In the Chinese Space station, she decides to vent the oxygen, and choose to die at a time of her own choosing. Then, Buzz Lightyear George Clooney reappears, opens the hatch, let himself in, and gives her a peptalk. He then disappears.
It is clear it is a hallucination, caused by oxygen deprivation. Then she comes back to earth through the soyuz-style chinese pod re-entry, lands in a lake, and swims to shore.
some people think instead, everything after her hallucination is her journey to the afterlife. She dies, goes through purgatory (ie fire), and then is reborn in heaven (with the landing in the lake being indicative of the River Styx). There is no contact with anyone else ie Houston, other astronauts, anyone - after this point in the film, so there is no evidence to support whether or not she is actually on Earth, or if it figurative.
The lake she lands in is Lake Powell in Arizona, which is where the spacecraft in Planet Of The apes lands. You might consider this a clue. Is it a rebirth or a different, imagined Earth? (Like the bit in Solaris (Clooney) where he talks about being back on Earth but "remembering it wrong")
You can interpret it anyway you like. Not saying I agree with it though...
As for it being implausible -
i) The mission number is given as STS-157 (actual shuttle missions finished at STS-132)
ii) The space Shuttle is called the Explorer (not a real shuttle name)
iii) One of the space stations seen isn't scheduled to be finished until 2022.
iv)Differing orbits of space stations and space debris mean no, it couldn't happen as it is presented in the film.
v)The JetPac that Clooney uses is no longer in use and doesn't have the same fuel capacity as shown in the film.
vi)Debris would have been travelling at 6miles per second. You would never even see it, it would just rip through you in a nanosecond.