I don't know - they've been racing ahead with the installation of plumbing, buffer tanks, etc... they might be aiming to get it done in the the next month.

It's not like they ever work on only one thing at a time, and those panels are going to have to be able to come off reasonably easily anyway for maintenance and upgrades of the protected components.

I think a big issue was simply that using an NTP on Earth is a really, really bad idea - I seem to recall hearing of an idea from that era of flying one over the USSR to essentially carpet-dirty-bomb them into submission.

And off Earth... we haven't had any national interest in doing that. The moon race was good cold-war propaganda and ritualized combat... bringing nukes into it was exactly what we were trying to avoid. Going to other planet's though? Where's the profit, power, or security in that? Especially after determining that the moon was just a big dead rock we were nowhere close to being ready to usefully colonize, and photos from the 1965 Mariner mission established that Mars was probably the same.

Yeah, the long-term dreams are inspirational - but it's likely to be generations before anything except mostly-automated asteroid mining (which was completely out of reach at the time) will be able to turn a profit, and colonialism is all about getting rich. Not having your great-grandkids get rich, assuming they can even maintain a position to cash out on your investment rather than someone else's grandkids being the ones to do so.

No reason to launch anything significantly radioactive. It's the waste you have to worry about, the fission fuel itself is (comparatively) safe. It has to be, if it were seriously radioactive it wouldn't still exist after almost 5 billion years in the ground.

Still not exactly *safe*, but so long as you stay well away from critical mass the heavy metal poisoning will probably do more damage than the radiation.

And I think I recall hearing that NASA is moving strongly away from using legacy pre-physics units for anything, in large part because it opens the door to stupid conversion errors like that, and someone inevitably walks through.

So long as they're well within each other's Hill Spheres I don't think the sun should be an issue any more than it is with our moon. (The moon's orbit is about 1/4 of the way to Earth's Sphere) And Earth already orbits our combined center of mass with the moon, it's just that the size discrepancy (it's only ~1% of our mass) means that's still within Earth's volume, about 3/4 of the ways up from the core.

Forming would be a different question, but e.g. if Theia had been considerably bigger (or faster?) when it hit proto-Earth the "splash cloud" might have coalesced into two much more similarly-sized sister-planets.

Exactly the same size (to how many significant digits?) would indeed take crazy long odds. But within 10% or 20% is probably not too outlandish.

Why closer? Gravity is a measure of acceleration, not force - replace the moon with an Earth-sized planet and it would accelerate towards us at exactly the same speed.

Of course, we'd accelerate towards it about 50x faster than towards the moon, which would rapidly destabilize things, but as long as we gave Earth a good strong sideways kick at the same time so they didn't collide on the first pass, the two should orbit their mutual center of mass, just as we currently do with the moon - even if that center is currently 1700km below Earth's surface. And over time tidal energy transfer would circularize the orbits, and tidally lock us to each other. Assuming we didn't give just the right kick to start with.

Is there a minimum stable distance for tidally locked binary planets?

They can't escape without one getting a huge outward kick of energy, and they can't collide without a huge inward kick. Tidal energy transfer is no longer happening, and I think you have to be pushing spiraling neutron star densities and speeds before you can shed much energy through gravitational waves. And so long as the sun is many orders of magnitude more massive, you're not going to get any pesky chaotically complex three-body problems cropping up. Just don't try to add a moon.

I suppose if they cross each other's Roche limit, where tidal stress will tear moons apart into rings, things might get complicated... but a quick search says that's only ~11,000km for Earth - a twin planet would actually be touching the surface before its center of mass crossed the line, but I think even such a "dumbbell planet" would still be stable so long as it wasn't spinning fast enough to throw stuff off the surface at the outer tips (you *know* that's where the spaceports would be!)

Keep in mind the moon is only ~1% the mass of the Earth, it's not doing much.

I would imagine the time to tidal locking decreases at *least* linearly with increasing gravity from the "lock-er" (e.g. twice the force = half the time, maybe less. That's normal for most systems), in which case if the moon were Earth-mass, 100x larger, Earth would lock to it 100x faster, and the 50 billion years until tidal locking (~55 with time served) would be closer to 550 million - almost before liquid water appeared on our surface.

12 hours? Definitely. That's only about half a degree away in our orbit around the sun. The impact wouldn't be long in coming.

6 months though? Then it would be exactly on the opposite side of the sun, in our L3 point.

It was actually once speculated that there might be a "Mirror Earth" there, but it wouldn't be a particularly stable location, and our probes have long since confirmed there's nothing significant there.

If density is constant then surface gravity increases linearly with radius: an r^(3) mass divided by gravity's r^(2) falloff.

So 10x the gravity means 10x the diameter and 1000x the mass. Crazy!

Density skews things, but even ignoring it is good enough for a sanity check. (I'm pretty sure 2x the density = 2x the gravity at the same size)

After all? Since when was it believed that Mars once had an oxygen-rich atmosphere?

This "refutation" is the first time I've heard it seriously suggested. There's a reason an oxygen-rich atmosphere is considered a bio-signature: it's so volatile that life continuously producing more free oxygen for a very long time is one of the only explanations for how it could exist in the first place. And last I heard the idea that Mars ever had life, even without photosynthesis to produce oxygen, is still very much under discussion, with minimal evidence in either direction (an "No" being the default answer)

Define complex life. The Earth was teeming with multicellular plants and animals long before the Great Oxygenation Event. The Cambrian Explosion didn't happen until much later, but there's not really any evidence that there was any sort of causal connection between the two. Not with hundreds of thousands of years of near-steady ocean oxygen levels between the two.

That's why Earth evolved life - to keep its spices properly separated. Not every planet is that clever.

On Earth it took something like a billion years after photosynthesis evolved before there was any more than trace amounts of oxygen in the atmosphere - oxidation stripped it out of the atmosphere just as fast as it was produced, until there was nothing left to oxidize.

Meanwhile, as others have said, oxygen is one of the most abundant elements in the universe, and makes up a huge percentage of the mass of all rocky planets (ballpark of 40% based on available samples) , it's very possible that Mars was well and truly oxidized just as soon as the surface cooled down enough for stable oxides to exist. Heck, the proto-planetary cloud that eventually became Mars was almost certainly already incredibly rich with oxides long before the planet formed - but the heat of formation would have driven off the oxygen from all but the most stable oxides until temperatures cooled again.