After the failed Progress M-27M / Progress 59 resupply mission in late April 2015, Roscosmos and NASA have been working on ISS schedule changes to accommodate loss of supplies to the station, while an investigation into Progress M-27M failure is still ongoing. Yesterday, May 12, 2015, the following schedule changes were announced:
- Return of Soyuz TMA-15M (Virts, Cristoforetti, Shkaplerov) in early June instead of on May 13,
- Progress M-28M launch in early July instead of August 6,
- Launch of Soyuz TMA-17M with Kjell Lindgren (NASA), Kimiya Yui (Japan Aerospace Exploration Agency) and Oleg Kononenko (Roscosmos) in late July instead of on May 26.
Not surprisingly, this includes changes to the crew rotation scheme. But it also delays Soyuz TMA-15M crew return for at least 18 days, possibly more. And that is somewhat surprising to me, considering that Progress M-27M supplies weren't delivered. While the station isn't running out of supplies, having a crew of six instead of a crew of three aboard for an extended period should thin them out faster. So to my question:
Given slightly extraordinary circumstances, is keeping a crew of six aboard the station for longer and maintaining tight crew rotation scheme better than extending the period during which the station only hosts a single Soyuz TMA crew of three?
In other words, I'm asking why wouldn't the Soyuz TMA-15M crew return as previously scheduled, and then launch Soyuz TMA-17M later, keeping only the Soyuz TMA-16M crew of three on the station for a few weeks longer? Is Soyuz TMA-17M launch still considered too tentative, pending conclusion to the Progress M-27M failure investigation, or something else prompted Soyuz TMA-15M crew return delay?