So, continuing from Back to the Moon, Steven was blown out into space. He awoke in a bubble, spinning on a trajectory that took him farther and farther away from Earth. He eventually smashes into Eyeball and the two have some discourse, with Eyeball talking about Rose and how everything the Crystal Gems do is trickery. Steven is trying to convince her that he is Rose.
Their trajectory takes them through an asteroid field and they bounce around, with a rock cracking Eyeball right in the gem, which cracked. She's knocked out, and Steven brings her into his bubble, where he heals her cracked gem and she finally realizes that yes, Steven is (partly) Rose Quartz.
So, Eyeball snaps. Her mission, as she's the oldest Ruby of the Ruby Squad, was to track down Rose and shatter her for shattering Pink Diamond. As they struggle, Steven is trying to convince Eyeball that he wasn't the one who did it, that he isn't like his mother, which confuses and enrages Eyeball even more. Finally, Steven releases his bubble, Eyeball flies off into space, and Steven encapsulates himself, alone.
As he drifts, his bubble shrinks and he curls up, knowing that he will die out there...
The Roaming Eye finds him, pulls him in, and he awakes in the arms of his three moms. On their way back to Earth, Steven asks for confirmation, disbelieving that his mother, Rose Quartz, could ever shatter a Diamond. How could his loving mother, whom Pearl had gushed endlessly over, how could that big soft belly Gem kill another Gem? Perhaps it was an accident...
Garnet, however, confirmed that it was no accident. That Rose shattered Pink Diamond on purpose. That it was necessary to protect the Earth.
The trip to Earth continues in silence.
And now... for a fridge logic tangent!
For Steven to be blown off of the Moon and into space, he would need to achieve escape velocity. Lunar escape velocity is 2.4 km/s (1.5 m/s). In order for Steven to be thrown into space, he would need to be traveling 5,400 mph!
The force necessary to send him flying into space, which was mostly a sudden drop in atmospheric pressure from the airlock, and a whack out the door (which wouldn't be enough to reach escape velocity), would be so great as to cause Steven to explode.
At most, Steven would bounce away from the base for several miles until gravity took over.