What space station crews call our “mission” is a bit more complicated than what you might think. Under normal operations, there are six crew members living on board station. We send up a three-person crew in the Russian Soyuz spacecraft four times a year, and the launches and landings are generally timed for spring and fall, to avoid severe weather in Kazakhstan.* This results in Soyuz crew overlaps of either four months or two months, with each three-person crew staying for about six months.
There are a number of advantages in this scheme, particularly during handover, when the newly arriving crew (we’re expecting one tonight) learns from the seasoned crew all the onerous nuances impossible to know except by being onboard.
Crews on space station are called “Expeditions,” a fitting name for a collection of explorers living on the frontier. Since there are two possible three-crew overlaps for each expedition, there are two possible expedition numbers that span a set of nine individuals. In addition, each crew of three arrives in a Soyuz with a designated engineering number, plus a space station mission number and a crew-chosen call sign. Thus, for my mission, I am Expedition 30 for four months, Expedition 31 for two months, and a crew member for Soyuz TMA-03M and Soyuz 29s, with call sign Antares.
This all gets multiplied by two, since we automatically function as backup crews for the mission that flies six months before us. So I am also backup crew for Expedition 28/29, on Soyuz TMA-02M and Soyuz 27s, with call sign Eridianus.
Then there are the management teams on the ground. These are people who work relentlessly through weekends and holidays to support the lucky crew members on space station. These management teams are called “Increments,” and they have numbers that usually correspond to the expedition numbers. Sometimes, though, these can get shifted to adjacent mission numbers. Of course, the nomenclature for increments, like expeditions, also gets multiplied by two, since every prime crew participates as backup crew for an earlier increment. When talking to crewmembers, people will speak in expeditions; when talking to NASA planners, they will speak in increments. Like the blind men feeling the elephant, we tend to describe our work from our immediate perspective. It is understandable that these subtleties can lead to confusion.
That’s why, when someone asks me what mission I am flying, the answer might lead to a conversation something like this: “I am backup crew for Expedition 28/29, also known as Increment 28/29, in Soyuz TMA-02M, or Soyuz 27s, called Eridianus, but am prime crew for Expedition 30/31 in Increment 30/31 for Soyuz TMA-03M, or Soyuz 29s, called Antares.” This kind of answer baffles even my fellow astronauts. I have decided that my mission identity is simply going to be dictated by the one with the largest three-crew overlap. Hence, I call myself Expedition 30. If you want the details, be prepared to settle in for a long conversation.
*There are exceptions. Expedition 29 (also known as Expedition 30, Increment 29, Increment 30, Soyuz TMA-22, or Soyuz 28s, with call sign Astraeus) slipped two months and launched in a November snowstorm so severe that from the viewing station only 1½ kilometers away, neither the rocket nor the launch pad were visible. At engine ignition, the TV cameras discovered they were pointed in the wrong direction, and quickly panned to the rocket, which appeared like a giant, slowly moving road flare—which was visible for perhaps 15 seconds before becoming completely obscured.