I get how it works, how you can do it, but it's still kind of incredible that they actually have 3 times in a row.
Edit: it was 8000km/h at MECO, not 3000km/h.
As for keeping it clean, apparently they added a system after they realized its tendency to fog/ice up. I'd love the details on it too. The technique of using a rolling bit of plastic might work but given the weight penalty you probably wouldn't get more than a couple of lens "changes" in the tank.
Some ballpark math: The 1st stage was travelling at 8,300 kph at MECO, and almost of that velocity would have been horizontal. There was no boost-back burn, so it would have kept that velocity until re-entry. Furthermore, it would be re-entering with some vertical velocity. They mentioned that apogee -- when the vertical velocity is zero -- was two minutes prior to the start of the re-entry burn. 2 minutes of freefall would add another 4,200 kph of vertical velocity. So that's 12,500 kph, which is less than 1/2 of a typical orbital re-entry of 26,000 kph. Because the energy grows with the velocity squared, it's more like a quarter of the heat of a typical orbital re-entry.
Finally, as I understand it, much of the point of the re-entry burn is to deflect the re-entry shockwave, effectively creating a shield of "cooler" rocket exhaust around the vehicle.
Still not anything you'd want to take a leisurely stroll in, but nowhere near as stressful as a proper orbital re-entry.
Also your calculation is not correct: You add another vertical 4,200 kph of free fall starting from apogee, but didn't factor in that of course the vehicle also slows down as much in the 2 minutes before reaching apogee. Since MECO is at about 65 km height at 8,300 kph, it will also have 8,300 kph at 65 km height when falling down again (minus a little atmospheric drag).
You also can't just add 8,300 kph horizontal (and it's not actually all horizontal, look how quickly the altitude is rising around MECO) and 4,200 kph vertical speed. Those are not scalar values but vectors, so you need to apply pythagoras for vector addition. 8,3 km/s horizontal plus 4,2 km/s vertical speed is just 9,3 km/s total speed.
and thats not including the fact that someone else pointed out that it would go with velocity cubed.
No, the drag force grows with the square of velocity. https://en.wikipedia.org/wiki/Drag_(physics)#Drag_at_high_ve...
Force * distance / time = power, so the heating will scale with the velocity cubed. Therefore at 1/2 the speed, it would experience 1/8 the heating.
Beyond that, the glass in front of that camera's probably formulated to take extreme heat.
And of course, as jdblair points out, it's starting at a slower speed to begin with; faster than just about any aircraft (except maybe the X-15), but well short of orbital velocity.
Re-entry from orbit wouldn't just destroy the camera, it would burn up much of the craft as there is no heat shield.
I also keep thinking about the book childhood's end, where aliens land and basically force us to run our things properly, cleaning up the planet's problems in a matter of a few years. I always wonder if we got put to the challenge as a species, whether we could meet it.
That said we do have good treatments for the vast majority of forms of cancer, the problem is that your survivability is directly tied to how early it's detected, surgical removal, radiation and more notably modern chemo regiments are very effective in early stages of detection.
Many type of cancer today have 90%+ survivability rates if they are detected early enough, detection isn't always easy allot of them can not turn symptomatic till later stages, and many of them won't show up in blood work / body scans clearly either.
Let's not assume that nobody else knows what they are doing.
They know what they are doing all right. For instance, ITER's managerial staff surely enjoys the jet-setting between Barcelona and the French Riviera. But their goals are not aligned with making progress.
We detached this subthread from https://news.ycombinator.com/item?id=11791424 and marked it off-topic.
I am mentally handicapped though, so you got that right.
The moral is: don't assume other's backgrounds, and don't over-assume your own. We all come at this stuff from different directions and a pointer can help some of the audience, even if its redundant for others.
Glibness aside, it's a private enterprise and is profitable, so even without mentioning Musk's ultimate goal of colonizing Mars, its existence needs no further justification. It makes money and its shareholders are happy.
And before someone jumps in with the "SpaceX-is-propped-up-by-the-government" canard, the U.S. government also saves money when it launches military satellites with SpaceX. According to  (among many other sources), SpaceX quotes around $60m, compared to the United Launch Alliance price of about $125m (or even, depending on how you count, $200m).
People always make these super slippery arguments about how space research leads to earth inventions and humans need frontiers to stay mentally healthy, all kinds of strange stories to make space exploration seem like a humanitarian pursuit. I think it's just leftover zeal from the U.S. government story that space exploration was important for the country...
... which it was. We needed to stay up on our rocketry game for national defense reasons.
I feel it just muddies the true rationale for space when you try to make it about something it's not.
I look forward to the breathless comments on "SpaceX successfully lands rocket for 38th time!"
Which is and will continue to be often.