Docking Compartments for the ISS

An isolated view of the Docking Compartment/Mini-Research Module-2 in orbital configuration

The Docking Compartment, also known as Pirs ("Pier") or Docking Module, provides the airlock for the spacewalks from the Russian segment of the International Space Station, ISS. The module features a large hatch, 1 meter in diameter (see photos on this page), allowing spacesuited cosmonauts to exit and return to the station. (NASA developed its own, larger airlock for the US segment of the outpost.)

The Russian Docking Compartment also provides a third docking port for the arriving Russian spacecraft. Without Docking Compartment, the Russian manned Soyuz and Progress cargo ships can dock either to the aft port of the Zvezda service module, or to the Earth-facing (nadir) port of the Zarya control module.

Two other existing docking ports onboard the Zvezda module (so-called zenith and nadir ports) have the interface designed for larger and heavier modules and it is not compatible with the standard hardware normally installed on the Soyuz spacecraft. With only two regular docking ports available on the Russian segment, the Progress supply ship has to be undocked from the station, every time the new Soyuz spacecraft arrives to the outpost to replace its older predecessor.

After the Soyuz exchange is completed the Progress cargo ship has to be either discarded or redocked to the station with the use of the TORU remote-control system operated manually from the inside of the ISS. In 1997, the Progress slammed into the Mir space station during the docking exercise with the use of the TORU remote control -- the accident, which could cost the Mir crew its life.

With the Docking Compartment-1, occupying the Earth-facing nadir port of the Zvezda module, the incoming Soyuz spacecraft will be able to dock to the compartment's free Earth-facing port. (see photo)

On its way to the station, the Docking Compartment will also serve as a cargo spacecraft, delivering the third space suit and the second Strela deployable boom, which will facilitate EVA work onboard the station.

Finally, additional supplies and consumable items needed by the crew at the time of the module's launch will be loaded in the free volume inside the spacecraft.

RKK Energia in Korolev originally developed the Docking Compartment as one of the elements of the Mir-2 space station, the Russian successor to Mir. With the merger of the Mir-2 and Freedom projects in 1993, the Docking Compartment became the part of the Russian segment of the ISS. As several other Mir-2 modules, the Docking Compartment is designed to maneuver in orbit and dock with the station with the help of the instrument and propulsion section, PAO, borrowed from the Progress cargo ship. Combined with PAO section, the Docking Compartment-1 fits under standard payload fairing of the Soyuz launcher. The maximum diameter of the module does not exceed 2.2 meters -- standard for the Progress spacecraft.

After the Docking Compartment docks to the ISS, the PAO section of the module jettisons to expose the standard docking port on the spacecraft, intended for the Soyuz docking.

According to the construction plan of the International Space Station, the Docking Compartment-1 has to be eventually replaced with a much larger Universal Docking Module, UDM. Launched by the Proton rocket, the UDM would provide life-support capabilities for as many as six crew members onboard the Russian segment of the ISS. Prior to UDM arrival, the Docking Compartment-1 would be discarded to free the Earth-facing "nadir" port of the Zvezda service module for the UDM.

The new Docking Compartment-2, as well as Russia's future science modules, would then be able to dock to multiple ports onboard the UDM. However, the development of the UDM fell far behind schedule, due to severe underfunding of the Russian segment by the federal government.

Changing role

During a redesign of the Russian segment of the ISS in 2001, RKK Energia decided to use Docking Compartment-1 as an "interface" between simplified NEP platform and the zenith (upward looking) docking port of the Zvezda service module. The construction of the Docking Compartment-2 would be abandoned altogether.

To play its new role, the Docking Compartment would be equipped with a grappling device for the station's robot arm, which would be used to place the module to its new location prior to the arrival of the FGB-2 to the station. Some minor modifications inside the docking compartment would be required as well.

In 2001, the transfer of Docking Compartment from nadir to zenith docking port of the Zvezda service module was expected at the end of 2004.

Mission chronology of Docking Compartment 1

2001 Jan. 9: RKK Energia started final testing of the Docking Compartment, the next Russian element of the International Space Station, ISS. The module's construction was lagging behind schedule due to financial problems. Although there were still some technical and financial issues to resolve, RKK Energia hoped to ship the module to Baikonur in March or April in preparation for the launch onboard the Soyuz rocket in June 2001.

Officially, the launch date is still set for June 1, 2001, however, a manager of the project said on January 4, it couldn't take place before the end of June. RKK Energia still needed $1 million in funding from Russian Aviation and Space Agency, Rosaviacosmos, to purchase subsystems for the orbital tug, which would deliver the Docking Compartment from its initial orbit to the station. This figure did not include the cost of the Soyuz launch vehicle and its payload fairing, which Rosaviacosmos, procures directly from its manufacturer.

The Russian federal government approved the budget for the space program, however, Rosaviacosmos was yet to allocate funds for particular projects, RKK Energia officials said.

In the meantime, the work on two larger Russian elements for the station remained at a virtual stand still due to lack of funds, RKK Energia managers said. The development of the so-called Science Power Platform, which was expected to carry solar panels, radiators and a robot arm for the Russian segment, was frozen at the initial construction stage.

RKK Energia tried to find "non-government" solutions to the problem of the Russian segment, by paying for the construction through commercial deals with its Western partners. A scaling down of the elements of the Russian segment was also under consideration.

2001 July 13: The Docking Compartment-1, was to leave Moscow for Kazakhstan today in preparation for its launch then scheduled for September 2001.

Late that night, the new module, known as Docking Compartment-1, or Pirs (Pier), was to leave the RKK Energia's manufacturing and testing facility in Korolev, on the northeastern outskirts of Moscow for a journey by rail to its launch site at Baikonur Cosmodrome in Western Kazakhstan. The module was expected to arrive to Baikonur's Site 254 on July 16 for its final processing before its launch, then scheduled for September 15, 2001.

According to RKK Energia sources, the Docking Compartment was to be used for a "commercial" spacewalk in the fall of 2001 paid for by a Japanese company. The purpose of the activity was yet to be disclosed, however it was expected to have an advertising nature.

The module's departure for Baikonur coincided with the Shuttle mission to install a larger airlock on the US segment of the station.

2001 July 16: The Docking Compartment arrived to Baikonur.

2001 July 18: The Docking Compartment was installed into a processing facility inside Site 254 in Baikonur. The preparation for a two-week testing of the electrical system of the module was scheduled to start on July 19.

2001 Sept. 15, 03:35 Moscow Time: A Soyuz rocket successfully launched the Docking Compartment toward the ISS; (previous launch date: Aug. 25)

2001 Sept. 16: The Docking Compartment-1, dubbed Pirs, docked automatically to the nadir port on the Zvezda service module at 8:05 p.m. CDT as the station orbited 250 miles above Mongolia.

The docking went according to plan, with the automated docking system controlling a Progress-style instrumentation and propulsion system attached to the rear of the Pirs compartment itself.

The 16-foot-long, 8,000-pound module approached the station from below and behind, beginning its automated docking sequence shortly after 5:30 p.m. About 20 minutes later, the station's thrusters moved it to the proper orientation for docking. The station's large solar array wings were positioned to eliminate contamination from the jets on Pirs as it made it final approach.

After the probe-and-drogue docking system completed capture of the incoming module and pulled the two spacecraft together, 12 active latching hooks were driven to their closed position, locking the module securely in place.

After docking, the Expedition 3 crew checked to make sure there was a good seal between the station and its new module, then began to equalize pressure between the two craft prior to the first opening of the hatch to Pirs, which was scheduled later that evening. The aft instrumentation and propulsion system locked onto the docking compartment itself was to be jettisoned later that month to set the stage for spacewalks by the crew to install and activate key systems for the compartment's future operation.

Three space walks were to be conducted in October and November 2001 from Pirs by the Expedition 3 crew - two by Pilot Vladimir Dezhurov and Flight Engineer Mikhail Tyurin and one by Culbertson and Dezhurov - to electrically mate the Docking Compartment to Zvezda and install more equipment on the outside of the module.

2001 Sept. 26, 3:36 a.m. Moscow Time: Mission control in Korolev commanded the propulsion and instrument section, PAO, to jettison from the Docking Compartment of the Russian segment of the ISS. The PAO section then fired its engines to deorbit. The maneuver freed the nadir (Earth-facing) docking port on the Docking Compartment, which will be used by the ISS-bound Soyuz spacecraft. The PAO section originally served as an orbital tug for the Docking Compartment during it delivery from the initial orbit to the ISS.

From Docking Compartment to Research Module, MIM-2, Poisk

By 2008, Docking Compartment-2 was back on the list of the future modules of the Russian segment, but it was bearing a new name: Mini-Research Module-2, MRM-2, or MIM-2 in Russian. The 4,000-kilogram module was scheduled for launch in 2009, (as early as Aug. 12, 2009), as part of a specialized custom-built Progress cargo ship, designated M-SO2 (No. 302). The total mass of the Progress M/MIM-2 stack is 7,150 kilograms. The MIM-2 would be docked to the zenith (upper) docking port of the Zvezda service module.

Like its older twin -- Docking Compartment-1 -- MIM-2 would feature a passive docking port for the Soyuz and Progress ships on its outer end and provide 12.5 cubic meters of internal volume. It could also serve as an airlock for spacewalking cosmonauts. On its forward end, MIM-2 would carry an active hybrid port SSVP-M G8000, enabling docking with passive hybrid ports of the service module.

However, unlike the DC-1, the new module would sport power-supply outlets and data-transmission interfaces for two external scientific payloads to be developed by the Russian Academy of Sciences, thus justifying its name as a research module. On its way to the station, MIM-2 was to carry a ton of cargo.

In the meantime, the Docking Compartment 1 would remain on the nadir (Earth-facing) docking port of the Zvezda service module until the arrival of the Multipurpose Laboratory Module, MLM, then scheduled for launch in 2011. A Progress cargo ship would be used to haul away and deorbit the Docking Compartment 1 at the end of its mission.

Russians jump-start station construction

Published: 2009 Nov. 9; updated: 2009 Nov. 10; 2010 Jan. 3

After almost a decade-long hiatus, Russia resumed the construction of her share in the International Space Station with the launch of a new module. The Soyuz-U rocket carrying Poisk ("Quest") Mini-Research Module-2, or MIM-2 in the Russian abbreviation, lifted off from Baikonur Cosmodrome in Kazakhstan as scheduled on Nov. 10, 2009, at 17:22 Moscow Time. According to reports from Russian mission control, the spacecraft reached its intended orbit successfully.

The spacecraft is essentially a twin of the previous Russian module, the Pirs Docking Compartment added to the outpost in September 2001. In the intervening years, economic problems kept further Russian pieces of the station on the ground and forced a significant scale down of the Russian segment in comparison to its originally conceived architecture.

The MIM-2 module was only first of three long-term components, which Russia planned to add to the station in the following three years. At the time, another module, Mini-Research Module-1 or MIM-1, was undergoing final checkups at RKK Energia in Korolev, near Moscow, Russia’s prime contractor in the manned spacecraft. In December 2009, MIM-1 was shipped to Cape Canaveral, Florida, from where it would be launched to the station in the cargo bay of NASA Space Shuttle in May 2010. The MIM-1 spacecraft was recycled from the habitation section of the aborted science and power-supply platform, NEP, which had never been completed due to lack of cash.

MIM-2 was scheduled to dock to the International Space Station, ISS, on Nov. 12, 2009, at 18:43:30 Moscow Time. (The actual docking took place at 18:41 Moscow Time). The crew was scheduled to enter the module a day later, and on Dec. 8, 2009, the Progress propulsion module of the MIM-2 module was to be discarded.

To prepare the MIM-2 for receiving Soyuz and Progress transport ship, cosmonauts Oleg Kotov and Maksim Suraev were scheduled to conduct a spacewalk out of the Pirs docking compartment on Jan. 14, 2010. They were to deploy AR-VKA and 2AR-VKA antennas and a docking target and plug the new module's Kurs antennas into the Kurs docking system circuitry instead of antennas on the zenith port of the Zvezda service module, which would no longer be need after the docking of the MIM-2 to that port. Following the spacewalk, on January 20, 2010, the Soyuz TMA-16 spacecraft would be re-docked to the zenith-facing port of the MIM-2 module.

Progress undocking

A propulsion section of the Progress spacecraft, which delivered MIM-2 to the station was undocked from the module on Dec. 8, 2009, at 03:16 Moscow Time (00:16 GMT. It was Dec. 7, 6:16 p.m. in Houston). The module was deorbited four hours later, with the reentry burn initiated at 07:48:30 Moscow Time (04:48:30 GMT).


Docking Compartment-1 and Mini-Research Module-2 technical specifications:

Mass at launch
4,350 kilograms
Mass at orbit insertion
3,580 kilograms
Available mass for cargo delivery
800 kilograms
Length with docking probe extended
4.91 meters
Maximum body diameter
2.55 meters
Pressurized compartment volume
12.5-13 cubic meters

Page author: Anatoly Zak; Last update: June 1, 2021

Page editor: Alain Chabot; Last edit: October 20, 2008

All rights reserved




A full-scale mockup of the Docking Compartment used for cosmonaut training in Star City can be seen attached to the Zvezda service module. In reality, the module will face Earth, while in orbit. Copyright © 2001 Anatoly Zak


The Docking Compartment 1 (SO1) with its Progress M SO1 orbital tug is being unloaded from a railway after its arrival to Baikonur. Copyright © 2001 Oleg Urusov


The Progress M SO1 spacecraft carrying Docking Compartment-1 docks to the nadir (Earth-facing) port of the Zvezda service module of the International Space Station on Sept. 16, 2001. Credit: NASA


A discarded instrument module of the Progress M SO1 spacecraft, which delivered the Docking Compartment-1 to the station, drifts away on Sept. 26, 2001. Credit: NASA


The Pirs docking compartment as seen by the Space Shuttle crew during the STS-108 mission. Note that only one Strela ("arrow") crane (seen on the left) was installed at the time. An interface cover for the second boom can be seen on the right side of the module. Credit: NASA


The Pirs docking compartment photographed by the Space Shuttle crew during the STS-114 mission. Two Strela ("arrow") cranes are already in place. Credit: NASA


Crewmembers from Expedition 14 onboard the ISS, pose in Russian Orlan spacesuits inside the Pirs docking compartment. The hatch for spacewalks can be seen on the right. Credit: NASA


Russian cosmonaut Sergei Volkov from Expedition 17 works on the exterior of the Pirs docking compartment during a spacewalk on July 15, 2008. A transfer boom of the Strela crane can be seen extending toward the foreground on the right. Credit: NASA


Docking Compartment's hatch open into vaccum of space during a spacewalk. The Soyuz spacecraft can be seen docked on the left. Credit: NASA

MIM-2 launch

The Soyuz-U rocket carrying the MIM-2 Poisk module lifts off on Nov. 10, 2009. Credit: TsENKI

MIM-2 approach

MIM-2 approaches ISS on Nov. 12, 2009. Credit: NASA


The MIM-2 module as seen by the crew of ISS on Nov. 21, 2009, days after its docking to the zenith-facing port of the Zvezda service module. Click to enlarge. Credit: NASA

PAO undock

Unlike poor quality images from Docking Compartment-1 mission in 2001, a departing propulsion section of the MIM-2 module was photographed at high resolution on Dec. 8, 2009. Click to enlarge. Credit: NASA


A Russian cosmonaut Oleg Kononenko, a member of Expedition 31 on the ISS, poses inside the Pirs Docking Compartment on May 12, 2012. Credit: NASA