Here's the scenario: Spacecraft #1 is launched into a nice, circular,
low-earth orbit. Shortly thereafter, spacecraft #2 is launched into
the _same_ orbit. At this point in the story, spacecraft #2 is keeping
pace with #1, and is simply 90 km in trail.
The flight plan calls for the two craft to rendezvous. At time t,
spacecraft #2 fires its thruster, thereby giving itself a delta v
of 60 km/h, directed toward spacecraft #1 in the obvious way.
Here's the question: Within a km or so, how far apart are the
spacecraft at time t + 90 minutes?