doctor who - The Cracks in the Universe: Why are they lethal to some, but not to others?

During the Doctor Who Season 5 story arc, the cracks in the universe absorb a number of creatures, erasing them completely from time:

However, there are 5 characters/species that are able to be close to the cracks, in some cases for long periods of time, without getting erased from time:

• Amy Pond, who had a crack in her room for years but never got absorbed by it, even though her parents did.



• The Doctor, who reached into a crack to pull out a piece of the exploding TARDIS in "Cold Blood".


• Prisoner Zero, who was able to escape prison by fleeing into Amy's house through the crack in her wall.


• An Atraxi, who was peering at the Doctor and Amy through the crack in her wall in "The Eleventh Hour".


• The Saturnynians, who fled their damaged world for Earth through a crack in "The Vampires of Venice".

Are there any potential in-universe explanations for why the cracks in the universe are not lethal to these few characters?

Answer

The answer from abcooper makes a very good point that didn't dawn on me: that the cracks can separately exhibit spatial or temporal properties. It then reminded me of something Angel Bob said in the episode "Flesh and Stone":

There is a rupture in time. The Angels calculate that if you throw yourself into it, it will close and they will be saved.

This led me to a potential in-universe explanation:

• When a crack first appears, it connects that point in time and space directly to the time explosion that is destroying the universe. It behaves more like a temporal rift, absorbing objects around it and erasing them from time.


• Upon absorbing enough, the crack is disconnected from the time explosion, connecting to a different point in space (and possibly time). It behaves more like a spatial rift, allowing individuals to pass through unharmed.

The initial appearance of a crack as a temporal rift which then transitions to a spatial rift helps explain some of the apparent discrepancies:

• Being an exceedingly complicated space/time event (and a Lord of Time) makes the Doctor able to get near an "active" temporal rift for at least a short period of time without being absorbed by it, allowing him to reach into one in "Cold Blood", suffering some pain in the process. However, he is absorbed and erased from time when he ultimately plunges himself directly into the time explosion in "The Big Bang".


• When the crack first appeared in Amy's home, it absorbed her parents (and maybe other things) then went "dormant", transitioning to a spatial rift. This allowed Amy to remain near it without being erased from time, although small amounts of residual time energy may have been leaking through that affected her. Later, Prisoner Zero was able to escape through the crack, and the Doctor was able to force it open to communicate with the Atraxi on the other side.



• The crack that appears in the Byzantium seems much more powerful, requiring that it absorb more "complicated space/time events" to close it. After absorbing some soldiers and all of the Weeping Angels it transitions to a dormant state.


• 
  

  In "The Vampires of Venice", the Saturnynian Rosanna Calvierri says:

  
  

  
  

  There were cracks. Some were tiny... some were as big as the sky. Through some we saw worlds and people and through others we saw silence... and the end of all things. We fled to an ocean like ours and the crack snapped shut behind us... and Saturnyne was lost.

  
  

  
  

  The cracks that appeared presumably absorbed and destroyed much of the planet, transitioning to spatial rifts which allowed a small group to flee through them to Earth.