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In this answer @RussellBorogove discussed the frequency of dockings of each space shuttle with the Mir space station. I've summarized those frequencies, total number of missions flown, and year of first and last flight below.

Not counting the Enterprise and the Challenger (lost early in its expected life) there are four shuttles that span the period over which Mir docking missions were carried out.

Of the 9 dockings, 7 were made by the Atlantis. I've done a rough calculation tossing nine balls into four equal bins repeatedly, and then calculated roughly how many times the maximum number of balls in a given bin would be $N_{max}$. The probability that 7 of 9 balls would end up in any one bin is less than 0.5%. So I think the high frequency of Atlantis-Mir docking is not likely to have been just a statistical fluke, although it's not impossible.

Were there any reasons why NASA would have used Atlantis for such a large fraction of all the Shuttle-Mir dockings? 

Enterprise: 0 /  0 (1977-2012, model only)
Columbia:   0 / 28 (1981-2003, lost)
Challenger: 0 / 10 (1983-1986, lost)
Discovery:  1 / 34 (1984-2011)
Atlantis:   7 / 33 (1985-2011)
Endeavour:  1 / 25 (1992-2011)


With 1,000,000 tries:
Nmax  %
---- ----
 3   37.2
 4   43.2
 5   15.6
 6    3.5
 7    0.5
 8   <0.1 

import numpy as np
from collections import Counter

n_tries = 100000 # I used a million for the question

# shamelessly inefficient way to do this
bob = np.random.randint(4, size=9*n_tries).reshape(9, -1)
qq  = [max(Counter(thing).values()) for thing in bob.T]
a,b = np.histogram(qq, bins=range(11))

for A, B in zip(a, b[:-1]):
    print B, float(A)/a.sum()
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    $\begingroup$ Note that balls in bins isn't a good model, here. It assumes that missions were assigned to shuttles independently and uniformly at random but that defintely wasn't the case. For example, in the 100+ missions flown from 1985 onwards, there were only a handful of instances where the same shuttle flew two consecutive missions. $\endgroup$
    – David Richerby
    Commented May 25, 2019 at 10:55
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    $\begingroup$ @DavidRicherby In order to head off bothersome comments like "Why do you think it is 'so frequently?'" I made a quick estimate of what "random" might look like precisely to show that the real distribution isn't random! So I think your comment misses the point. $\endgroup$
    – uhoh
    Commented May 25, 2019 at 12:19
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    $\begingroup$ But all you showed is that the distribution doesn't match a model that is already completely implausible. You don't need statistics to refute somebody who suggests "Maybe they just picked a shuttle uniformly at random, independently of any previous flights." And your refutation says nothing about a much more plausible "it's random" hypothesis such as "Maybe Mir missions just happened to be performed at times when Atlantis was the next shuttle to fly." Anyway, I don't to get into a big argument about this -- it doesn't affect the underlying question (which is a good one) or the answer (ditto). $\endgroup$
    – David Richerby
    Commented May 25, 2019 at 12:58
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    $\begingroup$ @DavidRicherby I think you are reading something into the question that's not there. $\endgroup$
    – uhoh
    Commented May 25, 2019 at 14:29

1 Answer 1

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Looking at old manifest documents, it seems pretty clear that Atlantis (OV-104) was outfitted with all the hardware for Shuttle-Mir missions and they used her exclusively for that until they couldn't any more.

The rules for orbiter mission scheduling were really byzantine but one major consideration was that every X flights and/or time interval, the orbiter had to go back to Palmdale (later KSC) for an Orbiter Maintenance Down Period (OMDP). From the attached charts you can see that Atlantis (OV-104) came out of a Palmdale OMDP in Oct 1994 and flew STS-66. Then was down for a long maintenance period, presumably being outfitted with the docking system and external airlock, although this is not explicit.

This orbiter then flew Shuttle-Mir missions and nothing else for the next 2+ years. Then it had to go back to Palmdale in Nov 1997. The final 2 Shuttle-Mir flights were flown by OV-105 and OV-103, presumably setting OV-105 up for the first ISS mission and OV-103 for the 2nd. (Maybe there were 2 docking systems by then?)

Shuttle-Mir missions are shown on the schedule as SMM-XX

enter image description here enter image description here

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    $\begingroup$ +(n+1) for going through all of the history and deducing cause and effect. Instead of explaining why so many of them were done with Atlantis, you've now explained instead why two of them were not with Atlantis, and why those two were split between two other shuttles. These schedule diagrams are fascinating - how much information is squeezed into each plot. Very nice, thank you! $\endgroup$
    – uhoh
    Commented Mar 12, 2017 at 15:02
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    $\begingroup$ +1 for the detailed explanation. Where did you get those diagrams? $\endgroup$
    – Vikki
    Commented Mar 29, 2018 at 21:05

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