Season 1
"Into The Abyss"- Ed and Molly landing on the Moon on Apollo 15. Ed came so close but had to back off when flying Apollo 10, while Molly never even had the chance when the Mercury Thirteen program was canceled. They both finally got a second chance and accomplished something that they'd been waiting years for.
"Rupture"
- The death of Ed and Karen's son, Shane.
- What's worse is that Ed isn't told so as not to distract him from the mission, which results in the Soviets unknowingly setting him off when they try to offer their genuine condolences.
"A City Upon a Hill"
- Ellen tries coming out to Deke after they both think they're as good as dead. Deke, who has previously been shown as a stern Father to His Men, lashes out. What's even worse is that he later makes amends and encourages her to remain in the closet, fearful that people will react just as he did.
- After Ellen successfully captures the fuel canister to refuel Apollo 24, she excitedly asks Deke if he saw what she did when she returns to the CSM. Unfortunately, she finds Deke slumped over in his seat, as he has died of his injuries. Then, while we don't hear Ellen relaying the news of his death to Houston, we do see Gordo receiving the news. While everyone around him is celebrating, the smile quickly fades from his face and he goes to tell Margo. We see Deke's wife Marge inside the observation gallery celebrating with everyone else and then she sees the serious looks on Margo and Weisner's faces and realizes that something is wrong. Weisner comes in to the gallery to inform Marge that Deke has died and she immediately breaks down.
Season 2
"And Here's to You"- Ed helping Gordo into his suit prior to Columbia's launch becomes very sad in light of Gordo's death, as neither know it at the time but it's the last time Ed and Gordo see each other. Ed makes sure to tell Gordo how proud he is of him for working hard to get back into space.
"The Grey"
- Doubles as an awesome moment, but the deaths of Gordo and Tracy in the Season 2 finale. As heroic as the circumstances are, it is devastating to see them succumb to the vacuum of space and to see their frozen, bloodshot bodies when Rossi opens the airlock. It's especially heartbreaking for their sons, who lose both their mother and father at once.
- The funeral is equally devastating, especially when Ed begins to break down and cry openly.
- The Apollo-Soyuz docking is an amazing moment but it becomes a tear jerker when it becomes clear that almost no one was able to watch it due to the threat of imminent nuclear war causing everyone to take shelter.
Season 3
- Ed realizing that he's over the hill with nothing to show for it. It gets worse as the season goes on as he goes from getting removed from the Mars mission to seemingly a prominent position at Helios, only for that to then end in shambles when he's locked out of his own ship after trying to go against orders to save the failing Russian Mars-94 craft.
"Seven Minutes of Terror"
- As Ed and Danny begin the descent to Mars, Ed has flashbacks to the time he and Gordo almost landed on the surface of the Moon. His second chance is null due to a rapid descent that causes Ed to activate the abort sequence and return to the Phoenix. While it's satisfying to see Dev and Helios get taken down a peg as NASA lands first on Mars, it's easy to forget that Ed was desperate to land on the Moon. Seeing him get robbed of what is likely to be his only other chance is utterly crushing.
- After he hits the abort, the camera keeps going and shows that they were just meters from the ground. However, with no visibility and no altitude readings, Ed probably would have been killed trying to be first.
- The reaction of Helios mission control when Popeye aborts and Sojourner-1 lands. While Dev is seething, the other Helios employees look genuinely despondent at the fact that they were so close to winning the race for Mars and ultimately will have to settle for second place.
"New Eden"
- The predicament of Will Tyler. After landing on Mars, he soon comes out as gay in a heartrending speech. The response? He's shunned by the crew, especially Rolan who was previously a close friend, and the For All Mankind equivalent of "Don't Ask, Don't Tell" is enacted by Ellen. Someone give the poor guy a hug.
"Bring It Down"
- Ellen finally learns that Pam left her due to her growing career.
"The Sands of Ares"
- Ellen and Pam's confrontation is painfully dour as Ellen actually seems prepared to abandon everything to be with Pam, who brushes her off and indirectly accuses her of being a Boomerang Bigot. It's a depressing contrast from their previous relationship, which was shown to be very loving.
- While everyone is trying to figure out how to rescue Ed and Danny, Dev is sitting alone in the viewing gallery of Mission Control. He tells Margo that everyone knows the story of how his dad came to America with nothing and became a success. Unfortunately, what he leaves out is that his dad worked for the company which produced the faulty component that destroyed Apollo 23, and he lost his job as a result.
- Trapped underground with Ed, Danny finally confesses that he's to blame for Shane's death in season 1, and that he's spent his whole adult life trying to make up for it, only to ruin everything for his family and his shipmates in the process.
- Alexei's death, and Kelly's (and Danny's) reactions.
- What's worse is that he was shown to be incredibly excited to meet Kelly's father. Imagine how happy he would've been to learn he was going to be a father?
"Stranger in a Strange Land"
- Lt. Col. Lee's entire mission up until meeting Poole and Kuznetsov. He's sent on a mission to Mars in a tiny craft that barely has enough room for him and his co-pilot. Given that it's North Korea, it's entirely possible that neither of them was a willing participant. They crash land and the co-pilot dies. Their communication equipment is damaged to the point that he has no way of communicating back to Earth that he survived. As far as anyone in North Korea knows, including his family, he didn't survive the trip. He somehow manages to survive on the meager supplies on the ship but the isolation and the knowledge that he was likely sent on a one-way trip drives him close to a Despair Event Horizon. He is ready to shoot himself with the gun on board the ship (which was likely for that purpose to begin with) before he sees the NASA rover driving over the actual horizon and he realizes that he is not alone. At the end of the episode, we see that he gets to communicate with his family back on Earth and he smiles, knowing that they know he's still alive. It's likely the first time he's smiled for real in months, since the laugh when he drew the smiley face in the dirt was clearly part of a mild Sanity Slippage.
- Jimmy gets cold feet at the last second and tries to stop the conspiracy theorists from bombing the JSC. Unfortunately, he fails and Karen is one of the casualties. Jimmy is clearly distraught over what he's done and does his best to comfort Karen in her last moments after he finds her in the rubble. Also, after all of the turmoil on Mars, Karen dies without knowing that both Ed and Kelly are alive and well and that Kelly gave birth to Karen's grandson.
- Ed learning of Karen's death is more heartbreaking when you remember he lost Shane whilst stationed on the moon, and now he's lost Karen whilst stationed on Mars. Ed felt immense guilt for years after Shane died because he wasn't even on Earth when it happened and blamed himself for not being there. Now it's happened again, and he will probably have to experience that feeling once more.
- The aftermath of the JSC bombing is heartrendingly realistic—the appearance of the building, the wall of photos and mementos in tribute to victims combined with flyers begging for information on those still missing.
Season 4:
"The Bear Hug"- The reveal that Bill Strausser was paralysed by the JSC bombing and is now confined to a wheelchair. When Aleida visits him for the first time in what is implied to be years, their meeting initially teeters somewhere between heartwarming and resentful, as Aleida never found the time to visit him because she was busy struggling with her own PTSD.
"Brazil"
- During Margo's half of the intercut between her and Sergei in their respective hotel rooms, the information she's pulling up on her computer is about the Brazilian space program, despite the asteroid capture mission being less than two days away and her insistence to Sergei that she needed to see the mission through before considering the idea of fleeing.
