> Based on Tiangong-1’s inclination, however, we can confidently say that this object will reenter somewhere between 43° North and 43° South latitudes.
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?
Imagine an orbit that went around the Earth exactly parallel to the direction of the Sun, it could be in sunlight all the time. If you tilt such an orbit a little then part of it will fall into the shadow of the Earth, but perhaps only just a small part. There are other factors at play as well, such as the geometry of the Earth's shadow against an orbit and refraction of sunsets through the Earth's atmosphere.
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.
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.
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.
"Space weather" is actually a thing. It doesn't really have anything to do with clouds, though. Most of the influence on the extreme upper atmosphere comes from solar phenomena, and their interactions with the earth's magnetic field.
Solar flares and other similar events happen all the time. They're not really noticeable at ground level, because they don't appreciably affect the sun's visible brightness, and the density of matter involved is tiny compared to the atmosphere at ground level. But at the heights where satellites orbit -- where the atmosphere is already extremely tenuous -- solar and geomagnetic phenomena can have a huge relative effect.
But we're not talking about space. We're talking about the earth's atmosphere.
The proportion of reflected light is called the "albedo", which ranges from a theoretical maximum of 1 (a perfect mirror, or a theoretical white surface with no absorption) to 0 (a theoretical "black body").
White clouds reflect a lot of the incoming sunlight (have a larger albedo). Light that strikes the ground from a clear sky causes more ground heating, which causes more air heating, which causes the atmosphere to expand.
ATW, the Earth's albedo can range from about 0.9 (fresh snow on the ground) to about 0.06 (open ocean). White clouds can approach about 0.8.
Edit: the height of the tropopause (the boundary between the troposphere and the stratosphere) varies from about 9 km (over the poles) to 17 km (over the equator) due largely to the greater temperature in equatorial regions. So that layer of the atmosphere, at least, is nearly twice as thick in warm regions than it is in cold regions. See:
The Sky Guide iOS app (and likely others -- it's just the one I have) shows the path if you search for Tiangong-1. From my location here in MN, the next time it is will be above the horizon is Monday morning around 4:50am. It leaves only a very slim chance of seeing anything from here, but I'll have a camera running on my roof just in case.
Still, I wonder if the data in the app is updated regularly enough to account for the recent changes, or if it's just showing the original orbit?
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.
Right, but what they've said is "we've lost TM and it's going to deorbit sometime in the future. We'll watch it and keep you posted." Which is all well and good until you realize what they haven't said, and they've had more than a year to say it:
What they haven't said is "the reason it's going to deorbit at an unspecified time instead of an exact time is because we've lost control so we can't do a controlled reentry burn to have it splash harmlessly in the Pacific. So if it puts a few redhot hunks of ceramic through an apartment building there's nothing we can do. Sorry."
Everyone knows they've lost control, and it's not the first time a country has lost control of a satellite. But maybe they get some legal benefit out of not admitting the loss of control. Because otherwise they should confess, it's not like this is the first time a country has lost control of a satellite.
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.
China, in typical China fashion, denied they lost control of the station right up to the end.
"Western space experts say they believe China has lost control of the station. China's chief space laboratory designer Zhu Zongpeng has denied Tiangong was out of control, but hasn't provided specifics on what, if anything, China is doing to guide the craft's re-entry."
what if that is their design and everything is going well with the plan? so well it didn't require a noticeable correction since the orbit move a year ago...
That's probably impossible to do. Slight variations in atmospheric densities even in the last days/weeks leading up to the final reentry can change the final reentry time by 1/6th or more of an orbit. Wired did an article about a small simplistic model check it out. So managing a 'controlled' reentry from 2+ years out is just too complex to be done now, maybe ever it depends on a lot of different variables.
For further example: as of right now ESA still gives a 4 hour window for the reentry which is a really wide swath of the Earth that it could fall onto.
could be fun if someone picks up some debris and put it on ebay for auction.
given that the international treaty explicitly states that the nation which launched such stuff should be responsible for its possible reentry damages, is there any international laws or treaties stating that the debris still legally owned by the nation that launched it?
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.
I'm just curious how is that information useful at all. A bolt falling from space will penetrate any house, so I guess you'd need an underground bunker to protect yourself - but then the chances are so incredibly low, is it worth bothering with?
For me, it's not so much get out of the way, as it is to look up and observe it directly. On its penultimate orbit it should be observable all along its orbit and when it does de-orbit there is probably a 2500 mile wide swatch where you will get to see some portion of its disintegration. These were pictures of Mir : http://news.bbc.co.uk/2/hi/asia-pacific/1238009.stm for example.
Honest question; there is a nuclear reactor about 10 miles from new york city; how is it protected against such sattelite smashing at the top of the dome? Wouldnt that be Chernobyl times 1,000 ? And does new york city have a solid plan on evacuating 8 million people off the island in some reasonable amount of time?
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.
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?
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.
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.
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.
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