Editing Rosetta (spacecraft) (section)

=== Reaction control system problems ===
In 2006, ''Rosetta'' suffered a leak in its [[reaction control system]] (RCS).<ref name="FAQ" /> The system, which consists of 24 [[bipropellant]] 10-[[newton (unit)|newton]] thrusters,<ref name="OCM" /> was responsible for fine tuning the trajectory of ''Rosetta'' throughout its journey. The RCS operated at a lower pressure than designed due to the leak. While this may have caused the propellants to mix incompletely and burn 'dirtier' and less efficiently, ESA engineers were confident that the spacecraft would have sufficient fuel reserves to allow for the successful completion of the mission.<ref name="bbc20140521">{{cite news |url=https://www.bbc.co.uk/news/science-environment-27498534 |title=Rosetta comet-chaser initiates 'big burn' |work=BBC News |first=Jonathan |last=Amos |date=21 May 2014 |access-date=24 May 2014}}</ref>

Prior to ''Rosetta''{{'s}} deep space hibernation period, two of the spacecraft's four [[reaction wheel]]s began exhibiting increased levels of "bearing friction noise". Increased friction levels in Reaction Wheel Assembly (RWA)&nbsp;B were noted after its September 2008 encounter with asteroid Šteins. Two attempts were made to relubricate the RWA using an on-board oil reservoir, but in each case noise levels were only temporarily lowered, and the RWA was turned off in mid-2010 after the flyby of asteroid Lutetia to avoid possible failure. Shortly after this, RWA&nbsp;C also began showing evidence of elevated friction. Relubrication was also performed on this RWA, and methods were found to temporarily increase its operating temperature to better improve the transfer of oil from its reservoir. In addition, the reaction wheel's speed range was decreased to limit lifetime accumulated rotations. These changes resulted in RWA&nbsp;C{{'s}} performance stabilising.<ref name="McMahon2017">{{cite conference |url=http://esmats.eu/esmatspapers/pastpapers/pdfs/2017/mcmahon.pdf |title=Inorbit Maintenance of the Rosetta Reaction Wheels (RWAs) |conference=European Space Mechanisms and Tribology Symposium. 20–22 September 2017. Hatfield, United Kingdom. |first1=Paul |last1=McMahon |first2=Rene |last2=Seiler |first3=Andrea |last3=Accomazzo |first4=Sylvain |last4=Lodiot |first5=Patrick |last5=Van Put |first6=Roberto |last6=Port |display-authors=1 |date=2017}}</ref>

During the spacecraft's Deep Space Hibernation flight phase, engineers performed ground testing on a flight spare RWA at the [[European Space Operations Centre]]. After ''Rosetta'' exited hibernation in January 2014, lessons learned from the ground testing were applied to all four RWAs, such as increasing their operating temperatures and limiting their wheel speeds to below 1000&nbsp;rpm. After these fixes, the RWAs showed nearly identical performance data.<ref name="McMahon2017" /> Three RWAs were kept operational, while one of the malfunctioning RWAs was held in reserve. Additionally, new on-board software was developed to allow ''Rosetta'' to operate with only two active RWAs if necessary.<ref name="FAQ" /><ref name="sfnow20140129">{{cite news |url=https://spaceflightnow.com/rosetta/140129update/ |title=ESA says Rosetta in good shape after 31-month snooze |work=Spaceflight Now |first=Stephen |last=Clark |date=29 January 2014 |access-date=29 July 2014}}</ref> These changes allowed the four RWAs to operate throughout ''Rosetta''{{'s}} mission at 67P/Churyumov–Gerasimenko despite occasional anomalies in their friction plots and a heavy workload imposed by numerous orbital changes.<ref name="McMahon2017" />