Engineers Find A Way
Corridor Testing Proba-3
With the recent launch of Proba-3 by the European Space Agency (ESA), it’s clear that engineers had to come up with innovative ways to balance scientific requirements against project costs. The main scientific requirement for Proba-3 was to artificially create Solar Eclipses for scientists to study the Sun’s corona. The biggest cost to balance was that it was impractical to launch a single satellite to create this solar eclipse. The answer was to launch two smaller satellites that can create solar eclipses on a schedule.
This is an overview of technical challenges for engineers to overcome and the testing needed to confirm instruments would work before launch.
Scientific Requirements
It is vitally important for humanity to have a better understanding of the Sun’s corona as this is the region that’s responsible for space weather and solar winds. Without this knowledge, we can’t fully understand how our local star works and what impact it can have on our daily lives. But this is a region that we’re not able to study in detail as the Sun’s brightness hides it from our instruments. Looking directly into the Sun is never a good idea. Pointing a telescope directly at it is an absolutely sure shot way to lose your eyesight.
The scientists for now have to wait for a total solar eclipse when the Sun’s center would be covered by the Moon (which in itself is an absolute fluke of nature). But these total solar eclipses don’t happen that often. Making it really hard for scientists to study this region. Or scientists can use Earth based options which have limited success due Earth’s atmosphere.
So we need a way to block the Sun’s center in space.
To do this on a single satellite would be cost-prohibitive. The next option is to send a pair of satellites, one to block the Sun’s center (occult) and the other to observe the Sun’s outer atmosphere (coronagraph).
Technical Challenges
Flying two satellites locked in formation increases the complexity of the mission by an order of magnitude more. Let’s review some of those challenges -
The satellites cannot be in exact formation all the time as the fuel costs would be too much. So they have to go out of sync and then reacquire each other again while orbiting Earth.
The satellites will fly in a highly elliptical orbit that brings them as close as 25m and then takes them apart by 250m every 24 hours. Their orbit takes them out of sync as they speed through in one part of their orbit, too fast to maneuver, and then eventually slows them down where fine adjustments can be made. They have to make these adjustments relative to each other autonomously.
The Occulter satellite cannot be off by even a millimeter. If that happens then it would not perfectly block the Sun and the second satellite would not be able to observe anything useful. Within this orbit, for about 6 hours, they need precision maneuvering to be in exact distance from each other to create the eclipse.
There are other sources of light, like Earth’s bright reflection, that have to be avoided because instruments on one satellite might not see the LEDs on the other satellite.
Vision-based instruments on the Occulter satellite helps it to keep track of the other satellite and achieve the necessary millimeter-scale accuracy. Controlling thrusters allow the satellites to autonomously maneuver in order to reestablish the 6 hour observing window.
This dance of these satellite pair in space brings in the era of precision flying that will benefit other missions in the future.
Instrument Testing
With those requirements and technical challenges comes the fun part. We need to test these instruments at a distance of about 2 football fields (official distance scale in the US). Additionally, we want to perform these tests indoors to avoid all the ambient sources of light, wind and other human activity. Before you say “let’s build a test facility for this”, remember cost is a factor.
So, the engineers at ESA decided to convert one of the longest corridors in ESA’s largest building as their testing space. This testing took place at ESA’s ESTEC technical centre in Noordwijk, the Netherlands, using its 230-m-long main corridor, which links project offices with technical laboratories and the establishment’s satellite Test Centre.
Lights were dimmed and exhibits removed to allow test-versions of the cameras to observe a flight-like target bearing LED displays down the entire length of the corridor.
This was absolutely critical to confirm the vision-based system will work before being launched into space. As the ESTEC’s website says — “Satellites are expensive, and once in orbit they cannot be fixed. This is why a satellite must be tested thoroughly before it is sent into space.”
The satellites were finally launched onboard the four-stage PSLV-XL rocket from Satish Dhawan Space Centre in Sriharikota, India, on Thursday, 5 December. Love how the ESAs testing team improvised and found a way to confirm that the instruments will work as expected.
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Proba 3: Shadowing the Sun By The Quantum Cat
An ambitious and complex project which has tremendous potential to gain knowledge about Sun’s Corona and its affect on our daily living. An interesting read about scientific achievements by human desire to unravel the mysteries of Sun …keep them coming 👍👍