Cupola hit by minor MMOD strike, shutter closed for evaluations

by Chris Bergin

Window 2 on the Cupola module has been hit by a minor MicroMeteoroid and Orbital Debris (MMOD) strike. With the window’s protective shutter closed, per flight rules, ISS managers are evaluating photos of the damage – downlinked from the International Space Station (ISS) – before they are expected to give the crew clearance to reopen the shutter.

Cupola Window:

After arriving at the ISS with Node 3, during Endeavour’s STS-130 mission, the European Space Agency (ESA) built Cupola has provided Station crews with a stunning view of the planet, often shared with the public via downlinked photography.

The module also hosts a Robotic Work Station (RWS), allowing crewmembers to actually see Visiting Vehicles (VVs) – such as SpaceX’s Dragon and Japan’s HTV – arrive for berthing, complimenting the camera views of their displays, allowing for increased situational awareness when operating the Station’s robotic assets.

The Cupola’s seven windows were exposed to space for the first time during STS-130’s EVA-3, following the removal of the module’s MLI blanket insulation by spacewalkers Bob Behnken and Nick Patrick.

After the spacewalkers removed the launch locks on the windows, the ISS crew cycled the window shields/shutters one at a time, providing them with the first view of the Earth from their new observation deck.

All of the windows weren’t open at the same time, with the task simply used to check the shutters opened without a problem. A few hours later, all of the windows were opened together, an event that is now commonplace on the ISS.

The module has hardly suffered from any issues during its tour of duty with the orbital outpost, with only a minor temperature issue noted ahead of Dragon’s arrival last month.

Through no fault of its own, the MMOD strike is a risk all spacecraft have to deal with, although exposed windows are built to withstand such impacts.

This latest strike, to Window 2 on the Cupola, looks more dramatic via close up photos, but is in reality very small and probably would not have been instantly obvious to crewmembers entering the module.

“Cupola Window MMOD Strike – On June 10, the crew reported an MMOD strike on Cupola window 2 and downlinked photos (full set available on L2) of it,” noted L2 level ISS Status (LINK). “The shutter for window 2 is closed now, as required by flight rules, until the MMOD strike has been evaluated. 

“Teams are working to determine the structural implications of the MMOD strike and clear the window 2 shutter for re-opening if possible.”

The flight rules are naturally cautious, in order to ensure a large amount of margin for crew safety. It is highly likely this will be cleared as an issue after the evaluations are completed.

Recent ISS Articles:

It is understood that this MMOD srike is the first to be suffered by a Cupola window.

In the event of the damage being more serious, on-orbit replacement of an entire window is a design feature. Such a replacement would require an EVA to fit an external pressure cover to allow for the changeout, with a pressure cover requiring a flight up to the ISS.

The window is made up of four panes – an inner scratch pane to protect the pressure pane from accidental damage, two pressure panes 25mm thick to maintain cabin pressure, and finally an outer debris pane.  The debris panes can be replaced individually, via EVAs. 


Strikes of these tiny pieces of debris are relatively commonplace and are only immediately noticeable when they impact on areas such as windows.

Recent MMOD related articles:

Such strikes were observed on a large amount of Space Shuttle missions, especially late into the mission when the orbiter had undocked from the protection of the ISS and was preparing to head home. However, they are taken seriously, given they are the third biggest threat to losing a vehicle during a mission – second only to launch and re-entry.

Extensive evaluations took place ahead of each shuttle mission, not least Atlantis’ STS-125 flight into space, a mission that provided one final servicing mission to the Hubble Space Telescope.

This mission held the largest risk to the orbiter, given it was the only post Return To Flight mission that did not have the “safe haven” option of docking with the ISS, ahead of a rescue orbiter being flown to pick up the crew.

STS-125 would also see an increase in the concentration of MMOD, due to the region of Low Earth Orbit (LEO) Atlantis flew in for the majority of her mission.

The Program Requirements Control Board (PRCB) results – which took into account satellite breakups and a variety of other components evaluated to be in Atlantis’ orbital neighborhood – showed that the overall risk of a Loss Of Crew and Vehicle (LOC/V) scenario due to MMOD impact(s) to the Thermal Protect System (TPS) was 1 in 185, with an error factor of 1.35 based on MMOD distribution, velocity, and density uncertainties.

As noted, most of the risk was associated with impacts on the TPS, as opposed to the windows. In the end, Atlantis completed her mission successfully and without any MMOD related concerns.

Examples of Mission Management Team (MMT) evaluations into window damage during a mission can be seen via STS-126’s flight, where then MMT Chair LeRoy Cain discussed the Mission Evaluation Room (MER) report into an impact on Endeavour’s Window 6 (Pilot Window).

“MMOD Window 6 Impact – crew reported 0.5” diameter to the PLT right window to the thermal pane, 1/32” depth,” noted the MER report. “Based on analysis the window should have no issues for peak heating M=8.0, but could form cracks but remain intact during peak loading M=0.7. Pilot visibility should not be hindered by this location if it cracks.

“No concern for orbit thermal cycles. Dynamic loading on the pane takes into account for the window frame thermal response.”

Meeting notes showed Mr Cain asked if there was a need to mitigate heat loads on the window – had it been determined that could have been a problem during entry – with his team reporting back that the thermal pane of the window would be able to handle peak heating.

Clarifying the impact showed no signs of risking a LOV/C during re-entry, it was noted that only a hole through more than just the thermal plane of the window would raise concerns “we might have to not come home,” which would have resulted in calling Discovery to provide a LON (Launch On Need) rescue.

Endeavour landed safely with no propagation of the MMOD damage during her reentry.

Both Endeavour and Atlantis would also receive a flesh wound from MMOD strikes to their radiator panels, a danger in its own rights, given the potential of an impact to one of the Freon-22 coolant plumbing inside the radiator panel. Such an impact could cause an entire coolant loop to be shut down and be declared as ‘failed’. This would have forced an immediate landing on the earliest US landing opportunity.

An example of the risk was seen during STS-109, when a small piece of debris was lodged in Columbia’s coolant loop 2 and restricted the flow of Freon-22 in that coolant loop. The amount of Freon-22 in the coolant loop was slightly below the flight rule red-limit, but after exhaustive analysis by the engineers on the ground, they decided to press on with the mission.

Usually spotted during post flight processing, Endeavour’s radiator damage showed as a “bullet hole” on her left hand aft radiator number 4. Engineers repaired the area during her Orbiter Processing Facility (OPF) flow.

Atlantis’ damage during STS-115 was the second largest debris strike in Shuttle history, causing a .108 inch diameter hole at entry point, with one-half of an inch thick damage to the honeycomb structure.

Interestingly, United Space Alliance (USA) engineers discovered a piece of circuit board embedded into the radiator panel during repairs, showing the MMOD strike originated from a piece of another vehicle, likely an Upper Stage from an expendable vehicle or a satellite.

(Images: L2 Content (all MMOD content), and NASA)

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