Marking the end of Expedition 24, cosmonauts Alexander Skvortsov, Mikhail Kornienko and astronaut Tracy Caldwell-Dyson successfully undocked their Soyuz TMA-18 at the second attempt, before landing at 5:23am GMT. The scheduled undocking was delayed 24 hours, following a problem related to the latch and hooks between the Soyuz and the MRM-2 (Mini Research Module-2) module.
Soyuz TMA-18 Return:
The Soyuz Vehicle consists of three modules: the Orbital Module, the Descent Module (DM), and the Instrumentation/Propulsion Module (IPM). All three modules nominally separate simultaneously, shortly after the deorbit burn is completed – at around 140 km altitude.
The two ‘off nominal’ re-entries in 2007 and 2008 were the cause of separation failures on the modules, thus initiating the ballistic return for their three person crews.
However, no issues have been suffered with the vehicle since an investigation found that the most likely cause of the previous failures related to an open the latch located at ‘Plane I interface, much to the relief of all concerned, not least NASA, who will be relying on the Russian vehicle for several years after the retirement of the space shuttle.
The investigation findings noted issues with the long-term exposure to electromagnetic emissions on-orbit, and their potential to cause issues with the pyro bolts, came after an extensive investigation that included the removal and return of one pyro bolt from Soyuz TMA-12.
Undocking for the second attempt took place at 2:02am GMT, allowing for a landing near the city of Dzheskazgan, Kazakhstan, at 5:21am GMT on Saturday morning.
The reason for the 24 hour delay was first noted via an issue with a microswitch on the hatch between the Soyuz and MRM-2, which failed to show the correct latch indication. Problems were also noted with the opening of the hooks on MRM-2.
The ISS crew then discovered two broken teeth on a gear wheel – associated with the undocking mechanism – under the jacket of an actuator on the related attachment point.
Expedition 25 Commander Doug Wheelock and Flight Engineers Shannon Walker and Fyodor Yurchikhin sent down video and still photos of a small star-shaped gear with the two broken teeth, prior to a plan to install electrical jumpers – which will mimic the required hatch closed indication.
MRM-2 was delivered to the ISS as a component of the “Progress M/MRM2” Cargo Vehicle/Module Complex, consisting of a total mass of 7,102kg. It was docked directly to the Service Module (SM) Zenith port of the ISS Russian Segment (RS).
“The purpose of MRM-2 is to: Add to the station a specialized compartment with EVA capabilities during ISS RS operation. Conduct scientific research. Serve as an additional port for docking and operating Soyuz and Progress vehicles while mated to the station,” according to Flight Readiness Review (FRR) presentations relating to the module (L2).
MRM-2 is a companion module to the recently added MRM-1 – carried to the Station by Atlantis during STS-132. Soyuz TMA-18 is only the second Soyuz to undock from MRM-2.
Skvortsov, Kornienko and Caldwell-Dyson only have a matter of hours to prepare for re-entry – unlike the several days it takes for a shuttle orbiter to return home. The crew were already wearing their Russian Kentavr anti-G suit under their Sokol suits and had already ingested fluid-electrolyte additives to prepare their bodies for re-entry.
Once back in the atmosphere, the pilot parachute (0.62 and 4.5 square meters), drogue chute (16 sq.m.) and main (518 sq.m.) chutes will slow the vehicle’s decent. Just before the landing six small rocket engines behind the heat shield will fire, resulting in a landing speed of around 10 meters per second.
Airplanes and helicopters are on duty around all estimated landing spots – 12 in total – covering the nominal landing zones, and also at greater distances, in the event of a ballistic landing.
The trio successfully landed at 5:23am GMT.
Earlier on Thursday, the traditional ‘Change of Command’ ceremony took place, with ISS command now transferred from Alexander Skvortsov to Douglas Wheelock. Approval from a NASA standpoint was given back on August 31, via the 24 Soyuz/Expedition 25 MOD FRR (notes on L2).
“Still assessing if to perform another Stage EVA prior to arrival of ULF-5 (STS-133),” noted highlights from the FRR. “Sabatier installation has rolled to E25 from E24. Continue to monitor the Soyuz 700 simulator status and crew training in prep for flying the Soyuz 701. Work to assess any updates with Neptun panel based on problems that have been seen on two previous Soyuz missions that utilized the panel.”