After accidentally extending its stay at the International Space Station (ISS), Europe’s Automated Transfer Vehicle (ATV-3) has successfully undocked from the orbital outpost on Friday. The extension of its stay nearly resulted in it conducting a Debris Avoidance Maneuver (DAM) for the ISS on Thursday morning, prior to managers deciding the debris threats would not pass close enough to the Station.
Europe’s resupply ship was scheduled to end its stay at the ISS on Tuesday evening. However, a problem with a required undocking command – sent just ahead of departure via the Russian Service Module “Zvezda” – resulted in the cancellation of ATV-3’s farewell.
As could be heard on the flight loop during live coverage of undocking events, Russian controllers noted issues with the command’s transmission, required to allow “Edoardo Amaldi” to depart.
(ATV-3 Docking Animation created from 70 hi res ATV-3 docking images acquired by L2 – LINK).
However, the root cause was only known the next day – following troubleshooting by ESA, NASA and Roscosmos teams.
“The joint ESA/CNES mission operations team reacted professionally and skilfully to this unexpected delay, and engineers immediately set to work to replan the undocking and reconfigure ATV into a safe mode while we and our international partners troubleshoot the issue,” noted Massimo Cislaghi, ESA’s ATV-3 Mission Manager.
The problem was found to be related to wrong spacecraft ID – 34 instead of 35 – being relayed through the system, an issue that was confirmed by telemetry analysis. ESA noted that ATV’s systems behaved perfectly until the interruption, and that the vehicle – currently in dormant mode – is in a stance to repeat an undocking attempt.
“Automated Transfer Vehicle (ATV)-3 Undocking Aborted: ATV 3 undocking was aborted, due to a failure of the ATV Control Panel and Russian Segment (RS) Laptop tests during the undocking operations. The problem was isolated to a configuration file issue on the Russian side,” noted L2’s rolling ISS internal update section – LINK.
“The Russians confirmed a good configuration file in their simulator and are ready to reattempt undocking again. The teams are working on the flight products to support an undocking reattempt on Friday.”
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ATV-3’s extended stay at the ISS was to include one final, additional role – as preparations were made to conduct a DAM – ahead of a new undocking date on Friday.
Given the DAM was cancelled, ISS managers are looking into the potential for an undocking to take place on Friday evening.
“ATV3 undock has been rescheduled for 9/28, 4:44pm local time. No open work is identified. ESA and RSC-E worked on the analysis of the commands and software setting between the ATV and SM to mitigate the issue that caused the malfunction of the ATV Proximity Communication Equipment (PCE) at the initial undocking attempt,” added L2′s ISS Updates.
“They identified the off-nominal status of the power busses connecting the SM and ATV that have led to a situation where we had a physical separation but with powered connectors. Both sides addressed this situation, and are ready to repeat the undock with the proper settings in place.”
ISS DAM Cancelled:
After accidentally extending its stay at the ISS, ATV-3 was set to help the Station conduct a Debris Avoidance Maneuver (DAM) on Thursday morning. However, the orbital dodge was cancelled, after the two pieces of debris were declassified from the status of “red” collision threats.
The additional role for ATV-3 was to fire its thrusters to help the ISS dodge two pieces of space debris – a piece of Russian COSMOS satellite debris and a fragment of an Indian PSLV rocket body – both of which were classed as red collision threats up until Thursday morning.
The time of closest approach for the Russian debris was calculated to occur on Thursday at 9:42 am Central time, around two and a half hours after the Station was to begin to move out of its path.
The DAM was also set to move the ISS out of the way of the Indian debris, which was due to pass by on Friday.
“ISS Debris Avoidance Maneuver (DAM): Trajectory Operations Officer (TOPO) console has been tracking two ISS conjunctions. They are Object 34309 (COSMOS 2251 Debris) with a Time of Closest Approach (TCA) on Thursday 09:42 AM CDT and Object 27107 (PSLV Debris) with a TCA on Friday, 12:47 AM CDT,” noted L2’s rolling ISS internal update section – LINK.
“The objects meet the Probability of Collision (Pc) red threshold requirement for performing the DAM. A 0.3 m/s posigrade DAM is being planned for Thursday using ATV 3 thrusters. The Time of Ignition (TIG) is scheduled for 7:12 AM CDT.”
However, by Thursday morning – seven hours ahead of TIG – both objects were no longer in the “Red box”, allowing for the cancellation of the DAM.
“ISS Debris Avoidance Maneuver (DAM) on 9/26: Updated tracking overnight confirmed that the objects being tracked are no longer in conjunction with the ISS. The DAM planned for today was cancelled,” added the ISS updates.
The Expedition 33 crew – led by commander Suni Williams – were never in danger, due to the advanced planning, and wouldn’t be required to close hatches and take to the safe haven of the Soyuz vehicles that double up as lifeboats on the ISS.
That procedure is only required via what is known as a “RED threshold late notice conjunction threat”, where tracking of a debris collision threat comes after it’s too late for the ISS to conduct a DAM.
The safe haven option has only been called for twice, the first time in March 12, 2009, when a “yo weight” – which was originally part of a Delta PAM-D stage used to launch GPS 37 in 1993, also known as Object “25090 PAM-D” – missed the Station.
This event led to additional reviews into the safe haven procedure, as overviewed in a presentation acquired by L2 in 2009. The document covered all elements of debris threats, including a rather morbid description of what would happen to the crew if the ISS pressure modules were breached by an impact.
The second safe haven event was in July of 2011, when an unidentified piece of debris passed just 250 meters from the Station during Expedition 28.
Red threats are not uncommon. However, once tracking models are refined over time, they tend to be projected on a path further outside of orbital corridor the ISS is sailing through, removing the need for either a DAM or safe haven procedures – as was the case this week.
(Images: L2’s ISS and ATV sections, plus NASA)
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