ISS shuffles docking ports ahead of busy schedule

by Pete Harding

Soyuz MS-17 has relocated to a new docking port in preparation for a spree of upcoming vehicle arrivals at the International Space Station. The relocation ensures that a Russian spacewalk in June can proceed, which will prepare the station for its first major addition in a decade.

Soyuz schedule

Soyuz MS-17 arrived at the ISS on 14 October 2020, with its crew of Russian cosmonauts Sergey Ryzhikov and Sergey Kud-Sverchkov, along with NASA astronaut Kate Rubins. It initially docked to Mini Research Module-1 (MRM-1) Rassvet.

Soyuz MS-18 is set to launch to the station on 9 April for a three-hour fast-rendezvous profile, with its crew of cosmonauts Oleg Novitsky and Pyotr Dubrov, and recently announced NASA astronaut Mark Vande-Hei.

This means that the MS-18 and MS-17 crews will have a week-long “direct handover” period prior to MS-17 departing on 17 April, as opposed to the usual “indirect handover” whereby one Soyuz departs prior to another arriving

This direct handover is necessary in order to maintain a Russian presence on the ISS between Soyuz flights, as the remainder of the station’s crew all arrived via a SpaceX Crew Dragon spacecraft, which does not yet fly Russian cosmonauts.

In order to allow MS-18 to dock at the MRM-1 Rassvet port (where MS-17 currently resides), MS-17 will perform a “flyaround” relocation to the MRM-2 Poisk module.

This relocation is necessary due to spacewalk safety protocols for an upcoming Russian EVA in June, which will begin preparing the ISS for the arrival of the Multipurpose Laboratory Module (MLM), marking the first major addition to the ISS since 2011.

Russian spacewalks have usually began at Docking Compartment (DC) Pirs, but now occur from the Poisk module. Pirs is in the process of being decommissioned in preparation for its undocking to make way for the MLM.

The spacecraft docked to Pirs are usually Progress cargo spacecraft. With a crew spacecraft now docked to Poisk, some additional precautions are necessary in order to maintain crew access to the Soyuz vehicle during contingency EVA scenarios.

In the event of a failed repressurization of Poisk at the conclusion of a spacewalk, the Zvezda Transfer Compartment (which is a spherical section below Poisk) is used as a back-up airlock in order to allow the EVA crew to re-enter the ISS.

However, in this scenario, access to the Soyuz docked to Posik would be cut-off by the depressurized module.

It is for this reason that Soyuz MS-17 has been relocated to Poisk, as it will allow Soyuz MS-18 to dock to Rassvet, which in turn leaves Poisk clear once MS-17 has departed the station. This eliminating any concerns about access to the Soyuz in the event of a failed repressurization of Poisk.

US Crew Vehicles

Hot on the heels of the upcoming Soyuz rotations, a number of crew rotations via US Commercial Crew vehicles will occur.

Boeing Starliner approaches PMA-2/IDA-2 while a SpaceX Crew Dragon is docked to PMA-3/IDA-3 – via Mack Crawford for NSF/L2

First, a relocation of the SpaceX Crew-1 Dragon Resilience from Pressurized Mating Adapter-2 (PMA-2)/International Docking Adapter-2 (IDA-2) on the forward port of the Harmony node, to PMA-3/IDA-3 on the zenith port of Harmony is planned.

This will clear PMA-2/IDA-2 for the arrival of the Dragon Endeavour on the Crew-2 mission, currently planned to launch on 22 April. Crew-1 and Crew-2 will then have a week-long direct handover period before Crew-1 Dragon returns to Earth on 1 May.

Another relocation of Dragon from PMA-2/IDA-2 to PMA-3/IDA-3 will then occur – unless it is decided to dock the Endeavour directly to IDA-3 upon arrival, which will eliminate the need for any relocations at all.

This will then set the stage for Boeing Starliner’s uncrewed Orbital Flight Test-2 (OFT-2), which is now not expected to occur until at least the Summer timeframe.

In June, the SpaceX CRS-22 Cargo Dragon flight will occur, bringing with it the first two ISS Roll Out Solar Arrays (IROSAs) which will subsequently be installed onto the P6 Truss via EVA.


On 15 July, the much-delayed Multipurpose Laboratory Module (MLM) is finally set to be launched, to be followed two days later by the undocking of the Pirs module, having been a part of the ISS since 2001.

Pre-launch checkouts of the MLM underway at the Baikonur Cosmodrome in Kazakhstan – via Roscosmos

This will mark the first major addition to the ISS since 2011, and the first major addition to the Russian Segment since 2001.

Once MLM docks to the ISS, a series of Russian spacewalks will occur to commission the new module, with the European Robotic Arm (ERA) requiring deployment, along with the MLM’s radiator and scientific airlock, both of which were launched attached to the Rassvet module on STS-132 in 2010.

On 24 November, the Prichal Node Module is scheduled to be launched, which will dock to the nadir port of the MLM to serve as a docking hub for the expanded Russian Segment.

Remainder of 2021

August will see the SpaceX CRS-23 Cargo Dragon flight, followed in September by the launch of the SpaceX Crew-3 mission, as well as the return of the Dragon Endeavour at the conclusion of the Crew-2 mission.

The Starliner Crewed Flight Test (CFT) mission is also tentatively planned for this timeframe, although that flight is likely to move to the right in line with the delays to the uncrewed OFT-2 mission.

The crew of SpaceX Crew-2, from left to right: Pilot K. Megan McArthur of NASA, Mission Specialist Thomas Pesquet of ESA, Mission Specialist Akihiko Hoshide of JAXA, and Commander Shane Kimbrough of NASA – via NASA

On 5 October, Soyuz MS-19 will launch to the ISS for what is planned to be a double space tourist flight, both of whom will return to earth aboard Soyuz MS-18 on 17 October

For this reason, MS-18 crewmembers Pyotr Dubrov and Mark Vande-Hei will likely be required to remain on the ISS for a full year, returning to Earth on Soyuz MS-19 in March 2022.

Soyuz MS-20 is scheduled to make another tourist flight to the ISS on 8 December, returning to Earth on 20 December.

It is not yet known how NASA’s desired plan to begin flying Russian cosmonauts on US crew vehicles may affect future Soyuz crew compositions, as that would require NASA astronauts to fly on Soyuz in order to maintain an American presence on the ISS, should a Commercial Crew vehicle be required to return to Earth.

There will be no visit of Japan’s HTV spacecraft to the ISS in 2021, as an evolution of that vehicle called HTV-X is currently in development and is not planned to fly until 2022.

Sierra Nevada Corporation’s Dream Chaser cargo craft is also not now planned to visit the ISS in 2021, with the schedule for that vehicle having moved into 2022.

The private Axiom-1 flight of a Crew Dragon to the ISS has also moved into early 2022.

(Lead photo of Soyuz MS-17 during relocation – via NASA)

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