Flying close to the sun

The Solar Orbiter, a joint project between NASA and the European Space Agency to study the Sun close up,  is scheduled for launch in 2018. It will get nearer to the Sun than we've ever been before.  Airbus Defence and Space were awarded the contract to build the Solar Orbiter, and so they have quite a challenge on their hands protecting it from the Sun's intense rays. Katie Hassell is a Senior Spacecraft Thermal Engineer with Airbus she explained to Connie Orbach just how close to the sun the probe will be going...

Katie - Solar Orbiter, as part of its mission, will going to a third of the distance from the Sun to the Earth, so the close third to the Sun - that's just inside Venus's orbit.

Connie - And what sort of temperatures is that then - that's a lot closer than we are?

Katie - Yes. So it has a really big heatshield on the front of the spacecraft and the front of that heatshield will see around 600 degrees. As an idea, if you've ever left a bar of chocolate in your pocket for a bit too long... it will do that to aluminium.

Connie - That's the front but the back must be quite a different temperature?

Katie - Yes, the back is cold. So the idea with the heatshield is that it works a little bit like a parasol in the sense it provides a shadow for the whole of the spacecraft and so, everything behind that heatshield is going to be cold.  And it can see dark space which we model that around -270 degrees, so the spacecraft itself could get down to temperatures of about -180.

Connie - Wow!. That's a really huge range.

Katie - Yes.

Connie - And I'm guessing, if we've not got the atmosphere of the Earth, then I'm guessing there's a radiation issue as well?

Katie - So radiation is an issue, particularly for Solar Orbiter, because it's going so close to the Sun it does mean there's going to be a lot of charged particles in the environment that it's going to be in.

Connie - When you're thinking about these problems, this huge range in temperature, lots of radiation, how do you go about choosing the materials to make the Solar Orbiter from to deal with these conditions?

Katie - So we have a good idea of how certain materials behave. We know radiation's going to be an issue, which then starts to narrow down what type of materials we can use. So for Solar Orbiter, we're looking for anything that is electrically conductive, so anything that isn't electrically conductive we then basically remove that from the options - it's not available to us.

Connie - Why does it need to be electrically conductive?

Katie - That's these charged particles. We don't want static to build up on any part of the spacecraft because, otherwise, we could end up with electrostatic discharge; it's like little lightning across the whole spacecraft, and that can damage the instruments.

Connie -   And so what about the other problems, the other materials?

Katie - So then, after that, it becomes quite a lot of a thermal issue. So what we're looking to do there is maintain the temperature of the spacecraft. Ideally, from a thermal point of view, it would be lovely if the spacecraft could be white. However, we couldn't find a white material that was going to maintain its whiteness during the lifetime of the mission so we're actually going to be using black, and by doing that it means that we can understand a lot more how the colour of the spacecraft is going to change over its lifetime. And the idea is we try to maintain a particular type of colour so that we're controlling the heat flow so from Sun, around the spacecraft, and then out to space.

Connie - So you kind of know what's going to happen right from the beginning as opposed to...?

Katie - That's the general idea.

Connie - OK. What are the materials then - what sort of things are you using?

Katie - Inside that shadow cone, we've got thermal blankets on there, they're a little bit like what you might see marathon runners being wrapped up in, but lots of layers of those. They're made out of aluminium foils and, again, they've got a black coating on the outside of those. And then the heatshield that I've already mentioned, that has a special paint on the very front of it that, it just so happens, is made out of burnt animal bones.

Connie - Ohh - that's quite surprising!

Katie - Kind of a random one!

Connie - Yes. There's a load of burnt animal bones going up into space?

Katie - Yes.

Connie - Why? Surely there's lots of things you could make which would do the job?

Katie - We've asked for a black paint that can meet all of our requirements and it just so happens that the black paint that has come back to us, that can maintain all of that electrostatic discharge, that can maintain it's colour, and isn't going to fall off once it's all painted on there, it's made out of charred animal bones.

Connie -   OK so, basically, this kind of a big box and it's got this big square sunshield in front of it, which isn't made of aluminium - what's it made of?

Katie - It's made out of titanium.

Connie - OK. Made of titanium and covered in this black paint made of all things... crushed animal bones. So how do you know it's actually going to work?

Katie - Good question!. During the whole development of the mission we do a lot of analysis, so different teams do their own types of analysis. I'm a thermal engineer so I've been doing thermal analysis. And then what we do is we build it and we put it in what's called a solar simulation chamber and that is, essentially, a really big box that has a zena light bulb. And the xenon light bulb is to simulate all of the different wavelengths that the Sun emits its light in, and we put our spacecraft in there and we see how the spacecraft reacts to that sunlight.

Connie - So that xenon light bulb is like a mini-sun?

Katie - It's a mini-sun, yes.

Connie - Oh Wow! And that's just looking at it right in the here and now. I'm guessing you have to then do some sort of analysis on this?

Katie - Yes, we do more analysis at that point. So we make a model of the test chamber and what we do with our spacecraft model is we make sure that the temperatures on the model match what we're getting out of the test chamber, and then we run lots of simulations to model the different types of environments that the spacecraft's going to be in during its mission lifetime.

Connie - And how long is that mission lifetime - how long is it going out there for?

Katie - It works out at around nine years. It's got three years where it's actually going to get to the Sun, and then we've got three years where it's doing the science and then, all being well, it will have a further three years doing even more science.