India launches two ESA sun-spotters that will fly in incredibly precise formation India’s space agency last week launched a pair of European Space Agency satellites that will hopefully demonstrate high-precision formation flight techniques that make it possible to create “virtual satellites”. The mission is called Proba-3 and its launch phase went so well that India’s space agency was able to show off video …
It would obviously be a lot easier to have a telescopic arm that can extend out to block the sun from the camera for the same effect, or even just use a filter on the camera lens itself. But, it'll be pretty cool if they can coordinate two spacecraft to do the job. I hope it works as expected, as at some point in the future we're going to need to know how to precision fly two spacecraft close together. Stuff like this is how we learn to do it.
Not necessarily depth of field, but the sun is not a point source which is why we get penumbras so the occlusion has to be far enough away and big enough for that.
But I think there are probably other reasons for the arrangement: having any kind moving arm is going to cause problems with alignment as both have constantly got to be adjusted to match each other. But there may be other benefits in being able to orchestrate this kind of synchronised stargazing™.
The ESA page concerning Proba-3 is here: https://www.esa.int/Enabling_Support/Space_Engineering_Technology/Proba_Missions/Proba-3_Mission3
A link gets to the following explanation of the reason why a 150m gap betwixt occulter and coronagraph helps:
"The Sun is a million times brighter than its surrounding corona, so eclipsing it is essential for coronal studies. This is what happens during a solar eclipse of the Sun by Earth's Moon. But that sporadic event lasts a for only a few minutes. A solar coronagraph is an optical instrument which reproduces artificial dark eclipse conditions by mean of an occulting disk. Previous Sun-observing missions such as SOHO incorporate ‘coronagraphs' to study the corona. But their effectiveness is limited by a phenomenon called diffraction, where stray light overspills the edge of the occulting disk.
Progress on this front requires moving the Occulter much further away while still preserving eclipse-like conditions for long periods of time – precisely the performance offered by the Proba-3 externally occulted coronagraph. The technique has previously been attempted during the 1975 manned Apollo-Soyuz mission, when an Apollo Command Module blocked light falling on a Soyuz spacecraft.
... targeting an increase in our close-up view of the Sun and its corona from three solar radii down to just 1.08 solar radii as a goal – that is not traditionally enabled by smaller-scale coronagraphs."
https://www.esa.int/Enabling_Support/Space_Engineering_Technology/Proba_Missions/Proba-3_Science_payloads
Assistance requested, please.
I am trying to understand how a 150m long satellite could be stabilised so well that it could act as a solar coronagraph. Even the ISS is not as long as 150m (https://iss.jaxa.jp/kids/en/station/03.html#:~:text=Once%20completed%2C%20the%20dimensions%20of,will%20weigh%20around%20450%20tons. Using Pythagoras' Theorem maximum dimension is about 130m.)
If it would end up needing precise stabilisation at both ends, then it might actually be simpler and easier to have two independent satellites rather than one extremely long thin one with a lump at each end and all the vibration and resonance issues that would involve.
Science Boffin icon, 'cos the answer may require this bear of very little brain to think quite hard to understand.
It would need to be incredibly stiff, and probably have active damping.
Maybe a three arm system with thin plates protruding well beyond the dimensions of the eclipse circle (like an extruded mercedes badge, but thinner)?
Maybe it just wouldn't be possible, I have no real doubt it would be more massive than the dual satellite system they've gone for.
Stiff but also telescopic? Nice if you can do it, but probably better to go for something that locks into place and has no ability to move independently so that you have only the main satellite to worry about. That's probably possible using the same kind of origami approach they used for James Webb. But, no matter how stiff the arm is, I'm sure it's still going to have more wobble in it than is acceptable. And, if it's really all about the occlusion, then best do away with the connecting arm and its potential for feedback loops, especially if you add in damping, because this is in space with Newton's first law keen to remind you that it's there all time – resonant vibrating bridges spring to mind – and use a second satellite, which is what they're doing.
Total launch mass is probably the least of their worries.
There are some amazing images of the solar coronosphere on APOD (Astronomy Picture Of the Day: https://apod.nasa.gov/apod/archivepix.html), so I'm looking forward to seeing some amazing pictures from this duo of satellites. Let's hope they can maintain accurate separation and formation flying as planned.
https://apod.nasa.gov/apod/ap240417.html
https://apod.nasa.gov/apod/ap240506.html
https://apod.nasa.gov/apod/ap240402.html
Of course, the sun is not perfectly spherical:
https://en.wikipedia.org/wiki/Solar_radius#:~:text=The%20solar%20radius%20to%20either,of%2010%20parts%20per%20million.
"The solar radius to either pole and that to the equator differ slightly due to the Sun's rotation, which induces an oblateness in the order of 10 parts per million."
But I doubt that will make much difference.
