With limited crew time available, NASA's fourth spacewalk to fix an aging dark-matter experiment on the outside of the International Space Station (ISS) may have to wait a while.
The agency has two big challenges as it schedules the last Alpha Magnetic Spectrometer (AMS) fix among other extravehicular activity (EVA) priorities: a lot of visiting vehicles crowding the schedule in December and a drop in crew numbers to only three people in February — half of the usual six.
"We will most likely not attempt an EVA while we're in the three-crew period," Space Station Program Manager Kenny Todd said in a livestreamed NASA press conference Tuesday (Dec. 3).
NASA, therefore, has a narrow window in January to finish off the work before half of the current Expedition 61 crew heads home. And with more-pressing spacewalks to perform, the final AMS repair may have to be put on the back burner for at least a few extra months. NASA needs to replace some batteries for the ISS power system, a more urgent task that requires two spacewalks to accomplish, agency officials have said.
There will be no adverse effects from leaving AMS dormant for a little longer, Todd said, although the team will do its best to complete the work in January if possible. The latest AMS spacewalk wrapped up successfully on Monday (Dec. 2).
"We'll see … what kind of plans that we're going to have for the batteries," Todd added. "Then, we'll make a decision on how to split up those three EVAs, if indeed we're going to be able to do three in January."
NASA spacewalks generally take up time from at least three crewmembers: two to perform the spacewalk and one to act as an "intravehicular crewmember," who assists the astronauts with their tasks, in cooperation with mission control on the ground. If only three people are on the space station, spacewalks leave little capacity to do other ISS duties or science experiments.
And there will be just three people onboard for a while starting in February, after Italian astronaut Luca Parmitano, NASA's Christina Koch and Russian cosmonaut Aleksandr Skvortsov depart for Earth.
The crewmember crunch is tied to delays in the development of private astronaut taxis, a project that NASA's Commercial Crew Program has been funding for nearly a decade. The agency wants these commercial vehicles to fill the crew-ferrying role formerly met by the space shuttle fleet, which was retired in 2011. Ever since then, NASA has been dependent on Russian Soyuz spacecraft to get U.S. astronauts to and from orbit.
SpaceX and Boeing are still working hard on preparing their vehicles — known as Crew Dragon and CST-100 Starliner, respectively — but neither provider is ready to send people into space yet. NASA hasn't purchased a Soyuz seat beyond April 2020, as far as we know, so crew numbers may stay low until one of the commercial craft is ready to fly people. (This milestone should come sometime next year, NASA officials have said.)
The crowded schedule of visiting vehicles this month complicates the picture. A used SpaceX Dragon cargo capsule launched to the ISS yesterday (Dec. 5), and a Russian Progress freighter did the same this morning (Dec. 6).
The Dragon will arrive at the ISS on Sunday (Dec. 8) for a roughly 30-day stay, and the Progress will get there on Monday morning (Dec. 9) for its seven-month visit.
Starliner is scheduled to launch on an uncrewed test flight to the orbiting lab on Dec. 19. The craft is expected to reach the space station on Dec. 20 and remain for four or five days, Todd said.
When cargo vehicles arrive at the station, crewmembers spend a certain number of hours unloading the equipment and supplies and then reloading the vehicles with trash or experiments to ferry away from the space station. So, that would leave little time in December to do any spacewalk, Todd said.
"We'll be looking to do that probably sometime after the first of the year, unless between now and then a window opens up," Todd said.
The battery work is a leftover job from when astronauts did two battery replacements in early October. A battery charge/discharge unit (BDCU) failed after the second spacewalk, and that unit was replaced during the first all-woman spacewalk, on Oct. 18. Though the BDCU is still working well, Todd said that NASA is investigating why the first unit failed, how that failure may be related to the battery replacements and how to prevent such an issue from happening again. "That team is still doing their job and coming up with their version of what might be happening," he said.
The best possibility for putting in all these spacewalks would be to start in early January, after the cargo Dragon leaves and clears out the supply-ship responsibilities, Todd added. Managers would then have until Feb. 6 to complete the spacewalk work before the beginning of Expedition 62. Later in 2020, there could be narrow windows of time to do the AMS spacewalk when the number of crewmembers jumps to six during changeovers, such as this coming April — but Todd said that isn't certain yet.
He added that doing the last AMS spacewalk with the current Expedition 61 crew would be convenient, since the spacesuits are currently sized for two of the crew astronauts: Parmitano and NASA astronaut Andrew Morgan. It can take about 12 hours of crew time to prepare the extravehicular mobility units (EMUs) that NASA uses for spacewalks; astronauts often need to switch out the torso section for larger or smaller body sizes and make other adjustments.
The AMS has been attached to the orbiting lab's exterior since 2011. The $2 billion experiment hunts for superenergetic cosmic rays, and its observations could help scientists better understand antimatter and mysterious dark matter and dark energy, NASA officials have said.
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