[cesarb]

> This might be a myth but when I was a kid I assumed (or was told?) that there was always another space shuttle ready to go if there was a problem with the one in flight.

I believe it was a myth. IIRC, one of the unusual things about the final Hubble servicing mission was said to be the sight of two space shuttles ready to go at the same time (one for the mission and a second one to be used as a rescue ship if necessary), implying that this wasn't done often (if at all) previously.

[rrmm]

Post Columbia accident, most of the shuttle missions were focused on the ISS where the crew could have taken refuge had there been a problem during ascent which prevented safe re-entry.[1]

As you note the Hubble mission (STS-125 with Atlantis) wouldn't have that option available due to its orbit, so NASA had the shuttle Endeavour ready to fly as STS-400 as a contingency.

[1] The plan would have been that crew stranded on the ISS would be picked up by a later shuttle, so NASA was obviously comfortable with the contingency of leaving a crew without transport off for a period of time during an emergency. These were designated STS-3xx missions and would have launched within 40 days of call up. The ISS could have supported the additional crew for 80 days.