This was a very good episode. I won't give it a 10 because 10 is perfect and this doesn't quite reach that. It deserves an 8.5. All three of the main actors were excellent. Some reviewers are complaining because it is "predictable". Admittedly, after Lana's solution to the issue after the "unimaginable tragedy" I saw the general place this was going. But I did not see the final results. And the writing and acting to get to that point were great. At all points in the process, what happened made sense.
I largely wrote this review to refute others who wrote bad reviews because of a "plot hole" that they do not realize does not exist. These reviewers were not paying attention and/or were not thinking.
(Now I become a little geeky. There are no real spoilers here. The specifics I give are either from early in the episode, or my own calculations of reality instead of things mentioned in the episode.)
These reviewers ask why they did not send Replicas to the space station, instead of sending humans, since the Replicas could do what the humans do. This is explained early on: When fans greet David's (Josh Hartnell's character's) Replica outside the movie theater, he says central to the mission is "the human experience, the survival of the human body, of life".
Remember, this is set early in space exploration. If the Netflix description is correct, 1969, the first time humans were, in the real world, on the moon. At one point in this episode, it was said that Cliff and David were to be in space for 6 years. This mission was to determine the effects on humans of being in space for years. This information would be needed as humans went further into space. (Many of the things the humans did when they were awake were not the purpose of the mission. What they did when awake was just so the mission could continue.)
Replicas need the connection to the human: what the Replica does is controlled by the human. If you think about, Replicas could not go significantly further into space while the humans remained on Earth. Assuming the communication between humans and their Replicas is at the speed of light (it cannot be any faster) humans on the space station in geostationary orbit could communicate with Replicas on Earth in a fraction of a second. So what we saw in this episode is possible. But what about when we want to explore Mars? Even when Mars is at its closest to Earth, it would take a few minutes to communicate that distance at the speed of light. Humans on Earth controlling Replicas could not get much done with that kind of delay. When NASA's probes are landed on Mars, the probe runs a preprogrammed process, and the engineers at NASA wait for several minutes until they find out if it worked.
