I come in peace... three times a night -- FOR ALL MANKIND (Apple TV Plus)

Lol, pretty much yeah

Can someone explain why Sergei was killed? Obviously it must have been the Russians that did it, but the Roscosmos lady didn't know Sergei was talking to Margo and Aleida, so why would she have ordered the hit?

Think was because he defected and they didn’t know where he was until he shows up in Houston.

But no-one saw him in Houston except Margo and Aleida?

I thought she did know, including their plans to escape and operate a space burger program in bolivia, and Margo sussed that she had Sergei killed, which factored into Margo's "revenge"

I must have missed that, but I have no idea how she found out.

the show never actually comfirms margo ever successfully evaded her minders so maybe they knew everything from the beginning?

Never explained how they knew about Sergei but very likely they had Aleida's house bugged/monitored since Margo was regularly talking to her and going there. In fact, my wife and I thought it was a bit silly that Margo met him there, but there's a bunch of stuff in the season that doesn't make sense

Real mars
https://www.youtube.com/watch?v=Vy_RPd0rblI

It looks way too terrestrial though! It's weird how the show fits my bad SF-addled concept of Mars more than real Mars.

Also the scene when Margo met Sergei outside a diner stretches credulity because she would never have been allowed there without her minders.