We might have issues with bone loss in low-g environments, we might not. The point is that we don't know. These are human beings we'll be sending. Taking a chance on "it'll probably be all right" just isn't acceptable.
And that's just one single issue out of thousands that needs to be resolved before we can start having people live off-Earth. Most of it is stuff we just don't know how to do. You're trying to build a rail network with iron-age know-how. We can't even get to the moon right now, much less build a Mars outpost.
And while it's true that testing stuff on Mars would be ideal, Mars is too far away. You can't respond to emergencies. You can't quickly iterate designs for systems. You can't easily commercialize tourism. If we build bases on the moon first, we can be much better prepared to take that larger step to Mars. Sure, send a couple Apollo-style "plant a flag, take pictures, hit a golf ball, go home" missions using professional astronauts, then maybe a few missions where you have a lab on the ground and you stay for a month. But if you're sending people to stay, you need to be very sure they aren't going to die on you. Dead astronauts are heroes. Dead colonists are a tragedy, and an excuse that will be used to justify never going there again. If we can keep a permanent settlement alive on the moon, Mars will be that much easier.
Either way, there's still a lot that needs figured out first. The book outlines just a small fraction of what needs to be done. A good chunk of that will happen here on Earth. Some will happen in orbit. Some will happen on the moon. And eventually, some will happen on Mars. And it'll be fucking cool! Seriously, space stuff is fun to geek out on. Lots of interesting problems to solve. If it was easy, it'd be boring.
Hitting on the most general issue first - you could spend the next million years trying to simulate every possible contingency and issue with Mars, but at the end of the day you're still going to have many things with a high level of uncertainty, let alone the unknown unknowns. This means that going with an acceptably high risk is simply a prerequisite - period. It worked during Apollo and it will work for Mars, simply because there is no other option. So yes, "it'll probably be alright" is absolutely going to be a norm.
And the Moon is a complete hell hole that shares essentially nothing in common with Mars. You're looking at 2 week long day/night cycles that oscillate between absurd extremes of temperature of something like -300f at night to +200f during the day. And there's also no atmosphere which is why the Moon's surface looks like a teen with chronic acne. Even the smallest pebble will pound into the surface and often at quite a high level of energy. Similarly this is a big part of the reason that Moon dust is some seriously nasty stuff. Mars has a similar issue, but orders of magnitude less severe owing to the nature of where its dust came from, which is more similar to terrestrial dust sans composition.
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As for the specific issue you mentioned - bone/muscle loss is not some unsolved mystery as the book implies. Your bones/muscles strengthen in accordance to the stresses they're under. In space there are 0 stresses so they deteriorate. The normal solution to this is weights, but in 0g that obviously doesn't work so astronauts are left doing largely ineffective and awkward elastic based exercises, which they have to spend 2+ hours a day doing. None of these are issues in low g where weights do work, and the bone/muscle loss will already be far less. These sort of arguments are like saying "Ok, we know this ship floats in 20 ft deep water, but how do we know it'll float in 50 ft deep?" Technically you don't, and you won't until you try it in 50ft of water, but ultimately there's no reason to think it won't.
And that's just one single issue out of thousands that needs to be resolved before we can start having people live off-Earth. Most of it is stuff we just don't know how to do. You're trying to build a rail network with iron-age know-how. We can't even get to the moon right now, much less build a Mars outpost.
And while it's true that testing stuff on Mars would be ideal, Mars is too far away. You can't respond to emergencies. You can't quickly iterate designs for systems. You can't easily commercialize tourism. If we build bases on the moon first, we can be much better prepared to take that larger step to Mars. Sure, send a couple Apollo-style "plant a flag, take pictures, hit a golf ball, go home" missions using professional astronauts, then maybe a few missions where you have a lab on the ground and you stay for a month. But if you're sending people to stay, you need to be very sure they aren't going to die on you. Dead astronauts are heroes. Dead colonists are a tragedy, and an excuse that will be used to justify never going there again. If we can keep a permanent settlement alive on the moon, Mars will be that much easier.
Either way, there's still a lot that needs figured out first. The book outlines just a small fraction of what needs to be done. A good chunk of that will happen here on Earth. Some will happen in orbit. Some will happen on the moon. And eventually, some will happen on Mars. And it'll be fucking cool! Seriously, space stuff is fun to geek out on. Lots of interesting problems to solve. If it was easy, it'd be boring.