> I find it extremely unlikely that the risk... was not discussed with McAuliffe....

It is far worse than you imagined, then. The extended except below is from On the Very Idea of Risk Management: Lessons from the Space Shuttle Challenger By Robert Elliott Allinson, published 12 September 2012 [1]

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Was Mrs. Christa McAuliffe made aware of the risk that she was taking? Grace Corrigan wrote:

With respect to the Challenger launch, ‘ … Christa felt no anxiety about the flight. ‘I don’t see it as a dangerous thing to do.’ She said, pausing for a moment. ‘Well, I suppose it is, with all those rockets and fuel tanks. But if I saw it as a big risk, I’d feel differently.’

Grace George Corrigan, A Journal for Christa, Christa McAuliffe, Teacher in Space, Lincoln and London: University of Nebraska Press, 1993, pp. 115, 118.

It was not only the case that Christa felt no pre-flight fears. She was never even informed that there was any real problem about which she should have been informed. Corrigan relates Christa McAuliffe’s account of what the President and the pilot from a previous launch told her in the White House:

‘They were told about the dangers of the space program. She said that one could be intimidated thinking of all that he had said until you realize that NASA employed the most sophisticated safety features, and they would never take any chances with their equipment, much less an astronaut’s life.’

When interviewed by Space News Roundup, she said that, ‘When the Challenger had the problem back in the summer with the heat sensors on the engine … and … one of Boston’s papers called me and asked me what I thought was wrong, … I said, ‘I have no idea. What has NASA said?’

Op. cit., p. 185.

It is obvious that Christa McAuliffe was not informed of any O-ring faults and the consequent life and death risk she was engaged and committed to taking by her participation. Is this a case where the statement from the Committee on Shuttle Criticality, Review and Analysis, that ‘The risks of space flight must be accepted by those who are asked to participate in each flight’ is even relevant when no one has been informed about the specific risks to which this particular flight will be prone? The fallacy of conflating the general, unknown risks of space flight with the specifically known risks of this flight makes “risk management” here into an unethical practice. There is, strictly speaking, no risk management that is being practiced. There is simply a wanton risk taking with human life.

The Nobel laureate physicist, Richard Feynman was shocked to learn that NASA management had claimed that the risk factor of a launch crash was 1 in 100,000 which they had arrived at through a subjective engineering judgment without relying upon any actual past performance data. If one calculated risk based upon actual past performance data, the risk was, according to Professor Feynman, 1 in 100.

Ibid., p. 183.

While management, in defending its decision to launch, pointed to the risk involved as being 1 in 100,000, there was no examination of how these figures were generated. If one took the actual performance data of rocket engines in the past, as the Nobel laureate physicist Roger Feynman did, the risk was far greater. When one does this, one can more clearly consider the case of the possibility of incidence versus the actuality of consequence. Does one wish to risk the lives of the astronauts and the civilians when the chance of their death is 1 in 100?
From the above example, one can generate the conclusion that whenever possible, when calculating risk, one should not calculate risk in any other way than from the conclusions generated from actual, real-life, past performance data. The lesson to be learned is that in risk assessment, past performance data, when available, must always be consulted. One should avoid guess work. Unless the risk estimates are based on past performance data as a data base, according to Professor Feynman, ‘it’s all tomfoolery’.

Ibid., p. 183.

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McAuliffe was misled with a false statistic off by 3 orders of magnitude, and was therefore coerced and denied the ability to give informed consent. And this highlights the problem, because it is not as effortless to do the same with a trained astronaut, physicist or military commander with decades of service. One should expect a bias towards that which one wishes were true. Who wouldn't want to visit space if it were completely safe? But it is absurd to ask this question, because it will never be even remotely safe, thus space travel requires only professionals, and must exclude lay persons (who are incapable of informed consent by definition) for it to be ethical.

[1] https://www.intechopen.com/chapters/38971