In a corner of #ESATech centre #ESTEC in #NL is what looks like a wall hanging – but is actually part of @HubbleTelescope’s space-flown solar array. How did it come to be here? Therein lies a tale… #Hubble30 1/28 
In 1977, NASA and ESA made a deal on @HubbleTelescope - ESA gained 15% of telescope observing time, and access to all data, in return for contributing an instrument (the Faint Object Camera), designing and building Hubble’s solar array & providing engineering support 2/28
Solar arrays? Sounds simple enough... but @HubbleTelescope presented special engineering challenges. To begin with, these solar arrays were going to be taken back to Earth, afterwards, to find out first-hand how orbital hardware endures in space – so needed careful design 3/28
Standard missions used rigid solar arrays that unfolded in orbit, but the @HubbleTelescope arrays were more like roller blinds, with a pair of flexible arrays covered with solar panels on each side. Flexible as they had to fit between Hubble hull & Shuttle cargo bay 4/28
#ESATech needed flexible solar cells that didn’t break when the arrays were rolled up. These @HubbleTelescope arrays also needed to be retractable, to be rolled back up and brought down to Earth again 5/28
The other main challenge was @HubbleTelescope’s place of operations: low orbit, around 600 km up, where the Sun rises & sets every 96 minutes & temperatures shift from plus to minus 100ºC.. The arrays had to stand up to around 30 000 thermal cycles over their 5-year lifetime 6/28
Imagine swapping household roller blinds between a cooker and a deep freeze 16 times a day. While the @HubbleTelescope solar arrays needed to be flexible, they also had to be kept taut by special booms, to stop them deforming & reducing their delivery of 4 kW of power 7/28
2 sets of solar arrays were contracted from European industry, plus electronics and Solar Array Drive Mechanisms, with (then) high-efficiency 12.7% silicon solar cells, silver connectors & protective Kapton. But the 1st Space Shuttle flights presented a problem 8/28
The 1st Shuttle came back with severely eroded thermal blankets - shocking everyone. The culprit was atomic oxygen: highly reactive, individual oxygen above the atmosphere. It would eat up @HubbleTelescope blankets in weeks. Designers had to start again 9/28
The redesigned @HubbleTelescope solar arrays used molybdenum-based interconnectors, which were oxygen safe, while the Kapton was protected by a silicone coating. The designers were able to incorporate newer 14% efficient solar cells into the updated design 10/28
The @HubbleTelescope was launched on 24 April 1990. But all was not well. Besides the famous ‘astigmatism’ due to a slight mirror error, the observatory experienced about 10 seconds of jitters each time it passed between night and day 11/28
The @HubbleTelescope’s solar arrays were flexing every time they passed through the day-night terminator, affecting the observatory's very-high angular resolution imaging. The source was the bi-stem booms used to tension the arrays, clicking against each other 12/28
The #ESATech team deduced this @HubbleTelescope jitter cause remotely – but eventually got to see they were right. Meanwhile they corrected the problem for the second set of solar arrays, which flew up in 1993 13/28
For the 2nd generation @HubbleTelescope arrays, the tensioning bi-stem booms were enclosed in a kind of Chinese-lantern shield, while the old compensation mechanism used to keep the array taut was replaced by a kind of 'bed-spring' system with a set of limit straps 14/28
The 2nd set of solar arrays flew on the 1st Hubble Servicing Mission, in December 1993. Of the original pair, one was indeed out of shape: its failed compensation mechanism made one boom bent like a bow. It couldn't be retracted as planned 15/28
So 1 of 2 @HubbleTelescope arrays couldn't be retracted. Instead astronaut Kathy Thornton released it into free space. It drifted for a couple of years before burning up. The other wing retracted like clockwork, and was taken back to Earth 16/28
#ESATech examined the returned @HubbleTelescope solar array. There was severe darkening of the silicone adhesive due to ultraviolet exposure, thousands of tiny impacts due to natural and man-made debris – helping to gather statistics on them – & thermal cracks 17/28
ESA’s 2nd set of solar arrays did duty on Hubble for more than 8 years. In December 1999 an emergency Servicing Mission (3A) was tasked to replace failed Hubble gyroscopes. The astronauts noted some alarming changes in the solar arrays during a routine inspection 18/28
Demonstrating the dynamic nature of low-orbit conditions, wire-like hinge pins holding different segments of the @HubbleTelescope solar blankets together had come loose after 8 years , extending up to 50 cm out from the array edge 19/28
#ESATech solar array team began emergency testing with the original, rejected solar array. Could these hinge pins penetrate a spacesuit? Might the solar blankets fall apart when they were retracted? After a tense 38 hours they demonstrated astronauts were safe 20/28
#ESATech testing showed the array’s power bridge pieces were also strong enough to hold the structure together for the remaining mission life & the planned retraction during the upcoming Servicing Mission 3B retraction 21/28
The 2nd set of solar arrays returned by Shuttle in 2002. They showed barely any power degradation.There were many impacts, but only 4 full penetrations per square metre – the largest hole only 5 mm in diameter 22/28
The 2nd-generation solar arrays were replaced by US-made models by 2002 Servicing Mission. These were smaller, rigid solar wings derived from Iridium satellite spares, because the idea of retrieving them by Shuttle had been abandoned. They were tested at #ESTEC 23/28
But the Solar Array Drive Mechanism (SADM) and electronics controlling these solar wings remain European - the only European hardware still aboard today’s Hubble, originally built by Dornier, now @AirbusSpace in Friedrichstafen 24/28
The Solar Array Drive Mechanism is crucial to @HubbleTelescope - the arrays move as the telescope shifts to image a target, producing the optimal amount of power during the observation 25/28
#ESATech and @AirbusSpace solar array team remained on call during the last Servicing Mission in May 2009. The arrays needed supervision during spacewalks to avoid contact with astronauts, and also because air from the airlock can blow the arrays out of position 26/28
#ESATech’s #HubbleTelescope experience laid the groundwork for future solar array designs. & the idea of flexible solar arrays is being returned to, as being lighter and more efficient for future satellites 27/28
The section of @HubbleTelescope array displayed at #ESATech centre #ESTEC comes from the 2nd generation arrays. Come see this well-travelled wall hanging at the next #ESAOpenDay, and see for yourself the impacts studding their surface #Hubble30 28/28
