for human spaceflight we want a lot more than "likely" (>50%). The standard is usually "extremely likely" (~1/100 to 1/1000 chance of failure)

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GMoromisato 12 hours ago | parent | next [-]

Maybe. What was the probability of Loss of Crew during Apollo? There were 9 crewed missions and 1 almost killed its crew (I will omit Apollo 1 for now). I could argue that Apollo had a 1 in 20 chance of killing a crew. Indeed, that was one reason given for cancelling the program.

The first Shuttle launch probably had a 1 in 4 chance of killing its crew. It was the first launch of an extremely complicated system and they sent it with a crew of two. Can you imagine NASA doing that today?

In a news conference last week, a NASA program manager estimated the Loss of Mission chance for Artemis II at between 1 in 2 and 1 in 50. They said, historically, a new rocket has a 1 in 2 chance of failure, but they learned much from Artemis I, so it's probably better than that. [Of course, that's Loss of Mission instead of Loss of Crew.]

My guess is NASA and the astronauts are comfortable with a 1 in 100 chance of Loss of Crew.

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kelnos 9 hours ago | parent | next [-]

> There were 9 crewed missions and 1 almost killed its crew (I will omit Apollo 1 for now). I could argue that Apollo had a 1 in 20 chance of killing a crew.

That's not how risk analysis works.

Let's say every Apollo mission had gone flawlessly, and no one even came close to dying. Would you then say that the risk of death for future missions would be zero? No, of course not.

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rjmunro 4 hours ago | parent | next [-]

I thought I'd look this up. If you've had 9 successful attempts, assuming nothing has changed between them and no other prior knowledge about success probability, then Laplace’s Rule of Succession says the probability of the next mission being a success is about 83.3%, i.e. there is a 1 in 6 chance of failing next time.

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jonahx an hour ago | parent [-]

> and no other prior knowledge about success probability

This phrase is misleading, as Laplace's Rule of Succession is equivalent to assuming a uniform Bayesian prior over all values of p. That is, before any experiments, a 50% chance of success. Depending on the situation, this may be roughly accurate or wildly wrong. You cannot appeal to this rule to resolve the situation.

	▲	falcor84 36 minutes ago \| parent [-]
		Well, obviously if we have a better prior, then that's better. But assuming no other knowledge, and especially if we think that other people's priors could be intentionally misleading, this rule seems to offer the best estimate.

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randomNumber7 6 hours ago | parent | prev [-]

If you only look at this data it would be the most reasonable guess.

Of course one should also analyze the technical sytems involved and then it is clear that 0% failure is not reasonable.

	▲	ballooney 5 hours ago \| parent [-]
		Bayes rule has existed for nearly 300 years, there is no excuse for ‘only look[ing] at this data’ and that is NEVER a reasonable thing to do.

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Someone 10 hours ago | parent | prev [-]

> I could argue that Apollo had a 1 in 20 chance of killing a crew.

NASA computed the chance of “landing a man on the Moon and returning him safely to Earth” as less than 1 in 20. I would think a lot more than 1 in 20 of those failures would result in killing crew members.

https://ntrs.nasa.gov/api/citations/20190002249/downloads/20...:

“Appreciating and deemphasizing risk in Apollo

Joseph Shea, the Apollo program manager, chaired the initial Apollo systems architecting team. The “calculation was made by its architecting team, assuming all elements from propulsion to rendezvous and life support were done as well or better than ever before, that 30 astronauts would be lost before 3 were returned safely to the Earth. Even to do that well, launch vehicle failure rates would have to be half those ever achieved and with untried propulsion systems.”

The high risk of the moon landing was understood by the astronauts. Apollo 11's Command Module pilot Mike Collins described it as a “fragile daisy chain of events.” Collins and Neil Armstrong, the first man to step on the moon, rated their chances of survival at 50-50.

The awareness of risk let to intense focus on reducing risk. “The only possible explanation for the astonishing success – no losses in space and on time – was that every participant at every level in every area far exceeded the norm of human capabilities.”

However, this appreciation of the risk was not considered appropriate for the public. During Apollo, NASA conducted a full Probabilistic Risk Assessment (PRA) to assess the likelihood of success in “landing a man on the Moon and returning him safely to Earth.” The PRA indicated the chance of success was “less than 5 percent.” The NASA Administrator felt that if the results were made public, “the numbers could do irreparable harm.” The PRA effort was cancelled and NASA stayed away from numerical risk assessment as a result.

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irjustin 12 hours ago | parent | prev [-]

1/100 is absolutely terrible. Shuttle had 1.5% failure rate. Bonkers.

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For comparison, commercial aviation has something like 1 in 5.8m or 6x 9's of reliability.

	▲	IshKebab 9 hours ago \| parent [-]
		It's not terrible for space flight. Flying a rocket to the moon and commercial aviation are obviously very different things.