Jeff Findley writes:

Because with Hubble, the shuttle crew controlled both the rendezvous
*and* the grapple. Hubble has no reaction control system. It can only
control its orientation (which it does with great precision).

With Dragon, there is no crew on board to control the rendezvous.
Automated rendezvous of this type isn't trivial. Remember the
Progress/Mir collision? I'm sure no one wants a repeat of that accident
with ISS.

The point is that this mission is just two missions folded into one.
Previously NASA wanted SpaceX to fly one mission in which Dragon would
fly several "virtual" approaches to the ISS in safe distance only and
then in a second mission actually repeat the same with the real thing.
Now SpaceX will do the first mission first and when this works out fine,
go to the ISS and berth there within the same flight.

So: Before this Dragon comes near the ISS it will have done all of what
it will do there in a safe distance. If anything goes wrong then it will
*not* come near the ISS and reenter instead.

It's surely not the easiest thing to do, but it looks as if this has
been planned well.


Jochem

--
"A designer knows he has arrived at perfection not when there is no
longer anything to add, but when there is no longer anything to take away."
- Antoine de Saint-Exupery