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?
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.
The even more extreme version of this JPL has been working on with their Starshade program. Not specifically a cornograph, but same concept of blocking light from a star to look for something more dim, in this case its looking for exoplanets. But it is a much larger scale. A 25 m deployable shade in formation 100 km away with the same level of mm precision.
Edit: Just to be clear on status of this, Starshade is still in early technology demonstration phase that they can actually build the shade and do the formation maintenance. This is not in full build or slated to launch any time soon.
Huh, very interesting- and counterintuitive that distance matters. If anyone here knows why I’d love to learn more!!
EDIT: maybe not so counterintuitive after all: if you scale everything up, you get a higher fidelity sensor and more signal to noise. Same reason telescopes want to be big: to collect more light especially from dim signals. Distance of the occulter then reduces perspective distortion that would eclipse the inner parts of the sensor more than the outer ones. Just my speculation though.
I'm speculating past my understanding here, but wouldn't some sort of diffractive effect around the edge of the disk explain it? Like the further you are into the far field of that diffraction the better or such?
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.
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.
Cold gas thrusters are as reliable (because of simplicity - basically just container of pressurised gas and a valve [1]) and as precise in their thrust pulses execution as it can get. Better precision in thrust execution probably only ion engines have but just because of extremely low thrust levels in general (milli-newtons).
I saw a really good YT video a few months ago that explained this very well. Went over all the fuel types and tanks and propulsion designs.
Started with what you said - a simple gas pressurized and a single valve.
Slowly more and more was added until we get to current rocket designs with multiple stages or active pressurization / fuel transfer and all that entails.
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.