESA's BepiColombo is launching to Mercury

Meet BepiColombo. It's the third human mission to planet Mercury ever and it'd due to launch at 1:45 AM GMT on 20 October 2018, which, as I am writing, is ~5h from now. It's a joint mission by the european space agency ESA and its japanese counterpart JAXA. And it's cool!

All images btw. are from ESA. I'll give a link to the full set of images at the end.

BepiColombo will launch on an Ariane 5 from Kourou in French Guyana and it's an amazing mission. Frankly, we know very little about Mercury. In part, because it is amazingly difficult to get there. One needs quite a lot of fuel to get so far towards the sun. More on that below.

So Ariane 5 will hopefully launch flawlessly. But that rocket is now well proved, recently the 100th Ariane 5 was launched. This is an artist's impression on fairing separation (we've all seen so many images like this now; thanks, Elon!) and then at some point BepiColombo will be just by itself.

Is that something unusual? Yes, because the satellite will travel with ion boosters. That means there is no chemical burning taking place, but it will draw its acceleration from Xenon atoms (ok, ions) being accelerated in an electric field, created from solar power. That helps, because one can accelerate the ions to much higher velocities than with a burn-thruster. And since momentum is mass x velocity, in order to accelerate BepiColombo forward, one has to eject some mass backward and the faster that happens, the less propellant mass one needs. Twice the velocity of the ejected propellant means only half as much propellant is needed.

Downside: the thrust of each of the 4 thruster is only in the range of 0.075-0.15 Newtons. What does that mean? That's the force that one to two Snickers bars on your hand excert due to gravity. Not much? Not much. But BepiColombo will travel for 7 years, will get some swing-by assists from Earth, Venus and Mercury and that small acceleration will add up to get it in sync with Mercury in the end.

Here's the timeline:

Yes, it will come back past Earth, then Venus twice, then several times Mercury, to arrive in December 2025. Then measure for 1-2 years. Yes, the solar system is pretty large compared to earthly things.

So, what will BepiColombo do? Actually, not that simple, because that mission is actually made of two orbiters: the Jaxa Mercury Magnetospheric Orbiter (nicknamed "Mio"), which, surprise, carries a number of instruments measuring magetic field and particles from/around Mercury, solar wind, and high 'atmosphere' composition.

This image shows Mio inside its sunshield.

And here's the other orbiter, ESA's part, the Mercury Planetary Orbiter, MPO with sadly no nickname. It has a number of cameras from X-ray to infrared on board, a laser altimeter to map the surface, a total of 11 cool science instruments.

They've got the power. Notice anything? That's a huge solar panel. Prize question: where is Mercury actually located?

Tick, tock, tick, tock.

Correct: really much much closer to the Sun compared to Earth. It is the innermost planet in our solar system. Shouldn't that mean there's enough sunlight and it doesn't need that large solar cells? Well, yes and no. The problem is that the Sun doesn't only provide 10x more visible light which is convertible into electricity, it also provides 10x more heat! That means lots of it. At the location of Mercury, BepiColombo will experience upward of +300°C = 570°F, hence sun-shields, radiators, etc.

The problem for the solar cells would be that they wouldn't really be able to operate. I'm not sure if they would simply break of if their efficiency would effectively go down to zero if they were directly pointed at the sun when near Mercury. But what the engineers came up with is to simply tilt them at an angle, I think 70° inclined, so that the sunlight only grazes the surface. In that way one will still get light and heat, but only substantially less per actual surface. So by tilting one basically decreases the 10x higher amount of sunlight to something closer to 3x more compared to Earth, which will work. Nice trick, but this is why the panels have to be so large.

Here's an artist's impression on how the stack of Mio, MPO and propulsion module will look with folded-up solar panels.

And this will be the two orbiters after separation on their separate routes around Mercury - after arrival in 7 years. Look at that dramatic sun with its wildly over-exagerated protuberance!

I don't know how many people worked on this mission. I guess around 1000 on the sat and instruments alone. So at some point some of them were able and allowed to sign the fairing inside which BepiColombo is already encased.

Quickly, an ESA mission sticker, also onto the fairing.

And if the launch is successful, which I truely hope and believe, then BepiColombo will add to the pretty impressive suite of ESA solar system missions that are either completed, flying, in the construction phase, or currently being planned.

So let's hope all goes well.

Credits and more images: http://sci.esa.int/bepicolombo/60769-outreach-resources
Media kid with more background info: http://sci.esa.int/bepicolombo/60754-bepicolombo-launch-media-kit
Live-stream: https://livestream.com/ESA/BepiColomboLaunch