[frederikvs]

I work at the company building these satellites, I've seen them sitting in our clean room.

If there's any questions, I might be able to give some answers. However, I'm not involved with Proba-3 directly, so no guarantees.

[_hl_]

I understand that part of the objective is to demonstrate precise autonomous navigation and choreography, but for the "main" scientific objective re. observing the solar corona, why not just stick a big stick with a disk on the observing satellite?

[frederikvs]

That's what's been done so far [0], both on the ground and in space. As I understand it, the further your disk is from your telescope, the better. But your stick needs to be rigid enough to keep the disk exactly in its place. And a rigid stick has weight.

For Proba-3 the goal is to have the two satellites more than 100m apart. If you want to do that with a stick, your stick has to be longer than the ISS. That should tell you a thing or two about the complexity and cost of building and launching that stick.

I do have to admit I'm not exactly sure what the advantages are of having the disk further away from the telescope. I suspect it's to do with the interaction between the light and the edge of the disk, but I'm not sure.

[0] https://en.wikipedia.org/wiki/Coronagraph

[verzali]

I worked on Proba-3 for a while. The original goal of the project was to demonstrate formation flying in preparation for another telescope (I forgot the name, but it was something like Xeus). That telescope would have a camera and lens on seperate spacecraft, and the idea was that by moving the lens backwards or forwards you'd be able to create a much bigger telescope than could be built with a single spacecraft. Unfortunately that telescope was cancelled, so although Proba-3 is still demonstrating a cool technology, it probably won't be applied elsewhere for a while.

[jsemrau]

Why did you use cold gas thrusters?

[frederikvs]

I'm not familiar enough with the project to answer why specifically cold gas thrusters were chosen. What I can say : if old, proven technology will get the job done, there's a strong preference to use that.

When needed to achieve the mission, new technology will be developed - sometimes the whole point of a mission is specifically to develop new technology. In this case, one of the major goals of the mission is to develop formation flying technology. Learn what the pitfalls and the tricky bits are, and make the technology available for future missions.

But when the mission can be achieved with old technology - technology with a long record of being used in space, where the problems are known and understood, where we know what works and what causes problems - then the mission will use old technology.

If you use newer technology, there's always a risk that you'll hit a new issue, previously unknown. Maybe you can work around it, maybe you can't. But this isn't web development where you can refactor, switch to a new framework and continuous deploy your way out of it. For the hardware, you get one launch and that's it. Why run the risk if you can avoid it? It'd be a shame if the mission can't achieve its primary objective (learning about formation flying) because it chose some new type of thruster, and encountered some new issue with it.

[dr_orpheus]

They need the milli-newton level of thrust control and so need tiny thrusters. Generally the simplicity of the cold gas thrusters in a small package is easy. Yes it is not as efficient, but moving up in efficiency and complexity to catalyzed monopropellant (generally hydrazine) thrusters the thrust ranges are usually > 1N. Same story with any bi-prop thrusters. Certainly electric propulsion has the levels of thrust needed, but then you would need a lot of power for it. And since they are not doing large delta V maneuvers there is less concern about the actual amount of propellant that you need to take with you.