The people on Earth was planning to push the ISS into deep space already, so why didn't Miranda and Jordan leave Calvin on the station when they got into their pods? They both could have gotten home safely and Calvin would still have gone to deep space....
shareI think Calvin had the ability to navigate the ISS back to earth.
shareThey planned to, but Calvin followed Jordan into his pod and there didn't seem to be any way to get it out again. Jordan closed his hatch because he wasn't planning on heading to Earth, and one way or another he wanted to be sure Calvin didn't either.
Except ... the pods were programmed to return to Earth, and land safely, unless manually overridden, which is what Jordan intended to do. Much is made of Calvin inexplicably being smart enough to guide the pod to land, but he really didn't need to. Having just watched it again, I'm pretty sure Calvin wasn't that smart -- it worked on prey instinct -- and probably had no idea where the pod was going. It simply attacked Jordan and infested him, as the next stage of its own life-cycle. Because that stopped Jordan being able to steer the pod away from the planet manually, the thing landed, carrying Calvin with it.
Miranda's pod, however, suffered a "systems failure" after its collision, and its guidance systems were screwed up. It flew away erratically, rather than land as it was intended to.
The ISS was put into a decaying orbit when they were firing the thrusters to stop Calvin getting back inside and the shuttle that was launched to put it into deep space was wrecked before it could finish pushing it out far enough so it was still heading for Earth that's why they couldn't leave Calvin on it.
Jordan deliberately led Calvin into his pod using the oxygen sticks so he could fly it into deep space himself.