Falling Chinese Space Station to Crash This Weekend

That's a surprisingly complete page, with lots of other interesting information. Currently:

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Reentry Information

Tiangong-1 is currently predicted to reenter the Earth’s atmosphere around April 2nd, 2018 02:00 UTC ± 7 hours.

This prediction was performed by The Aerospace Corporation on 2018 March 31.

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woaw this .png is updated in real-time http://www.aerospace.org/CORDSuploads/TiangongStoryboard.png

From 11:05 to 11:25 UTC the perigee dropped ~10km (165 to 153)

In the image on the bottom left, theres a graph that shows how much time Tiangong spends in the sun and in the shadows. Why does it seem like Tiangong spends a lot more time in sun than in the shadow when its a non polar orbit?

> Based on Tiangong-1’s inclination, however, we can confidently say that this object will reenter somewhere between 43° North and 43° South latitudes.

Whew! Glad they narrowed that down some!

Probably because it's hard to predict where it will reach an altitude after which it starts rapid reentry. This could basically happen anywhere in its orbit, since the station is uncontrolled, so there's not a whole lot of confidence in where it will end up.

It's a timescale problem. They can figure out when it might fall to within a couple of days, but, it orbits once every 90 minutes. As the descent becomes steeper the error bars become smaller than a day, but it's still orbiting about once every 90 minutes.

To estimate that they’d need very precise information about the atmospheric drag on the station. That depends on many factors including the precise geometry of the station and all it’s peripheral parts and attachments and their relationships to each other, it’s attitude, It’s rotation and angular momentum in every axis, it’s precise weight distribution, then there’s the atmospheric characteristics at every point through its trajectory across multiple latitudes, over sea and land and with varying underlying weather and temperatures. Then there is varying gravitational strength due to geological factors, and possibly even the influence of earths magnetic field on the structure. Bear in mind the vehicle is grazing through the atmosphere at an extremely shallow angle.

The possible trajectories are quite specific: https://twitter.com/Marco_Langbroek/status/97903409781211136... because it will re-enter basically over its own ground track. So although we don't know exactly where this will happen, we have a very good picture of where it won't happen (and that is most of the planet).

The alternatives occur because it is losing speed only very gradually with each orbit through the upper atmosphere. Tiangong-1 goes around the planet every 88 minutes, and one of those orbits will lose enough speed and altitude that the vehicle disintegrates due to heat or drag. It's just that due to variations in the upper atmosphere, we don't know which orbit exactly will finally tip the balance.

This is in contrast to a controlled re-entry vehicle e.g. Soyuz that washes off something like 150m/sec in a single engine burn to place it on an known and specific sub-orbital trajectory to get home.

I think the best analogy is a roulette wheel. You know about when it's going to stop, but that doesn't help much to analyze where.

You're looking at it's current orbital parameters - if you look further down you'll see how the orbit degraded from approx 350km in 2017

None really for this one. They're not controlling the descent at all as far as anyone has been able to tell, there's just been a steady degradation of the orbit since it's last reboost years ago.

As signatories of the Outer Space Treaty they should give a heads up to the international community, and are liable for any damage that's caused by this event even if they provide warning (but could probably get out of it with enough political double talk).

What's interesting to me is that China haven't officially admitted to losing control of the station, they have only admitting to lost telemetry- so we must rely on external sensors to track it. I suspect (but don't have any real information on why) that they are doing this to avoid some sort of admission of guilt in the event that it ends up hitting a populated area. I'm not entirely sure but admitting you lost control of an orbiting school bus might be seen similarly to a kid that accidentally breaks your window with a baseball- it wasn't intentional, but that kid is still paying for the window. If your window gets hit by something, but the kid never admits that it was his ball ("gee mister, I lost my ball yesterday but I'm pretty sure it didn't go in your backyard"), then maybe he can get away without paying for it.

If it falls in Australia, the Chinese will probably have to pay a fine. But they can just not pay, I think the Americans never did for Skylab.

I guarantee there will be fraudulent sellers though.

same, or a twitter profile or service that would notify everyone when it actually happens, and if it happens to be over a populated area, warn people below it in a timely manner that they might get hit with a bolt or something.

What do you believe the chances of it hitting that reactor are?

If we were to assume that falling satellites hit nuclear reactors with a frequency of once every 200 million years, how much money do you think should be spent preventing accidents of that type?

Satellites and space stations are neither aerodynamic nor dense. If anything survives reentry, drag will slow it down until it might still damage a regular building but will do nothing to an impact resistant concrete dome designed to resist plane crashes.

A big meteor could punch through both the concrete dome and the underlying steel containment. If that happens, we will probably be glad it hit the reactor and not nearby New York City. As long the meteor doesn't replace the steel containment structure with a crater but merely damages it (after obliterating the dome), we have at worst a second Fukushima. What made Chernobyl so bad was that the cooling water caused a giant steam explosion, carrying radioactive material high into the atmosphere. That failure mode is impossible in any reactor operated today.

The domes are supposed to be resistant against impact from small planes. And there's an air defence system which could break it into smaller pieces.

However, the chances of hitting it by accident are absurdly tiny. It would be pretty hard to hit from orbit deliberately; maybe the only people who could do that are SpaceX.

Why the downvotes? That's a totally legit question. Unexpected events happen all the time. Yes, the probability of this single event happening is small, but the sheer number of unexpected events that can happen around the world makes it likely that some of them will happen. I'd rather live in a place where asking these questions won't get you burned.

Objects re-entering generally hit the ground at around terminal velocity, which is only a few hundred kph usually. The reactor core is surrounded by a very sturdy reinforced concrete containment structure which should be able to survive an impact of orbital debris.

Additionally, the chance of orbital debris striking any particular location on Earth is nearly zero, so it's a bit foolish to over prepare for something that is less likely than winning the lottery several times in a row.